Geological processes and the mechanical aspects of rock squeeze

119

Tectonophysics, 91 (1983) 119-135 Elsevier

Scientific

Publishing

GEOLOGICAL

Company,

Amsterdam

PROCESSES

- Printed

in The Netherlands

AND THE MECHANICAL

ASPECTS

OF ROCK

SQUEEZE

T.R. HARPER

and J.S. SZYMANSKI

Leas Cottage, Callow Hi/l, Virginia Water, Surrey (United Kingdom)

Geoscience Services, 50 Divisian Avenue, Millington, N.J. (U.S.A.) (Received

January

21, 1982; accepted

May 25, 1982)

ABSTRACT

Harper,

T.R. and Szymanski,

J.S., 1983. Geological

processes

and the mechanical

aspects of rock squeeze.

Tectonophysics, 91: 119- 135. The

nature

commonly common

and

origins

of slow displacements

termed rock squeeze when hazardous origin of strain

development

which occurs

result of the stimulation

of geological

materials

of strain

are considered

energy

movements stimulate

which

on strain geological

energy materials

changes

of geological

to engineered

materials

structures,

in response

to natural

by man, is emphasised.

include

the effects

in near-surface

rocks.

near

the earth’s

surface,

are reviewed. The similarity geological

The origins

of present-day

processes,

and

and as a

and modes of release

differential

Some of the activities

vertical

crustal

of man which

may

are noted.

INTRODUCTION

Of the various

geological

hazards

to which engineered

the term rock squeeze has been used to denote rock masses. As a result of such progressive engineered

structures

has been focussed

such that their integrity

on certain

geographic

(e.g. Lee and Lo, 1976), yet analysis

facilities

the progressive

displacements, is impaired.

locations

may be exposed,

slow displacements

of

loads may be applied Attention

to

to rock squeeze

where the effects are conspicuous

of the mechanisms

involved

demonstrates

that

the phenomenon is more common than is normally recognised and that it is of widespread occurrence. This paper first summarises the various mechanisms which may be responsible for progressive, slow displacements of rock masses. A main objective is then to review, without detail, strain energy release and crustal movement as causes of the progressive displacements. The more general objective is to emphasise that both natural geological processes and the stimulation of geological materials by man give rise to a spectrum of overlapping and interacting displacement modes and displacement ~-1951/83/~-~/$03.~

0 1983 Elsevier Scientific

Publishing

Company

120

rates.

The displacements

quently,

if the hazards

potentially plants,

may affect

detrimental

which

may

to the public,

then a prior assessment

The intention

the integrity arise

of engineered

as a result

of structural

such as in nuclear

of the displacement

of this discussion

structures.

damage

power plants,

potential

is to review principles.

Conseare

LNG or NGL

is appropriate.

The presentation

of detail

is avoided. ROCK SQUEEZE

AND

GEOLOGICAL

PROCESSES

It is our belief that rock squeeze derives from natural geological processes, as well as from changes brought about by man’s activities. While the rates of progressive rock mass displacement may be increased (or reduced) as a result of man’s disturbance of a natural state of dynamic equilibrium, both geological and artificially-induced

displacement

rates are viewed as parts

of a continuum.

The various

rates of displacement arising from natural geological processes and the various rates of displacement arising from man’s disturbance of geological materials combine to form a spectrum

extending

from zero displacement

much higher than are necessary to damage traditional techniques. Not only are geological the earth’s

surface

frequently

dynamic

(Harper

and Szymanski,

in nature

(Hast,

rate (temporarily)

engineered facilities materials frequently

to rates very constructed by metastable near

1981), but the state of equilibrium

is

1966).

Not only do the strain rates of geological processes and artificially induced strain rates overlap, but so do the modes. Thus the strains developed during rock squeeze induced

by excavation,

occurring

for example,

in the undisturbed

may be similar

state. Bedding

in nature

to those which were

plane sliding is an example

of a form of

strain common to both natural and artificially stimulated rock mass displacements. (Such stimulation could arise by raising the water table, removal of overburden or by excavation

and change

of lateral

boundary

restraints).

