CHAPTER 8

Boreholes
A BOREHOLE DRILLING

1 Drilling for water 266 5.3.3 Removal of a drill pipe 291

1.1 Exploration 266 5.3.4 Common problems 292
1.2 Exploitation 266 5.3.5 Analysis of cuttings
1.3 Examples of borehole costs 267 and signs of water 293
2 Drilling techniques 267 5.4 DTH percussion drilling 293
2.1 Rotary drilling 267 5.4.1 Adjustment and lubrication
2.2 Down-the-hole hammer drilling 268 of the DTH hammer 293
2.3 Borehole parameters 269 5.4.2 Installation of the hammer 294
2.3.1 Rotation, pressure and lifting force 270 5.4.3 Drilling process 294
2.3.2 Drilling fluids 271 5.4.3.1 Starting the hole 295
2.3.3 Rotary drilling mud 271 5.4.3.2 Advance 295
2.3.4 Air in DTH drilling 272 5.4.3.3 Addition and removal
2.3.5 Drilling guidelines 273 of drill pipes 295
3 Lightweight drilling rigs 274 5.4.4 Difficulties and possible solutions 296
3.1 ACF-PAT 201 kit 274 5.4.5 Analysis of cuttings, signs of water
3.2 ACF-PAT 301 kit 275 and flow calculation 296
3.2.1 Technical characteristics 276 6 Borehole equipping 297
3.2.2 Working principle 277 6.1 Permanent casing 297
3.3 ACF-PAT 401 PTO kit 279 6.1.1 Choice of casing and screen 297
3.4 Other lightweight drilling rigs 279 6.1.2 Casing fitting 298
4 Borehole design 282 6.2 Gravel pack and grouting 300
4.1 Choice of casing 282 6.2.1 Gravel pack 300
4.2 Pre-casing 284 6.2.2 Grouting 301
4.3 Usual configurations 284 6.2.2.1 Preparation of the grout 301
5 Borehole drilling 286 6.2.2.2 Placing the grout 301
5.1 Choice of technique 286 7 Development 301
5.2 Site preparation 286 7.1 Borehole cleaning 302
5.2.1 Installation 286 7.2 Development processes 302
5.2.2 Mud pits 287 7.2.1 Air-lift development 302
5.2.3 Preparation of drilling mud 289 7.2.2 Other development techniques 304
5.2.4 Removal of cuttings 7.2.3 Pumping 305
in the DTH technique 290 7.3 Instantaneous flow 305
5.3 Rotary drilling 290 8 Monitoring and borehole report 305
5.3.1 Starting 290 9 Surface works 306
5.3.2 Advance: adding a drill pipe 290

This chapter is a practical guide to standard drilling techniques and implementing drilling pro-
grammes in places with a hydrogeological potential that are accessible with light drilling machinery.
The performance of this type of machinery makes it very versatile, and suitable for difficult contexts
of humanitarian operations. Three machines have been developed by ACF in collaboration with a
Thai manufacturer (PAT, see Annex 11A):

8A. Boreholes. Borehole drilling 265

 – ACF-PAT 201 is a light and fairly inexpensive machine, but limited to unconsolidated sedi-
mentary formations;
– ACF-PAT 301 is used to drill boreholes in consolidated or unconsolidated formations;
– ACF-PAT 401 is a more powerful machine than the ACF-PAT 301 with simpler implemen-
tation logistics.
Over the past few years, PAT have continued to improve these models on their own.

1 Drilling for water

In a difficult hydrogeological context (for example, with little or no alluvial aquifer, or in the
1.1 Exploration

presence of multi-layered aquifers with some saline water levels), it is advisable to drill prospection
boreholes. These indicate the presence and quality of groundwater and the nature of the aquifer, and
allow calibration of the readings taken during geophysical exploration. Generally such boreholes are
narrow, with small-diameter casing (43 to 100 mm). After the prospection phase they are either main-
tained as piezometers, or blocked up and abandoned. Simple pumping tests verify the presence of water.

The work carried out enables an underground aquifer to be reached and exploited, even if it is
1.2 Exploitation

situated at great depth (more than a hundred metres). In ACF’s programmes, most boreholes are
equipped with handpumps to provide drinking water to rural and/or displaced populations.
In some countries, national regulations impose boreholes rather than wells (for the preserva-
tion of groundwater quality). In addition, boreholes are particularly appropriate in the following cases:
– pollution of shallow aquifers (poor bacteriological or physicochemical water quality);
– digging wells is too long or costly to meet the needs of the populations (displaced people’s
camps);
– geological context which does not allow well-digging, due to formations that are too hard or
too deep;
– impossible to maintain an emergency water treatment station (not taken on by the community);
– boreholes which allow rapid response to urgent needs.
However, a certain number of technical, financial and logistical factors must be taken into
account before beginning a borehole programme, in order to ensure its feasibility:
– the hydrogeological potential of the zone must be assessed by a preliminary study to deter-
mine the type of drilling rig required, the foreseeable flows and the chances of success. These
may be low, so this must be provided for in the action plan;
– the choice of pump to be installed (manual or electric submersible), depending on the hydro-
geological potential and the required flow;
– the possibility of finding a drilling rig in working order locally, or the need to import it (air,
sea and/or land transport);
– local technical skills (driller, mechanic, geologist). It is possible to train a driller in drilling
techniques, but can it take some time at the beginning of the programme. Normally the use of
an ACF-PAT 201 (see Section 3) does not present problems;
– time required for import and starting (1 month minimum);
– local means of transport from site to site.

266 III. Water supply

 Since 1991, ACF has drilled more than 4 000 exploitation boreholes equipped with hand-
1.3 Examples of borehole costs

pumps in Asia (Cambodia, Myanmar) and Africa (Liberia, Sierra Leone, Ivory Coast, Guinea, Sudan,
Sudan, Uganda, Mozambique, Angola, Ethiopia, Honduras, Guatemala, Chad), using ACF-PAT 201,
301, 301T and 401 rigs.
In Cambodia, the cost of a 30-m borehole equipped with a handpump (suction pump type
VN6) is USD 300 (equipment only).
In Guinea, the cost of an equipped borehole with an average depth of 40 m, using a Kardia
handpump (USD 2 500), amounts to USD 4 000 (equipment).
A programme of 30 boreholes at an average depth of 40 m with an 80% success rate, taking into
account the depreciation of one ACF-PAT 301 rig, corresponds to a cost of USD 7 000 per borehole.
As a comparison, a borehole drilled by a contractor, without a pump, can be costed as follows:
– in Haïti, 35 m deep, 8” diameter: USD 8 500;
– in Mali, 120 m deep, 6” diameter: USD 12 000;
– in Angola, a programme of at least 10 boreholes at a depth of 60 m: USD 8 000 with cable-
tool drilling rig, and USD 13 000 with a rotary drilling rig;
– in Southern Sudan / Uganda, at a depth of 50 m and with a 6” diameter: USD 12 000-15 000.

2 Drilling techniques
Several techniques of drilling for water have been developed to suit the type of borehole requi-
red and the geological context.
Cable-tool drilling is the oldest technique. It is conceptually simple, and is especially useful in
coarse sedimentary formations (gravels, pebbles), which are excellent reservoirs. This technique is
not covered in detail in this book. Material removed is lifted to the surface mechanically, using a
cylindrical bailer or scoop (Beneto-type machines).
Rotary and ‘down-the-hole’ (DTH) hammer techniques are the most widely used and are the
most suitable in drilling for water. Certain rotary drills are very large and can drill down to several
hundred metres.
For the lightweight machinery used by ACF, the ACF-PAT 201 model comes only in a rotary
version, whereas the ACF-PAT 301 and 401 are DTH models (combined rotary and percussion).

The rotary technique (Figure 8.1) is used only in unconsolidated sedimentary formations with
2.1 Rotary drilling

lightweight machinery (high-power rotary machines such as that used for oil drilling can however
work in hard formations).
A rotary drill bit, known as a tricone bit, is driven from surface level via drill pipes. The drill
bit works by abrasion of the ground, without percussion, using only rotation and pressure. This is pro-
vided by the power of the machine but, above all, by the weight of the drill pipes above the drill bit:
when drilling large boreholes, weighted drill pipes, are used for this purpose.
At the bottom of the hole, the drill bit cuts away pieces of ground (cuttings). The circulation
of a liquid, the borehole drilling mud, brings the cuttings up to the surface. The drilling mud is injec-
ted down the centre of the hollow drill pipes (or drill string) to the level of the drill bit and returns to
the surface via the annular space between the drill string and the sides of the hole. While it is rising,
the drilling mud covers the borehole sides and stabilises them (cake). This drilling mud is made up of
water, a clay (bentonite) or a polymer, usually polycol. It moves in a closed circuit: when it arrives at
the surface, it is channelled into a series of pits which allow the cuttings to settle, and it is then
re-pumped and injected under pressure down the drill string.

8A. Boreholes. Borehole drilling 267

 Figure 8.1: Working principle
of rotary drilling.

This technique allows drilling in hard formations.

2.2 Down-the-hole hammer drilling

A cutter with tungsten carbide buttons, fixed directly onto a pneumatic hammer (DTH), is rota-
ted with a hammer action to break and grind the rocks. The hammer works like a pneumatic road drill,
using compressed air delivered by a compressor. The air flow raises the cuttings to the surface.
There are two phases, percussion and blowing (Figure 8.2).

A B

Figure 8.2: Working principle of DTH hammer drilling.

A: percussion: the compressed air operates the piston of the hammer, which strikes the drill bit pressed
on the rock; part of the air then escapes into the annular space, carrying the cuttings with it.
B: blowing, or removal of the cuttings: the drill bit is withdrawn slightly, so that all the air flow passes
around the hammer without operating it, and then escapes into the annular space.

268 III. Water supply

2.3 Borehole parameters
The parameters that control drilling progress depend on the technique used (rotary or DTH):
rotation and pressure on the drill bit (Box 8.1), and rising speed and pressure of the drilling mud or
air – see box 8.2. These factors have varying influences on the rotary or DTH techniques, and it is
essential to control them in order to achieve good working conditions: smooth progress, constant cut-
ting removal, and stabilisation of the borehole sides (Figure 8.3).
In rotary drilling, for a given rotational speed, the essential parameter for progress of a bore-
hole is the weight acting on the drill bit. Rotational speed is kept as regular as possible, depending on
the drill-bit diameter and the nature of the formation. Generally, rotational speed will be lower for
hard formations.
In DTH drilling, on the other hand, the determining factor is not weight, but the percussive
action of the drill bit on the rock caused by the pressure of the compressed air acting on the hammer.
However, insufficient weight can induce blind striking, which can cause serious damage to the ham-
mer and drill bit. Too much weight, on the other hand, damages the drill-bit buttons. In practice, and
with experience, the pressure on the drill bit is controlled by ear – a clear striking sound means that
the hammer is working correctly – so as to obtain a regular rotational speed and to prevent excessive
vibration.

Box 8.1
Calculation of pressure and rotational speed*.
Drill-bit loading
In rotary mode, the theoretical minimum pressure on a tricone bit is about 450 kg per inch of bit diameter
and about 225 kg per inch for a three-bladed bit, i.e. a minimum downward force of 1 350 kg for a 6” three-
bladed bit, and 2 700 kg for a tricone bit of the same size.
For a DTH hammer, the usual pressure is 100 to 200 kg per inch of drill-bit diameter, i.e. between 600 and
1 200 kg for a 6” drill bit.
Rotational speed
The calculated speed is that of a point situated at the periphery of the drill bit (tangential speed), that is, the
time the point takes to cover a given distance.
The following formula is used to calculate the number of revolutions per minute:
tangential speed (m/min)
revolutions per minute (rpm) = –––––––––––––––––––––––––
π . d(m)
where π = 3.14 and d is the drill bit diameter (m).
In rotary mode, the minimum tangential speed must be 60 m/min, and for a DTH hammer it must be 10
m/min, that is, for a 150 mm drill bit:
– in rotary mode, 127 rpm;
– in DTH mode, 21 rpm.
Torque
For rotary and DTH drilling, the minimum advised torque is 2 000 N-m per inch of diameter of drill bit used.
A safety factor of 1.33 is applied; that is, for a 6” drill bit, a torque of 16 kN-m.

