ROCK TUNNELING
(By: KU Internship team, Mid-Bhotekoshi HPP, 2015)
1. DRILL AND BLAST METHOD
This tunneling method involves the use of explosives. Drilling rigs are used to drill blast holes on
the proposed tunnel surface to a designated depth for blasting. Explosives and timed detonators
(Delay detonators) are then placed in the blast holes. Once blasting is carried out, waste rocks
and soils are transported out of the tunnel before further blasting.
Drilling and blasting method is suitable for tunneling in medium to high strength rocks. It can be
applied to a wide range of rock conditions. Some of its features include versatile equipment, fast
start-up and relatively low capital cost tied to the equipment.
1.1 Procedure
As the name suggests, drilling and blasting works as follows:
A number of holes are drilled into the rock face.
The holes are then filled with explosives.
Detonating the explosive causes the rock to collapse.
The smoke and dust produced after the explosion is removed.
The rubble is removed and the new tunnel surface is reinforced.
Repeating these steps will eventually create a tunnel.
Figure 1 Drill and Blast Cycle
1.2 Drilling
The tunnel face can be roughly divided into four sections for drilling. The main goal is to ensure
the optimum number of correctly placed and accurately drilled holes. This helps to ensure
successful charging and blasting, as well as produce accurate and smooth tunnel walls, roof and
floor.
Figure 2 Types of holes in tunnel face
1.3 Drilling Pattern
The drilling pattern ensures the distribution of the explosive in the rock and desired blasting
result. Several factors must be taken into account when designing the drilling pattern: rock
drillability and blastability, the type of explosives, blast vibration restrictions and accuracy
requirements of the blasted wall etc. When designing a drilling pattern in tunneling, the main
goal is to ensure the optimum number of correctly placed and accurately drilled holes. This helps
to ensure successful charging and blasting, as well as produce accurate and smooth tunnel walls,
roof and floor. A drilling pattern optimized in this way is also the most economical and efficient
for the given conditions.
1.3.1 Hole Size
Hole sizes under 38mm in diameter are often considered small, holes between 41mm - 64mm
intermediate, and those over 64mm large. Most tunneling operations today are based on hole
sizes between 38 - 51mm in diameter. Only cut holes are larger than 51mm. Rock drills and
mechanized drilling equipment used in tunneling are designed to give optimum performance in
this hole range.
1.3.2 Cut Hole
The blasting sequence in a tunnel always starts from the cut, a pattern of holes at or close to
the center of the face, designed to provide the ideal line of deformation. The placement,
arrangement and drilling accuracy of the cut is crucial for successful blasting in tunneling. A
wide variety of cut types have been used in mining and construction, but basically they fall into
two categories: cuts based on parallel holes, and cuts that use holes drilled at certain angles. The
most common types of cut today is the parallel and V cut.
Figure 3 V Cut
The V cut is the older of the two and is still widely used in construction. It is an effective type of
cut for tunnels with a fairly large cross-section and requires fewer holes than a parallel cut. The
parallel cut was introduced when the first mechanized drilling machines came on the market
making accurate parallel drilling possible.
1.3.3 Stopehole
The holes surrounding the cut are called stopeholes (blast holes). The diameter of a stopehole is
typically between 41 - 51mm. Holes smaller than 41mm may require drilling an excessive
number of holes to ensure successful blasting. Holes bigger than 51mm can result in excessive
charging and an uncontrolled blast.
Holes are placed around the cut section in an evenly distributed pattern using a space/burden
ratio of 1:1.1. Burden is the minimum distance from the axis of the blast hole to the free face,
and spacing is the distance between blast holes in the same row. If hole size is between 45 51mm, typical spacing and burden are both between 1.0m - 1.3m. Actual rock conditions and
ability to drill in the required positions are factors that can reduce or add to the number of holes
needed. The design of the drilling pattern can now be carried out and the cut located in the cross
section in a suitable way.
1.3.4 Contour Holes and Floor Holes
Floor holes have approximately the same spacing as stope holes, but the burden is somewhat
smaller; from 0.7m to 1.1m. Inaccurate or incorrect drilling and charging of the floor holes can
leave unblasted bumps, which are difficult to remove later. The contour holes lie in the perimeter
of the drilling pattern. In smooth blasting, contour holes are drilled closer to each other and are
specially charged for smooth blasting purposes. Spacing is typically from 0.5m to 0-7m and
burden varies between 1 and 1.25 times the space. This type of layout makes it possible to use
special smooth blasting explosives, which limits the width and depth of the fracture zone in the
walls and roof caused by blasting. In special circumstances, two or more smooth blasting rows
can be used.
In tunneling, however, contour holes are blasted with stope holes, but timed to detonate last. The
result in smooth contour excavation mostly depends on drilling accuracy. The required amount of
shotcreting and concrete casting can be significantly reduced by using smooth blasting,
particularly in poor rock conditions. Smooth blasting increases the number of holes needed for
the drilling pattern by roughly 10 - 15%.