It is not uncommon for near-surface geological materials to exist in a state of limit equilibrium, reflecting the slow progression of strain development arising from a continually adjusting balance between rock strength and the applied forces (Hast, 1966;

Harper

and

Szymanski,

198 1). Consequently,

in geological

materials,

the

engineer should not be considering whether or not the rock squeeze phenomenon will occur; it is more realistic to assess the rate at which it will occur. It may be that the strength behaviour, disturbance

of the rock is such that displacements and that displacement (e.g.

by

excavation).

rates are negligible On

the

other

result almost entirely

from elastic

except shortly after a period of hand,

response such as sliding may occur. These may be delayed compounded with the progression of natural geological engineering concern.

components

of inelastic

by viscous damping and processes to become of

In general, if Hast’s 1966 concepts of dynamic equilibrium are accepted, then a similarity and overlapping nature of natural geological strain development and that

LONG

TERM

INCREASED

RATE

OF

DISF’LACEMENT APPARENTLY CAUSED BY BLASTING OF TUNNELS BECK

1905

1910

1920

1930

1940

IQ50

1960

FOR

SIR

IL POW&R

ADAM PLANT

1970

1975

Fig. 1. Inferred inward movement of one wall of the Niagara Wheelpit excavation (movement inferred by halving the recorded closqe of the excavation). (After Bowen et al., 1976).

strain development which occurs as a result of the stimulation of geological materials by man is to be expected. This paper concentrates upon those mechanisms of rock squeeze whereby both natural and artificially-indu~d strains in geological materials are similar in origin and mode of devefopment. REVIEW OF THE MAIN CAUSES OF ROCK SQUEEZE

Progressive slow displacements of rock masses have commonly been termed “rock squeeze” because of their effeci on engineered structures. One of the most impressive demonstrations of the process describes the closure of an excavation in the Niagara region at the west end of Lake Ontario in North America. Figure 1 shows the approximately 70-year record of inward movement of one wall of this excavation. Many other examples of progressive displacements and transfer of strain to engineered structures have been recorded in this area. These include cracking of tunnel linings, buckling of steel struts placed between opposing walls of excavations,

122 Passive displacement in response to boundary displacements

Hydration

Chemical

swelling

reactions

generally not reversible e.g. hydration of anhydrite, pyrite oxidation

(take up of interlayer water or capillary pressure effects in clays and shales) 1

Strain

Energy

Release

(strain energy accumulated in response to geological boundary displacements*). Form of either or or

strains (a) (b) (c)

may be

:

Normal Distortional Both

Fig. 2. Mechanisms of rock squeeze. (* Finite strain also possible from

residual

strain

release:

not

quantified.)

cracking

of base mats and crushing

the necessity progressive gineered

for repeated buckling

facilities

dredging

* buildings. Rose (195 1) reported of a canal in Lockport, New York, resulting from

of the canal

would include

floor.

Analogous

the longitudinal

The view that these displacements with excavation

of in-ground

result

has been widely expressed

forms

compression

primarily

of deformation

of en-

of dams and bridges.

from stress relief associated

(e.g. Franklin

and Hungr,

1976). There

appears to be no recognition that the progressive displacements may occur as a natural geological process. Yet the presence of neotectonic structures (e.g. Franklin and Hungr, 1976; White et al, 1973) is a record of such displacements. Such structures have often experienced a transient phase of high strain rate (Szymanski and Laird, 1981), the results of which appear to dominate the character of the resulting finite strain state. High strain rate may occur, for example, during buckling, or when a structure first breaks through to the free (ground) surface * The term “in-ground” is used to denote a surface structure founded in an excavation (the base of which lies below ground surface).

123

(White

et al., 1973). At the other phases of development,

may best describe observed

today.

in relation

the process However,

more correct can occur activities

of the high-strain

as suggested

of natural

may be superimposed

earlier in this discussion,

geological

processes

state

rate phase is insignificant

of the period of slow strain rate development.

perspective,

as a result

a slow strain rate

most likely to have given rise to the finite strain

the timespan

to the duration

however,

alone.

Therefore,

a

is that rock squeeze Any effects

on these, and indeed greatly enhance

of man’s

the strain rates

observed, but it seems fundamental that natural adjustments are commonplace, and directed toward the attainment of an ever-changing equilibrium condition, while man’s activities Figure

may dramatically

2 shows

identified:

the main

modify

causes

the nature

of rock

(1) passive * displacement

and rates of adjustment.

squeeze.

in response

Three

principal

to boundary

causes

are

displacements;

(2)

strain energy release (this may contribute to the process either directly, or indirectly via a process of volumetric increase caused by capillary pressure changes); (3) chemical reaction (generally irreversible). This discussion is restricted to the relatively strain energy release and “passive” displacements ments. been