–––––––––––––––
* Raymond Rowles, Drilling for Water, a Practical Manual, Avebury/Cranfield University, 1995.

8A. Boreholes. Borehole drilling 269

 downward force too high

irregular
rotation

rotational irregular rotation, correct excessive rotational

speed too low drilling power reduced control drill bit wear speed too high

vibration,
drilling
power
reduced

downward force too low

Figure 8.3: Control of downward force and rotation.

The rotation is transmitted mechanically (motor, gearbox, clutch or kelly on large machines) to the
2.3.1 ROTATION, PRESSURE AND LIFTING FORCE

drill string by the drive head. It is calculated by simply counting the number of revolutions per minute.
The torque of the machine is expressed in Newton-metre and plays a fundamental role in
rotary rigs working in hard sedimentary formations, and at great depths. It plays a secondary role in
lightweight rigs, since the rotary technique has a limited application in hard formations. The values
expressed are well within the recommended ranges.
The pressure depends on the power of the rig itself and the weight of the drill string above the
drill bit. Consequently, the deeper the borehole, the heavier the weight on the drill bit induced by the
weight of the drill pipes. When the borehole is started, the pressure on the drill bit is therefore some-
times low, particularly with lightweight rigs. On the
other hand, at great depths, the drill string must be sup-
ported so as not to apply excessive pressure on the drill
bit (Figure 8.4). The pressure to be applied on a tricone
bit (rotary) is much higher than that applied on a DTH
bit, but rotational speed is reduced (Box 8.1).

Figure 8.4: Downward force applied as a function of

depth and weight of drill pipes.
The drill bit is 150 mm diameter, and the downward
force on the hammer is about 800 kg. The weight of
standard drill pipes is about 7 – 8 kg/m. The downward
force of the drilling rig (in addition to the drill string)
has to be increased at the start of drilling; after
a certain depth is reached, the drill string may have to
be supported as the weight of drill pipes increases.

270 III. Water supply

 The lifting force is provided by the machine power. Its value is given by the manufacturer and
is generally expressed in tonnes. Obviously, it lifts the drill string, but can also be used to free the drill
bit if the borehole sides collapse.

Drilling fluids are either lubricated air (with or without foam) for use with DTH hammers, or
2.3.2 DRILLING FLUIDS

water incorporating a given amount of drilling mud for rotary drilling. These fluids play several roles,
summarised in Table 8.I.

Table 8.I: Drilling fluids.

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Drilling Type of fluid Role of the fluid

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
technique

Rotary Drilling mud: Cuttings removal

– water Binding and stabilising the sides
– bentonite (cake formation)
– polycol Lubrication, cooling of the drill string and drill bit

DTH Lubricated compressed air Operating the hammer

Lubricated compressed air Improved cuttings removal (blowing)

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
+ foam (foaming agent) Lubrification of the hole

DTH drilling is often done without foam but this practice must be avoided. The use of foam
really improves the efficiency of drilling (cuttings removal) and strongly decreases the risk of getting
the hammer stuck.
Rotary drilling can be carried out using only air, without drilling mud. This technique allows
quicker drilling and can sometimes get to a good depth in dry and stable formations. This allows the
absence of water to be established without having to install any casing. If water is reached however,
drilling becomes more difficult, with increased risk of collapse, and here mud drilling must be used.
This technique is also often applied over the first few metres of the borehole (10-20 m), as it
avoids the need for drilling mud if drilling is to be continued with a DTH hammer (bedrock near the
surface). However, if the surface layers are not stabilised by the cake, the risk of the sides collapsing
is higher (erosion by the air flow). Furthermore, the wet cuttings tend to agglomerate and, being too
heavy to rise to the surface, they remain in suspension in the borehole until they form a plug in the
annular space.

Drilling mud plays an essential role in the drilling process: it brings the cuttings to the surface,
2.3.3 ROTARY DRILLING MUD

stabilises the sides, and lubricates the drill bit. The intrinsic characteristics of drilling mud (density,
viscosity) are regularly checked and modified during the drilling process, thinning or thickening as
necessary:
– density influences the transport of the cuttings to the surface and the stabilisation of the bore
hole sides. Heavy drilling mud has better transport properties, and the cuttings float better;
– low temperature cools the drill bit;
– viscosity influences the lubrication of the drill bit as well as the transport of the cuttings
(thrusting effect).
Note. – Polycol is a polymer which gives a spiral movement when circulating in the hole, and
this improves the rise of the cuttings.

8A. Boreholes. Borehole drilling 271

 Hydrodynamic parameters (flow, pressure) also have an effect:
– the flow of the pump influences the circulation rate of the drilling mud (rate of rise), and
directly influences the removal of the cuttings (Box 8.2). For the cuttings to pass within the
annular space, it is necessary to maintain a minimum speed suitable to the density of the fluid.
For constant flow, the velocity of the fluid (m/s) decreases as the annular space increases;
– the pressure of the drilling mud offsets head-losses in the drill string, since the circuit is
balanced (open U circuit at atmospheric pressure at the surface). In theory, no pressure is
necessary to ensure the return of the drilling mud. Nevertheless, high pressure is very useful
in the event of a blockage in the annular space.

Air has two different functions: to operate the hammer, and to bring the cuttings to the surface.
2.3.4 AIR IN DTH DRILLING

Therefore, several essential parameters must be checked.

The minimum air supply necessary to operate the hammer (several litres per second) and also
to provide air flow high enough to carry medium-size cuttings (several millimetres – Figure 8.5 and
Table 8.II) must be determined. The addition of foam to create an air/foam emulsion increases the
transport capacity, and the emulsion can carry cuttings with diameters of about one centimetre, for
low rise rates of about 10 to 15 m/s. By lubricating the rocky sides of the hole, the foam decreases the
risk of getting the hammer stuck and it must be systematically used in deep boreholes.
The pressure of the air injected has a direct effect on the ability of the hammer to crush the
rock, and therefore on the drilling advance speed (Table 8.III). The lubrication of the air must be per-
manent, because it lubricates the hammer piston liner.

Table 8.II: Air velocity necessary in DTH drilling, without the addition of foam, to bring spherical
cuttings of a specific gravity of 2.8 to the surface.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Cuttings diameter (mm) Air velocity (m/s)

0.1 1
0.5 5
1 8
5 18

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
10 24

Figure 8.5: Air velocity as a function of compressor flow and drill bit diameter.

272 III. Water supply

Table 8.III: Drill advance speed at a site in northern Uganda in gneiss formations with an ACF-PAT 301
rig equipped with a 150 mm drill bit and a 8 or 12 bar compressor.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Compressor 8 bars – 175 l/s 12 bars – 125 l/s

Borehole number 10 825 10 823 10 829 10 828 10 832 10 836

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Average time
per 2 m drill pipe (min) 44 52 69 23 18 36

Box 8.2
Calculation of rise rate of the fluid.
To calculate the fluid velocity in the annular space, pump flow is divided by passage cross-sectional area,
e.g., for a flow of 19 l/s, a 150 mm borehole, and 76 mm drill pipes:
Q
––––––––––––––– = V
πid /4 – πiD2/4
2

0.019
––––––––––––––––––––––––––––––– = 1.4 m/s
3.14 x (0.15)2/4 – 3.14 x (0.076)2/4
where d is the external diameter of the drill pipes (m), D the diameter of the borehole (m), Q the flow (m3/s)
and V the speed (m/s).
Raymond Bowles (1995) gives minimum annular flow speeds required for various fluids: 0.6 m/s for water,
0.35 m/s for drilling mud (water + bentonite) and 15 m/s for pure air (without foam).
He also gives maximum permissible flow speeds: 1.5 m/s for water and 25 m/s for air. At speeds above this,
erosion of the sides of the borehole may occur, which could lead to loss of the borehole.

The values of these parameters are merely indicative (Table 8.IV) and correspond to recom-
2.3.5 DRILLING GUIDELINES

mendations for standard drilling rigs; they are therefore much larger than the values used with light-
weight rigs.

Table 8.IV: Drilling parameters (guidelines).

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Rotary DTH

Pressure on the drill bit Per inch of drill bit

– three-bladed bit 225 kg
– tricone bit 450 kg
– DTH-hammer bit 100-200 kg
Rotation 10-150 rpm 25-50 rpm
Torque 2 000 N-m per inch of drill bit
Coefficient of 1.33 in addition
to be applied
Fluid flow speed Drilling mud Air (pure)
– minimum 0.35 m/s 15 m/s
– maximum 1.5 m/s 25 m/s
Minimum pressure of the fluid (in bars) Drilling mud Air
for a 4” borehole 1 bar 12 bars

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Depends on diameter Depends on diameter

8A. Boreholes. Borehole drilling 273

 The characteristics of the ACF-PAT rigs do not meet the specifications given in Table 8.IV
(pressure, torque) in rotary technique, which simply means that these drills have very precise limita-
tions in application, and that it is necessary to adapt the technique to the formations encountered. A
rotary technique with light drilling machinery is not applicable in hard formations, where DTH ham-
mers are required.

3 Lightweight drilling rigs

The three drilling rigs described below have been developed by the firm PAT, based in Thai-
land. ACF has adapted these machines (originally designed for working in Asian contexts, i.e. sedi-
mentary formations) to the context of African bedrock zones. Three drilling kits have been manufac-
tured: ACF-PAT201, ACF-PAT 301 and ACF-PAT 401 PTO.

3.1 ACF-PAT 201 kit

This very simple machine is a rotary rig composed of a frame and a rotary motor (Figure 8.6
and Annex 11A), a mud pump, and a small light compressor for borehole development.
The main advantages of this machine
come from its light and mobile structure,
which enables drilling to be carried out in iso-
lated zones without needing to transport a
complete site kit, which is very heavy*. A
standard pick-up is enough to transport it from
site to site with the rest of the equipment. The
whole kit can be transported by light aircraft,
which is an asset in many inaccessible places
with humanitarian crises. The very simple
operation of this type of machine allows local
teams to become technically independent very
quickly.
Boreholes are equipped with hand-
pumps, or sometimes 4” submersible pumps,
depending on demand and available flow.
Maximum exploration depth is about 45-60 m
in all unconsolidated formations (sand, clay,
and fine gravel). For depths greater than 60 m,
the machine is limited by the configuration of
its drill string in its standard version (60 m),
by its manual lifting winch, and by the capa-
city of the mud pump. However, it is possible
to use some optional equipment which allows
drilling to a depth of 80 m.

Figure 8.6: The ACF-PAT 201 drilling rig.

–––––––––––––––
* Net weight of the complete site kit: 787 kg. Gross weight of the complete kit: 949 kg. Packed volume (8 boxes):
5.5 m3.

