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Annals of Glaciology 2 1981© International Glaciological
Society
THE CONTRIBUTION OF DISCONTINUOUS
ROCK-MASS FAILURE TO GLACIER EROSION
byKen Addison
(The Polytechnic, Wolverhampton WV1 1LY, and St Peter's College,
Oxford OX1 2DL, England)
PRINCIPAL GEOMECHANICAL ROCK-MASS PROPERTIESA major problem in
assessing the erodibility
of rock stems from the gross inequality betweenthe internal
shear strength of rock and glacierice; this, is partially resolved
for abrasion byregarding the glacier sole as an ice-rock-debrismix,
but large-scale block removal or "quarrying"is not so effectively
explained.Intact rock-mass strength (IRMS)
The failure criteria for rock is usuallydefined by a simple
linear Mohr-Coulomb equation(Fig.l):
by freeze-thaw loosening (Jahns 1943, Lewis1954, Chapman and
Rioux 1958, Battey 1960,Whi11ans 1978). Although the age,
formation,and significance of the resultant sheetingstructures
(supposedly formed irrespective ofany existing anisotropy) are
sometimes qualified,preoccupation with "pressure release"
demonstra-tes a simplistic and frequently inaccurate viewof
rock-mass properties and response to stress,as other authors have
indicated (Harland 1957,Twidale 1972, 1973, Brunner and Scheidegger
1973).Occasionally, absence of serious attention torock-mass
performance under stress has led tomany omissions. The "melt-water
hypothesis" ofcirque erosion (Lewis 1938, 1940) is one
clearexample, where the effect of water pressure onstability is
ignored, often despite substantialevidence of its abundant presence
in rock mass;another is the notable avoidance of suggestedfailure
mechanisms in studies which otherwisedemonstrate intimate
structural control overerosion (Haynes 1968, Sugden 1974).
Bedrock performance has been examined inmore detail recently.
Broster and others (1979)give qualified support to the fracture of
intactrock by glacier ice; this is questioned later inthis paper.
Although primarily not investigatingnor modelling rock properties,
Morland andBoulton (1975), Morland and Morris (1977), andBoulton
(1979) consider that a jointed bedrockmodel responding to basal ice
shear would bemore appropriate. Up to now, studies of
bedrockresponse to specific stress conditions inducedby glacier ice
lack an understanding of theprinciples of rock mechanics and
resultingfailure mechanisms. Those principles consideredby the
present ai/thor to be important win nowbe outl ined.
ABSTRACTGeomechanica1 rock-mass properties control
the response of bedrock to applied stresses andcan be summarized
in a linear Mohr-Coulombequation, which defines the principal
parametersdetermining failure. Nevertheless, in studyingthe erosion
of bedrock by glacier ice, littleattention has been paid to failure
criteriaalthough a coincidence of erosional landformswith fracture
systems at regional and localscales has been demonstrated. Few
studies haveanalysed the precise nature of the fracturegeometry, or
proposed its mechanical impact inassociation with glacier ice.
This investigation proposes that, sincealmost all bedrock
possesses identifiablefracture systems, the properties of
discontinuousrock mass (ORM) be regarded as defining
primaryconditions of stress and stability which aresubsequently
modified directly and indirectly byglacier ice. Consequent
rock-mass failure modesare prescribed by discontinuity geometry
andapplied stresses, and evidence from North Walesconfirms the
validity of the theoretical treat-ment of rock-mass properties, and
explains theaccordance of landforms with structure.INTROOUCTI
ON
Glacier erosion of hard rock is a complexprocess controlled by
material properties inter-'acting at what Weertman (1979) termed
the bed-water-ice interface. Bedrock propertiesdetermine
exclusively the first of thesecomponents and hydraulic conductivity
influencesthe second. Although many ice-flow problems havebeen
resolved, rock-mass performance under stressbeneath and adjacent to
glaciers has receivedlittle attention, although the extent to
whichthe close control on 91acier erosion exerted bygeological
structure has been recognized,especially by geomorpho10gists.
