SITE PREPARATION

Introduction:

It involves the investigations that are carried out by an architect, a builder or a contractor. It
covers the soil characteristics, regulations required by the local authority concerned, the labour
force, equipments, tools, materials required and safety precautions to be observed during the
construction activities.

Site investigation

It’s the work carried out on the proposed site for putting up a building after the client has
selected the site.

Basically there two types of investigations:

a) Those carried out by an architect or somebody with building experience ( usually the
clients agent)
b) Those carried out by a builder or a contractor

Investigation carried out by an architect or somebody with building experience.

An architect carries out an investigation for the purpose of determining:

i) Physical features on the site (land topography): the surface of the site should have a
gentle slope to facilitate natural drainage to prevent flooding.
ii) Vegetation: the trees should be planted to provide shades and protect the building
from being damaged by wind. The grass and flowers need to be provided to improve
the surrounding and stop soil erosion.
iii) Types of soil: an architect needs to know the characteristic of the soil so as to enable
him to design a suitable foundation for the house and material required.
iv) Service: an architect needs to know the availability of water, telephone, electricity
and road so as to be in a better position to advice the builder.

Investigation carried out by a builder

A builder would carry out his investigation for the purpose of:
 i) Planning the offices, stores, workers hutments, car packing bays, latrines and water
points.
ii) Fencing the plot to keep away animals and an unauthorized people to minimize theft.
iii) Checking on the availability of the local skilled and unskilled labour to help the
builder in his estimates for the labour.
iv) Identifying suitable tools and equipments for the construction work.
v) Planning access roads: the access roads when being constructed should not interfere
with the existing building and protected trees.

Regulations governing preparation of site

Regulations and by laws which are set out in the Kenya building code, for site preparation must
be observed and met before the construction starts.

These regulations include:

a) Position of building lines

b) Provision of access roads
c) Erection of hoarding
d) Precautions while demolishing
e) Damage to streets
f) Closing and obstruction of streets
g) Disposal of rain water from the site

Types of soil

- Soil is the top most layer of the earth’s crust. It occurs in the form of particles of
mineral and vegetable matter.
- The stability of a foundation of a house depends very much on the type of soil it is to
be put on.
- Some soils are more expensive to build on than others. That is why it is important for
a builder to know the type of soil he intends to build a house on.
Common types

a) Clay soil: is composed of very fine particles. It breaks down easily under water. When
wet it sticks and expand. When dry it shrinks and cracks. This behavior of Cray soil
makes it unsuitable and expensive to build on.
b) Sandy soil: is composed of slightly larger particles than clay soil. Clean sand breaks
down completely when dry and individual are gritty to fingers. When cleaned and graded,
sand is used for building purposes. Putting up a house on a sandy soil is not as expensive
as it is on clay soil. This is because a house laid on a sandy soil may not be as deep as
that on clay soil.
c) Loam soil: is a mixture of sandy soil and clay soil with high content of humus. It is used
for dressing in readiness for flower beds or tree planting on site. It is also called vegetable
soil.
d) Silt: it is a finer variety of soil whose particles have smooth texture. It is relatively
impervious. Silt deposits may contain some organic matter. It dries quickly and can be
dusted off fingers when dry. The settlement on account of the loading may also be
significant.

Determination of ground profile

The amount of investigation into soil conditions carried out on the site depends to a large extent
on the site and type of structure which is proposed. For small sites, the location and the depth of
the bore or trial holes should be such as to build up a profile of those soils which are likely to
affect the stability of the foundation. The depth of the trial hole will depend upon the type of
foundation proposed.

A good guide is that the depth investigation hole should extend at least 1.5 times the foundation
width below the foundation level.

Method of investigation

a) Trial holes
b) Shallow bore holes
c) Deep bore holes

Trial hole
 - The hole is excavated by hand or mechanical excavator to the required depth which
allows good visual inspection.
- Hand auger or other hand tools are used when the hole is dug manually. The trial hole
is dug in order to determine the ground profile.

Hand tools used for excavating trial hole

Gravel auger drive shaft clay auger

Jembe pick spade

Typical section of a trial hole

Ground level

Silt

Fine sand

Coarse sand

Sand with gravel

Safety on site
Safety precautions must be observed throughout construction activities.

Protective devices: workers must be advised strongly to wear safety goggles, helmets, gloves,
and clothing’s to protect their bodies from injuries.

Tools and equipments: care must be taken when using the tools during trial holes preparation so
as to prevent injuries. The trial holes should be guarded to prevent people from falling in them.

SETTING OUT

This is the name given to measuring out the building and its parts on the site. The positioning of
the building is usually described as the setting out drawing which are produced by the architect.
The actual location can be indicated using the survey stations, road kerbs or related to permanent
objects.