274 III. Water supply

Table 8.V: Characteristics of the ACF-PAT 201 kit (1998 version).
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Frame Total height of crossbar: 2.9 m
Manual lifting winch equipped with 5 pulleys (cable length 11.5 m, ratio 4:1)
Installation chassis for a pick-up

Drive head Engine: Honda GXV-140 petrol 5 HP, 3 600 rpm + gearbox + clutch
Rotational speed 80-120 rpm

Drill pipe, Length 1.5 m x 40 units –thread 2” 3/8 API reg.

standard drill bits Exterior dia. of the drill pipes 54 mm – 4 mm pitch – weight 16 kg – total 45 m
and accessories Three-bladed bits: 1 pcs 8” (103 mm) – 2 pcs 6” 1/2 (165 mm) – 2 pcs 3” 1/2 (89 mm)
1 three-bladed bit 165 mm for clay
Adaptor: 2 pcs 201 A – 2” 3/8 API female
2 pcs 201 A – 3”1/2 API female
Complete toolbox

Pressure Manual winch

torque 196 Nm
lifting force Manual winch max. 400 kg

Mud pump Pump: Taki – TGH max. 42 m – max. flow 19 l/s

Engine: Honda GX 390 – 13 HP petrol
Suction pipe 3’’ x 4 m
Delivery pipe 1”1/2 x 6 m
Development Engine: Honda GX 390 – 13 HP – 3 600 rpm
compressor Compressor: FUSHENG model TA 80 – 3 compression cylinders

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
– max. flow 7.5 l/s – max. pressure 100 m
80 m air hose on reel

The drilling power is given by the weight of the drill pipes above the drill bit, and is limited
by the surface formation encountered (a cap of laterite, for example).
Its configuration and cost also make it very suitable for exploration to optimise a well-digging
programme, thus avoiding expensive dry wells.
Numerous borehole programmes have been carried out by humanitarian organisations with
this machine in South-East Asia, in Africa by ACF, in very isolated areas such as Southern Sudan,
Liberia, Sierra Leone, Mozambique or Angola.
The speed of implementation depends essentially on the geological context and the conditions
of access to the site. In a very favourable context (shallow water table), it is possible to drill one bore-
hole per day. However, during a drilling programme, the time invested in transport, installation and
packing up, choice of site, construction of boreholes, and maintenance must also be taken into
account. Normally, in a difficult context, it is possible to drill one borehole per week.
The technical specifications of the ACF-PAT 201 kit are given in Table 8.V.

The ACF-PAT 301 rig (Figure 8.7 & Annex 11A) is a combined rotary and percussion (DTH)
3.2 ACF-PAT 301 kit

drill, developed to drill in all types of hard sedimentary formations. ACF has adapted it to much har-
der formations, such as bedrock.
In rotary mode, it can drill deeper than the ACF-PAT 201 in fairly hard formations. It can the-
refore be limited to rotary mode. Its investigation depth is about 100 m for 6” and up to 150 m for 4”
holes (ACF, Myanmar, 1996). In percussion mode, it can be used for boreholes of 40 to 60 m depth
and 150 mm diameter in rocks and weathered formations.
This rig has most of the advantages of the lighter ACF-PAT 201, but much wider application.
It is available as an air-transportable kit, and can be used for emergencies; its technology is relatively

8A. Boreholes. Borehole drilling 275

Figure 8.7: The ACF-PAT 301 drilling rig – working principle.

simple and accessible for a trained local team. The assembly can be fixed on a pick-up or a flat-bed
truck, or mounted directly on the ground. The frame can also be towed (mounted on two wheels – not
advisable in rough conditions).
The ACF kit includes a chassis adapted to the dimensions of the bed of a Land Cruiser pick-
up, which allows the machine to be fixed. It is important to fit jacks to the back of the pick-up or truck
in order to be able to drill vertically and stabilise the vehicle during the drilling process.
Installation of this rig directly on the ground is the simplest technique, and allows a very quick
start while waiting for a possible mounting on a vehicle.
Since 2002, PAT has developed a new concept of the 301, the PAT 301T, by installing the rig
on a trailer. The PAT 301T is easier to install and more stable (and efficient) than the standard 301.
As the trailer is independent of the vehicle, the rig doesn’t prevent use of the vehicle during drilling
as with the 401, giving operational flexibility.

Table 8.VI: Site configuration depending on intervention contexts.

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Context Characteristics Site configuration

Borehole in camps, Fairly short distances Machine on the ground or on a pick-up

in an urban area or near the base between drilling locations Towed compressor
Village borehole Long distances, Machine and compressor on a truck

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
in a scattered zone (Sahel type) Very bad roads
Only rotary drilling Machine on pick-up, machine on ground

The machine is composed of a frame, a drive head and hydraulic power unit, a pumping unit
3.2.1 TECHNICAL CHARACTERISTICS

(mud pump), a small compressor (for development), and an air compressor to work with a DTH ham-
mer. The technical specifications of the standard kit are given in Table 8.VII and 8.VIII. The applica-
tions of the PAT in rotary mode are below the advised standards: the use of the tricone bit, which
requires a high pressure, is not very advisable. In practice, for fairly hard sedimentary formations, the
DTH is more suitable, because it needs much less pressure. The air-rise rate is limited by the flow of
the compressor used and the drilling diameters.

276 III. Water supply

Table 8.VII: Characteristics of the ACF-PAT 301 kit.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Frame Height: 3.15 m, with crossbar of 2.25 m of useful run, equipped with 2 wheels
Weight 320 kg – drilling table dia. 200 mm

Drive head Hydraulic motor

Rotational speed 0-40 rpm – torque 136.5 kgf.m (1 320 Nm)

Drill pipe Length of the drill pipes 2 m x 50 units – exterior dia. 76 mm

& standard drill bits Thickness 4 mm – weight 16 kg
Screw thread: 2”3/8 API reg.
Three-bladed bits: 2 pcs 9” (228 mm) – 1 pcs 8” (203 mm) – 2 pcs 6”1/2 (165 mm)
– 2 pcs 4” (101 mm)
1 three-bladed bit 61/2” for clay
3 adaptors 2”3/8 x 2”3/8 API reg. (female – female)
2 adaptors 2”3/8 x 3”1/2 API reg. (female – female)
Hammer: Stenuick Challenger 5 – drill bits: 1 x 150 mm – 2 x 165 mm

Hydraulic unit Engine: Honda 13 HP petrol, 3 600 rpm (portable chassis)

Hydraulic oil reservoir 60 l
Hydraulic pump 250 bars

Feed system Drive head raised and lowered by a hydraulic cylinder and heavy-duty transmission chain
Lifting capacity: 1 590 kg, max. speed: 15 m/min

Mud pump R Engine: Honda GX 390 – 13 HP – 3 600 rpm – petrol

Standard (or Yanmar 10 HP diesel, hand-start, 3 600 rpm, air-cooled)
Pump: Taki 65-33/2 (168 kg), 1 000 l/min at 30 m head, 600 l/min at 50 m head,
pressure max. 4 bar

Screw compressor ATLAS COPCO XAH 12 bar -175 l/s*

for drilling Weight 1.5 T – 2 wheels – Diesel engine: DEUTZ 115 HP

Development Engine: GX 390 HONDA 13 HP – 3 600 rpm

compressor (Yanmar 10 HP diesel, hand-start, 3 600 rpm, air-cooled)
Compressor: FUSHENG TA 80 (3 cylinders) – max. pressure 10 bar– max. flow 125 l/s
1/2” x 5 m flexible rubber hose with connections from reel stand to borehole development
compressor
1/2” x 50 m (optional 80 m) flexible rubber hose with connections and attached air probe
Tubular steel frame with detachable steel handle for rolling up and storing the hose
after well development

Foam pump Engine: Honda GX 120 petrol – 3.8 HP 3 600 rev/min

(also available: 4.0 HP Yanmar diesel engine, hand-start , 3 600 rev/min, air-cooled)
3-cylinder piston pump (triplex piston pump)
Max. pressure 35 bar – max. flow 20 l/min
25 mm x 6 m lift pipe

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
25 mm x 2 m suction pipe

* Other compressors are now available:

– XAS-186 7 bar, 186 l/s
– XAHS-186 12 bar, 186 l/s

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
– XAHS-236 12 bar, 236 l/s

A hydraulic circuit drives a motor for rotating the drill string, and a hydraulic jack for raising
3.2.2 WORKING PRINCIPLE

or lowering the drill pipes and providing pressure on the drill bit (Figure 8.7). This jack moves the
drive head up or down the pillar by means of a chain. The machine is operated from a control panel
(Figure 8.8).

8A. Boreholes. Borehole drilling 277

Table 8.VIII: Characteristics of the PAT 301T.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Frame Height: 3.15 m, with crossbar of 2.25 m of useful run, equipped with 2 wheels
Weight 320 kg – drilling table dia. 200 mm
Mast raised hydraulically to vertical from storage position
Mast equipped with 2 floodlights for night-time operations

Drive head Hydraulic motor

Rotational speed 0-45 rpm – torque 205 kgf.m (1 980 Nm)
Able to swing aside for easy casing installation

Drill pipe Length of the drill pipes 2 m x 50 units – exterior dia. 76 mm

& standard drill bits Thickness 4 mm – weight 16 kg
Screw thread: 2”3/8 API reg.
Three-bladed bits: 2 pcs 9” (228 mm) – 1 pcs 8” (203 mm) – 2 pcs 6”1/2 (165 mm)
– 2 pcs 4” (101 mm)
1 three-bladed bit 6”1/2 for clay
3 adaptors 2”3/8 x 2”3/8 API reg. (female – female)
2 adaptors 2”3/8 x 3”1/2 API reg. (female – female)
Hammer: Stenuick Challenger 5”

Feed system Drive head raised and lowered by a hydraulic cylinder and heavy-duty transmission chain
Lifting capacity: 2 300 kg, max. speed: 19.5 m/min
Drive-down capacity: 3 480 kg, max. speed: 14.5 m/min

Hydraulic unit Engine: Yanmar 20 HP diesel, 2 800 rpm (portable chassis), 3 cylinders, water
cooled, electric start
Hydraulic oil reservoir 70 l
Hydraulic pump 250 bars max.

Mud pump R Engine: Honda GX 390 – 13 HP – 3 600 rpm - petrol

Standard (or Yanmar 10 HP diesel, hand-start, 3 600 rpm, air-cooled)
Pump: Taki 65-33/2 (168 kg), 1 000 l/min at 30 m head, 600 l/min at 50 m head,
pressure max. 4 bar

Screw compressor ATLAS COPCO XAH 12 bar -175 l/s*

for drilling Weight 1.5 T – 2 wheels – Diesel engine: DEUTZ 115 HP

Development Engine: GX 390 HONDA 13 HP – 3 600 rpm

compressor (or Yanmar 10 HP diesel, hand-start, 3 600 rpm, air-cooled)
Compressor: FUSHENG TA 80 (3 cylinders) – max. pressure 10 bar– max. flow 125 l/s
1/2” x 5 m flexible rubber hose with connections from reel stand to borehole development
compressor
1/2” x 50 m (optional 80 m) flexible rubber hose with connections and attached an air probe
Tubular steel frame with detachable steel handle for rolling up and storing the hose
after well development

Foam pump Engine: Honda GX 120 petrol – 3.8 HP 3 600 rpm

(also available: 4.0 HP Yanmar diesel engine, hand-start, 3 600 rpm, air-cooled)
3-cylinder piston pump (triplex piston pump)
Max. pressure 35 bar – max. flow 20 l/min
25 mm x 6 m lift pipe

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
25 mm x 2 m suction pipe

* Other compressors are now available:

– XAS-186 7 bar, 186 l/s
– XAHS-186 12 bar, 186 l/s

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
– XAHS-236 12 bar, 236 l/s

278 III. Water supply

Figure 8.8: Control panel functions on the ACF-PAT 301 (1996 model).
The hydraulic pressure provided by the hydraulic pump is 210 – 220 bars maximum, and is controlled
at the inlet to the control panel by the hex-head control screw P1. If the chain breaks too often,
it may be that this screw is incorrectly set.
Pressure on the feeder F3: this imparts correct rotation in automatic setting. The control range of screw
F3 is adjusted as follows: – turn the control screw for feeder F3 to minimum; – adjust the hydraulic
pressure to 400 psi (manometer F2) using valve F4.

