Tunneling : Drill and Blast Method Prepared by Jitin Malhotra
08515g
Tunnelling : Drill-and-blast Method
The advancement of long tunnels through hard rock well
beforetunnel boring machines(TBMs) were invented relied entirely on
thedrill-and-blast method. Today, the drill-and-blast method is
still widely practiced and used in building tunnels through hard
rock where the use of tunnel boring machines is not justified and
too expensive. The drill-and blast-method is also used in
combination withfull face drillingwith tunnel boring machines.
PROCEDUREAs the name suggests, drilling and blasting works as
follows:A number of holes are drilled into the rock, which are then
filled with explosives.Detonating the explosive causes the rock to
collapse.Rubble is removed and the new tunnel surface is
reinforced.Repeating these steps will eventually create a
tunnel.
The positions and depths of the holes (and the amount of
explosive each hole receives) are determined by a carefully
constructed pattern, which, together with the correct timing of the
individual explosions, will guarantee that the tunnel will have an
approximately circular cross-section.
CYCLE TIME:-Cycle time is the time taken between two consecutive
blast. That is sum of time taken to all the activities between two
consecutive blasts.
Progress of work < 1 Cycle time (if length of blast is
same)
DIFFERENT ACTIVITIES OF CYCLE TIME1. Drilling and blasting2.
Defuming3. Mucking4. Scaling5. First layer shotcrete6.
Rockbolting7. Wiremesh stiching8. Lattice girder fixing9.
Forepoling10. Second layer shotcrete 11. Bottom cleaning and
survey12.Final rock bolting13.Face drilling.14.Charging and
blast.15.Intrruptions.
DRILLING AND BLASTINGDrilling is done with help of drill jumbo
(tamrock) with two or three booms. It has drilling capacity of 2m
per minute , after drilling gelatine is loaded.
DefumingGases produced after blast are allowed to go out.
Defuming is carried out to about half an hour. Duct pipe is used
for this purpose which is also used for ventilation.
MuckingMuck produced during blast is removed in dumpers and
dumped at dumping site. To clear the face quickly the muck is
disposed at nearby points ,can be in a tunnel or outside and then
later taken to dumping site.
Scaling After mucking, scaling is done to remove the loose rock
left after blast. It is done with help of tamrock, twin cutter and
excavator/backhoe.
Twin Cutter
Tamrock
First layer of shotcreteShotcreteing is done immediately after
the blast to increase the standup time of rock and to prevent any
loose rockfall. Its thickness is usually 50 mm. Its done with
shotcrete machine.
RockboltingRockbolting is done after first layer of shotcrete 25
mm or 30 mm dia rockbolts are used initially and their spacing
depends on the rock conditions. It is done with tamrock.
Rock Bolt
WiremeshWiremesh is placed between the shotcrete to increase its
strength. Wiremesh of size 100x100x4 is used .
Lattice girder fixingLattice girders of 3 sets are fixed after
wire mesh installation. These are basically made of steel bars
having different sizes like 25, 25, 32 dia. They are fixed to walls
with help of bolts.
Forepoling Forepoling is the process in which steel bars of 6m
length , dia 32 mm are inserted into the face from crown to
bottom.
FOREPOLES
Final layer shotcreteFinal shotcreting is done finally with
executed after forepolling.
Bottom cleaning and surveyFinally cleaning of bottom surface is
done and survey is done with the help of total station.
Final rock boltingFinal rock bolting is done depending upon the
type of rock to give more strength.
InterruptionsInterruptions are the time taken in activities
which, without completion of other work cannot be completed or
started. Holidays are also the part of interruptions.
Paper TargetTotal StationInterruptions!
LINING AT DIFFERENT ADITS
During lining reinforcement is provided at places where
needed.No reinforcement is provided upto rock class (iii).Lining is
done in different thickness it may be 40mm or 60mm.For lining
gantry is used which also act as form work and vibrators are also
attached with it for the compaction of concrete.M20 concrete is
used for lining.
