Slope stability engineering : developments and applications : proceedings of the International Conference on Slope Stability

430 Pages·1991·48.659 MB·English

by  Chandler, Richard John

The Institution of Civil Engineers
Slope stability engineering
developments and applications
Proceedings of the international conference on slope stability
organized by the Institution of Civil Engineers and held on the Isle
of Wight on 15-18 April 1991
Thomas Telford
Conference organized by the Institution of Civil Engineers
Organizing committee: Professor R. J. Chandler (Chairman), Professor E. N. Bromhead,
Professor J. D. Geddes, R. G. Mclnnes, Dr G. Walton
British Library cataloguing in publication data
Slope stability.
624.1
© The Institution of Civil Engineers, 1991, unless otherwise stated.
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Published on behalf of the Institution of Civil Engineers by Thomas Telford Ltd, Thomas Telford House, 1 Heron Quay. London E14
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ISBN 978-07277-1660-6
Foreword
Landslides and slope stability problems have caused, and will continue to cause, many problems
world-wide, threatening lives and property in an ever more populated world.
The intention of the committee organizing the conference was to highlight recent developments in
studies of slopes and landslides to help guard against these problems. The world-wide interest in
landslides and problems of slope stability, ensured that, in the event, these objectives were easily
attained. This interest is reflected in the number and variety of papers submitted, the conference
attracted well over 50 papers, and delegates from all over the world. The topics covered include the
mechanics of landslide processes, the discussion of planning problems (both social and technical) posed
by landslides and the consideration of the means of stabilizing slopes and landslides. Even the casual
reader will not fail to be impressed by the wide range of papers and by the stimulating discussion they
generated.
We were fortunate to be able to hold the conference on the Isle of Wight, which has a well exposed
and interesting geology, and also has its own slope stability problems, the consequence of extensive
coastal landslides. Two afternoons of the four-day conference were put aside for field visits; these gave
delegates the opportunity to see and learn from the landslides at first hand. These field visits gave the
conference a unique flavour and contributed considerably to its undoubted success.