The pneumatic hammer is driven by a compressor (235 to 283 l/s, 12 bars). The compressed
air used for the hammer must be permanently fed with oil (lubricator placed between the compressor
and the air admission valve). A foam pump allows a foaming agent to be injected in addition, to faci-
litate the transport of cuttings to the surface.

3.3 ACF-PAT 401 PTO kit

The ACF-PAT 401 PTO rig is a development of the 301. Its applications are the same as those
of the 301, but it offers easier drilling conditions, because it is more powerful. Its characteristics are
given in Table 8.IX.
The rig is powered by the engine of the vehicle which transports it: in the standard version, a
Toyota Land-Cruiser or a Dyna truck. A power take-off drives the hydraulic pumps: lifting/lowe-
ring/rotation pump, mud pump, stabilising jack pump and foam pump.
The equipment is therefore powered by a single engine, totally controlled from the board situa-
ted at the back of the vehicle (see Annex 11A). Getting on the road, installing the rig and starting work
are simple jobs: the vehicle is positioned, and the platform stabilised. The whole kit is air-transpor-
table (1 vehicle + 1 compressor). The drilling platform can be mounted on the vehicle by a team of
mechanics in a few days. The total weight of the kit is 4.5 tonnes.

3.4 Other lightweight drilling rigs

There are other drilling rigs on the market similar to the ACF-PAT. Table 8.X compares the
characteristics of the main lightweight rigs used in drinking-water supply programmes. The Eureka
and Dando machines are British. The Stenuick (BB) is not very widely used in drilling for water, but
has some useful characteristics for DTH-hammer drilling; its special feature is the fact that it is enti-
rely pneumatic, which simplifies maintenance. Table 8.XI compares the range of Pat drilling rigs with
other similar models.

8A. Boreholes. Borehole drilling 279

Table 8.IX: Characteristics of the ACF-PAT 401 kit.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Drilling platform Composed of a hydraulic unit, a control panel, oil reservoirs, 35 drill pipes,
(to be installed a mud pump and a foam pump, lighting and electrical system 12 volts DC
on a Land-Cruiser*) Total weight of the kit with 70 m of drill pipes: 2 t

Drive head Rotational speed 0-60 rpm

Torque 2 460 Nm max
Able to swing aside for easy casing installation

Drill pipe Length of the drill pipes 2 m x 35 units – exterior dia. 76 mm

& standard drill bits Screw thread: 2”3/8 API reg.
Weight of each drill pipe: 15.2 kg
Three-bladed bits: 2 pcs 9” (228 mm) – 1 pcs 8” (203 mm) – 2 pcs 6”1/2 (165 mm)
2 pcs 4” (101 mm)
1 tricone bit 6”1/2
1 three-bladed bit 6”1/2 for clay
Adaptors 2”3/8 API reg. female x 2”3/8 API reg. female
2 adaptors 2”3/8 API reg. female x 3”1/2 API reg. female
Toolbox, spares, lubricants
Hammer 4” and 5”, bits 165 mm

Hydraulic unit Deck engine: Yanmar diesel, 4 cylinders, 30 HP, 2 800 rpm, water cooled, electric start,
driving hydraulic pumps
System pressure 250 bar max.
Reservoir capacity 125 l

Feed system Drive head raised and lowered by a hydraulic cylinder and heavy-duty transmission chain
Pull-up capacity: 3 500 kg, max. speed: 25.5 m/min
Drive-down capacity: 2 560 kg, max. speed: 34.5 m/min

Stabilising jacks 2 front / 2 rear

Lifting power of 6 t per jack

Mud pump Engine: Honda GX 390 – 13 HP – 3 600 rpm – petrol

(or Yanmar 10 HP diesel, hand-start, 3 600 rpm, air-cooled)
Pump: Taki 65-33/2 (168 kg), 1 000 l/min at 30 m head, 600 l/min at 50 m head,
pressure max. 4 bar

Foam pump Engine: Honda GX 120 petrol – 3.8 HP 3 600 rpm

(also available: 4.0 HP Yanmar diesel engine, hand-start, 3 600 rpm, air-cooled)
Triplex piston pump driven by a hydraulic motor, 450 rpm, 10 l/min max., 30 bar max.

Compressor ATLAS COPCO XAH 12 bar – 175 l/s

(as 301) Weight 1.5 t
Other compressors are available as for the 301

Development Engine: Honda GX 390 – 13 HP – 3 600 rpm

compressor (or Yanmar 10 HP diesel, hand-start, 3 600 rpm, air-cooled)
Compressor: FUSHENG TA 80 (3 cylinders) – max. pressure 10 bar– max. flow 125 l/s
1/2” x 5 m flexible rubber hose with connections from reel stand to borehole
development compressor
1/2” x 50 m (optional 80 m) flexible rubber hose with connections and attached an air probe
Tubular steel frame with detachable steel handle for rolling up and storing the hose

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
after well development

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
* The complete kit can also be assembled on a trailer.

280 III. Water supply

Table 8.X: Comparison of several lightweight drilling machines.
P: service pressure. Q: air flow. W: power.
–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Weight of Lifting Lifting Rotation Torque Drilling Compressor Comments

–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
rig only force speed speed (Nm) mud pump (DTH)
or kit (t) (kg) (m/min) (rpm)

Eureka 1.5 750 40-75 1 000 No Rotary

drill system

Dando 1.85 7 000 No No Options:

Buffalo 1.71 Motor 20 hp rotary
3 000 + winch 3 000 DTH
cable-tool
machine

Stenuick* 2 600 70 1 800 Q: 250 l/s Rotary + DTH

BB P: 12 bar pneumatic

ACF-PAT 1 (kit) 400 Manual 80-120 196 Q: 19 l/s No Kit

201 Manual P: 4.2 bar – Rotary

ACF-PAT 3.5 (kit) 1 590 Max: 15 0-40 1 320 Q: 19 l/s Q : 125-236 l/s Kit
301 Intermittent Nor: 10 P: 4.2 bar P : 12 bar W: 13 HP
Rotary + DTH

PAT 301T 3.5 (kit) 2 300 Max:19.3 0-45 1 980 Q: 19 l/s Q: 125-236 l/s
Intermittent P: 4.2 bar P: 12 bar

ACF-PAT 4.5 (kit) 3 500 Max: 25.5 0-60 2 460 Q: 15.5 l/s Q: 175-236 l/s PTO kit
401 Land Cruiser Intermittent Min: 1.2 P: 4 bar P: 12 bar W: 40 HP
PTO Rotary + DTH

ACF-PAT 4.5 (kit) 3 500 Max: 25..5 0-60 2 460 Q: 125-236 l/s PTO kit
401 Dyna Min: 1.2 P: 12 bar W: 40 HP
PTO Rotary + DTH

–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
PAT 501 5.5 (kit) 4 350 Max: 25.5 0-50 4 840 Q: 19 l/s Q: 175-236 l/s Trailer
Intermittent P: 4 bar P: 12 bar

–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
* BB drill, equipped with a F624 pneumatic motor for rotation and two F575 motors for raising and lowering.

8A. Boreholes. Borehole drilling 281

Table 8.XI: Comparison of PAT drilling rigs.
–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––

–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Description PAT-Drill 201 PAT-Drill 301 PAT-Drill 301T PAT-Drill 401 PAT-Drill 501

–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Physical formation Alluvial, soil, clay All kinds All kinds All kinds All kinds

Drill system and operation Manual Hydraulic Hydraulic Hydraulic Hydraulic

Rig assembly Separate kits, Separate kits, 1 complete unit, 1 complete unit, 1 complete unit,
3 engines driven 3 engines driven main engine driven main engine driven main engine driven

Mobility No 2 -wheel trailer 2- wheel trailer 2 -wheel trailer 4- wheel tandem trailer
Mounted on Toyota Mounted on 3-t truck
Land Cruiser

Transport Load on/off pick- up Load on/off pick- up Towed by pick -up Towed by pick- up Towed by pick- up
Mounted on Toyota Mounted on 3-t truck
Land Cruiser

Drill depth* 0-60 m 0-100 m 0-100 m 0-120 m 0-150 m

Drill pipe 50 mm x 1.5 m 76 mm x 2 m 76 mm x 2 m 76 mm x 2 m 76 mm x 3 m

(diameter x length)

Borehole diameter in alluvia, 3 1/2” – 6 1/2” 4” – 8” 4” – 8” 4” – 9” 4” – 9”

rotary drilling, mud flushing*

Borehole diameter in hard rock, Not 4 1/2” – 6” 4 1/2” – 6” 4 1/2” – 6” 4 1/2” – 6”

DTH-hammer drilling, recommended
air flushing*

Air drilling, DTH-hammer — 3”– 4” hammer 3”– 4” hammer 4”– 5” hammer 4” –5” hammer

Button-bit diameter — 90-150 mm 90-150 mm 115-165 mm 115-165 mm

Air compressor 250-400 cfm, 250-400 cfm, 300-400 cfm, 300-400 cfm,
7-12 bar 7-12 bar 7-12 bar 7-12 bar

Foam pump — Engine driven, Hydraulic driven, Hydraulic driven, Hydraulic driven,
separate kit mounted on rig mounted on rig mounted on rig

Weight 250 kg 700 kg 1 450 kg 1 860 kg 3 080 kg

(without compressor)

Shipping information 1.1 t, 3 m3 Mud and air drilling, Mud and air drilling, Mud and air drilling, Mud and air drilling,

–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
for a complete set one 20 ft container one 40 ft container one 20 ft container one 40 ft container

–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
* Depends on geological conditions and skills of operators.

4 Borehole design
4.1 Choice of casing
The depth and diameter of the casing and location of the screen depend on the hydrogeologi-
cal context (depth of the aquifer, exploitation flow) and the type of pump to be installed (handpump
or submersible pump). The choice of casing diameter depends on the size (diameter) of the pump,
which in turn depends on the flow it can provide (Table 8.XII).
A 4” pump normally passes through a casing of 100 mm diameter. However, it is advisable to
leave one inch between the pump and casing and it is therefore advisable to use casing of 113 mm
internal diameter for an 4” pump. This clearance must be carefully considered when an electrical sub-
mersible pump is installed. It must be large enough to limit the head-losses (especially for large head-

282 III. Water supply

Table 8.XII: Flows and diameters of submersible pumps.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
External diameter of the pump (inches) Usual flow range (m3/h)

3” 1–3
4” 3 – 10
6’’ 10 – 50

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
8” 50 – 150

losses flows), but narrow enough to allow for a sufficient flow across the sides of the motor to cool
it. When a motorised pump is placed below (or in) the screen a shroud must be installed in order to
direct the flow across the motor to ensure cooling.
Logically, the external diameter, and therefore the thickness, of the casing depends on mecha-
nical forces to be resisted (horizontal pressure of the ground and weight of the suspended casing).
PVC casing, the most widely used for water boreholes of medium depth (no corrosion, easy to handle
and install etc.), will be considered later.
The borehole diameter selected (Table 8.XIII) must allow the casing to pass freely, without
force, and leave a space for the gravel pack around the screen.

Table 8.XIII: Corresponding diameters of PVC casing and drill bits in order to ensure good working
conditions.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
External diameter of the casing Minimum diameter of the drill bit to be used

4” – 110 mm 6” – 152 mm
4”1/2 – 125 mm 6”1/2 – 165 mm
6’’ – 165 mm 8” – 203 mm
6”1/2 – 180 mm 8”1/2 – 215 mm

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
7” – 195 mm 9”5/8 – 245 mm

The quality of a borehole (sustainability, quality and turbidity of the water, exploitation flow)
depends greatly on the installation of the equipment, the location of the screen relative to incoming
water, the placing of the gravel pack, and finally the cementing of the annular space to avoid surface
infiltration.
The size of the screen slots determines their maximum hydraulic discharge capacity. Table
8.XIV gives an example of this information for PVC screens. The table gives the upper theoretical
limits although slot size is initially determined by the nature of the formation encountered during the
drilling operations (see Section 6.1.1).