Setting out the building

To mark the position of a building a series of profile are used.

The profile: a profile consists of posts and a board as shown in diagram 1. The advantage of
profiles is that the position of walls, trenches and their thicknesses can be relocated at any time.
To mark those positions a string is stretched across two profiles and using a level or plumb bob
the positions can be obtained (diagram 2)

NB/ profile should be placed at all corners and firmly placed on the ground. The pegs should be
kept vertical and boards kept horizontal. They should also be placed one metre away from trench
to avoid disturbance during excavation.

Setting out a right angle

There several ways of obtaining a right angle. The most commonly used in building are:

1. Using the 3, 4, 5 method. The base line is marked 3 units long and then 4 units and 5
units one crossing the triangle so formed is 90º
2. Using the builder’s squire. This is a triangle made from timber with side lengths of
3metres or 2metres long. The triangle is placed in the corner and strings are stretched
both ways

Frontage line required measurement to road / boundary

 A C 3 D B

Large square 5 4

Diagonal checks

G H

E Fig 1 F

Nail ranging line

Peg

Fig 2

Profiles

When pegging out has been cheeked, profiles are set up clear of the trench runs and the positions
of the ranging lines transferred to them. Figure3. Profiles consist of a horizontal boards fixed to
posts or pegs. The position of the wall and trench being marked on them (boards) by saw cuts or
nails. Profiles are placed at all corners and at the ends of cross walls figure 4. The positions of
walls can be obtained from lines strained between them. Figure5.

Setting out peg

Profiles

Trench Lines wall

Peg for marking position of drawing

 Fig 3

Profiles

Trench

Fig 4

Alternative arrangement for corner profile

Foundation width

Lines

Drop bob

Level

Position of wall

Peg fixing level of concret

FOUNDATION
- Foundations are generally regarded as the lowest part of the building which evntually
transmits load into the ground in conjunction with the immediate ground (natural
foundation) that receives the loading.
- The artificial foundation must be able to safely do the transmission without causing
unnecessary movement of the structure.
- There are different types of foundations, the type chosen depending on.
- a) the type of structure used
- b) the soil bearing capacity
- c) the load imposed

Types of foundation

1) Strip foundation
These are suitable where there are continuous load bearing walls from the foundation.
The strip may either be made from concrete (mass or reinforced) or from masonry
construction e.g. brickwork. For mass concrete, the load is transmuted at an angle of
45º hence making the effective width of the strip is three times more the thickness of
the wall. The depth should be equal to this thickness as well. If it exceeds this the
foundation would have a tendency to crack along the line of shear unless
reinforcements are included.

Load bearing wall line of shear axial load

Line of load transmission
t t
o
45 Mass conc strip
3t tendency to fail if strip exceeds
3t with no reinforcement

2) Wide strip foundation

 This type is used where the soil has a low bearing capacity and the aim is to distribute
load over as much an area as possible. The strip in this case should be reinforced so as
to arrest the shear failure together with the cracking (flexural) failure at the bottom
due to bending moment.

Distribution bar

Main bar
Tendency to pull due to bending moment

3) Deep strip foundation

These are used where the suitable bearing is way down and since concrete is good in
compression strength, it is mostly done in mass concrete.

Deep strip

4) Stepped foundation
 These are done where the site is sloping for an economical solution. The strip is
stepped in series according to the land and fall and the lap between two sections of
the strip must be equal to the depth of the strip.
Where the step is substantial, the foundation is designed like column. The height of
the step should be such that a whole number of courses for the foundation walling fit
to avoid a fraction of a course.

Land fall
Whole number of wall course

If the height is > 600mm it should be H

designed as a column

Stepped foundation strip

Leveling of trench bottom for stepped foundation

Same as that for ordinary strip foundation except for gauging the levels for steps
Factors to consider:
1. Slope of land
2. Depth of foundation at the highest point on site
3. Thickness of foundation concrete
4. Level of next step
Procedure of leveling
A gauge rod with the thickness of the foundation and the total height of walling units
required to reach the step marked on it is used. The gauge rod is used to get the level of
the next step. Cut and plumb the vertical part of the foundation step, and then continue
leveling the bed (horizontal) as for ordinary stripped foundation.

5) Pad foundation/ isolated area

 This is done for isolated load bearing elements (column) and its mostly in reinforced
concrete since the tendency for failure is to cup in, the reinforcements are placed in
either direction.

Reinforced pad
Foundation

Main bars in both direction

6) Combined foundation
This is where two or three pad foundations are combined so as to distribute the load
effectively. This may be due to may be site boundary where the load cannot be
transmitted to the neighbors plot, a second column inside the site may be combined
with the first one so as to ease the loading on it.

