Types of rock bolts and the UCS test

1. SIMON MADIRO
R167149W

2. ROCKBOLTS
• In underground excavations rock masses are weakened and poses a great danger to
workers. To protect workers stability of these excavations is improved by rock
support. The most dominant roof support system is the use of rock bolts. 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 work by joining the rock mass together sufficiently before it can move
enough to loosen and fail by separating pierce by pierce. They can also be used for
supporting mesh wire but this is just a part of their purpose

3. .
• Rock bolting is a more effective and efficient support system because it is an
active support system utilising the rock to support itself by applying internal
reinforcing stresses
• They are installed in a pattern that is entirely dependent on rock quality and
the type of excavation that has been undertaken.
• The installation can be fully mechanised. The length and spacing of these
rock bolts can vary from place to place all depending on reinforcement
required.

4. TYPES
• There are many types of rock bolts used in mining operations across the
world. Rock bolts can be:
Mechanically anchored,
Grouted anchor bolts and
Friction anchor bolts.

5. MECHANICALLY ANCHORED
• Mechanically anchored bolts consist of smooth bar with a thread and to
which a shell anchor is attached and used to anchor the system.
• A toque is applied to the bolt, the bolt drives the plug against outer shell
which then expands and is set the in the borehole walls.
• Once anchored the bolt is tensioned and holds the rock mass together but
overtime this tension reduces due to creepy or rock failure. For this reason it
is advised to use rock bolts in strong rock mass.

6. MECHANICALLY ANCHORED BOLT

7. .
• Mechanical rock bolts are active short term support systems.
• They are recommended for:
Hard strong rock
Temporary reinforced system
Areas away from blast sites were bolt tension may be lost
Areas where tension can be checked regularly


8. RESIN POINT ANCHOR BOLTS
• They works same way as mechanical, the only difference is that the
expansion shell is replaced by fast setting resin. It therefore means that this is
an active support system
•

9. POINT RESIN ANCHOR BOLT.

10. FULL GROUTED ANCHOR BOLTS
• There are two forms of grouted bolts, the resin-grouted and cement grouted
rockbolts. They are relatively easy to install. The bolt is inserted into the
borehole and a fast setting resin or cement is pumped into the area between
rod and borehole.
• The resin sets and cement together the weaker rock fragments to the interior
stronger walls.

11. FULLY GROUTED ROCKBOLT

12. .
• Fully grouted bolts are passive long terms support systems.
• They are recommended to be used for:-
Areas where mechanical are not recommended
Long term support for thinly bedded root strata
Rock without wide fractures or voids heavy loads weak rock mass

13. FRICTIONAL ROCK BOLTS
• These are generally passive bolts because they cannot be pre-tensioned they
wait for the load to come to them.
• The most common one is the split set rock bolt. They are installed by being
forced into an undersized hole.
• This give rise to radial forces which are dependent on method of installation
• and those forces provide enough strength to hold together the rock mass.

14. SPLIT SET

15. .
• Frictional anchor bolts are relatively subject to higher rates of corrosion since
much of their length will be exposed.
• For this reason they can not be used for long term support of underground
excavations.
• Their major disadvantage is cost, they are expensive, easily corroded and also
installation of longer bolts is difficult
• Advantage is on easy fitting, very useful in moving and bursting grounds.

16. • Frictional bolts are used
Helping to secure wire mesh to roof or walls
Short term light loads
Burnt coal ribsides thinly laminated layers

17. THE UCS TEST
• Introduction
• Unconfined Compression Test (UCT) is a simple laboratory testing method to
assess the mechanical properties of rocks and fine-grained soils. It provides a
measures of the undrained strength and the stress-strain characteristics of the
rocks.
• The primary purpose of the Unconfined Compression Test is to quickly
determine a measure of the unconfined compressive strength of rocks or fine-
grained soils that possess sufficient cohesion to permit testing in the
unconfined state.

18. TYPICAL UCS TEST DEVICE

19. PROCEDURE
1. In the unconfined compression test, the sample is placed in the loading
machine between the lower and upper plates.
2. Before starting the loading, the upper plate is adjusted to be in contact with
the sample and the deformation is set as zero.
3. The test then starts by applying a constant axial strain of about 0.5 to 2%
per minute.
4. The load and deformation values are recorded as needed for obtaining a
reasonably complete load-deformation curve.

20. PROCEDURE CONTINUED
5. The loading is continued until the load values decrease or remain constant
with increasing strain or until it reaches 20%.
6. At this state, the samples is considered to be at failure.
7. Maximum axial stress unconfined compressive strength.
•S = σc / 2
Where S is undrained shear strength and σc is the unconfined compressive
strength

21. SOURCES OF ERRORS
i. One of the largest sources is the use of an unrepresentative sample of rock.
The rock may be unrepresentative because it is not the same as, or perhaps
even similar to the bulk of the rock mass found underground.
ii. If the sample is too small there will be significant end effects. These are
caused by the top and bottom plate. They can increase the strength of a
rock sample by preventing the formation of the weakest failure plane.
iii. Systematic errors from the equipment used
iv. Random error from taking readings.