Table 8.XIV: Maximum yield (m3/h) per metre of screen pipe.

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Screen diameter (mm) Slot size

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
0.5 mm 0.75 mm 1.0 mm 1.5 mm 2.0 mm 3 mm

110 2 2.8 3.4 3.7 4.2

125 2.2 3 3.9 4.2 5 5.7
160 3 4.1 5.4 5.8 6.5 7.5
180 3.2 4.6 5.8 6.1 7.2 8.1

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
200 5 6 6.4 7.6 8.6

8A. Boreholes. Borehole drilling 283

 Note. – Increasing the borehole diameter does not lead to much of an increase in its capacity.
A series of tests was established in the USA to illustrate this, and results are shown in Table 8.XV.
Table 8. XV: Yield versus borehole diameter.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Diameter of borehole D 2D 3D 4D 6D 8D

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Yield* Q 1.12Q 1.19Q 1.25Q 1.35Q 1.43Q

* Increasing the diameter has the same influence (same coefficient) on the specific capacity (m3/h/m) of the bore-

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
hole as on the yield (m3/h).

4.2 Pre-casing
Pre-casing is not normally required, but it may be necessary if the sides of the borehole are
unstable: surface formations are not usually very consolidated, and pre-casing stabilises them for sub-
sequent drilling. It is advisable to fix the pre-casing base with a layer of cement in the case of signi-
ficant erosion and collapse problems (e.g., in granitic sands, the flow of air can create a cavity at the
base of the pre-casing), or in the case of infiltration of surface pollution (e.g. a contaminated surface
aquifer which needs to be isolated).
In DTH mode, it is quite likely that the sides of the borehole in the first few meters of soil will
collapse (before the rocky layer is reached), especially when using foam, because of the water. As a
result, the risk of getting the hammer stuck is very high. Consequently, considering the price of a ham-
mer, it is highly advisable to install pre-casing when drilling with DTH.
In rotary mode, the risk of erosion of the borehole sides and collapsing is reduced, even at great
depths (50 to 80 m), because the drilling mud stabilises the sides by caking. Also, the circulation rate
of the drilling mud is not very high.
The surface formation may be loose (sand, soil), which may require several metres of pre-casing.
Non-cemented PVC pre-casing can be removed if it is less than 20 m deep. Beyond that depth
it becomes impossible to remove without risking breakage. The use of steel pre-casing allows extrac-
tion from any depth, assuming enough lifting force from the machine (weight of the casing plus fric-
tion). Lightweight drilling machines such as the ACF-PAT range are not powerful enough to carry out
this kind of operation beyond 20 m.
The internal diameter of the pre-casing must be several millimetres larger than the diameter of
the drill bit used to drill through the underlying terrain. For example, to pass a 165 mm (6”1/2) drill
bit, the pre-casing will need to have an internal diameter of 178 mm. Pre-casing of 167 mm internal
diameter can also be used with care and in shallow boreholes.

4.3 Usual configurations

These examples are taken from boreholes intended to take 4” handpumps or electrical sub-
mersible pumps (Figures 8.9 and 8.10).
Normally, handpumps pass through casing with a 100 mm internal diameter. Kardia K 65
pumps are an exception, with their 96-mm external diameter cylinder and centralisers: casing of 113
mm internal diameter must therefore be used. Pre-casing, if necessary, will then be of 178-195 mm or
167-180 mm.
It is strongly advisable to case boreholes throughout their depth to increase life of the screen,
and ensure filtering of fine particles by the gravel pack.
Sometimes some boreholes in bedrock are not cased in their lower sections (especially boreholes
equipped with handpumps). The borehole is then only partially cased, to protect the less consolidated
upper part, with a casing of 125 mm diameter or more. The lower, fractured section is not cased.

284 III. Water supply

 A B

Figure 8.9: Rotary drilling (A) and DTH hammer (B).

A B

Figure 8.10: Mixed drilling using both rotary and DTH techniques.
A: complete equipment. B: partial equipment for partly consolidated formations.

This technique is not advisable, because it affects the longevity of the borehole, even if the
fractures are clean and the pumped water appears to be clear at first.
Exceptionally, when using a lightweight drilling rig in very hard formations, where drilling is
very slow, the only solution is to drill to a smaller diameter borehole (100 mm) and leave it uncased.
The most usual diameters are shown in Table 8.XVI.
Table 8.XVI: Choice of drilling diameters and equipment.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Geological Technique Pre-casing 3-bladed bit Piping 3-bladed bit DTH bit
context (mm) (mm) (mm) (mm) (mm)

Sedimentary Rotary 167 – 180 228 103 – 113 165

(DN 165, 6”1/2)(9”) (6”1/2)
Sedimentary Rotary 178 – 195 245 113 -125 165
(DN 175, 7”) (9”5/8)
Consolidated DTH 167 – 180 228 103 – 113 150
(ND 100, 4”) (5”7/8)

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Consolidated DTH 178 – 195 244 113 – 125 165
(ND 115, 4”1/2) (6”1/2)

8A. Boreholes. Borehole drilling 285

5 Borehole drilling
The examples and tips mentioned below refer especially to drilling boreholes with the ACF-
PAT 301 machine, but the technique is applicable to other machines with similar characteristics.

5.1 Choice of technique

The behaviour of the formations to be drilled will depend, obviously, on their nature, but also
on their water content (Figure 8.11). Only experience allows cuttings removal and drilling progress to
be correctly evaluated, depending on the method used. Beyond a certain depth, the air rotary method
is of no further use, because it is difficult to control (poor cuttings removal). In relatively unconsoli-
dated sedimentary formations, the best technique is rotary drilling using drilling mud.

5.2 Site preparation

The organisation of the site (Figure 8.12) must allow the driller to see the overall picture, and
5.2.1 INSTALLATION

therefore act quickly if problems arise. Practical measures taken must include:
– a safety barrier around the site;
– access for vehicles;
– water supply (water tanks);
– easy access for filling the pits;
– a sheltered area for writing work;
– an area for spoil (cuttings);
– a levelled area to facilitate setting the machine vertical;
– location and digging of drilling mud pits;
– positioning the compressor so that it is not exposed to drilling dust (do not locate it down
wind of the borehole);
– installation of all pumping units, hydraulic pressure units, and engines on a horizontal surface;
– clearly delimited work zone, with a fence if necessary.

Figure 8.11: Flow chart for selection of drilling technique.

286 III. Water supply

 compressor drill-pipe stack

mud pump
access
foam pump

mud pit
drill
or DTH exhaust

hydraulic water tank

pump tank, bladder
sampling
debris
drilling log
cuttings
compilation

Clearly delimited work zone, with enclosure if necessary

Figure 8.12: Site organisation.

To ensure better stability of the machine on the ground, it is advisable to fix 6-mm steel cables
to the upper corners of the pole frame and to pegs firmly anchored in the ground. It is also advisable
to place sandbags on the anchor arms of the machine.
The hydraulic pump unit (power pack) must be protected from the sun and placed in a well-venti-
lated area in order to avoid overheating, which could mean a power loss (critical oil temperature 60 °C).
The ACF-PAT 301 and the hydraulic pump are linked by two pipes which carry the hydraulic
oil. The male and female connectors for these pipes cannot be wrongly connected to the pump or the
control panel. The hydraulic unit must not be started before having made the connections, because
that would pressurise the links and block the circulation.
Pipes must be connected during prolonged storage (a closed-circuit pipe on the hydraulic
pump and one on the control panel).
The pipe storage rack divides the drill pipes into in two groups, which helps to avoid counting
errors, and therefore errors of depth drilled. It is always advisable to number them so as to differen-
tiate them. The threads must be protected by plugs/caps and systematically greased (drill pipes and
drive head) with copper grease every time they are used, to ensure the drill string is watertight and to
prevent seizing.
If the machine is mounted on a vehicle, the site should be set out in the same way. On a light
vehicle such as the Land-Cruiser 4x4, the hydraulic pressure unit and drill pipes are on the back, and
the compressor is towed by another vehicle which transports the rest of the equipment. On 5-t trucks,
it is possible to mount the compressor as well.
Setting up a site with a machine fixed on a vehicle is quicker. The jacks are used to stabilise
the rig in the vertical plane, and lift and the vehicle. Beams must be placed under the jacks to distri-
bute the weight over a larger ground area.

Mud pits form a reservoir of drilling fluid, and allow recycling after settling of the cuttings.
5.2.2 MUD PITS

For shallow boreholes (20-30 m) in unconsolidated formations, the dimensions given in Figure 8.13
and Box 8.3 can be used.

8A. Boreholes. Borehole drilling 287

 A

Figure 8.13: Mud circulation.

A: plan view; B: section view.

A first channel of 2 m in length and 0.20 x 0.20 m cross-section is dug from the location cho-
sen for the borehole, emptying into the first pit. It must be long enough for the pit to be beyond the
edge of the slab of the future water point in order to avoid differential settling under the slab.

Box 8.3
Mud pit design.
The dimensions of the mud pits are calculated bearing in mind the depth of the borehole to be drilled.
Ideally, the total volume of the pits must be equal to three times the volume of the borehole, with (dimen-
sions in m):
– for the settling pit:
width = 3√(volume borehole in litres x 0.57)
length = 1.25 x width
depth = 0.85 x width
– for the pumping pit:
width = as for the settling pit
length = 2.5 x width
depth = 0.85 x width

288 III. Water supply

 The first pit (settling pit) facilitates the sedimentation that is started in the channel. Its volume
is 0.2 m3 (0.6 x 0.6 x 0.6 m).
The axis of the second channel must be offset from that of the first one, so as to deflect and
attenuate the flow to facilitate settling.
The second pit (pumping pit) is a reservoir from which the drilling mud is pumped to be injec-
ted into the drill string; its volume is about 1 m3 (1.5 x 0.8 x 0.8 m). The pits and channels are regu-
larly scraped out and cleaned of sediments formed in the course of the drilling process.

In clay formations, it is preferable to drill with water only, to avoid blocking the aquifer. The
5.2.3 PREPARATION OF DRILLING MUD

water will become loaded with clay from the ground as drilling proceeds.
If there is no reliable information about the nature of the formations to be drilled, the drilling
water must be mixed with bentonite or polycol to increase its density and to prepare drilling mud
which can be thickened or thinned, as follows:
– polycol is a polymer which is very widely used in rotary drilling. It must be mixed in a pro-
portion of 2.5 to 5 kg per m3 of water. The water-polycol mixture is more homogenous than the mix-
ture of water and bentonite, and it needs less attention in its use. There are many types of polycol with
different characteristics, suitable for different contexts (biodegradable, anti-colloidal, suitable for a
saline environment, suitable for different climates etc.);
– bentonite is a powdered clay which must be mixed in a proportion of 15-30 kg per m3 of
water. It risks sealing the aquifer, but this sealing property makes it better for very permeable forma-
tions (gravels, sands), where the losses of drilling mud and the risk of collapse can be significant.
Clean water should be used for drilling mud. It is essential to have a 5 – 10 m3 water store
(bladder or water barrel) for the site, to be able to make up any loss of drilling mud as quickly as
possible.
The density of the drilling mud must be adjusted as the drilling advances. With experience, and
depending on the formation being drilled, the driller adjusts the density according to the feel of the
mud. Clay has the effect of thickening the mud, therefore it is necessary to thin it by adding water. In
loose or sandy formations, it is necessary to use quite dense mud, as ingress of groundwater can thin
it excessively.
To obtain a homogenous mixture, the polycol or bentonite must be sprinkled over the water jet
while filling the pit. A mixer can be made with some fittings: a venturi tube is made, and then connec-
ted to the bypass on the discharge side of the mud pumps (Figure 8.14).
The drilling mud is circulated from pit to pit so that it remains homogenous before the effec-
tive start of drilling.