No further labelled

comments

chemical

will be noted

reactions

common mechanical processes of in response to boundary displace-

concerning

in Fig. 2. For the interested

chemical changes have been discussed by Einstein and Vogan (1970). Volumetric changes associated have been discussed BOUNDARY

the release

has

examples

of

and Bischoff (1975) and Quigley with capillary pressure changes

et al. (1979).

first the passive

displacement

of stored

energy.

because

is therefore

strain

unlikely

category

substantial

This category

it is probably quantities

that rock mass displacement “Boundary”

does not involve a totally

artificial

of stored strain energy;

will not be accompanied ** displacements

it

by some

are presented

as a

solely for ease of explanation.

Figure 3 shows a series of possible ment. Taking

category.

In reality,

most rocks contain

degree of relief of the stored strain. separate

reader,

which

DISPLACEMENTS

Consider category

by Harper

the mechanism

model 2, the relative

models of differential

lateral

displacement

mass may be deduced for a given magnitude ment, relative to an element at a differing

vertical

crustal

of an element

of differential vertical elevation. (This could

displace-

within

a rock

crustal moveapply to any

engineered structures founded at a given level, yet subject to lateral movements at another level. For example, it relates to a structure founded on the bottom of an

* The term passive

is included

** The term “boundary” convenient considered

term

rather

a boundary

to emphasise

is placed than

a true

an absence

in inverted mechanical

yet only represent

commas

of strain energy to indicate

boundary

a discontinuity

release.

that the boundary

(e.g. a fault

plane

of strain and stress state).

may

may well be a be conveniently

124

3. DISTRIBUTED

1

_/A

4. LOCALISED

BENDING

BENDING

5. JUXTAPOSITION OF MOVEMENTS OF OPPOSING SENSE

\

Fig. 3. Models

_I

----

of differential

crustal

movement.

z.

0

GD”NDARY------r

Fig. 4. Model of amplification

a’;-=

ae

NO-DISPLACEMENT

MAGNITUDE OF RESULTANT SHEAR DISPLACCYENT ON ;:NE;EE BEDDING

I

I I

I

I I

I

I

NO EXTENSIONAL STRAIN DILATION IN VERTICAL PLANES

displacements.

I. ASSUMES 2.lGNORING

of geological

NOTES.

I

1

I

\

SHEAR

I

u

I\ I I \I

0

4:

STRAIN

E

126

113y1 NI NOllVA313

F 1

3

* *

I

127

excavation, nearer

whose walls are affected

to the ground

movement

reveal

negligible

surface).

that

for typical

relative

would

of strata

which are located

describing

such differential

be very

small

and

usually

structures.

however,

such that the displacements

computations

displacements

in-ground

It may be possible,

by displacements

Simple

that a form of geometric

are considerable.

Figure

amplification

can occur

4 shows an example

of such a

possibility. The effect of this geometrical arrangement is such that displacements one bed relative to an adjacent bed can be enhanced by perhaps two orders magnitude.

Figure

5, a neotectonic

structure

observed

in flat-lying

may be a product of such a geometrical arrangement, release of strain energy accumulated and stored during

sedimentary

in combination the geological

of of

rock,

with the history of

these strata. STRAIN

ENERGY

RELEASE

The release of stored strain energy is probably the most universal of the mechanisms responsible for rock squeeze. As indicated in Fig. 2, this can also trigger another mechanism. Because strain energy release corresponds to a reduction of confining pressure, swelling can occur in response to strain energy release. In turn, the dilative strain of the swelling process may then increase the strain energy of the system if limited-displacement boundary conditions pertain. This may be a significant factor in the maintenance of equilibrium conditions in geological materials and, in certain

rocks, in the maintenance

of substantial

magnitudes

of stored strain,

even

near the free (ground) surface. In general, it is often the weaker rocks which tend to demonstrate pronounced swelling (argillaceous rocks); perhaps this swelling characteristic

provides

a mechanism

whereby

in the weak rocks of a sedimentary Consider associated

the swelling

release

similar

sequence

of stored

strain

magnitudes

of strain can be stored

as are stored in relatively energy,

effects that may occur. Nichols

per

se,

strong rocks.

in isolation

(1980) has inferred

from

any

that changes

in the large-scale * dimensions of rock materials may result from the release of residual strains **. One can imagine that such a process could contribute to rock squeeze if the elastic moduli of the locked-together materials (e.g. elastic grains and cementing material) were different. However, further investigation of this possibility may be appropriate before this possible mechanism is accepted to be of realistic engineering significance. Moreover, some authors have claimed that residual strains are only released in the vicinity of free surfaces (e.g. Friedman, 1972). (Also, inelastic processes

such as rupture

* The term “large” engineering

in this

at grain boundary

context to denote

scales

contacts

of the order

could be encour-

of the dimensions

of an

excavation.