Randall (1961),Ho1tedahl (1967), and Nilsen (1973) consideredthat
large-scale fractures or joints ofCaledonian age determined fjord
and otherglacially-eroded valley ali9nment in Norway;Trainer (1973)
described incipient joints openedby ice flow in a wide range of
rock from Cali-fornia, Maine, and New York State, and
Zumberge(1955) demonstrated structurally-aligned glacierscouring
around western Lake Superior. In allcases, structural elements
involved are clearlyof pre-glacial tectonic origin,
whereasrelaxation of overburden-confining stresses wasbelieved by
some authors to cause rock fractureimmediately preceding or during
glaciation, andhence to facil itate "quarrying", often enhanced
T = c + a tan ~ ,where '[ and a are shear and normal
stresses,respectively, c is the value for internal
(1)
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Addison: Contribution of rock-mass failure to glacier
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the failure criteria are for dry rock and afurther significant
modification afforded bydiscontinuous rock mass (DRM) is high
secondarypermeability (Witherspoon and Gale 1977). Aswell as
contributing to shear stress, waterreduces normal stress 0 to
effective normalstress On by a value u (Hoek and Bray 1974), andfor
practical purposes the modified Mohr-Coulombequation may be written
as
'[ = c + (0 - u) tan (~ + ~f) , (2)or, in a simplified form,
as
where ~r is the effective friction angle.Roughness increases the
friction angle ~ by anamount ~f, and the plane possesses,
morecorrectly, bi-linear shear strength, whichassumes the lower
(residual) value upon shearingof asperities (Witherspoon and Gale
1977).
In practice, all rock mass possessesinternal fractures which
normally demonstrate astrongly preferred geometry, determined by
geo-logical (principally tectonic) history (Attewelland Farmer
1976). Marked planar anisotropyrenders DRM liable to simple failure
in a co-axial stress field and to complex failure,involving
multiple planes, elsewhere. Compoundfailure along discontinuities
and cracks propa-gated across rock bridges was reported in
labora-tory tests (Brown 1970). Brown also suggestedthat
discontinuous rock-mass failure (DRMF) only
IT
(f----
Fig.l. Linear Mohr-Coulomb relationshipsbetween shear stress and
normal stress forintact rock mass (IRM); the progressivereduction
in required shear stress is shownfor dry discontinuous rock mass
(DRMd) whereC = 0, and wetted discontinuous rock mass(DRMw), where
a given friction angle ~ isreduced to an effective friction angle
~r.
cohesion, and ~ is the angle of friction alongthe eventual
failure plane (Hoek 1970). Adistinction is made between peak shear
strength,beyond which the rock deforms, and the residualshear
strength of the deformed mass. These maybe similar for soft rock,
whereas residualstrength may be as little as half the peakstrength
for hard igneous rock (Hoek and Bray1974). Typical intact rock
shear strengths andother properties are shown in Table I.
'[ = c + 0- tan ~ ,n r (3)
TABLE 1. SOME TYPICAL ROCK-MASS PROPERTIES (after Hoek and Bray
(1974), Kulhawy (1975), among others)Uni-axial Friction angle ~
Residual friction Cohesion C
compressive strength (tri-axial load) angle
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Addison: Contribution of rock-mass failure to glacier
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FAILURE MODES
Plane failure (of single or multiple slabs)occurs when
Fig.2. Failure modes and their related discontin-uity stereonet
(equatorial equal-area lowerhemisphere projection).
of gravity overhangs a pivot point (de Freitasand Watters
1973).
It is emphasized that, although actual slopestability may be
complex, practical applicationof theoretical analysis is generally
successful(Hoek 1973). Also, whilst its primary applica-tion is for
gravitational loading, an extensionof principles to dynamic glacier
loading may beappropriate and theoretical modifications tostress
relationships induced by glacier ice arenow proposed.
o/s < o/d,i Z ~ . (9)STRESS MODIFICATIONS BY GLACIER ICE
A glacier must activate inherent rock-massinstability for
erosion to occur (Terzaghi 1962);this investigation restricts
itself to suggestedde-stabilizing processes and not to the
necessaryresultant entrainment or incorporation with theglacier.
Two quite different domains are recog-nized; (i) rock mass confined
by ice and underdynamic load, and (ii) rock mass unconfined
andprimarily loaded by gravity.Rock mass confined by ice
This is the more difficult to analysebecause of the relative
inaccessibility of theice-rock contact, and also because DRM
isinherently stable in its primary valley-floorposition. Equations
(7) and (8) do not apply,and discontinuity-stress relationships
show
Although gravitational load does not meet limit-ing equilibrium
requirements, two factorsincrease shear stress. First, ice flow
generatesa low-magnitude shear stress of about 0.1 MN m-2(Weertman
1979) enhanced for an ice-rock mix.Second, melt water at the
ice-rock mass contactpenetrates discontinuities, contributing a
forceV and reducing normal stress by a value u(Equation 6).
