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Page 1

I BULLETIN of the International Association of ENGINEERING GEOLOGY L

Paris - No 50 -

de rAssociation Internationale de GEOLOGIE DE L'INGI~NIEUR

A SUGGESTED PROPOSITION DE TERRAIN

METHOD D'UNE

FOR

MF, T H O D E

REPORTING POUR

LANDSLIDE

RENDRE

COMPTE

0ctobre

1994

r ]

CAUSES DES CAUSES

DES GLISSEMENTS

M.E. POPESCU*

Summary A brief list of [andsIide causal factors is presented and a format for reporting landslide causes is suggested. They make useful additions to the Landslide Report proposed by the International Geotechnical Societies;' UNESCO Working Party' on World t.andslide Inventory. Rdsumd L'auteur pr~scnte une br~ve liste des causes des glissements de terrain et sugg~.re un syst~:me de rdtdrencc destind h rendrc compte de ces causes. Cette listc et ce syst~me constituent un compldment utile au - Compte rendu sur un glissement de terrain ,, propoad par le Comi,'e de Travail de I'UNESCO pour I'[nventaire mondial des glissements de terrain, 6tabli par les Socidtds Internationales de Gdotechnique.

1. I n t r o d u c t i o n The international G e o t e c h n i c a l S o c i e t i e s ' U N E S C O Working Party on World Landslide Inventory ( W P / W L I ) has been formed from the I A E G ' s C o m m i s s i o n on Landslides and Other Mass M o v e m e n t s , the I S S M F E ' s Technical C o m m i t t e e on Landslides and nominees of National Groups on the [international Society for R o c k Mechanics. As a contribution to the International Decade for Natural Disaster R e d u c t i o n (1990-2000), the Working Party is assisting the establishment of a World Landslide [inventory by suggesting standard t e r m i n o l o g y for describing landslides. The Working Party has suggested a method for reporting a landslide ( W P / W L I , 1990) and for preparing a landslide summary ( W P / W L I , 1991). The Party's working definition of a landslide is " T h e m o v e m e n t of a mass of rock, earth or debris d o w n a s l o p e " (Cruden, 1991). In August 1989. the Working Party set up Working Groups to suggest methods of classifying the rates of m o v e m e n t of landslides, their causes, their geology, their activity and the distribution of m o v e m e n t within landslides. When preparing a landslide report for a particular site, it is of primary importance to r e c o g n i z e the conditions that caused the slope to b e c o m e unstable and the processes that triggered the m o v e m e n t . Only an accurate diagnosis makes it possible to properly understand the landslide m e c h a n i s m s and to propose e f f e c t i v e remedial

measures. The great variety of slope m o v e m e n t s reflects the diversity of factors that may disturb slope stability (Popescu, 1984). This report provides an operational approach to the classification of landslide causes for use in reporting historic landslides, as p r o p o s e d by the Working Party on World Landslide I n v e n t o r y (1990, 1991.). The suggested method is s u m m a r i z e d in Tables 1 and 2.

2. L a n d s l i d e

causes

and

the factor

of safety

In e v e r y slope there are forces which tend to promote d o w n s l o p e m o v e m e n t and opposing forces which tend to resist m o v e m e n t . A general definition of the factor of safety, F, of a slope resu[ts from c o m p a r i n g the d o w n s l o p e shear stress, "c, with the shear strength, "rt, of the soil, along an assumed or known rupture surface : F = "q-/'c. Starting from this general definition. Terzaghi (1950) d i v i d e d landslide causes into external causes which result in an increase of sheraing stress (e.g. geometrical changes, unloading the slope toe, loading the slope crest, shocks and vibrations, drawdown, changes in water regime) and internal causes which result in a decrease of the shearing resistance (e.g. progressive failure, weathering, seepage erosion). H o w e v e r , Varnes (1978) pointed out there are a number of external or internal causes which may be operating either ko reduce

* Chairman of the Working Group on Causes of Landstides. The International Geotechnical Societies' UNESCO Working Party on World Landslide Inventory. Civil Engineering Institute, P.O. Box 2.45, 78172 Bucharest 2, Romania.