Columns carrying equal loads

7) Raft foundations
 These are used where the building has a basement, the bearing capacity of the soils
are quite low.
There are three types of raft foundations namely:
- Solid slab raft foundation
- Slab and beam raft foundation
- Cellular raft foundation
Solid slab raft foundation
This consists of a continuous slab of reinforced concrete forming the foundation.

Foundation wall

Solid R.C raft (maximizes the load bearing area)

Slab and beam raft foundation

Beams are combined with the slab to give more rigid base for heavier loading. These
beams may either face downwards or can be upwards on which the floor rests.

Floor finishing on beam

Slab Foundation
Basement wall

Space created may be used as horizontal ducts

Cellular raft foundation
This is where the up stand beams are large as to form walls (basement walls) in lattice
manner (cells) these cells can be utilized i.e. used as rooms (i.e. for boilers or
storage). This type is done where very rigid bearing for heavy loading is required.
The cellular raft foundation may be combined with raft beam type and have down
facing beams.

R.C wall forming lattice cell

Slab

PILES (foundation)
These are underground columns done so as to span between the bearing stratum and
the building so as to transfer the load. They are similar to those in underpinning but
they are done together with the building.
Piles are classified according to:
a) Mode of bearing load
b) Mode of placing load
a) Mode of bearing load
i) End bearing piles: these simply bears load by transferring it to the lower
end where the bearing stratum is.

Load weak soil

Strong stratum
 ii) Friction piles: these bears the load as a result of the frictional forces
created between the sides of the pile and the ground.

Load

Frictional force

b) Mode of placing
i) Displacement piles: this includes those piles that are driven into the
ground hence displacing the ground material in its way.

Driving force

Material being displaced

ii) Replacement piles: These piles are casted into pre-drilled or bored holes
to replace the grounds material be casted insitu concrete piles.

Concrete

Reinforcement
Driven piles

This type of piles consists of rigid materials designed to be able to take the hammering often
done by mechanical means (and the design load). They are normally circular or hexagonal in
section so as to allow for easy penetration to the ground. The driving action is normally noisy
and causes a lot of vibration into the ground. This make it unsuitable in a heavy built up locality
where the vibration can easily destabilize existing foundation

Hammer

Supporting frame

Pile

Bored piles

These are typically replacement piles and involve casting or inserting piles in predrilled holes.
The process involves little vibration and so the more suitable in heavily built environment than
driven piles

Piles are designed to take certain load (i.e. bearing capacity) and since they should not be so
bulky so as to necessitate easy driving where more load is anticipated the piles are combined
(grouped

Pile grouping
These grouping are joined at the top by means of a slab (pad) which intern supports the ground
beam on which the building rests.

Driven tube piles

These piles consist of a hollow tube made from either precast concrete or steel, driven into the
ground and then filled with cast insitu concrete. There are two basic types:

i) Open end tube Pile; the hollow tube is simply plugged by means of hammering and
once driven filled up.
ii)

Space filled after drilling

Concrete/steel tube
iii) Closed end tube pile; the lower end in this case is closed by means of a steel cap and
again once driven, the hollow space is filled up say with reinforced concrete

Hollow tube

Steel cup

Structural stability
This concern the change in the soils caused by the imposed loading from the building
(dead & live loads) due to the variations in the soil, the nature of the soil must be
investigated before the design of a suitable foundation that would affect maximum
stability and to avoid overstressing the soil.
Bearing capacity
- This is the property of the soil dictating the maximum loading that it can take. This
measure is force per m². in determining this, soil sample is subjected to a load and the
point of failure determined. The bearing capacity is also a function of the
compactness of the soil.
- If the bearing capacity of the soil is known and the load imposed by the building also
known, the safe bearing area of the foundation can be worked out by using the
relationship

Bearing capacity = load imposed

Foundation area
NB/ as long as the ratio L/A does not exceed the bearing capacity, the soil would be
able to support the load without excessive settlement.
 Foundation area = length × width for strip foundation
 While for raft foundation = area of the raft
 Distribution of contact pressure
This is an effect which results from a combination of varying soil conditions and
foundations stiffness which causes unequal pressure distribution underneath the
foundation. It creates points of maximum pressure.
The pressure bulbs (diagrams)

Strip foundation

BC = load
area

Maximum pressure

Minimum pressure

Maximum pressure

Distribution of pressure

This is given by load

Area at any depth

The pressure distribution would more or less be uniform across any horizontal section and would
reduce as you go deeper since most of the loading is counteracted by the soil resistance. For strip
foundations, it would define a triangle at an angle of 45.
 Pressure distribution with depth

45º 45º

Mass concrete foundation

) 45º

Increased area also reduced pressure

Distribution of shear stress

Soils under the foundation are always subjected to shear stress due to resistant to the downward
force of the foundation. This is normally not uniformly distributed. Stress bulbs like those of
pressure can also be determined each bulb linking points of equal shear stress.