However, it is recognized that acomplex feedback relationship
exists here,whereby secondary penneability afforded by DRMmay alter
critically the basal pressure-meltingbalance. Two further
qualifications are made tothis domain. Once begun, block removal
permitt-ing low-angled planes to "daylight", and alsoremoving the
restraining presence of the block,may reduce sliding resistance
sufficiently underdynamic load, where o/d j
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Addison: Contribution of rock-mass failure to glacier
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*To be published as "The instability of chalkslopes".
fracture geometry (Fig.4). In both cases,structurally controlled
DRMF beneath and adjacentto the glacier bed is believed to have
been theprincipal mechanism of excavation.
Fig.3. Frequency orientation diagrams of regionalfracture
systems for each of the six mountaingroups of Snowdonia, mapped
from stereographicair photograph cover. Circular scale shows 5
km.Inset outline shows location of Figure 4.
REGIONAL STRUCTURALDISCONTINUITIES
5'
? ~ ~ 'kmIT""TI mileso 1 2 3
FIELD EVIDENCE OF GLACIER-INDUCED DISCONTINUOUSROCK-MASS
FAILURE
A combination of inferred and calculatedconditions identifies
failure in a previously-glaciated environment. The difficulties
ofobserving rock-mass failure and erosion aroundexisting glacie~s,
especially under the ice,justify the study of failure at
previouslyglaciated sites (where a comprehensive survey ofthe
fracture systems is possible), provided thatthe evidence for a
glacial origin of failure isconvincing. Location and mode of
failure iseasily recognized by residual rock-wall
elementsrepresenting the release surfaces, block debriswhere
present, and visual comparisons of theslope and discontinuity
geometries (de Freitasand Watters 1973, Addison, unpublished,
Causayand Farrar, in preparation*). Detailed confir-mation may be
calculated from measurement of thediscontinuity geometry (Silveira
and others 1966,Young and Fowell 1978) with typical values
.ofgeomechanical properties, or specific valuesobtained from in
situ and laboratory tests, and
the back-wall zone of cirque glaciers must beconsidered to make
a major contribution to DRMFwithout recourse to freeze-thaw
mechanisms.
Again, further qualifications may be madewith respect to the
destabilizing mechanism.(i) Unconfined failure can only apply
toglaciers confined within bedrock channels,limiting the mechanism
almost entirely to valleyand cirque glaciers, and thus accounting
in partfor their significantly greater erosive power.(ii) Once
initiated, progressive failure iscontrolled primarily by the DRM
geometry; unlikeengineering applications, where the requiredslope
and discontinuity geometries mayor maynot coincide favourably, it
is suggested thatglacier erosion will always show close
conforma-tion. Principal applied stress will seek outthe most
closely related potential failure planes,and hence structure
controls glacier erosion.
Geomechanical principles, modified for theglacier environment,
are now investigated forapplication to glacier-eroded rock mass in
Snow-donia, North Wales, after reviewing the generalstructural
geology of the region.
STRUCTURAL GEOLOGY OF SNOWDONIA, NORTH WALESClastic marine
sediments, progressively
interstratified with ignimbrites and laterintruded by dolerites,
rhyolites, and microgran-ites, form a lmost all of the 1 200 km2
study area,and represent a complex Lower Palaeozoic (lateCambrian
to late Ordovician) synclinal accumula-tion whose axis forms the
mountain core above1 000 m.
The pile was subjected to pronounced poly-phase deformation of
Caledonian tectonic origin(late Silurian-early Devonian (Shackleton
1954)),and four distinct structural components arerecognized,
represented by four fold axes( Fl - If) and associated syngenetic
axial-planarcleavage (Sl _If) (Helm and others 1963, Lynas1970).
The regional structure is dominated byF2' S2, with their typical
Caledonoid NNE-SSW
strike, mainly dipping steeply north-west.A primary fracture
geometry of steep-
vertical discontinuities, which confirms theCaledonoid tectonic
stress field, has beendescribed (Addison, unpublished). Over 2 900
kmof principal fractures were recorded. Thesystematic regional
("master") fracture pattern(Fig.3) corresponds to expected tectonic
config-uration (Badgley 1965, Fookes and Wilson 1966,Price 1966,
Causay 1977); in the field, thethree-dimensional discontinuity
geometry measuredin bedrock outcrops replicates the
establishedregional pattern, and continues within rock slabswith
close facsimile planar anistropy. Strengthand spacing depend on
lithology at the smallerscales, but otherwise the fracture network
dis-regards lithological boundaries.