Figure 8.14: Venturi mixer made from PVC fittings.

8A. Boreholes. Borehole drilling 289

5.2.4 REMOVAL OF CUTTINGS

The cuttings (and the water with

IN THE DTH TECHNIQUE

foam) are raised to the surface by blowing

compressed air, and then channelled to allow
sampling (and the estimation of the flow).
When the machine is installed on a
vehicle, the mixture of water and cuttings
strikes the underside of the deck. It is neces-
sary to create a circular area on the ground
which directs the flow towards a drain. In
practice, the most effective means of chan-
nelling the cuttings and avoiding splashing is
to place 1-2 m of casing (Figure 8.15) under
the drilling table. A bucket is placed under
the shower of cuttings. Figure 8.15: Collection of cuttings.

5.3 Rotary drilling

It is absolutely essential to follow the procedures given in Figure 8.16.

5.3.1 STARTING

Drilling progress is regulated by the rotational torque and pressure on the drill bit controlled from
5.3.2 ADVANCE: ADDING A DRILL PIPE

the control panel. The possible solutions to drilling problems are explained in detail in paragraph 5.3.4.
The borehole must be drilled down to the end of the drill pipe passage in order to create a space
between the bottom of the hole and the drill bit when the drill pipe has to be changed. When the end of a
drill pipe is reached, it is raised and lowered by its full length in order to check the hole and clear the bore-
hole sides. A drill pipe can be added as long as the drilling mud is not too full of cuttings (Figure 8.17).

Figure 8.16: Fitting the first drill-pipe.

290 III. Water supply

Figure 8.17: Unscrewing a drill pipe.

After switching over the mud-pump outlet to circulate mud from pit to pit (slowing the motor),
it is possible to change the drill pipe. Stopping and restarting the flow of mud must be done as
smoothly as possible in order to avoid any destabilisation of the borehole sides.
The locking shoe holds the drill pipes suspended in the borehole during the addition or remo-
val of drill pipes: it engages at the level of the drill pipe flats (Figure 8.18).

Figure 8.18: Locking drill pipes in suspension.

The drive head is lifted into the high position and the shoe engaged on the flat of the lower
5.3.3 REMOVAL OF A DRILL PIPE

drill pipe. To remove a drill pipe, it is necessary to unscrew the upper thread using the drive head first,
then the spanner, and then to unscrew the lower thread (Figure 8.19).

8A. Boreholes. Borehole drilling 291

Figure 8.19: Removing a drill pipe.

Many difficulties may appear in the course of the drilling process, but most of them are simple
5.3.4 COMMON PROBLEMS

to overcome with a little experience (Table 8.XVII). Success depends on constant monitoring of all
the factors which may influence the progress of the operations, on precise observation of the cuttings,
and on ‘listening’ to the machine: experienced drillers are very attentive during the key phases of the
drilling process, to detect the slightest problem.

Table 8.XVII: Frequently-encountered problems in rotary drilling and recommended solutions.

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Observations – difficulties Recommended solutions

Significant losses Increase the density of the drilling mud

and/or dilution of drilling mud Use bentonite rather than polycol
Install pre-casing

Thickening of drilling mud Empty and clean pits

Add clean water

Sides of borehole not stabilised Increase density of drilling mud

and collapsing, Reduce fluid circulation rate
erosion of sides Reduce cleaning and circulation time
Case immediately
Install pre-casing

Sides collapsing, Increase pressure and flow of drilling mud

circulation stopped, Lift drill string until circulation returns to normal
rotation blockage Install pre-casing

Sealing of aquifer Clean with water to break the cake

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Use polycol rather than bentonite

292 III. Water supply

 The geological section of the ground being drilled is established by the hydrogeologists or
5.3.5 ANALYSIS OF CUTTINGS AND SIGNS OF WATER

their assistants as the borehole goes along, and described in detail in the borehole report. The cuttings
which come to the surface with the drilling mud are a source of essential information: their geologi-
cal analysis helps to identify the formations traversed, and to know their nature, whether they are per-
meable (indicating a reservoir) and capable of providing water. Samples are taken with each change
of drill pipe and formation, by hand, just at the borehole outlet, and placed in a box with different
compartments in order to visualise the geological section. They are then preserved in plastic bags mar-
ked with the name of the borehole and the depth at which the sample was taken. The samples are
always covered in drilling mud, which makes them difficult to interpret, so they have to be rinsed with
clean water.
In rotary drilling, there is nothing that clearly indicates the presence or absence of water during
the drilling process: only water tests (direct blowing) and pumping tests, carried out once the bore-
hole has been equipped, allow the presence of water to be confirmed, and the exploitation flow to be
evaluated. Nevertheless, during the drilling process, there are several signs of water that allow an
aquifer area to be located:
– analysis of the cuttings, as noted before, indicates the presence of an aquifer by revealing
layers of permeable material (sands, gravels), supported by cross-checking with the informa-
tion gathered in other boreholes drilled in the same area and which have turned out to be posi-
tive;
– loss of drilling mud, which means leakage of drilling mud into the surrounding ground,
becomes evident from rapid decrease in the levels in the pits during circulation, or in the bore-
hole after the circulation has stopped (for example, while a drill pipe is being changed). These
phenomena indicate that the borehole is passing through layers of permeable material;
– traces of oxidation and visible alterations on the grains of quartz and feldspar (ochre/rusty
aspect) are signs of groundwater movement. However, these may be old signs, relating to
water movements in the past, and they may not reflect the current situation (e.g. if the static
level has dropped);
– thinning (i.e. dilution) of drilling mud indicates groundwater ingress. But this phenomenon
is rarely detected, because the pressure of the drilling mud is usually higher than that of the
aquifer, and the aquifer is usually plugged by the cake.

5.4 DTH percussion drilling

The DTH hammer is a precision tool, consisting of a piston which slides in a cylinder due to
5.4.1 ADJUSTMENT AND LUBRICATION OF THE DTH HAMMER

the passage of compressed air through a set of cavities. The piston strikes the drill bit during the per-
cussion phase and releases compressed air during the blowing phase (Figure 8.20).
It is essential to keep the hammer lubricated, and so the injected air must itself be lubricated
throughout the length of the drilling. A lubricator is located between the compressor and the air-
admission valve of the drilling rig, injecting biodegradable drilling oil. The operation is checked by
blowing the lubricated air onto a small board placed under a suspended drill pipe. Optimum flow
(0.2 l/h) occurs when the board is lightly and evenly sprayed. Adjustment is carried out with the screw
situated on the lubricator: the screw at the base is turned fully to the right (closed), then unscrewed a
quarter turn to the left. If foam is added, the quantity of oil used must be higher. Every time a drill
pipe is changed, it is essential to check the presence of oil in the air coming out of the drill head.
Finally, when the hammer is completely dismantled, it is necessary to oil it (by direct introduction of
hydraulic oil) and to grease all the threads (with copper grease).

8A. Boreholes. Borehole drilling 293

Figure 8.20: Hammer function.

In bedrock areas, weathered layers are drilled in rotary mode using either air or drilling mud
5.4.2 INSTALLATION OF THE HAMMER

until the bedrock is reached, when drilling continues using a hammer (Figures 8.21 and 8.10).
Certain precautions must be taken for the installation and lowering of the hammer to the bot-
tom of the borehole:
– all drill pipes must be cleaned with air from the compressor to remove all drilling mud resi-
due before the hammer is connected (to avoid damage to the hammer);
– before storage, all dry drilling-mud residue must be cleaned from the drill pipes used for
rotary drilling, with water from with the foam pump in high-pressure washer position;
– before lowering the DTH hammer, the depth of the borehole must be measured (with a dip-
per) in order to check for possible collapse;
– when each drill pipe is added, it is cleaned after being screwed to the drive head and before
being connected to the drill string (place a board on the drill pipe locked in the shoe and blast
with air to flush out contaminants);
– drilling mud contained in the borehole is regularly flushed (by air blowing) as the hammer
descends. If the hole is pre-cased and the space between the casing and the drill bit is small
(just a few millimetres), there is always a risk of putting the hammer into percussion mode if
it rubs against the sides of the casing, which would damage it.

Before starting percussion, clockwise rotation is commenced and then maintained during rai-
5.4.3 DRILLING PROCESS

sing or lowering of the drill string. It is only stopped when all other operations cease.

294 III. Water supply

Figure 8.21: Fitting the hammer.

Any anticlockwise rotation can unscrew the drill string or the hammer completely, causing
them to fall to the bottom of the borehole; this can be aggravated by the vibrations caused by percus-
sion. The recovery of a drill bit or part of the drill string requires specific tools and is a delicate ope-
ration. Anticlockwise rotation during percussion is therefore to be avoided at all costs.

With the air flow shut off, the drill bit is placed several centimetres above the ground to be
5.4.3.1 Starting the hole

drilled, and clockwise rotation is started. The air is turned on, and the hammer is gradually pressed
onto the ground until percussion starts.
Initially, air flow is opened half way, percussion is relatively weak, and rotation slow, until the
drill bit penetrates the ground. The air valves are progressively opened to increase percussion.
Pressure and rotation are then controlled to give regular advance.

Good drilling involves a balance between pressure and rotation, giving constant penetration
5.4.3.2 Advance

speed and regular, smooth rotation (Figure 8.3).

The borehole is regularly purged (every 50 cm) by blowing in order to remove cuttings and
avoid blockage. Large cuttings tend to remain in suspension above the DTH hammer during the
drilling process. If air circulation stops, cuttings can fall back onto the hammer and block it.
To purge the borehole, the hammer is slightly withdrawn (stopping percussion) and set to the blo-
wing position. The total air flow provided by the compressor must allow the borehole to be purged of all
cuttings. It may be necessary to sweep up the height of the drill pipe to purge the borehole thoroughly.

The procedure explained in Sections 5.3.2 and 5.3.3 for rotary drilling can also be consulted.
5.4.3.3 Addition and removal of drill pipes

Before unscrewing the drill pipes, the residual pressure in the drill string is checked with a
manometer. This pressure remains high if a plug of cuttings is formed in the annular space (see Sec-
tion 5.4.4 for precautions to be taken to avoid this): in this case the drill pipes must be unscrewed care-
fully to release pressure gradually.
Air lubrication is checked with the addition of each drill pipe.

8A. Boreholes. Borehole drilling 295

 Air drilling of formations covered by unconsolidated material that has not been pre-cased can
5.4.4 DIFFICULTIES AND POSSIBLE SOLUTIONS

present difficulties:
– in the first stages of drilling, the pressurised air can erode and undermine the soil around the
borehole, thus endangering the stability of the drilling rig;
– during the drilling process, the movement of cuttings to the surface erodes the sides of the
borehole, which may cause collapse and block the drill string;
– air losses in very loose formations can lower the rise-rate of the cuttings.
If the surface formation does not have a minimum of stability, and if the cuttings do not rise
to the surface correctly and so create a blockage, the rotary technique, using drilling mud rather than
air must be used. If the surface formation collapses, pre-casing is essential before continuing drilling
with the hammer.
For the problems usually encountered during the drilling process, there are several solutions,
as recommended in Table 8.XVIII. The addition of foam (polymer) appreciably modifies the charac-
teristics of the circulating air, and solves a certain number of problems (return of cuttings, filling and
air losses in the ground).