R. J. Chandler
Chairman,
Organizing Committee
Contents
Mechanics
1. Keynote paper. Mechanics of landslides. P. R. VAUGHAN 1
2. Determination of soil strength parameters for the analysis of highway slope failures.
G. I. CRABB and J. H. ATKINSON 13
3. Stability analysis of a complex landslide under static and dynamic conditions. A. CANCELLI,
G. CROSTA and P. ROMANI 19
4. Residual strength of volcanic clay. D. H. CORNFORTH and K. F. FUJUANI 23
5. Tension cracks and slope failure. R. N. CHOWDHURY and S. ZHANG — 27
6. Observation of Graben geometry in landslides. D. M. CRUDEN, S. THOMSON
and B. A. HOFFMANN 33
7. A model for prediction of piezometric levels in landslides. R. FELL, T. G. CHAPMAN
andP.KMAGUIRE 37
8. Calculation procedures for slope stability analyses involving negative pore-water pressures.
H. RAHARDJO and D. G. FREDLUND 43
9. The influence of the pore pressures within the slide mass: a re-examination of the slide in
Lodalen 1954.1. M. MORRISON 51
10. Improved multi-wedge landslide analysis. I. B. DONALD and P. S. K. GIAM 55
11. The Sargent landslide and long-term ground-water fluctuations in California USA hillsides.
W. B. WIGGINTON and D. HICKMOTT 61
12. Designing the shape of soil slopes stable during seepage. N. B. ILYINSKY, A. R. KACIMOV
andN.D.YAKIMOV 67
13. Reliability index versus safety factor for coal mine spoil pile stability. D. J. WILLIAMS, J. Z. ZOU
and J.GRAHAM 71
Discussion on Papers 1-13 77
14. Keynote paper. Planning aspects of slopes in Britain.D. BROOK 85
15. Management of landslides within Shropshire. A. M. CARSON and J. FISHER 95
16. Environmental impact of a large landslide near Lausanne, Switzerland. F. NOVERRAZ,
C. A. BONNARD and A. GIRAUD 101
17. An interesting tool in highway planning in landslip-prone areas. T. COLLOTTA, R. ENRICI,
C. LONGARETTI, O. CIANCIOSI and P. C. MORETTI 107
18. Building a residential complex on a slow deep-seated slope instability.F. OBONI and Z. HLOBIL 113
19. Cliff management: a photographic monitoring system. P. GRAINGER and P. G. KALAUGHER 119
20. Debris flows in the Campanian volcaniclastic soils. F. M. GUADAGNO 125
21. Instability of Cobb Road, Lyme Regis, Dorset. A. B. HAWKINS 131
22. Development of a methodology for landslide potential mapping in the Rhondda Valley.
H. J. SIDDLE, D. B. JONES and H. R. PAYNE 137
23. A comparative study of indirect methods of landslip potential assessment. P. J. JENNINGS,
H. J. SIDDLE and S. P. BENTLEY 143
24. Planning, phasing and implementation of rock slop remedial works in the Scottish
Highlands. T. P. DAVIES and D. W. HUGHES 149
Discussion on Papers 18-24 155
Isle of Wight
25. Keynote paper. The landslides forming the South Wight Undercliff. J. N. HUTCHINSON 157
26. Investigations of the landslides at St Catherine's Point, Isle of Wight. J. N. HUTCHINSON,
E. N. BROMHEAD and M. P. CHANDLER 169
27. The natural evolution of the soft rock cliff at Shanklin, Isle of Wight and its planning and
engineering implications.M. E. BARTON 181
28. The recent history and geotechnics of landslides at Gore Cliff, Isle of Wight. E. N. BROMHEAD,
M. P. CHANDLER and J. N. HUTCHINSON 189
29. A preliminary landslide hazard zonation of the Undercliff of the Isle of Wight.
J. N. HUTCHINSON and M. P. CHANDLER 197
30. The assessment of ground behaviour at Ventnor, Isle of Wight. E. M. LEE, R. MOORE,
H. J. SIDDLE and D. BRUNSDEN 207
31. The geomorphology of the landslide complex at Ventnor, Isle of Wight. J. N. HUTCHINSON,
D. BRUNSDEN and E. M. LEE 213
32. Strategies for managing the landslide complex at Ventnor, Isle of Wight. E. M. LEE,
R. MOORE, D. BRUNSDEN and N. BURT 219
33. The impact, causes and management of landsliding at Luccombe village, Isle of Wight.
R. MOORE, E. M. LEE and R LONGMAN 225
34. The distribution, frequency and magnitude of ground movements at Ventnor, Isle of Wight.
R. MOORE, E. M. LEE and N. H. NOTON 231
35. A review of instability on the southern coasts of the Isle of Wight and the role of the local
authority. G. McINTYRE and R G. McINNES 237
36. Living with landslip: Ventnor. N. H. NOTON 245
Discussion on Papers 25-36 251
Coastal stability
37. Keynote paper. Landslide hazard management — experience in the United States.
R.L. SCHUSTER 253
38. The role of coast protection in coastal slope stabilization. M. G. BARRETT and J. L. ANDREWS 265
39. The observation and analysis of a failure in a cliff of glacial clay till at Cowden, Holderness.
A. P. BUTCHER 271
40. The mechanics of first-time slides in the London clay cliff at the Isle of Sheppey, England.
N. DIXON and E. N. BROMHEAD 277
41. The Whitby cliff stabilization and coast protection scheme. A. R. CLARK and S. GUEST 283
42. Ground movements of the Encombe landslip at Sandgate, Kent. M. J. PALMER 291
43. Landsliding in the coastal slopes of Capo Spulico area in the Gulf of Taranto (Italy).
B. D'ELIA, G. LANZO and M. ROSSI-DORIA 297
44. Coastal stability and its impact on adjacent structures. B. WAREHAM, P. N. HYDE
and L.K. RODGER 303
Discussion on Papers 37-44 309
Remedial measures
45. Keynote paper. Slope stabilisation experience in South Wales, UK. B. JONES 313
46. Stabilization of a landslide with submerged motor-driven pumps. A. OLCESE,
CVESCOVO,S.BONIandG.GIUSTI 321
47. Slope stabilisation by vertical soil reinforcement. D. H. BARKER 327
48. Slope stabilisation by new ground anchorage systems in rocks and soils.
A. D. BARLEY KELLER 335
49. Large landslide stabilization by deep drainage wells. B. BIANCO and D. A. BRUCE 341
50. Influence of tectonic structure on landslipping: Thrace motorway, Turkey. T. GORDON,
J. A. LORD and T. STATHAM 349
51. Embankment construction over a landslip in coal measures colluvium. J. M. W. HOLDEN
and S. J. HODGETTS 355