Previous research on glacier erosion inSnowdonia concentrated on
the significance of theorientation and elevation of nearly 50
cirques inreconstructing Pleistocene glacio-climatology(Seddon
1957, Unwin, unpublished), and, whilstdistribution conforms to a
north-west Europeanpattern, it also corresponds intimately to
thefracture geometry (Addison 1977). Moreover, arecently
reconstructed Pleistocene "Merioneth"ice cap, centred to the
south-east of Snowdonia(Addison, unpublished, Foster,
unpublished,Rowlands, unpublished), was shown to have beenthe
source of radial outlet glaciers whichbreached the mountain axis
with transfluenttroughs up to 600 m deep. As with the cirques,they
too show a marked conformity to the regional
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Addison: Contribution of rock-mass failure to glacier
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REGIONAL STRUCTU-RAl DISCONTINUITY CONTROL
Fig.4. Primary fractures in the Snowdon (SW) andGlyder (NE)
groups and principal glacier-erodedcirques and troughs. Fractures
are shown bybroken lines, cliffs by toothed shading, andlakes by
stippling.
expressed as a factor of safety F where F =represents limiting
equilibrium (Hoek 1970,1973) .
A glacial origin for DRMF is inferred fromthe glacial history of
the site, contemporarystability in the absence of
glacier-relateddisturbing forces, and, in particular, absenceof the
failed mass at the toe of the slope.Site examples from Snowdonia
are now presented.Confi ned failure
Slope-failure criteria do not apply soreadily here as stated
earlier. At the threechosen sites, DRMF was compound, being
induceddynamically in otherwise stable rock mass bybasal shear, and
then, once block separationbegan, local small-scale slab, wedge,
andtoppling failures occurred along destabilizeddiscontinuities.(
i) Culm Stwlan
Excavation for the upper dam foundations ofthe Ffestiniog
pumped-storage hydro-electricscheme, constructed on the bedrock
threshold ofa glacial cirque, revealed considerably disturbed,hard,
unweathered rhyolite dislocated along pre-existing discontinuities
to a depth of 13 macross a front 150 m wide, which
necessitateddesign modifications. Anderson (1969:193) con-sidered
the dislocation to have been caused byglacier drag across the
threshold: " ...facili-tated by the presence of five faults in the
partmost affected and by joints almost at rightangles to the
rock-lip ... The affected zone doesnot tail off but ends abruptly
on both sides.
The limits may be partly related to the faults.but they may also
mark the width within whichthe glacier was thick enough to exert
drag".(ii) Ogwen
Bedrock floor in a major glacier-breachedwatershed is shown in
Figure 5. Ice flowing fromleft to right first abraded the rock,
followedby "quarrying" (displaced blocks show striationson one
surface only) which removed some blocksaltogether and displaced
others; thereafter,gravitationally loaded secondary failure
furtherdislocated the rock mass, at least in part sub-
Fig.5. Ogwen valley floor. Sub-glacier confinedDRMF, with
secondary failure evident in excava-ted sections. Slab failure
occurred along twoplanar sets (a, b) and toppling failure awayfrom
(c).
Fig.6. Toppling failure in Nant Peris; de-stabilization caused
the parting of blocksalong arrowed discontinuities.
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Addison: Contribution of rock-mass failure to glacier
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glacially since many blocks are missing. Failuresurfaces were
entirely controlled by the discon-tinuity geometry, and primary and
modified con-fined failure is indicated by the displacement
ofdebris down-glacier and down-slope.(iii) Nant Peris
Figure 6 shows an example of topplingfailure towards the valley
floor generated bythe removal of adjacent rock mass under
glacierconfinement close to the valley floor. Cliffelements such as
these are common, with at leastthe greater part of the failed rock
mass absentfrom the toe; by comparison, the few remaininginstable
blocks which have recently toppled fromthe now unconfined face are
all present at thetoe, and exhibit less-weathered contact
planes.Unconfined failureCWm Graianog
This cirque basin affords one of the finestsite concentrations
of all modes of rock-massfailure in Snowdonia. It is excavated
inFfestiniog grit, and failure modes are discussedwith reference to
a standard structural presenta-tion (Hoek and Bray 1974) shown in
Figure 7.