Table 8.XVIII: Frequently-encountered problems in DTH-hammer drilling and recommended solutions.

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Observations – difficulties Recommended solutions

Poor rise of cuttings to the surface Longer blowing time

Injection of foam and water
Decrease in air flow at the Injection of foam and water
borehole exit – blockage of cuttings Care while unscrewing drill pipes
High residual pressure in drill pipes
Air losses in surface formations Injection of foam
Pre-casing if necessary
Blockage with dry Injection of water and foam if necessary
or slightly damp cuttings Frequent high-pressure blowing
Formation of small balls Pulling up drill string
Erosion of the borehole sides due to air flow Decrease in air flow
and return of cuttings to surface Use of foam
Pre-casing
Formation of a large cavity Stop drilling
Immediate casing, or pre-casing
Drill bit blockage by falling debris Sharp rotation, raising and lowering to crush the debris
High-pressure blowing with water and foam
Blockage of rotation Percussion and sharp restart of rotation
Light anticlockwise unscrewing to increase amplitude of jolts

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Drilling in a cavity Left to the drillers’ judgement

As the fluid used in DTH drilling is air, the cuttings will be clean and not mixed with drilling
5.4.5 ANALYSIS OF CUTTINGS, SIGNS OF WATER AND FLOW CALCULATION

mud, facilitating analysis. Even the use of foam doesn’t hamper the observation of the cuttings. Gene-
rally, the bigger the cuttings, the more friable the drilled formation is, and the finer they are (dust), the
harder the drilled rock is. The presence of fractures is usually identified by larger cuttings. Additio-
nally, any signs of erosion on these cuttings could indicate a water flow (current or historic).

296 III. Water supply

 In drilling with air, ingress of water is visible and quantifiable in most cases (return of a mix-
ture of water and cuttings when blowing). However, some water ingress may not be noticed because
it is blocked by the cuttings that form a cake on the sides of the borehole.
It is easy to estimate water flow during the drilling process to decide when it should be stop-
ped, and the borehole equipped. Flow measurements are taken at each significant water ingress (blo-
wing). In addition, all water emerging from the borehole is channelled to an outlet equipped with a
pipe to facilitate measurement using a bucket.
Ingress of water in the finished borehole must be regular and continuous. The measured flow
is generally less than the borehole capacity, because the cuttings block some supply zones, and the
borehole is still not fully developed. Nevertheless, ingress of water is generally progressive: it appears
first in the form of traces of dampness, and then, as the borehole advances, in the form of a cumula-
tive flow coming from various fissures or fractures. In some cases, crossing a well-supplied major
fracture causes a sharp increase in flow.

6 Borehole equipping
Equipping the borehole (installing casing and screen) is an essential stage in the construction
of a water borehole. The casing plan and the position of the screen have a great influence on the yield
of the borehole, as well as its longevity. The aquifer must be protected from surface pollution which
can enter down the side of the casing by the surface works and the cement plug (Figure 8.22).

6.1 Permanent casing

PVC is the most suitable material for shallow boreholes. It is preferable to use proper reinfor-
6.1.1 CHOICE OF CASING AND SCREEN

ced casing with screw joints. The mechanical strength of the casing can be calculated (Box 8.4). It
must be strong enough to avoid pipe deformation during installation, as holes are not always circular,
and during pumping, which applies pressure on the pipe.
Strictly speaking, the screen slot size depends on the grain size of the aquifer (Table 8.XIX).
Before drilling starts this is not always known, but usually the slots should be between 0.5 and 2 mm wide.

Figure 8.22: Borehole equipment

(based on ACF, Uganda, 1997).

8A. Boreholes. Borehole drilling 297

 Box 8.4
Resistance of the casing to crushing.
For water boreholes, an important mechanical characteristic of the casing is its crush resistance. Sometimes
this information is provided by the suppliers, but it can be calculated using the following simplified formula:
Re = K . E . (e/D)3
where Re is the crushing resistance (bar), K a non-dimensional coefficient (for PVC K = 2.43 (Tubafor®),
for steel K = 2.2), E the modulus of elasticity (bar) of the material at 20°C (for PVC 3 x 104, for steel 2 x
106), e the thickness of the casing wall, and D its external diameter.
For slotted casing (screen), calculated values should be multiplied by the coefficient (1 – F), where F is the
fraction of voids. For particular cases, consult the supplier.
The forces, i.e. the lateral pressure on the casing, are generally calculated from:
– the specific weight of the ground, with the lateral forces equal to half the vertical forces, which are them-
selves equal to the weight of the unconsolidated, dry or saturated ground above (specific weight of 2 to 2.5).
Therefore, it is usually considered that hard formations do not exert any lateral pressure;
– the pressure due to the presence of water or drilling mud in the borehole (hydrostatic pressure in bar
P = H d/10).
For example, if the static level is near the surface and the formation is unconsolidated, the horizontal pres-
sure is then 20 bars for each 100 m of depth (weight of the ground divided by 2 + 10 bars of hydrostatic
pressure).
However, bearing in mind the small cross-sectional area of the boreholes, the vault effect of their sides pro-
tects the casing from lateral ground pressure. In practice, only the static and dynamic hydrostatic pressures
are taken into account. These pressures are increased during pouring of cement grout, lowering the casing,
and sharp reduction of water level during development.
For shallow boreholes in hard bedrock, experience has shown that it is enough to consider a horizontal pres-
sure of 0.75 bars per 10 m section. Therefore, casing with a strength of 7.5 bars can be installed in a 100 m
borehole.

During drilling of the first boreholes, the grain size of the aquifer can be easily identified by
analysing the cuttings with a sieve. Table 8.XIX gives the grain size of gravel pack and the screen slot
size recommended for different aquifer grain sizes encountered underground.

Table 8.XIX: Choice of screen slot and gravel pack per aquifer grain size.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Aquifer grain size Gravel pack grain size Screen slot size

0.1 to 0.6 mm 0.7 to 1.2 mm 0.50 mm

0.2 to 0.8 mm 0.1 to 0.5 mm 0.75 mm
0.3 to 1.2 mm 1.5 to 2.0 mm 1.00 mm
0.4 to 2.0 mm 1.7 to 2.5 mm 1.50 mm

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
0.5 to 3.0 mm 3.0 to 4.0 mm 2.00 mm

The risk of collapse could be high, and the casing is therefore fitted as quickly as possible. The
6.1.2 CASING FITTING

borehole must not remain unprotected for any length of time, because there is always the risk of losing
the borehole through collapse of the sides.

298 III. Water supply

 The casing plan (length and position of casing and screen) is established according to the geo-
logical profile of the borehole where the different strata and points of ingress of water are noted. Dia-
graphy’s tests (electrical resistance, gamma ray, neutron) can be carried out before casing to improve
the casing plan, especially in sedimentary formations, with the use of rotary drilling, where it is some-
times difficult to identify the aquifer horizons (see Chapter 5).
The screen is placed so that its bottom is level with the bottom of the aquifer or zone of water
ingress, with its length chosen according the following rules:
– confined aquifer: 80 to 90% of the thickness of the aquifer
– unconfined aquifer: 30 to 60% of the thickness of the aquifer
The issue is to find a good compromise between having screens long enough to reduce the velo-
city of the water moving towards the borehole, and short enough to allow the installation of the pump
above the screen, to avoid draw-down of the water table below the level of the screen.
Placing the pump within the screen itself can damage the screen: the high velocity of water
causes erosion of the slots, and the pump hits the screen when starting. The gravel pack and the aqui-
fer around the screen are also destabilised in the long term. The pump may also be damaged by dra-
wing fine material into it and by causing cooling problems (water flow must arrive down the pump to
cool the motor; a shroud can be used to orient the flow).
The de-watering of the screen presents several risks; oxygenation of the aquifer can favour the
precipitation of metals (Fe, Mg etc.) that clogs up the screens and encourages the development of bac-
teria (see Chapter 8B). It also accelerates the compaction of the aquifer.
These rules have to be strictly applied for a motorised borehole (the pump should be installed
above the screen or should have a shroud if installed below it). For handpumps the velocity of the
water is lower and the longevity of the borehole is not so affected by having the pump within the
screen (in this case, centralisers around the pipe of the pump must be used to avoid the pump cylin-
der hitting the screen during pumping).
Moreover:
– the bottom section of the casing must be a length of unslotted casing of about 0.5 m, plug-
ged at the bottom (settling pipe);
– since the casing does not always reach the bottom of the borehole (cuttings suspended in the
drilling mud falling back when circulation stops, or collapse of the material from the borehole
sides), it is necessary to reduce the length of the casing by 0.5 to 1 m in relation to the actual

A B

Figure 8.23 Centralisers.

A: commercial model. B: in Angola, centralisers were made by fixing small pieces of PVC pipe secured
with fasteners around the casing. One centraliser was installed on each screen section, and one per two
or three casing sections.

8A. Boreholes. Borehole drilling 299

 depth drilled (after drilling several boreholes in the same area, the driller should be able to esti-
mate the height lost during drilling and adapt accordingly);
– the top of the casing must extend to about 0.5 m above the surface of the ground.
The lengths of casing could vary with the joint threading, so it is advisable to measure each length.
The casing must pass freely down the borehole under its own weight. If the borehole is not
vertical, friction between the casing and the borehole sides can block the installation. Light pressure
on the casing can facilitate its descent, otherwise it is necessary to return it to the surface and rebore
the hole.
An alternative method consists of lowering the casing without a bottom plug so that it can
scrape down the sides. In this case, it is recommended to seal the bottom of the borehole with cement
grout pumped down from the surface once the casing is in place.
In order to guarantee correct positioning of the casing in the borehole and an even gravel pack
around the screen, it is recommended to install centralisers. Various suppliers produce centralisers, but
they can be easily made in the field (Figure 8.23).

6.2 Gravel pack and grouting

The gravel pack allows for a larger screen slot size to be used, increasing the yield from the
6.2.1 GRAVEL PACK

borehole by reducing the velocity of the water entering the screen (therefore reducing head-loss). The
gravel pack also helps in the stabilisation of the surrounding aquifer.
The gravel pack must be reasonably uniform, calibrated, clean, round and preferably siliceous,
to guarantee good porosity and longevity. It must not be calcareous, lateritic or crushed.
In practice, the gravel pack grain size is defined by the grain size of the aquifer and the slot
size of the screen: the gravel must be as fine as possible without passing through the screen (Table
8.XIX).
The gravel is passed down through the annular space between the casing and the sides of the
borehole. The use of a funnel (sheet metal, plastic sheet or pipe) facilitates its introduction.
If the falling gravel blocks the annular space, circulation of water can clear it.
Mud rising up through the casing indicates that the gravel is falling correctly. When the level of
gravel reaches the top of the screen, the mud no longer comes up through the casing, but through the
annular space. The gravel filter must then go on a few metres beyond the height of the screen (com-
paction may occur after installation). This level can be checked with a dipper in shallow boreholes.
The volume of gravel required can be defined theoretically (volume of the borehole minus volume
of the casing) or empirically (Box 8.5), but in practice, more gravel is always needed than is estimated
(non-rectilinear hole, formation of cavities etc.). Table 8.XX gives some approximate volumes of gravel
required for various borehole and casing diameters, in litres per metre height of gravel pack.

Tab 8.XX: Volume of gravel in relation to borehole and casing diameter.

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Borehole diameter Casing diameter Volume of gravel (l/m)
Theoretical volume Likely real volume

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
(empirical formula)

3”3/4 1”1/2 6 10.16

3”3/4 2” 5.1 9.5
5”3/4 4” 8.65 12.67
6”1/2 4” 13.3 19

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
6”1/2 4”1/2 11.15 15.83

300 III. Water supply

 Box 8.5
Volume of gravel.
Empirical calculation of the volume of the gravel filter:
V = h x π x (D2 – d2) x 0.16
where V is the volume of gravel (l), h the height of the gravel filter (m), D the diameter of the borehole
(inches) and d the diameter of the casing (inches), π = 3.14. The factor 0.16 is here to correct the difference
of units.