52. The use of geogrids in landslide control works: case history from Valtellina A. CAMBIAGHI
andP.RIMOLDI 365
53. The use of bored piles and counterfort drains to stabilize a major landslip — a comparison
of theoretical and filed performance. J. A. ALLISON, J. MAWDUT and G. T. WILLIAMS 369
54. Landslide and remedial works in Wadhurst clay. R. S. PUGH, A. G. WEEKS and
D. E. HUTCHINSON 377
Discussion on Papers 45-54 383
55. Keynote paper. Landslide control by means of a row of piles. M. E. POPESCU 389
56. Slope stabilisation using lime. CD. F.ROGERS and C J.BRUCE • 395
57. Stabilisation of a landslide on Etruria Marl. R. P. THOMPSON 403
58. Rockfall containment measures at Springdale, Newfoundland. R. D. BOYD 409
59. Methods of slope stabilization by retaining walls. L. K. GINZBURG 415
60. Slope stabilization by means of cuts and fills. V. R. GRECO 421
61. Getting closer in advancing access and data collection techniques for rock face investigation
and stabilization. P. McMILLAN and W. A. WALLACE 427
62. Causes and effects of a block slide of aliano sands in Basilicata/Southern Italy. M. DEL PRETE,
E. FAVIA and N. TAFUNI 435
Discussion on Papers 55-62 439
1. Stability analysis of deep slides in brittle soil — lessons from
Carsington
P. R. VAUGHAN, Professor of Ground Engineering, Imperial College of Science, Technology and
Medicine, UK
Slope stability is usually analysed by limit equilibrium methods in two dimensions. Investigations
of the failure of Carsington embankment in 1984 have illustrated limitations to such analyses. These
are discussed. They include a reduced average strength due to progressive failure in two dimensions
progressive failure due to lateral load transfer in three dimensions, and lateral load transfer
after the slide such that the residual strength could not be deduced from back-analysis.
INTRODUCTION In such soils the peak strength can only be
1. The stability of actual or potential mobilised along a rupture surface at collapse if
slides is usually examined using limit equil- oading produces the appropriate stress
ibrium analysis, This approximate method is well distribution, which is unique. Normally, this
developed and there is extensive experience of does not occur, and the average strength
its use. However, it suffers limitations, mobilised at collapse is less than the peak
C a] It presumes that there is a complete strength. The possibility of loss of strength
rupture surface within the soil, and that move- through progressive failure has been appreciated
ment along it is sufficient for a particular for many years, but quantification of its effect
strength to be mobilised. Otherwise it does not has been hampered by lack of unambiguous field
consider deformation of and within the sliding examples from which its effect might be deduced
mass. Thus it cannot predict deformations or and of a method of analysis from which its
deal with the condition where a sliding surface potential effect could be quantified.
is not fully developed. 4. The failure of the Carsington embankment
Cb3 It is usually applied in two dimensions, provided the field example and stimulated the
when it is assumed that there is no shear on development of a method of analysis. The field
cross-sectional planes, and no transfer of load studies and the limit equilibrium analyses have
longitudinally along the slide. been described in ref. 1 and 2, and the finite
Cc3 It cannot deal specifically with pro- element analyses which were developed to analyse
gressive failure in brittle soils, as, once the the effect of progressive failure are described
peak strength is reached, the subsequent loss of in ref, 4 and 5. The limit equilibrium analyses
strength depends on displacement. The average showed that the average strength at failure was
strength at rupture must lie between peak and approximately mid-way between peak and residual
residual, but it cannot be evaluated more for the materials involved in the rupture. The
precisely by limit equilibrium analysis, use of peak strength parameters overestimated
although an arbitrary correction can be made to the safety factor by more than 20%. The finite
the strength adopted to allow for progressive element analyses were able to recover the effect
failure. Since the movement of a first-time of progressive failure by reproducing strain-
slide to a flatter slope automatically indicates softening and the development of the shear zone,
loss of strength and brittleness, this 5. The results obtained at the section
uncertainty arises in most slides. through the initial failure of the Carsington
2. The embankment of Carsington Dam slipped slope are shown on fig. 1, taken from ref. 6.
in 1984, just before it was complete. The The failure [fig. Ia3 occurred through the core
investigations into this failure Cref. 1, 2 & and core extension [the 'boot13, which were of
33 have provided new information concerning the rather weak clay and which were undrained
limitations of limit equilibrium analysis, in during construction, and along a layer of clay
both two and three dimensions, for the 'first on top of the foundation, which was fully
time' slide and for the slide as it came to consolidated throughout construction. Fig. lb
rest. The findings from the investigation will and lc show the computed situation prior to and
be reviewed here, and the implications for the at the moment of collapse. Construction of the
analysis of cutting and natural slope failures embankment in layers was simulated, and the
will be summarised. correct height at collapse was predicted. Other
field measurements of displacement, pore press-
FIRST TIME SLIDES IN TWO DIMENSIONS ure, etc. were successfully reproduced.