Fig.8. Cwm Graianog (Ffestiniog grit). Northside wall (right)
with D1 planar surfaces;head wall (1eft) wi th D1- D2 wedgi ng and
D2planar surfaces.
CWM GRAIANOG
Fig.9. Cwm Graianog. Single D1 major planar-slide release
surface, (showing ripple marks).
slab towards the western end of the basin mayhave failed to the
full height of the rock wall(200 01) and across a width averaging
60 m. Theremaining rock wall is clean, being devoid ofresidual
blocks, overhanging elements below whichrock mass have been
released, and stable laterally-confining units. With a principal D1
spacing of3 01, this would have yielded a single failure of36 000
m3 of the side wall, all of which wasremoved by the glacier. D1
planes in the wallat this point are marked uniquely by
large-scalebedding-plane ripples (Fig.9), and the only debris
-4-Plane
DEGRADED
ROCKWALL/MORAINE
BOUNDARY
LOWER ROCKWALL
LIM ITUPPER ROCKWA\..\..liMITr7l. :.> Moel PerfeddU
Granophyre
1II11anVirn Slate
@ Ffest ini ogl2:;l Grit --
f........ -~~----~~~--~------~--O-----125--250m
I I I
04~
Sl!a£i.!!g01 3m02 2m03 2m04 I-3m05 ~5m
(b) Discontinuity Slereollel
Fig.7. Bedrock map and discontinuity stereonetfor Cwm
Graianog.
The north sidewall consists almost entirelyof a spectacular
series of D1 surfaces (Fig.B)and the slope angle is effectively the
same as thediscontinuity dip of 38-40° to the south-east.Slab
failure down D1 was released along D2'which frequently possesses an
injected quartzfill, and D3' and the vertical extent ofindividual
slabs is limited by D" with the samestrike, but opposite dip, as
D1. One entire
blocks so marked are found several hundred metresaway in, and
resting upon, cirque moraines inpositions where they could have
been depositedonly by glacier ice.
The irregular strength and spacing of D5renders it 1ess apparent
as a control, but D 2and D" become more important as the back wall
isapproached. The "curved" transition is effectedby D1- D2 wedge
failure (1/Ji=39°, orientated at130°) producing a series of
buttresses, and theextent to which the geometry
predeterminedfailure on excavated slopes is completed by thesuperi
mpos it ion of topp 1ing fa ilure releasedfrom D1 and low-angled D"
(itself below tileassumed friction angle) on more complex slab
and
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Addison: Contribution of rock-mass failure to glacier
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wedge failure in the south wall. A rapid assess-ment of current
stability (after Hoek 1970)suggests that 01 has a factor of safety
F >- 1for typical assumed mechanical values, which hastwo
important implications. (i) Excavation atthe toe would rapidly
cause 01 to "dayl ight" andgenerate further slab failure; this is
consideredto be typical of the effect of 91acier erosion.(ii) Other
modifications to Mohr-Coulomb para-meters would result in failure;
locally smallcontemporary slides are evident, considered to bethe
smoothing effect of weathering on roughnessesalong the 01
planes.There is a dramatic decline in side-wallheight at the
Llanvirn slates-granophyre contact;it is suggested that the lower
ORMS of slatespermitted greater excavation of the side wall,and it
is further noted here and elsewhere thatcleavage planes do not
appear to have providedsignificant failure surfaces during
glaciererosion.
CONCLUSIONGeomechanical rock-mass' properties have
been neglected in examining processes of glaciererosion, and it
is proposed that alteration ofstress relationships in discontinuous
rock massdirectly or indirectly by glacier ice provides arealistic
principal mechanism for the study ofbedrock excavation by glaciers.
