ACTIVE
SUPPORTS
RECAP
1. What happens when the virgin ground is first disturbed by an
excavation?
2. Mention three importance of supporting the excavation.
3. What is ground reinforcement?
4. Define bolt-torque.
5. Mention three factors to be considered when designing
underground mine support system.
6. Differentiate between active and passive support.
7. Why does passive support do not work on weak ground?
ACTIVE SUPPORT SYSTEM
INTRODUCTION
• Refers to a system of supports which immediately upon installation
against a rock face exerting restraint or loading forces against the rock. Or
as a systems where by element of support become part of the rock mass in
much the same way as reinforcing steel become part of reinforced
concrete.
• Active support system includes rock bolts, cable bolts, and shotcrete
ROCK BOLTING
• For short term applications the bolts are generally left ungrouted. For
more permanent applications or in rock in which corrosive groundwater is
present, the space between the bolt and the rock can be filled with cement
or resin grout.
TYPES OF ROCKBOLTS
1. MECHANICALLY ANCHORED ROCK BOLTS
They generally consist of plain steel rods with a mechanical anchor at one end and
a face plate and nut at the other. They use expansion shells on the end of the hole to
connect the bolt to the rock which are always tensioned after installation.
The holes are drilled in advance, and the bolt with the expansion shell is placed into
the hole. Make the hole about 100 mm longer than the bolt.
Once the bolt is inserted, pull on it sharply so that it will expand and dig into the wall
of the rock. Also tensioning of rock bolts is important to ensure that all of the
components are in contact and that a positive force is applied to the rock.
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Area of application
• This kind of bolt is best for moderately hard to hard rocks. They are not very
effective in closely jointed rocks and in soft rocks, because of deformation and
failure of the rock in contact with the wedge grips.
• Also grout can be used to prevent rock bolt failure by rusting or corrosion and
to lock the mechanical anchor in place, so that it cannot be disturbed by
blasting vibrations and rock mass displacement
2. GROUTED-RESIN ANCHORED BOLT
• Resin anchored rock bolts, are sealed using a resin. A cartridge full of the resin in
separate compartments is placed at the end of the hole, and the bolt is stuck in the hole
after it. The bolt is then "drilled" through the hole, puncturing the cartridge and causing
the resin to mix by this spinning action. The resin is then released into the hole, and it
slowly hardens seal the bolt and keeps the bolt in place. Settling of the resin occurs
within a few minutes (depending upon the specifications of the resin mix) and a very
strong anchor is created.
• The fast- settling resin is inserted first followed by slow-settling resin.
• Cement materials are sometimes used as grouted materials on anchored rock bolts.
Area of application
For 'permanent' applications such as bolting around shaft stations or crusher
chambers, etc.
This type of anchor will work in most rocks, including the weak shale’s and
mudstones in which expansion shell anchors are not suitable.
Resin grouted rock bolt is considered a permanent ground support with a lifespan
of 20–30 years.
3. FRICTION ANCHORED ROCK BOLTS-SPLIT SETS
Are types of rock bolts which are much more easier to install than mechanical bolts or
grouted bolts (Bolt lengths < 3 m). For split-sets, the bolt is hammered into the drill
hole, which has a smaller diameter than the bolt. Pressure from the bolt on the wall
holds the rock together.
Area of application
Split-sets are particularly susceptible to corrosion and rust from water unless
galvanized split sets are used.
CABLE BOLT
A cable bolt may be defined as the steel cable cemented into a drill hole so as to
help support blocky ground. The major advantage of cable bolt is that it can suspend
long broken rock mass.
Cable bolts are much longer than standard rock bolts, usually between 10–25 meters
long.
TYPES OF CABLE BOLT
1. BULBED CABLE BOLTS
2. GROUTED CABLE BOLTS
BULBED CABLE BOLT
Bulbing is an effective means to center the bolt in the drill hole to develop bond
for anchoring the bolt. It may also be fully grouted with resins, or cement grouts.
GROUTED-CABLE BOLTS
Are used to bind large masses of rock in the hanging wall and
around large excavations.
Cable bolts are grouted with a cement grout.
Cables can be grouted into holes by:
Breather tube method or
Grout tube method
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For breather tube method ， the grout usually having a water/cement ratio ≥
0.4, is injected into the bottom of the hole through a large diameter
tube,typically19 mm diameter.
The air is trapped through a smaller diameter tube which extends to the end
of the hole and which is taped onto the cablebolt.
Both tubes and the cablebolt are sealed into the bottom of the hole by means
of a plug of cotton waste.
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GROUTED-CABLE BOLTS
For grout tube method ， a large diameter grout injection tube extends to
the end of the hole and is taped onto the cablebolt.
The cablebolt and tube are held in place in the hole by a wooden wedge
inserted into the hole collar. Note that care has to be taken to avoid
compressing the grout tube between the wedge and the cablebolt.
Grout is injected to the top of the hole and is pumped down the hole until it
appears at the hole collar.
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Summary of the
development of
cable bolt
configurations