72 Table I : A brief list of landslide causal factors_ 1. GROUND CONDITIONS 1) Plastic weak material 2) Sensitive material 37 Collapsible material 4) Wheathered material 5) Sheared material 6) Jointed of fissured material 7) Adversely oriented mass discontinuities (including bedding, schistosity, cleavage) 8) Adversely oriented mass discontinuities (including faults, unconformities, flexural shears, sedimentary contacts) 9) Contrast in permeability and its effects on ground water 10) Contrast in stiffness (stiff, dense material over plastic materials) 2. GEOMORPHOLOGICAL PROCESSES t) Tectonic uplift 2) Volcanic uplift 3) Glacial rebound 4) Fluvial erosion of the slope toe 5) Wave erosion of the slope toe 6) Glacial erosion of the slope toe 7) Erosion of the lateral margins 8) Subterranean erosion (solution, piping) 9) Deposition loading the slope crest 107 Vegetation removal (by erosion, forest fire. drought) 3. PHYSlCAL PROCESSES 17 intense, short period, rainfall 2) Rapid melt of deep snow 3) Prolonged high precipitalion 4) Rapid drawdown fClowing floods, high tides or breaching of natural dams 5) Earthquake 6) Votcanic eruption 7) Breaching of crater lakes 8) Thawing of permafrost 97 Freeze and thaw weathering 10) Shrink and swell weathering of expansive soils 4. MAN-MADE PROCESSES 1) Excavation of the slope or at its toe 2) Loading of thc slope or at its crest 3) Drawdown (of reservoirs) 4) Irrigation 5) Defective maintenance of drainage, system 6) Water leakage from services (water supplies sewers, stormwater drains) 7) Vegetation removal (deforestation) 8) Mining and quarrying (open pits or underground galleries) 9) Creation of dumps of very loose waste 10) Artificial vibration (including traffic, pile driving, heavy machinery)

the shearing resistance or to increase the shearing stress. There are also causes affecting simultaneously both terms of the factor of safety ratio. In order to facilitate a better understanding o f landslide causes, reference is m a d e to Figure 1 which shows an e x a m p l e of factor of safety variation as a function of time, for a given slope. Seasonal rainfall and evaporation is reflected in seasonal variations in the factor of safety. Should there be a l o n g - t e r m trend in g r o u n d w a t e r levels, or changes in strength due to weathering, these will show as a trend imposed on the seasonal variation. Sudden changes will be due to short-term variation in either the strength o f the materials or the forces applied to the slope. This demonstrates that seldom, if ever, can a landslide be attributed to a single causal factor. The process lead-

ing to the d e v e l o p m e n t of the slide has its b e g i n n i n g with the formation of the r o c k itself, when its basic properties are d e t e r m i n e d and includes all the subsequent events of crustal m o v e m e n t , erosion and weathering (Varnes, 1978). Table 2 : Landslide Report Section on Landslide Causal Factors. Example for the Hudson Slide (Terzaghi, 1950). Preparatory causal factors: 4.2. 4.7 Triggering causal factors : 3.3

The c o m p u t e d value of the f a c t o r of safety is a clear and simple distinction b e t w e e n stable and unstable slopes. H o w e v e r , from the p h y s i c a l point of view, it is better to visualize slopes e x i s t i n g in one of the f o l l o w i n g three stages : stable, m a r g i n a l l y stable and actively unstable (Crozier, 1986). Stable s l o p e s are those where the margin of stability is s u f f i c i e n t l y high to withstand all destabilising forces. M a r g i n a l l y stable slopes are those which will fail at some time in response to the destabilising forces attaining a c e r t a i n level of activity. Finally, a c t i v e l y unstable s l o p e s are those in which destabilising forces produce c o n t i n u o u s or intermittent movement. The three stability stages p r o v i d e a useful f r a m e w o r k for understanding the causal factors of landslides and classifying them into two g r o u p s on the basis of their function (Fig. 1) : 1. Preparatory causal factors w h i c h make the slope susceptible to m o v e m e n t w i t h o u t actually initiating it and thereby tending to place the s l o p e in a marginally stable state. 2. T r i g g e r i n g causal factors w h i c h initiate m o v e m e n t . These causal factors shift the slope from a m a r g i n a l l y stable to an actively unstable state. A particular causal factor m a y perform either or both functions, d e p e n d i n g on its d e g r e e of activity and the margin of stability. A l t h o u g h it may be possible to identify a single triggering process, an e x p l a n a t i o n of ultimate causes of a landslide invariably i n v o l v e s a number of preparatory conditions and processes. B a s e d on their temporal variability, the destabilising p r o c e s s e s may be grouped into slow changing (e.g. w e a t h e r i n g , erosion) and fast changing processes (e.g. earthquake, d r a w d o w n ) . In the search for landslide causes, attention is often focused on those processes within the slope s y s t e m which p r o v o k e the greatest rate of change. A l t h o u g h slow changes act o v e r a long period o f time to r e d u c e the resistance/shear stress ratio, often a fast c h a n g e can be identified as having triggered m o v e m e n t . 3. A n operational approach to the classification o f l a n d s l i d e causal factors B e c a u s e landslide cause a s s e s s m e n t is c o m p l e x and landslides are not always i n v e s t i g a t e d in great detail, it


73

I-.= I-1 t-A

weatherin= 0

<~

E Pro~aratory causal factors

I[

causal factors

I

/

i

...............................................

