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1
In Situ Rock Failure
at the Surface and Underground
John A Hudson
Lecture 6
We have already looked at
• the rock stress
• the stress path during excavation
• the failure of intact rock
• the occurrence/frequency of fractures
Let us now consider the modes of rock
failure at the surface and underground that
can occur during engineering activities
PAST PRESENT FUTURE
Structural Geology
Rock Engineering
Interpretation of natural
processes that have
created the rock
structures we see today
Prediction of natural
geohazards, such as
volcanic eruptions,
earthquakes, landslips
Interpretation of past
engineering practice:
past successes, and
past failures
Prediction of the rock
mass response to
engineering
perturbations
PAST PRESENT FUTURE
Structural Geology
Rock Engineering
Interpretation of natural
processes that have
created the rock
structures we see today
Prediction of natural
geohazards, such as
volcanic eruptions,
earthquakes, landslips
Interpretation of past
engineering practice:
past successes, and
past failures
Prediction of the rock
mass response to
engineering
perturbations
2
F1
F2
F3
Fn
Fractures
Intact rock
Boundary
conditions
Excavation
Water flow
We now need to
be able to
predict what will
happen when
the tunnelling
engineer makes
an excavation in
this mechanical
environment
3
Diagram from Dr Erik Eberhardt
4
In a blocky
rock mass
such as
this, it is
easy to see
what could
happen if an
excavation
were to be
made in it
5
Such as a
small
tunnel…
6
…or a surface
rock face
7
Plane failure in a granodiorite quarry
8
Wedge
failure in a
granodiorite
quarry
9
Large wedge failure at
the Teutonic Bore Mine,
Western Australia
10
Large wedge failure at the Teutonic Bore Mine, Western Australia
11
Spalling due to high horizontal stress Spalling due to
high vertical
stress
Depth of spalling
12
Spalling in a
mine ore pass
13
Rock slabbing/spalling at the
JinPing II hydroelectric
project in China
14
Worst case scenario – slabbing/spalling in the South African gold mines
15
Civil
Engineering
Mining
Engineering
Objective: Creation of
underground space
Objective: Obtaining
the excavated rock
Geometry specified
by engineering
function and location
with emphasis on
integrity of remaining
rock. Limited scope
for design.
Mine geometry
specified by orebody,
with emphasis on
excavated rock - but
many mining meth-
ods possible. Large
scope for design.
Borehole
cross-section dictated
by rotary drilling;
depth and orientation
determined by oilfield
access and production
strategy. Limited
scope for design
Petroleum
Engineering
Objective:
Transporting oil
TunnelMine stope Borehole
Objectives
16
The main mechanical stability problems are related to the
release of rock blocks and stress induced spalling
from Prof Derek Martin 17
?
from Prof Derek Martin
The effects of
rock stress and
fracturing on the
stability of
underground
openings
18
Pictures of underground rock failure
from Table 7.7 in
19
The
20
The
21
The
22
The
23
The
24
The
25
The
26
The three primary effects of
excavation are:
a) displacements occur because
stressed rock has been removed,
allowing the remaining rock to move
(due to unloading);
(b) there are no normal and shear
stresses on an unsupported
excavation surface and hence the
excavation boundary must be a
principal stress plane with one of the
principal stresses (of magnitude
zero) being normal to the surface.
Generally, this will involve a major
perturbation of the pre-existing
stress field, both in the principal
stress magnitudes and their
orientations; and
(c) at the boundary of an excavation
open to the atmosphere, any
previous fluid pressure existing in
the rock mass will be reduced to zero
(or more strictly, to atmospheric
pressure). This causes the
excavation to act as a 'sink', and any
fluid within the rock mass will tend to
flow into the excavation.
27
The construction of shafts, tunnels and caverns will lead to
changes in the rocks surrounding the excavation.
Excavation will result in localised mechanical deformation,
alteration in the stress distribution and changes in the water
flow and hydraulic properties of the surrounding rock
volume. The zone of altered properties is termed the
Excavation Disturbed Zone.
Some people prefer the term
“Excavation Disturbed Zone - EdZ” to the term
“Excavation Damaged Zone - EDZ”
because ‘disturbance’ can be described directly from the
mechanics, whereas ‘damage’ depends on the engineering
interpretation of the mechanics.
28
The inevitable disturbance is the result of removing
part of the rock mass, e.g. removing a horizontal
cylinder of rock to create a tunnel. Such excavation
not only removes the rock but reduces the
mechanical and hydrogeological resistance of the
region to effectively zero.
The additional disturbance is any extra disturbance
above this inevitable threshold disturbance caused
by the particular mode of excavation, blasting or
TBM.
29
Rock
movement
Disturb-
ance
Stress
redistri-
bution
Water
flow
INEVITABLE
DISTURBANCE
ADDITIONAL
DISTURBANCE
Rock
movement
Disturb-
ance
Stress
redistri-
bution
Water
flow
INEVITABLE
DISTURBANCE
ADDITIONAL
DISTURBANCE
EXCAVATION OF A TUNNEL BY TUNNEL
BORING MACHINE
EXCAVATION OF A TUNNEL BY
BLASTING
The inevitable and additional disturbances during rock excavation
30
Factors relating to the Excavation Damaged Zone
from Rolf Christiansson
31
A categorisation of rock reinforcement and rock support in continuous and discontinuous rock is required because rock reinforcement and rock support are not the same. (a) the block displacements are occurring because the rock mass
is a discontinuum, and hence the rock is reinforced so that it behaves like a continuum; or
(b) direct support elements are introduced into the excavation in
order to maintain block displacements at tolerable levels. The first option is known as rock reinforcement; the second is known as rock support.
32
33
Reinforcement Support
34
Experimental shaft in chalk (in preparation for the Channel Tunnel):
Rockbolt reinforcement
35
Cast concrete segmental linings for the Channel Tunnel:
Support
36
The different types of possible
Ground Response Curves
37
38
The influence of support stiffness
39
40
Problems up above
40
End of Lecture 6
41