1.4 Charging and Blasting
In tunnel excavation, blasting works outward from the first blast in the holes in the cut. Each
blast provides more space for the following ring of blast holes. Successful blasting of the cut
section is critical to the success of the whole round. Because the cut holes initially have only one
direction in which to expand, the blast charge in the cut is considerably higher than in the rest of
the tunnel.
Contour holes should be blasted almost last. It is important to blast each smooth blasting section
(walls or roof) simultaneously to achieve a smooth and even surface. Bottom holes are blasted
last right before the bottom corner holes. This lifts the loosened rock pile a little, which makes
mucking easier.
1.5 Ventilation
In tunnel excavation, a ventilation system is required to provide an acceptable working
environment for the people in the tunnel. The environment is affected by the concentration of
impurities in the tunnel air. The impurities are mostly created by blasting and traffic in the
tunnel. Ventilation is provided to dilute the explosion gas so that toxic gas concentration is
acceptable, and get the next stages in the drill & blast cycle started.
1.5.1 Blowing ventilation is the easiest and most used method in tunneling. Fresh air from the
outside is blown through a duct into the tunnel, relatively close to the face. The fresh air dilutes
the gas plug and starts to move it backwards out of the tunnel. In longer tunnels with larger
cross-section areas, blowing ventilation is not adequate, or requires too long a ventilation time
before the cycle can continue
Figure 4 Blowing ventilation
1.5.2 Two-way ventilation is becoming a common method in tunnels that are longer than 1000
m. Two-way ventilation removes the explosion gas plug from the tunnel fast, providing an
improved working environment in the tunnel. In two-directional ventilation, the explosion gases
are sucked from the tunnel through a duct to the outside of the tunnel. Substitutive air is led to
the tunnel through a blowing duct (two-duct system), or through the tunnel (one-duct system).
The two-duct system is practical in long tunnels (> 4 km). The system removes explosion gases
fast and effectively. After the explosion gases are removed, both ducts can be used for blowing
ventilation to get even more fresh air into the tunnel during loading and transportation.
Figure 5 Two-way ventilation using two ducts
The one-duct system is practical in tunnels up to 4-5 km in length. With this system there is only
one ventilation duct in the tunnel. At the tunnel face end, there is a two-fan system which
controls the ventilation according to the stage in the drill & blast cycle. During drilling, charging,
loading & hauling, the system is used for conventional blowing ventilation. After blasting, a
transverse fan is used to remove explosion gases through the duct while the other fan blows fresh
air towards the face to ensure that all explosive gases are mixed and removed. The one-duct
system removes explosion gases fast and effectively, and is more cost-effective than the two-duct
system.
Figure 6 Two-way ventilation by single duct
1.6 Scaling
The purpose of scaling is to clear loose rock from walls and surfaces after blasting. Manually
done it is hard work involving many safe hazards such as falling rock and dust, and requiring
awkward working positions. Scaling is often very time consuming when done manually. Today,
modern mechanized scaling equipment is used whenever possible.
1.7 Mucking
Mucking is done after the fumes have been removed. Relevant loading and hauling devices can
be used for removing the muck. Two basic types of transportation system available for moving
materials in the tunnel are railroad tracks and various rubber-tired vehicles.
1.7.1 Rails: From the standpoint of energy consumption, rail haulage provides by far the most
efficient handling of materials in the tunnel. Rail-mounted vehicles are moved, singly or in
trains, by locomotives powered by either internal combustion engines or electric motors.
1.7.2 Rubber-tired Vehicles: Transportation with rubber-tired vehicles offers great flexibility
because its operation is not restricted to locations having fixed facilities, as is the case with
railroad track. A wide range of vehicle sizes and configurations, all typically diesel-powered, are
available. Standard front end loaders can be used for transport. Although primarily intended for
loading, such units may be economically used for short haul distances. Dump trucks can be used
to haul materials at greater distances.
1.8 Lighting
In the past, an evenly spaced string of light bulbs was the usual type of general lighting. On some
projects, no general lighting is provided, and personnel are supplied with flashlights or cap lamps
for emergencies and general use. Safety regulations may require specific lighting standards in
tunnels and should be consulted when planning a particular job. Floodlights are used for lighting
work areas, and are mounted in strategic locations on jumbos and other working structures.
1.9 Rock Support
As the tunnel is incrementally excavated the roof and sides of the tunnel need to be supported to
stop the rock falling into the excavation. The philosophy and methods for rock support vary
widely but typical rock support systems can include rock bolts or rock dowels, shotcrete and ribs.
1.9.1 Rock bolts or rock dowels
A rock bolt is a long anchor bolt, for stabilizing rock excavations, which may be used in tunnels
or rock cuts. It transfers load from the unstable exterior, to the confined (and much stronger)
interior of the rock mass.
Rock bolts are almost always installed in a pattern, the design of which depends on the rock
quality designation and the type of excavation. Rock bolts work by 'knitting' the rock mass
together sufficiently before it can move enough to loosen and fail by unravelling (piece by
piece). Rock bolts may also be used to support wire mesh, but this is usually a small part of their
function.
Figure 7 Typical rock bolting pattern for a tunnel
1.8.2 Shotcrete
Shotcrete is concrete conveyed through a hose and pneumatically projected at high velocity onto
a surface, as a construction technique. It is reinforced by conventional steel mesh, and/or fibers.