** Self-equilibriating, Barron,

is used

and sliding

1972.)

balanced

within

very small volumes.

(Terminology

according

to Bielenstein

and

128

Fig. 6. Asymmetric

excavation

closure

resulting

from the relief of distortional

aged by the release of such a form of stored strain energy).

strains.

Consequently,

while it is

theoretically feasible that residual strains may contribute to the rock squeeze process *, the matter is not considered further in this discussion. The subsequent and main

part of our discussion

energy

which

remanent

of strain

has accumulated

gravitational

Figure 2 records

energy release concerns

in response

or remanent

tectonic

to boundary

only release of strain displacements,

or by active tectonic

whether

displacement.

that the types of strain energy release that may be involved

rock squeeze process

are the release of normal

strains,

of distortional

strains,

in the or of

both. The release of normal strains has received widespread recognition by the geotechnical profession (e.g. Lee and Lo, 1976). A typical example of normal strain release is the inward The effects significance

movement

of distortional

would

of the sides of an excavation. strains

seem to be equally

have received

far less recognition,

great. The only field example,

yet their

to which the

effects of distortional strain are attributed, known to these authors, relates to the distortional strains, acting on vertical planes, associated with differing magnitudes of horizontal principal stress (Quigley et al., 1978). These authors reported sliding on vertical joints in the sidewall of an opencut excavation. Moreover, one can readily envisage that a distortion of excavation shape can result if the horizontal stresses are not aligned parallel and normal to the walls of an excavation.

principal

The significance of shear stresses acting in horizontal planes does not seem to have received the attention merited by their engineering significance. Only geological * Such displacements

have also been shown in the laboratory

(Price,

1966).

129

aspects ski,

have been discussed

(e.g. Szymanski

1981). Figure 6 shows

closure

an example

of displacements

of layer-parallel

shear strain, released

1981; Harper

of the asymmetric

which can result from the relief of distortional

the distribution (about

and Laird,

associated

profile

strains.

of excavation

This example

with an essentially

from an additional

and Szyman-

uniform

shows

distribution

short section of excavation

4 m) at the base of a shaft.

If rock

displacements

are instantaneous,

or short-term

only,

then

it can

be

feasible simply to wait for the displacements to cease before commencing construction of the buildings. (Implicit in the suggestion of this expedient is the assumption that the phenomenon is recognised by the constructors or the regulatory body.) However,

as the Niagara

Wheelpit

record indicates,

the long-term

nature

of the rock

squeeze process can prohibit the elimination of the problem by the simple procedure of waiting for stability to occur. Possible reasons for this time-dependence include the following: argillaceous

viscous behaviour materials);

effects of crustal or changes POSSIBLE

of the strata (in particular,

the interaction

movements;

of strain

and, changes

energy

perhaps, release

of rock strength

and

induced

the viscosity

of

swelling;

the

by weathering

of water table elevation. SOURCES

OF STRAIN

ENERGY

The sources of the strain energy, the release of which is responsible for these long-term progressive displacements, are indicated on Fig. 7. This may be either “fossil” (remanent) or may be continually replenished by crustal movements. Fossil strain energy can derive from two processes of quite different scale: regional processes and local processes. For example, strain energy may accumulate as a result of the downwarp and uplift of a sedimentary basin (e.g. Price, 1974). This would be termed essentially normal

remanent gravitational strain energy. However,

shear stresses (Harper

and Szymanski,

sliding

in a sedimentary

of one horizon

modifying example normal

the effects of progressive

and distortional

of layer-parallel displacements

strain energy. Conceivably, even a slight dip can impart

198 l), which may subsequently sequence normal

over the adjacent stress

which probably

relief. occurred

it could be layer-parallel encourage

horizon,

Figure

6 shows

in response

the

thereby an

to both

strains.