Theoreticalfailure criteria applied to specific rock-massproperties
are sustained by field evidence, andsupport the following
conclusions.1. Failure of rock slabs occurs along pre-existing
discontinuities, and the relative dis-position and strength of the
discontinuitygeometry provides an exclusive framework forexcavation
and is manifest in structurally-controlled erosional landforms at
all scales.2. Failure in confined rock is due primarily tothe
dynamic loading potential of the conditionsat the ice-rock mass
contact, and in unconfinedrock to the activation of gravitational
loadingon otherwise stable slopes.3. ORMF re-defines in more
appropriate mechanicalterms conditions which in certain
circumstanceshave been identified as erosion processes involv-ing
"pressure release" and "melt-water sapping".It is contended that
many quoted instances ofpressure release in fact describe parallel
slopefacets determined by pre-existing discontinuitygeometry, where
forms of glacial erosion havebeen controlled by structure rather
than viceversa.4. Stress conditions in discontinuous rock masscan
be incorporated usefully into theoretical andpractical examination
of ice flow patterns andbehaviour at the rock-ice interface.
ACKNOWLEOGEI4ENTSThe author wishes to thank M.V. Barr,
British Petroleum Research Centre, Sunbury, forcommenting on the
draft; Peter Masters and JackLandon, School of Geography,
University of Oxford,for photographic work; Brenda Cartwright,
ThePolytechnic, WOlverhampton, for typing the manu-script; and The
Polytechnic, Wolverhampton, forfinancial support in presenting the
paper.
REFERENCESAddison K 1977 The influence of structural
geology on glacial erosion in Snowdonia,North Wa 1es. x INQUA
Congress, Birmingham,1977. Abstracts: 5
Addison K Unpublished Aspects of the glacia-tion of Snowdonia,
North Wales. (OPhilthesis, UniverSity of Oxford, 1975)
Anderson J G C 1969 Geological factors in thedesign and
construction of the Ffestiniogpumped storage scheme, Merioneth,
Wales.Quarterly Journal of Engineering Geology2 (3) : 183-194
Attewell P B, Farmer I W 1976 Principles ofengineering geology.
London, Chapman andHall
Badgley P C 1965 Structural and tectonicprinciples. New York,
Harper and Row
Barton N 1973 Review of a new shear-strengthcriterion for rock
joints. EngineeringGeology 7(4): 287-332
Battey M H 1960 Geological factors in thedevelopment of
Veslgjuv-botn and Vesl-Skautbotn. In Lewis W V (ed)
Investigationson Norwegian cirque glaciers. London,Royal
Geographical Society: 5-10 (RGSResearch Seri es 4)
Boulton G S 1979 Processes of glacier erosionon different
substrata. Journal ofGlaciology 23(89): 15-38
Broster B E, Oreimanis A, White J C 1979 Asequence of glacial
deformation, erosion,and deposition at the ice-rock interfaceduring
the last glaciation: Cranbrook,British Columbia, Canada. Journal
ofGlaciology 23(89): 283-295
Brown E T 1970 Modes of failure in jointedrock mass. In:
Proceedings of the secondCongress of the International Society
ofRock Mechanics, Belgrade, [1970?} 2: 293-298
Brunner F K, Scheidegger A E 1973 Exfoliation.Rock Mechanics 5:
43-62
Causay 0 1977 The measurement of fracturepatterns in the chalk
of southern England.Engineering Geology 11: 201-215
Chapman C A, Rioux R L 1958 Statistical studyof topography,
sheeting and jointing ingranite, Acadia National Park,
Maine.American Journal of Science 256(2): 111-127
Fookes P G, Wilson 0 0 1966 The geometry ofdiscontinuities and
slope failures inSiwalik clay. Geotechnique 16(4):305-320
Foster H 0 Unpublished. The glaciation of theHarlech Dome. (PhD
thesis, University ofLondon, 1968)
Freitas M H de, Watters R J 1973 Some fieldexamples of toppling
failure. Geotechnique23(4): 495-514
Harland W B 1957 Exfoliation joints and iceaction. Journal of
Glaciology 3(21):8-10
Haynes V M 1968 The influence of glacialerosion and rock
structure on corries inScotland. Geografiska Annaler
50A(4):221-234
Helm 0 G, Roberts B, Simpson A 1963 Polyphasefolding in the
Caledonides south of theScottish Highlands. Nature 200
(4911):1060-1062
Hoek E 1964 Fracture of anisotropic rock.Journal of South
African Institute ofMining and Metallurgy 64: 501-518
Hoek E 1970 Estimating the stability ofexcavated slopes in
opencast mines.Transactions of the Institution ofMining and
Metallurgy 79 (A): 109-132
Hoek E 1973 Methods for the rapid assessmentof the stability of
three-dimensional rockslopes. Quarterly Journal of
EngineeringGeology 6(3-4): 243-255
Hoek E, Bray J W 1974 Rock slope engineering.London, Institution
of Mining and Metallurgy
Holtedahl H 1967 Notes on the formation offjords and
fjord-valleys. GeografiskaAnnaler 49A(2-4): 188-203
9
Downloaded from https://www.cambridge.org/core. 04 Jul 2021 at
15:33:28, subject to the Cambridge Core terms of use.