Ultimate bearing capacity

This is a measure of the soil strength. A point where the soils fail to support a load (imposed) due
to the safe bearing point or shear resistance bearing exceeded

Maximum bearing capacity

At this point the soil shear resistance is not exceeded though considerable settlement takes place.

Safe bearing pressure (capacity)

A point where the soils are loaded without any shear failure and settlement is within acceptable
limits.

PREPARATION FOR THE FOUNDATION

After site investigation it may be found that the top soil is soft and has poor bearing
capacity. It is usually economical to dig down to a layer of soil firm enough and of
good bearing capacity on which to lay foundation.
Before the start of any foundation trench work on the site the outlines of the
foundation concrete and the foundation wall has to be set out. After the setting out is
done and the diagonals checked for accuracy, work on trench excavation can began.
For small jobs work for excavation is done manually to a layer of good bearing
capacity. A width of at least 600mm is required for a man to be able to work in.
The sides of the trench should be trimmed vertically by hand and supported for the
safety of the people who will work in them. This is done so as to get a straight,
vertical and a fair finished face of the sides.

Supports for deep foundation

Supporting or propping of trench sides is done to keep the sides in place in case of
loose soil or wet sides which may cave- in.
Timbering is used to refer to temporary supports to the sides of the excavations. It is
sometimes referred to as plunking and strutting.
The main factors to be considered whether timbering will be provided or not are:
a) Safety of the men working in the trench. This is to protect them from the
collapsing sides.
b) The safety of the surrounding property and roads.
c) The nature of the sub soil.
d) The length of the time the trench will remain open before backfilling.

Terminology

Poling boards: boards varying in length depending on depth of excavation, it is

placed vertically against the face of the ground.

Walling: horizontal timber member which supports the polling boards between struts,
sizes vary from 75mm × 75mm to 225mm × 75mm

Struts: horizontal member holding the poling board apart, usually placed 1.8m to2m
depending on the type of soil and to allow for working space.
 225 x 50 polling board

225 x 75 walling board

150 x 50mm struts @ 180 c/c

Leveling of the trench bottom

The base of foundation concrete should rest of a firm and level base for it to distribute
the loads uniformly on to the adjoining soil below.

Transfer of datum to the bottom of trench

The practice usually is to fix level pegs in the trench one at each corner of the
building. The use of a gauge rod together with a spirit level may only be used easy in
shallow trenches while in deep trenches other methods such as the use of water levels
or dumpy levels may be adopted.

The first peg is driven into the bottom of the foundation and with the aid of water
level (water filled in a clear hose pipe) and a gouge rod; the peg is driven down until
the required height of the foundation is attained. One end of the hose pipe is held
level with top of the site datum which is at the same level with the proposed floor
level. The other end is held level with the height of the foundation wall up to damp
proof course. The heights of the courses are marked on the gouge rod.

Water level

Straight edge on datum peg

Datum peg

Gauge rod

Establishing levels

After the first peg has been driven to the required level, the levels for the other pegs
can be transferred from the established one. This may be done by the use of a long
leveling board and a spirit level.
 Spirit level straight edge board

Level peg

Trench bottom

NB: care must be taken to reverse the leveling board each time you work from peg to
peg so as to minimize errors in leveling.

Bottoming

This is referred to as the leveling of the bottom of the trench. It is done by cutting the
high positions, filling the low areas and compacting the loose earth as may be found
necessary after establishing the level pegs. Preparation of the bottom is done to ensure
that the foundation rests on an even firm bed.

Procedure for laying the foundation

1) Leveling: any of the methods of leveling the bottom trench may be used ( hose
leveling, spirit level with board )
2) Pegging: for deep strip foundation, it is necessary to transfer the level of the site
datum to some other pegs at corners and along the foundation trench. These level
pegs are also placed at junctions of wall. The pegs will help in obtaining the
leveled top of the mass concrete in the foundation trench. The desired level of
concrete can be obtained by transferring levels from these pegs to the top of
concrete by using a gauge rod and a straight edge.

3) Wetting: before the mixed concrete is poured into the foundation trench, during
dry and hot season, the trench must be wetted to minimize rapid loss of water
from the concrete to dry soil. This rapid loss of water would weaken the
foundation.
4) Placing the concrete: concrete should be poured into the foundation trench
carefully in uniform layers not exceeding 300mm. these layers should be
thoroughly compacted and leveled until the required thickness is achieved in
order to avoid segregation and air pockets forming in the concrete, it should be
poured at low heights.
When pouring manually the concrete should be lowered to the bottom in mortar
pans (karais) and buckets. At no time should the concrete be poured from a height
greater than 1.5m. The poured concrete should be cured for not less than three
days by wetting the top twice a day.