Long-term displacements can also result from the release of strain energy which is associated with localised strain energy accumulation. Such accumulation is associated with folds and fault structures. In both fold and fault formation, for example, bending stresses develop. Very significant quantities of strain energy can accumulate in association with only apparently minor flexure. Thus, even minor faulting can be associated with the storage of substantial quantities of strain energy (e.g. Harper and Szymanski, 1981). Figure 8 shows an example of progressive displacements which followed the excavation of part of a drag fold adjacent to a fault in a sequence of sandstones and shales of Carboniferous age in the United Kingdom.

130 STRAIN

ENERGY

EVOLVING (balanced accumulation/dissipation)

FOSSIL (remanent)

Nature of change 1. progressive, 2. intermittent

(a) Regional; (b) Local structure. (modified by strain gradients associated with free surfaces)

:

::;g,,m;;;:“,ts:::lHu_

3:

Released by .: (a) decrease of material strength (e.g. erosion, rising water table); of equilibrium !b) disturbance by man (excavation, impoundment of water); strength Cc) time-dependent properties; of vertical crustal (d) effect movement on equilibrium state; (e) earthquake triggering.

may be steady; release

lation (e.g. oscillatory vertical crustal movement superimposed on a lon term trend). intermit ! ent accumulation/dissipation (oscillatory crustal movements).

4.

Fig. 7. Origins of strain energy in geological materials.

The question might naturally arise in the reader’s mind: but are geological materials able to store strain energy for long periods (of the order of lo8 years)? It is not our intention

to debate

the observations

of residual

provide

that strain energy can be stored in geological

evidence

of geological

proportions.

not

to represent

purport

this matter strain

here. However,

energy

(This, of course, an

identical

release

represents

magnitude

Price (1974) has noted

which

he reported materials

a specific of strain

example to that

that

in 1966 do for periods and does which

was

introduced about lo8 years before present.) The strain energy released in rock squeeze processes may not be confined to “fossil” origins. The deformation associated with differential vertical crustal movement (Fig. 3) can be associated both with the accumulation and the dissipation of strain energy. This may result in an even progression of displacements. On the other hand, an intermittent release may occur, stick/slip in nature. This may be a result of the strength characteristics of the material. Reversals of displacement sense could even occur, associated with an oscillatory form of vertical crustal movement. At time scales of lo*-104, natural processes can cause boundary displacements of major significance with respect to the induction of strains in geological materials. Glacial loading is perhaps the most obvious example. Tilting of strata caused by

STRUCTURE AOJACENT Of

1 0

CONTOURS TO

A FAULT

ON AND

A HORIZON LOCATION

INCLINOMETER

J 1

2

3

TIME

4

5

6

7

8

(MONTHS)

Fig. 8. Displacements resulting from disturbance of a drag fold.

glacial loading has been held responsible for the generation of shear forces sufficient to develop neotectonic structures (Szymanski and Laird, 1981). Even during shorter time scales, IO’--IO2 years, the boundary displacements associated with differential vertical crustal movement can be influential in that they can disturb the equilibrium state (Harper and Szymanski, 1981) and thereby promote strain energy release.

132

200 600

/

’ 7’

Years

Fig. 9. Water level changes

As noted,

7z’ 1’ a

-5

5000

10000

before

present

at the southeast

the decrease

(6.P)

margin

of strength

of Lake Ontario.

of geological

material

is directly

with the change of boundary conditions brought about by uplift, proximity to the ground surface where no loads are applied. Reduction

associated erosion and of confining

pressure reduces the strength of the material, and thereby its ability to store strain energy. In particular, frictional sliding becomes easier. (Buckling is also facilitated and high strain-rate features such as pop-ups may be witnessed *.) Proximity to the ground surface may also be responsible for another form of strength reduction. In materials which may be dry at depth, such as granites, the increasing frequency of

* In reality

a phase

rate) adjustments

of high-strain

rate (buckling)

before and after buckling.

is probably

accompanied

by progressive

(slow strain

133

joint

development

as the rock approaches

is associated

with strain energy changes.

distribution

of the pore

weakening surface

the medium.

strength

Apart associated

residing

Naturally,

surface weakens

the rock and

in near-surface

the effects

fracture

of weathering

promote

networks,

thus

can also cause near-

reduction.

from

progression

water

the ground

What is more, this can presumably

these

changes

of displacements with changes

of water table geomorphological

associated

with

proximity

may be profoundly

of pore pressure.

elevation associated evidence. A rapid

to the free surface,

influenced

by changes

Figure 9 shows an example

with drop

the

of strength

of the changes

a period of glaciation, inferred from of water level was associated with the

melting of the ice which was blocking the outlet of the proglacial lake. It can be speculated that this caused an initial phase of high pore pressure as the water load was removed, and pore pressures were unable to dissipate in the strata underlying the relatively impermeable boulder clay left by the retreating glacier. At this stage the rocks became rapidly weakened. Subsequently, caused the strata to become progressively stronger.

the fall of water table must have Finally, the slow progressive rise

of the water table was associated with a reduction of effective stress (and possibly a slaking of partially-saturated strata). A renewal of the progression of slow strain-rate displacements

may be inferred.

