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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

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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

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Diagram from Dr Erik Eberhardt

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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

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Such as a

small

tunnel…

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…or a surface

rock face

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Plane failure in a granodiorite quarry

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Wedge

failure in a

granodiorite

quarry

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Large wedge failure at

the Teutonic Bore Mine,

Western Australia

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Large wedge failure at the Teutonic Bore Mine, Western Australia

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Spalling due to high horizontal stress Spalling due to

high vertical

stress

Depth of spalling

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Spalling in a

mine ore pass

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Rock slabbing/spalling at the

JinPing II hydroelectric

project in China

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Worst case scenario – slabbing/spalling in the South African gold mines

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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

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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

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Pictures of underground rock failure

from Table 7.7 in

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The

20

The

21

The

22

The

23

The

24

The

25

The

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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.

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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.

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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.

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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

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Factors relating to the Excavation Damaged Zone

from Rolf Christiansson

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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.

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Reinforcement Support

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Experimental shaft in chalk (in preparation for the Channel Tunnel):

Rockbolt reinforcement

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Cast concrete segmental linings for the Channel Tunnel:

Support

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The different types of possible

Ground Response Curves

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The influence of support stiffness

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Problems up above

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End of Lecture 6

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