Grouting is an essential operation which protects the borehole from external pollution; even if
6.2.2 GROUTING

a slab is cast around the casing, only proper grouting can prevent water filtering down the side of the
casing. Grouting can be done with clay or with a mixture of bentonite and cement.
A clay plug must be placed on top of the gravel pack in order to stop the grout from plugging
the gravel pack. The bentonite continues to swell over time, guaranteeing the seal even if the grout
becomes damaged.

The operation consists of filling the annular space above the gravel filter with a mixture of
6.2.2.1 Preparation of the grout

water and cement (grout) up to ground level. When the borehole is deep, one plug can be put above
the gravel pack with another in the last two metres, the intermediate space being filled with clay
(cuttings).
The proportion is about 50 l of water for 100 kg of cement, which gives 75 l of grout. If ben-
tonite is available, the following mixture is used: 70 l of water, 4 kg of bentonite and 100 kg of
cement. This second mixture stops the water from filtering out of the grout, but its setting time is
slightly longer.

The procedure involves filling the annular space up to ground level, and then leaving to set for
6.2.2.2 Placing the grout

a minimum of 12 h before starting development and pumping tests.

Generally, grouting must be carried out before pumping tests. Nevertheless, if it is not possible
to wait for 12 h, it can take place after the development operations and pumping tests, as long as a
clay plug has been placed above the gravel filter.

7 Development
The development of a borehole is a very important step, which removes the majority of fine
particles from the aquifer and gravel pack that have entered the borehole, as well as the remaining
drilling-mud cake, and sorts the aquifer around the screen in order to increase its permeability.
This operation allows borehole yield to be increased significantly. The aquifer is progressively
brought into production and freed from fine particles, with a consequent increase in permeability and
water flow.
As the maximum yield of a borehole in use should be around two thirds of the yield obtained
at the end of the development process, it is important to estimate maximum yield during development.
If the yield during use is higher than the maximum obtained during development, there is a danger of
drawing fine material into the borehole and damaging the pump.

8A. Boreholes. Borehole drilling 301

 Box 8.6

An aquifer put into production via a borehole is automatically developed by pumping.

Self-development of the aquifer.

Lowering of the water table is a maximum at the borehole, but decreases with distance from the borehole
axis, forming a depression cone. The size of this cone depends on the nature of the ground and the supply
of the aquifer or its limits, as well as on pumping time and flow.
It has been demonstrated that the speed of the water decreases with distance from the borehole (according
to Darcy’s law) and therefore that the materials around the borehole are sorted under the influence of the
pumping. The coarser materials settle around the screen, and the finer ones settle at the limit of the area
affected. Fine particles are therefore drawn into the screen over time by a slow process which causes pumps
to deteriorate.
‘Sand bridges’ are also formed, fine materials which accumulate under the effect of the flow. To break them,
it is necessary to reverse the flow, through the development operation, which involves causing suction fol-
lowed by pressure (Figure 8.25B).

In rotary drilling using mud, cleaning consists of washing the sides of the borehole with clear
7.1 Borehole cleaning

water to eliminate the cake. It is best to make the drilling mud as thin as possible without risking a
collapse of the borehole, at the end of the drilling phase, before casing. Once the casing has been
introduced, the injection of clean water from the surface thoroughly rinses the screen and the gravel
pack blocked with drilling mud. The phases of rinsing and air-lift pumping are alternated in the bore-
hole until clear water emerges.
DTH-hammer drilling does not seal the aquifer. On the contrary, the borehole is developed by
successive blowing while it is being drilled. However, it is still possible to suck in a great deal of sand,
damaging the pumping equipment and causing the ground around the screen to sink. It is therefore
necessary to carry out the development of the borehole.

7.2 Development processes

Air-lift development is the most effective and widely used process of development. Its main
7.2.1 AIR-LIFT DEVELOPMENT

advantage is avoiding damaging pumps with sand. At the intake level, quite strong pressure and suc-
tion forces are created by the introduction of large volumes of air. Through alternate phases of air-lift
pumping and direct blowing of air at the screen level, sand bridges are destroyed. Air lift is the most
effective development technique for destroying sand bridges.
In practice, two pipes are introduced in the borehole (Figure 8.24):
– a 11/2” PVC or G.I. pipe, called the water pipe, through which the pumped water returns to
the surface;
– a polyethylene pipe with a smaller diameter, mounted on a drum and called the air pipe, intro-
duced into the water pipe, which allows compressed air to be injected. Depending on its position
inside the water pipe, it pumps water out of the borehole water, or blows on the inside the casing.
The different phases of development are given in Table 8.XXI. The method consists of blo-
wing from the base of the borehole, in successive phases, to just above the screen. Development is
not finished until the water coming out of the borehole is perfectly clear: this operation can last for
several hours, and sometimes more than one day. To verify whether the water is clear, it should be
collected in buckets and checked for any suspended matter (bucket or stain test). By spinning the
water one can observe the suspended particles concentrated in the centre of the bucket. If the circle
created is as big as a coin then the development must be continued.

302 III. Water supply

Table 8.XXI: Air-lift development process.
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
Pumping Lower the water pipe foot to about 0.6 m from the borehole bottom
Introduce the air pipe into the water pipe, locking with a self-grip wrench when its bottom end
is about 0.3 m above the bottom of the water pipe (pumping position)
Install a tee at the exit of the water pipe to channel the jet
Ensure a seal between the two pipes with a rag or a piece of rubber compressed by the
weight of the air pipe
Open the air valve and let the pumped water flow until it is clear
Blowing Shut off the air and lower the bottom end of the air pipe to about 0.3 m below the bottom end
of the water pipe, i.e. 0.6 m lower than previously (air-cleaning position)
Open the air valve, which expels the water contained in the casing
Close and reopen the air valve sharply and repeatedly
Pumping Raise the air pipe 0.3 m inside the water pipe: eject very cloudy water (reversal of flow with
consequent turbulence around the screen)
Renewal Clarify the water, raise water pipe by 1 m and restart operations (alternating pumping and
blowing) up all the height of the screen
Restart from the base, and continue the process until the water is totally clear
Cleaning Once above the screen, lower the device to the bottom of the borehole and pump
the casing out the sand deposited there
Borehole Position the pipes at the bottom of the borehole, in blowing position (air pipe below
plugged by clay water pipe)
or bentonite Connect the mud-pump delivery pipe to the water pipe
Pump air and clean water into the borehole at the same time that the air is being blown
The flow created rinses the borehole: the water descends through the water pipe and returns

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
through the casing

A B

Figure 8.24: Air-lift development.

A: pumping phase. B: blowing phase.

8A. Boreholes. Borehole drilling 303

 A B

Figure 8.25: Air-lift.

A: design. B: sand-bridge destruction.

For small-diameter boreholes (11/2” or 2”), a simple blowing test allows the presence or
absence of water to be confirmed.
Note. – If the water column isn’t high enough then the air-lift won’t be able to elevate the
water, for physical reasons. Approximately, the system will function efficiently if BC ≥ 0.60 x AC
(Figure 8.25A).

Other development techniques can be used, depending on the characteristics of the boreholes
7.2.2 OTHER DEVELOPMENT TECHNIQUES

and the equipment available. These different methods can be also combined with each other:
– Over-pumping: this is the easiest method and consists of pumping at a higher rate than the
planned exploitation yield. It complements the air-lift method and is necessary when the planned abs-
traction rate is greater than the one obtained by air-lift. This method can also be coupled with the
‘alternating pumping’ or ‘pistoning’ (surging) methods. Used alone, it has no effect on sand-bridges.
– Alternating pumping: the objective is to create pressure variations within the installation by
alternating phases of pumping and resting. A high pressure is created by the water column falling
down in the rising main when the pumping stops.
– Pistoning: this consists of moving a piston vertically within the casing to create, alternately,
suction (water and fines move from the aquifer to inside the casing) and compression (water and fines
are pushed out of the casing); this destroys sand-bridges. The borehole can be emptied with a bailer
or scoop.
– Pressurised washing: this consists of injecting pressurised water within the borehole. It can
be useful and fast especially in sandstone formations where the drilling operation often obstructs the
porosity. This cheap method can be coupled with the action of chemicals for cleaning the borehole
and its surroundings (see Chapter 8B).

304 III. Water supply

 Carrying out pumping tests (see Chapter 6) after development with air lift generally allows the
7.2.3 PUMPING

development of the borehole to be completed, through alternating pumping (see Section 7.2.2).
Note. – The yield of the pumping test must be higher than the planned exploitation yield.

The characteristics of the aquifer are defined by long-term pumping tests, which are usually
7.3 Instantaneous flow

difficult to carry out. When equipped with non-motorised pumps, the characteristics of the borehole
are determined by pumping tests in flow steps, which are easier to carry out in ACF programmes (see
Chapter 6).
In order to prepare the pumping test steps, the instantaneous flow of the borehole and corres-
ponding drop in water level are measured at the end of development:
– the flow is estimated when the air-lift device is in pumping position (note: the size of the
device influences the flow of water blown);
– then the water level is lowered (avoiding de-watering the borehole), and a fairly long period
is allowed for the flow to stabilise;
– finally, the flow (time to fill a 20 l bucket) and the corresponding lowering of water level are
measured.

8 Monitoring and borehole report

Monitoring borehole construction does not necessarily have to be carried out by a hydrogeo-
logist. Experience has shown that a rigorous site manager can very well be in charge of this job after
a training period with the hydrogeologist responsible for the borehole programme. Afterwards, the
site manager technician will regularly (daily, by radio if necessary) provide the hydrogeologist with
all the major information and important decisions taken during the drilling process. The monitoring
cards and borehole report are given in Annexes 11B to D.
All information related to the borehole must be noted:
– name of the site or village, GPS coordinates whenever possible;
– working dates, starting, stopping and restarting times;
– name of the drilling firm and, where necessary, of the driller;
– time counters of the machines (compressor, engine);
– technique used, progress by drill pipe or metre, drill-pipe addition;
– any major incidents or important operations such as pulling up a drill string, stopping machi-
nery, equipping the borehole;
– estimated yield and drawdown through development
– casing plan, with the exact lengths of casing and screen, their diameters, the position of the
gravel pack, clay and cement plugs.
Essential geological information is also included, e.g. nature of the ground drilled, signs of
water and flow estimated after each ingress of water. Finally, the driller keeps an up-to-date log book
which collates all information on consumption of materials (cement, casing, bentonite), fuel and lubri-
cants, machinery maintenance, mechanical problems encountered, and their solutions.
When the borehole is completed, essential information is summarised in the borehole report,
whether the borehole is dry or wet. The hydrogeologist in charge is responsible for writing this docu-
ment. These reports are an invaluable source of information for the project, and also provide a hydro-
geological data bank. They must therefore be centralised at project level and also delivered to the rele-
vant local authorities, who may, in certain cases, recommend a common approach for all organisations
involved in the same area.

8A. Boreholes. Borehole drilling 305

 These reports are filed with all the technical information on the water point: field surveys, data
and interpretations of geophysical exploration tests, pumping-test data, site plan etc.

9 Surface works
An example of a borehole equipped with a handpump is given in Figure 8.26. Construction
details are given in Annex 14.

Figure 8.26: Borehole surface works (ACF, Kampala, Uganda, 1996).

306 III. Water supply

 ACTION CONTRE LA FAIM
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HERMANN ÉDITEURS DES SCIENCES ET DES ARTS

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ISBN 2 7056 6499 8

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