DISTURBANCE

OF THE EQUILIBRIUM

OF GEOLOGICAL

MATERIALS

The activities of man, of course, disturb the equilibrium of the geological materials. Although tierhaps contrary to one’s instinctive reaction, rocks are extremely sensitive to disturbance. First proposed by Hast (1966) a dynamic condition of limiting equilibrium should be regarded as relatively commonplace, not a rarity. Thus, when man creates an excavation, even a small one, some readjustment is to be expected.

All the effects of excavation

effects of water level changes The impoundment

may not be immediately

obvious,

such as the

on swelling.

of water is best known

in terms of high strain-rate

displace-

ment (induced seismicity). However, there is no reason not to assume that in certain circumstances a process of slow strain energy release is promoted by water impoundment. Certainly, such artifacts

the equilibrium as reservoirs

of the geological

geological

modified

by

(e.g. Snow, 1972).

Man is not the only agent which may advance is part of natural

system is significantly

evolution.

the rate of strain dissipation

For example,

one part of a geological

which system

might influence another part by means of energy released in the form of vibratory ground motion. This might disturb the delicate equilibrium pertaining at locations other than at the location of energy release, although strain release might be inhibited in such a situation by transient pore pressure changes.

134

CONCLUSIONS

Progressive impair

slow displacements

the structural

is termed

integrity

rock squeeze.

release associated We propose,

however,

stimulated

have been attributed

that slow displacements

material

of geological

observed

to

this phenomenon to strain

energy

by excavation. materials

(of signifi-

of engineered structures) can also occur as a result of natural This proposition derives principally from observations of a

state of limit

development

have been

Sometimes

of the geological

equilibrium

in geological

from the effects which may be inferred of the magnitudes

materials

structures.

These displacements

with the disturbance

cance to the integrity geological processes. natural

of geological

of engineered

observed

of neotectonic by the activities

even in intraplate structures.

materials

near

to result from differential regions,

Accepting

of man, a continuous

the earth’s crustal

and from

that geological spectrum

surface,

movements

analyses materials

of modes

of the may be

and rates of

displacement may be envisaged, extending from totally natural to predominantly artificial. This includes a range of overlapping relationships in terms of both the rates and the modes of displacement. Consequently, if the malfunction of an engineered structure is potentially detrimental to public safety, an evaluation of the potential for rock squeeze is warranted. Because natural geological processes cause slow displacements of geological materials, it is unreasonable to evaluate the matter on the basis of whether or not rock squeeze occur.

will occur;

a more logical

While it has been accepted for rock appear

squeeze,

approach

is to assess the rates at which it will

that the release of normal

the significance

to have been recognised.

of distortional Perhaps

strains has been responsible

strain

energy

release

this is a result of the common

does

not

assumption

that horizontal planes are principal planes. However, shear stresses in the plane of bedding of flat-lying sedimentary strata have been recorded, and distortional strain relief observed to follow excavation. The nature of the strain energy which promotes slow displacements of geological materials may be eitherremanent (“fossil”) or currently evolving, and it may exist at regional or local scales. Hydration swelling may be a mechanism whereby strain energy levels can be maintained in soft and weak (argillaceous) materials. The release of strain energy is promoted by a reduction of confining pressure and other factors associated with uplift and erosion as geological materials approach the ground surface.

ACKNOWLEDGEMENTS

Many individuals have been involved in the collection of the data presented in the figures in this paper. In particular, the authors whish to acknowledge the exceptional

135

efforts

of Gordon

Appel,

Scott

Laird,

Bob Aten,

Martha

Pendleton,

and

John

Beneway. REFERENCES Bowen,

C.F.P.,

Hewson,

Can. Geotech. Bielenstein,

F.I., MacDonald,

J., 13:

D.H., Tanner,

R.G.,

1976. Rock squeeze at Thorold

tunnel.

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