~tY"'- ~=>"L-\f'v-"-'~ /u~pers'~ \-

1.0

- / f ' x ' - ' X - - - ~ ,-,--, ~ ,

!

............................................................. !..=.!.."..f..=.J.!........7.................... ..'..:.!..".f.-.=.}.'........... . 7_[................................. .~ ........................................................................................................................................ ;7s

overloading

................................

TIME Fig. 1 : An example of changes in the factor of safety with time.

Class 1

PREPARATORY

CROURD CORDITIORS Class 2 --I GEOIORPIIOLOGICAL }___PROCESSES Class 3 PIIYSICAL

CAUSAL FACTORS

~_~ __J

PROCESSES

TRIGGERINC CAUSAL FACTORS

Class 4

__

IAN-MADE PROCESSES

1

Fig. 2 : Types of causal factors of landslides.

appears reasonable to adopt a simple classification system of landslide causal factors. When assessing landslide causes it is necessary to make a distinction between ground conditions and processes. Ground conditions are the specification of the slope system, the setting on which a process can act to prepare or trigger a failure (Brunsden, 1979). The proposed operational approach to classification of landslide causal factors is intended to cover the majority of landslides. It involves the consideration of the available data from a simple site investigation and information furnished by other site observations. The classification proposed in Figure 2 and Table 1 divides landslide causal factors according to their effect (preparatory or triggering) and their origin (ground conditions and geomorphological, physical or man-made

processes). Ground conditions may not have a triggering function, while any ground condition or process may have a preparatory function.

Ground conditions or the material and mass characteristics of the ground, can be mapped on the surface of the landslide and the surrounding ground and explored in the subsurface by drilling, trenching and adits. Mechanical characteristics can be determined by testing.

Geomorphological processes or changes in the morphology of the ground, can be documented by pre-existing maps, aerial photographs, surveys of the landslide, or careful observation over time by the local population. Physical processes concern the environment arrd can be documented at the site by instrumentation, such as rainfall gauges, seismographs or piezometers. Careful local observations over time of water wells or damage


74 f r o m e a r t h q u a k e s m a y be a c c e p t a b l e substitutes. Variations in m e c h a n i c a l p r o p e r t i e s with d i s t a n c e from the surface may, in s o m e c i r c u m s t a n c e s , i n d i c a t e c h a n g e s of these p r o p e r t i e s w i t h time.

We p r o p o s e t h a t a s e c t i o n o n l a n d s l i d e c a u s a l factors be i n c l u d e d in the L a n d s l i d e R e p o r t ( W P / W L I , 1990) as o u t l i n e d in T a b l e 2. T a b l e I w o u l d m a k e a useful c h e c k list a t t a c h e d to the L a n d s l i d e R e p o r t form.

M a n - m a d e p r o c e s s e s c a n be d o c u m e n t e d by site o b s e r v a t i o n s a n d from c o n s t r u c t i o n or e x c a v a t i o n records at the site. S e p a r a t e i d e n t i f i c a t i o n of artificial a n d natural 1ands[ides is useful for b o t h a d m i n i s t r a t i v e and t h e o r e t i cal reasons.

T h e structure o f the l a n d s l i d e causal factors list (Table 1) and the f o r m a t o f the l a n d s l i d e causal factors report (Table 2) are s u g g e s t i o n s w h i c h can be m o d i f i e d b y d i s c u s s i o n s f r o m w i t h i n o r w i t h o u t the W o r k i n g G r o u p . The W o r k i n g G r o u p w e l c o m e s c a r e f u l l y d o c u m e n t e d p r o p o s a l s for a d d i t i o n s or a m e n d m e n t s to the Suggested Method.

We p r o p o s e to i n c l u d e a n e w s e c t i o n o n l a n d s l i d e c a u s e s in the L a n d s l i d e R e p o r t ( W P / W L I , 1990). T h i s s e c t i o n w o u l d h a v e two h e a d i n g s , n a m e l y " P r e p a r a t o r y causal f a c t o r s " and " T r i g g e r i n g c a u s a l f a c t o r s " . U n d e r e a c h h e a d i n g there will be a list of causal c o n d i t i o n s and p r o c e s s e s b e l o n g i n g to e a c h class, w h i c h are r e l e v a n t to the r e p o r t e d l a n d s l i d e (Table 2). For i n s t a n c e , T e r z a g h i (1950, p. I05) s u g g e s t e d that a m o n g the c a u s e s o f the H u d s o n slide of 1915 were " t h e a c c u m u l a t i o n o f s t o c k p i l e s of c r u s h e d rock, with a total w e i g h t of a b o u t 2 5 , 0 0 0 tons a l o n g the u p p e r edge of the slope.., the d e f o r e s t a t i o n o f the o u t c r o p s of the gravel or of an a d j a c e n t a q u i f e r p r o d u c e d an u n p r e c e d e n t e d i n c r e a s e o f the h i g h e s t e I e v a t i o n o f the w a t e r t a b l e " . T h e s e are p r e p a r a t o r y causal p r o c e s s e s 4.2 and 4.7 in Table 1. T e r z a g h i also n o t e d ( 1 9 5 0 p. 105) " T h e H u d s o n slide was p r e c e d e d by u n u s u a l l y h e a v y r a i n f a l l " . T h i s is triggering p r o c e s s 3.3 in Table 1.