The need for ductility, toughness, and a residual strength is generally met by incorporating short,
thin pieces of wire or sheet steel into the mix (fiber shotcrete). Welded wire fabric (WWF) was
introduced into shotcrete usage to provide ductility; however, steel fiber now provides this
characteristic more effectively. Welded wire fabric is not recommended now a days due to
practical reasons. Even properly spaced fabric (4 in. X 4 in. or 6 in. X 6 in.) is quite stiff, making
installation time consuming, difficult, and therefore costly. When used in drill-and-blast tunnels,
considerable excess shotcrete may be required to fill overbreak to which WWF cannot be
properly formed. Sometimes, woven wire mesh is used in conjunction with rock bolts for safety
when rock is reinforced.
Shotcrete can be dry mix-shotcrete or wet mix-shotcrete. Dry-mix shotcrete consists of a
mixture of damp aggregate and cement fed into a placing machine, fed at a uniform rate into an
airstream to travel through a hose to the nozzle. The water of hydration is added at the nozzle
before discharge to the surface. Water is manually controlled, permitting adjusting to changing
surface wetness. Powdered accelerators are added to the dry mix as it is fed into the placer. If
liquid, the accelerator is mixed with the feed water before it goes to the nozzle.
The wet-mix process consists of mixing measured quantities of aggregate, cement, and water,
and introducing the resulting mix into a vessel for discharge pneumatically or mechanically
through a hose to final delivery from a nozzle. It has the advantage of rigidly controlling the
water/cement (W/C) ratio of the product. Successful methods have been devised to introduce
quick-acting accelerators to the delivery hose. Pumping low-slump concrete is commonly a
problem, and so a slightly higher than desirable water content is used. By use of accelerators,
such concrete can be made to adhere overhead, but ultimate strength usually suffers. However,
the method has been found convenient for use with less-skilled operators.
The strength of shotcrete should be tested and approved for used in projects. Shotcrete testing is
a three-part process. The first stage, compatibility checking, is required before the proposed
materials and their sources are approved. Cement-accelerator compatibility is of prime
importance. Similarly, compatibility of the entire mix and proportions must be established by
meeting the various requirements with proposed mixes prepared, cured, and tested in the
laboratory. The second stage, field trials, begins upon completion of the first part. Material from
the approved sources should be combined per approved mixes by the production equipment to be
used and then shot by a certified nozzleman into appropriate size boxes. After curing in the
manner proposed for the production work, samples should be taken and tested.
The third stage, production testing, has three parts. First, the field trial process should be
repeated at the heading during production shotcreting upon demand by the engineer. Second,
cores should be taken from the in-place shotcrete, at specified intervals. The primary purpose of
these is to check thickness and adhesion; however, compressive strength should also be tested.
The third part is the overall checking of the in-place concrete. In addition to a visual check for
defects, the shotcrete should be sounded at frequent intervals (locations) by striking with a
geologist's or similar hammer. Sound, adhering concrete will give a distinct ringing sound.
Laminated shotcrete or voids behind the shotcrete will result in a drummy or hollow sound. If
drummy, the area should be rechecked thoroughly and the approximate boundaries determined.
Cores should then be taken and examined. Defective shotcrete should be removed and replaced
with sound shotcrete.
1.8.3 Ribs
Steel ribs set close to the tunnel surface and blocked from it are normally used as the initial
support system for rock tunnels, especially those constructed by conventional drill-and- blast
methods. Wood, concrete, or steel lagging may be placed between the ribs to secure blocky or
raveling ground, or welded wire fabric can also be used.
ROCK SUPPORT FOR VARIOUS ROCK CONDITIONS
Dsfa
Support
Rock Mass Class
Very Good Rock
I
RMR: 81-100
Good Rock
II
RMR: 61-80
Fair Rock
III
RMR: 41-60
Poor Rock
IV
RMR: 21-40
Very Poor Rock
V
RMR:0-20
Rock bolts (Length
1/3 to Tunnel
Shotcrete
Steel Sets
Width)
Generally no support required except for occasional spot bolting.
Locally bolts in roof
10 ft. long, spaced 8
ft. with occasional
wire mesh.
Systematic bolts 12
ft. long, spaced 5-6 ft.
in roof and walls with
wire mesh in crown.
Systematic bolts 1215 ft. long, spaced 35 ft. in roof and walls
with wire mesh.
Systematic bolts 1520 ft. long, spaced 35 ft. in roof and walls
with wire mesh.
2 in. in roof where
required.
None.
2 to 4 in. in roof and
1 in. on walls.
None.
4 to 6 in. in roof and
4 in. on walls.
Light to medium ribs
spaced 5 ft. where
required.
6 to 8 in. in roof. 6 in. Medium to heavy ribs
on walls and 2 in. on spaced 2 ft 6 in. with
face.
steel lagging and
forepoling if
required. Close
invert.
Rock support for various Rock Mass Ratings can be chosen using the following chart as a
guideline:
Fig. Tunnel Support chart for rock bolts and shotcrete as a function of RMR