PROGRESSIVE FAILURE 6. The analysis showed that, at collapse,
loss of strength had occurred through strain
3. Progressive failure occurs in brittle softening in the core [A3, and in the inner part
soils which suffer loss of strength post-peak. of the clay foundation CB3 as the failure sur~
Slope stability engineering. Thomas Telford, London, 1991. 1
MECHANICS
[a]
AH
Observed rupture
surface
Foundation clay layer
[b] 198 m
Vectors of incremental displacement
at collapse .
"""Computed rupture surface atcollapse (201m)
CORE
— •— Post-peak
100
Pre-peak
-40
Fig. 1. Failure of Carsington Embankment. The influence of progressive failure on the
CaJ Section through initial failure. Cb] Failure benefits to be obtained by flattening a slope
as shown by finite element analysis, [cl Shear 8. A slope may be stabilised by reducing its
stresses mobilised on rupture surface at failure angle, or by adding a berm. It is usual to
from the finite element analysis. assess the improvement in stability achieved by
limit equilibrium analysis. If the slope is
subject to progressive failure, this may be
unconservative. The problem was examined as part
of a research programme which followed the
face propagated along it. This propagation had Carsington investigations, using the finite
started when the constructed level was some 5m element programme developed and calibrated for
lower than at collapse, yet the measured and these investigations Cref. 73.
predicted movements of the embankment remained 9. The passive resistance of three embank-
small and gave no warning of collapse. Near the ment shoulders was investigated. The first CC13
toe of the embankment CC3, where the final of fig. 23 was the same as the shoulder of the
rupture surface developed post-collapse, the Carsington embankment [fig. 13. To minimise com-
strength was not fully mobilised. putation it was 'built' in layers with the inner
7. The investigations and analyses indicated boundary, X - X, held at zero displacement. The
that the following factors contributed to the shoulder was then 'failed' by applying a
failure. triangularly distributed normal and downward
[a3 The weak undrained clay in the boot made shear stress to X - X. The resultant thrust was
deep seated failure critical. at 10* to the horizontal. Collapse occurred in a
Cbl The strain-softening,brittle soils all- manner similar to that of the full action [fig.
owed progressive failure to occur, 13. A strain- ioftening failure surface prop-
Cc] The stress concentration at the agated along the weak foundation layer.
transition from the weak undrained boot to the 10. Two other alternative shoulders [[23 and
strong drained foundation,, which can be seen on [33 of fig. 23 were analysed in a similar way.
fig. 1, promoted progressive failure. The first had an enlarged berm with a level
Cd3 The concentration of strain in the surface. The second had a flatter slope. The
foundation layer, partly because it was under- additional material 'added1 was the same for
lain by a much stiffer foundation, promoted both, as was the increase in lateral resistance
strain softening and progressive failure. [3O%3 predicted by drained limit equilibrium
PAPER 1:VAUGHAN
Carsington which will promote progressive
failure in an excavated slope is a high initial
lateral stress. The effect of this is
illustrated in fig. 3. Expansion must occur to
relieve this stress, or horizontal equilibrium
cannot be obtained. If this expansion is large
and the strains are concentrated by an
underlying stiff layer, or by bedding surfaces,
then peak strength may well be exceeded and a
shear surface formed on which strength is lost
due to strain-softening C fig. 3a]. This
mechanism has been observed Cref. 8].
14. Once a weakened surface has formed at
depth, it is possible for the movement to be
'captured' by a gravity-driven wedge, as shown
on fig. 3b, Such a mechanism may give rise to
deep-seated failures which cannot be explained
by the forces due to gravity alone.
15. Swelling of a slope excavated undrained,
either from the slope surface or from an inter-
X
nal permeable layer, may produce strain discon-
tinuities and so promote progressive failure.
Fig- 2. Improving stability by adding fill to a
slope - the geometries investigated. [Q]
analysis. The computed collapse mechanisms were
similar to those of £13. The resistances at col-
lapse increased by 2% [slope 21 and 15% [slope
33, compared to the increase of 30% predicted by Stiff layer
limit equilibrium analysis. The finite element
analyses showed that a substantial part of the
increase . in shearing resistance provided by the
berrrts was to the part of the final rupture [b]
surface towards the toe which formed post-
collapse. Thus the increase in resistance of
this part id not contribute fully to the total
resistance at collapse,
11. The better performance of slope £33 was
because this slope was significantly stiffer in / 7/7 ////////
the horizontal direction than slope C23. It is
self-evident that, if the shoulder was very Fig. 3. Progressive failure of an excavated
stiff, it would act like the top of a direct slope in stiff clay; Ca3 the formation of a deep
shear box and the load would be applied to the shear zone by horizontal expansion over a stiff
weak foundation layer uniformly along its layer, Cb3 Capture of the shear zone by a
length. Progressive failure would then be gravity driven slide.