4. Discussion The n e e d to p r o p e r l y r e c o g n i z e l a n d s l i d e causal c o n d i tions a n d p r o c e s s e s in o r d e r to u n d e r s t a n d landslide m e c h a n i s m s and to p r o p o s e e f f e c t i v e r e m e d i a l m e a s u r e s is a p p a r e n t . T h e s u g g e s t e d m e t h o d for r e p o r t i n g l a n d s l i d e c a u s e s in the L a n d s l i d e R e p o r t ( W P / W L I , 1990), s h o u l d s i m p l i f y a c q u i r i n g and a c c u m u l a t i n g i n f o r m a t i o n . A b r i e f list o f l a n d s l i d e causal factors is p r o p o s e d in Table I. M o s t o f t h e m are s e l e c t e d from a larger list in Varnes (1978) and t h e i r o p e r a t i o n is d i s c u s s e d in refere n c e s g i v e n by Varnes. H o w e v e r , there are s o m e landslide causal p r o c e s s e s w h i c h are not c o n s i d e r e d by Varnes, such as the t h a w i n g of p e r m a f r o s t slopes ( M c R o p e r t s , M o r g e n s t e r n , 1974) or b r e a c h i n g of c r a t e r lakes ( O ' S h e a , 1954) w h i c h h a v e b e e n i n s e r t e d in o u r classification.

Acknowledgements As Chairman o f the Working Group on Causes o f Landslides o f the UNESCO Working Parry on Worht LandslMe Inventor3; l am pleased to acknowledge stimulating discussions with the Working Group members (P. Anagnosti, D.M. Cruden, E Esu, J. Farkas, R. Fell, S. Novosad, K. Sassa. M. Watanabe). Discussions and corresponda/zce with Ch. Bomzard, D. Brunsden, H.H. Einstein, A. Federico. J. ttutchinson. G. Lefebvre. M. Rosenbaum, D.J. Varnes and 7~ gamagami are gratefully acknowledged. The VV?~rkingGroup is gratefitl to UNESCO Earth Sciences Division f o r funding the travel o f some members to meetings to discuss the Suggested Method.

References BRUNSDEN D., 1979: Mass movement, in Processes in Geomorphology, ed. C. Embleton and J. Thornes. Edward Arnold, London, p. 130- ~86. CROZIER M.J.. 1986: Landslides - Causes, consequences and environment, Croom Helm, London, 252 p. CRUDEN D.M., 199l: A simple definition of a landslide, Bulletin IAEG, No. 43, 27-29. McROBERTS E.C., MORGENSTERN N.R., 1974: The stability of thawing slopes, Canadian Geotechnical Journal. Vol. II. 44%469. O'SHEA B.E., 1954: Ruapehu and the Tangiwai disaster, New Zealand Journal of Science and Technology. B. 36, 174-189. POPESCU M., 1984: Landslides in overconsoiidated clays as encountered in Eastern Europe. Proceedings 4th International Symposium on Landslides, Toronto, Vol. 1, 83-106. TERZAGHI K., 1950 : Mechanisms of landslides, Geological Society of America, Berkey Volume, 83-123. VARNES D.J., 1978 : Slope movements and types and processes. In : Landslides Analysis and Control, Transportation Research Board Special Report 176, 11-33. Working Party on World Landslide Inventory, 1990: A suggested method for reporting a landslide, Bulletin IAEG, No, 41. 5-12. Working Pa~y on World Landslide Inventory, 1991 : A suggested method for a landslide summary, Bulletin IAEG, No. 43, 101-110.

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A SUGGESTED METHOD FOR REPORTING LANDSLIDE CAUSES  

A SUGGESTED METHOD FOR REPORTING LANDSLIDE CAUSES measures. The great variety of slope movements reflects the diversity of factors that may...

A SUGGESTED METHOD FOR REPORTING LANDSLIDE CAUSES  

A SUGGESTED METHOD FOR REPORTING LANDSLIDE CAUSES measures. The great variety of slope movements reflects the diversity of factors that may...

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