https://www.cambridge.org/core
-
Addison: Contribution of rock-mass failure to glacier
erosion
Jahns R M 1943 Sheet structures in granites:its origin and use
as a measure of glacialerosion in New England. Journal of Geology51
(2): 71-98
Kulhawy F H 1975 Stress deformation propertiesof rock and rock
discontinuities.Engineering Geology 9: 327-350
Lewis W V 1938 A melt-water hypothesis ofcirque formation.
Geological Magazine75(888): 249-265
Lewis W V 1940 The function of meltwater incirque formation.
Geographical Review30 (1) : 64-83
Lewis W V 1954 Pressure release and glacialerosion. Journal of
Glaciology 2(16):417-422
Lynas B D T 1970 Clarification of the polyphasedeformation of
North Wales Palaeozoic rocks.Geological Magazine 107(6):
505-510
Morland L W, Boulton G S 1975 Stress in anelastic hump: the
effects of glacier flowover elastic bedrock. Proceedings of
theRoyal Society of London A 344(1637):157-173
Morland L W, Morris E M 1977 Stress in anelastic bedrock hump
due to glacier flow.Journal of Glaciology 18(78): 67-75
Nilsen T H 1973 The relation of jointpatterns to the formation
of fjords in
. western Norway. Norsk GeologiskTidsskrift 53(2): 183-194
Price N J 1966 Fault and joint development inbrittle and
semi-brittle rock. Oxford,Pergamon
Randall BAD 1961 On the relationship ofvalley and fjord
directions to thefracture pattern of Lyngen, Troms,N Norway.
Geografiska Annaler 43(3-4):336-338
Rowlands B M Unpublished. The glaciation ofthe Arenig region.
(PhD thesis, Univer-sity of Liverpool, 1970)
Seddon B 1957 Late-glacial cwm glaciers inWales. Journal of
Glaciology 3(22):94-99
Shackleton R M 1954 The structural evolutionof North Wales.
Liverpool and ManchesterGeological Journal 1(3): 261-297
Silveira A Fda, ROdrigues F P, Grossmann N F,Mendes F de M 1966
Quantitative charac-terization of the geometric parameters
ofjointing in rock masses. In: Proceedingsof the first Congress of
the InternationalSociety of Rock Mechanics, Lisbon, 1966.Lisboa,
Laborat6rio Nacional de EngenhariaCivil 1: 225-233
Sugden D E 1974 Landscapes of glacial erosionin Greenland and
their relationship toice, topographic and bedrock conditions.In
·Waters.R $, Brown E H (eds) Progressin geomorphology. London,
Institute ofBritish Geographers: 177-195 (SpecialPublication 7)
Terzaghi K 1962 Stability of steep slopes onhard unweathered
rock. Geotechnique12(4): 251-263,269-270
Trainer F W 1973 Formation of joints in bed-rock by moving
glacial ice. us GeologicalSurvey. Journal of Research 1(2):
229-235
Twidale C R 1972 The neglected third dimen~sion. Zeitschrift fur
Geomorphologie NF6(3): 283-300
Twidale C R 1973 On the origin of sheet joint-ing. Rock
Mechanics 5: 163-187
Unwi~ D J Unpublished. Some aspects of theglacial geomorphology
of Snowdonia, NorthWales. (MPhil thesis, University ofLondon,
1970)
10
Weertman J 1979 The unsolved general glaCiersliding problem.
Journal of Glaciology23(89): 97-115
Whillans I M 1978 Erosion by continental icesheets. Journal of
Geology 86(4): 516-524
Witherspoon P A, Gale J E 1977 Mechanical andhydra.ulic
properties of rocks related toinduced seismicity. Engineering
Geology11: 23-55
Young R P, Fowell R J 1978 Assessing rockdiscontinuities.
Tunnels and Tunnelling10(5): 45-48
Zumberge J M 1955 Glacial erosion in tiltedrock layers. Journal
of Geology 63(2):149-158
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