largely, if not entirely, avoided. Thus the
geometry and lateral stiffness of the additions
to a slope in brittle soil may be more important
than their weight in determining their FIRST TIME SLIDES IN THREE DIMENSIONS -
contribution to stability. Analysis by limit PROGRESSIVE FAILURE BY LATERAL LOAD TRANSFER
equilibrium may give an unconservative assess-
ment of this contribution. 16. A number of features of the Carsington
slide were noticeable. It was of unusual length
Progressive failure in excavated slopes C5OOm3 relative to its depth. It started on one
12. Excavated slopes, whether made by man or side of the valley, and spread to the other side
by natural erosion, are subject to unloading and over two days. The extent of these movements is
swelling, rather than the loading involved at shown on fig. 4. The depth of the slide changed
Carsington. Such slopes have not yet been exam- along its length by a factor of nearly two. In
ined by strain-softening finite element analy- the valley centre failure occurred through the
sis, but some probable effects can be estimated, base of the fill rather than in the foundation
13. In general a sharp discontinuity in clay layer, which was absent. While the slide
strength on a potential rupture surface, with started where fill was being added, no fill had
its attendant stress concentration, is less been added for some time to the lengths of
likely within an excavated slope, although it embankment to which the slide subsequently
might occur in a slope to which a remedial berm spread. In view of ail these factors, it seemed
had been added. Concentration of strain can unlikely that 500m of slope had reached a factor
occur on top of a stiff layer, or on a weak, of safety of unity simultaneously.
planar bedding surface. A factor not present at 17, It is self-evident that if brittle fail-
MECHANICS
Crest level at collapse
200:
m Final length of slide
OD
Initial movement
190-
180-
bt toe
170-
500 600 700 800 900 1000 1100
_ . . ., Chainage m.
Post failure y
movement
20'
m
10'
0
. ^Failure in foundation Failure in fill Failure in foundation
Safety factor relative
to section A
1-2
o o
o
o A
_o_o
Fig. 4. Carsington embankment failure; a discrete slide. If Block B is 'dragged off1
Longitudinal section of slide, horizontal and and fails, then it may move out of balance and
vertical movement [see fig 1 for key], and exert a force on block C. If this occurs the
safety factor relative to Section A through failure may propagate laterally through sections
initial failure by limit equilibrium. of embankment which have a significant safety
factor as assessed by two dimensional analysis,
19. A simple analysis can be developed from
ure of a slope occurs and the slope moves to a fig. 5 and 6, by assuming that the maximum value
new equilibrium position, the sliding mass will of the out-of-balance force per unit length CF,
not be in static equilibrium during the of fig. 53 acts throughout block A. Such an
movement. The slope will be subject to a analysis shows that an end force sufficient to
disturbing force, as illustrated on fig, 5, for cause failure on the end plane is developed by a
a deep seated failure similar to that at block of similar width to its height. It also
Carsington. The slope may not accelerate signif- shows that an end force approaching this
icantly under this force, as it will be braked limiting value could overcome a substantial
by viscous effects and, probably, by the devel- additional resistance in the next block, B.
opment of temporarily depressed undrained pore However, such an analysis involves many
pressures in the part of the rupture formed assumptions.
rapidly post-collapse. Movement is then
controlled by the rate at which these pore Analysis of lateral load transfer
pressures increase, 20, A simple lumped parameter analysis
18. It is also self-evident that a slope involving continuity along the length of the
which has failed in a brittle manner but which slope was developed as part of the Carsington
remains longitudinally continuous is capable of investigations Cref. 91. The essential features
exerting a significant force on adjacent parts of this analysis are:-
of the slope, as yet unfailed, as shown on fig. Ci] The embankment is simulated as an elastic
6. If the resistance of block B is greater than beam, sliding on a rigid rough foundation. The
the force exerted by block A, then block A may forces acting on a unit slice of the beam are
be prevented from moving. If the force exerted shown on fig, 7a. An active force, P^, is
by block A is sufficient, one of two other assumed to act horizontally. Pre-failure dis-
things can happen. Either the extra load on placements are small, and it is assumed
block B will cause it to fail, even if it constant.
originally had a reserve of stability, or the [ii] The sliding resistance of the foundat-
force is sufficient to shear the embankment on ion, S, is a function of the displacement of
the cross-sectional plane, and block A moves as the beam, 5, as shown on fig. 7b. Note that this