a numerical classification of selected landslides of the debris slide blong

8/14/2019 A Numerical Classification of Selected Landslides of the Debris Slide Blong

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Engineering Geology, 7 1973) 99-114 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

A N U M E R I C A L C L A S S I F I C A T I O N O F S E L E C T E D L A N D S L I D E S O FT H E D E B R I S S L I D E - A V A L A N C H E- F L O W T Y P E

R. J. BLONGSchool o f Earth Sc iences, Macqua rie University, N ort h R yde , N .S. 14I. [AustraliaAccepted for publication April 5, 1973)

ABSTRACTBlong, R. J., 1973. A numerical classification of selected landslides of the d~bris slide-avalanche flowtype. Eng. Geol., 7: 99-114.

Numerous classifications of landslides have been proposed based on a variety of classificatorycriteria. Several writers have mentioned the difficulties of distinguishing accurately between landslidesclassed by Sharpe 1938) and Varnes 1958) as d~bris slides, d~bris avalanches, and d~bris flows. Asample of 92 such landslides from the greywacke hill country of the North Island of New Zealand isclassified on the basis of as many as 19 numerical and 43 disordered multistate attributes. The resultsof the agglomerative polythetic classifications do not help to distinguish these landslide phenomenaclearly. Until some distinctive criteria characterizing landslides of this type are identified the use ofunsatisfactory simple classifications is recommended.

INTRODUCTIONLandslides have been classified in a number of ways by workers in a variety of discip-

lines. In the present study, 92 landslides, mainly of the d6bris slide-d6b ris ava lanc he-d6bris flow type, are grouped according to a number of simple classifications. Subse-quently, a large numb er of quantitative and qualitative morphometric at tributes aredefined and the techniques of numerical taxo nomy are employed in an attemp t toiden tify the mos t suitable morphological parameters for classification purposes.

All 92 landslides examin ed were located in the upper Mangawhara catc hment , an areaof deeply dissected, red-weathered, and as h-mantled greywacke hill cou ntr y in the NorthIsland of New Zeal and. Nearly all of the landslides occurred during two hi gh-intensi tyrainstorms on February 28th 1966, and February 2nd 1967.LANDSLIDE CLASSIFICATION

A survey of the lit erature reveals that a variety of criteria has been used to classifylandslides.


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100 R.J . BLONGBases of c lass i f ica t ion inc lude : the l i tho logy o f the shear p lane (L add , 1 935; Za i ruba

and M enc l , 1969 ) , t he m echan ic s o f s l ope fa i lu r e (Te rzagh i , 1950 ; Ya t su , 1967 ) , t hefo rm in pa r t i cu l a r the geom e t ry o f t he f a il u r e r e la t ive t o t he th i cknes s o f t he mo v ingm a s s - ( W a r d , 1 9 4 5 ; S k e m p t o n , 1 9 5 3 ) , a n d t h e t y p e o f m a t e r i a l i n v o l v e d , t o g e t h e r w i t hthe shape o f t he su r f ace o f r up tu re , and t he a r r ange me n t o f t he d6b r is (Sha rpe , 1938 :Varnes , 1958 ) .

C la s s i f ic a t i on o f lands l ide s acco rd ing t o t he pa re n t ma te r i a l in wh ich f a i l u r e occu r sm ay n o t be ve ry succes s fu l . Ladd s ( 19 35 ) c l a s s if i c a ti on , u sing t h is c r i t e r i on , i nd i ca t e stha t s imi l a r t ype s o f f a i l u r e occu r i n a w ide va r i e ty o f l i tho log i e s . S imi l a r l y , w i thou t t hep ro lon ged i nves t iga t i ons u sua l ly on ly con duc t ed by so il eng inee r ing l abo ra to r i e s , c l as s if ic -a t i ons o f lands l ide s acco rd ing t o cause s o f f a il u r e a re no t succes s fu l. Wi thou t such i nves t i-ga t i ons , t he cause can f r equ en t ly on ly be e s t ab l i shed by e l im ina t i on o f some poss ib i li t ie s .F u r t h e r m o r e , m a n y l a nd s li d es a r e a t t r i b u t e d t o s e ve r al c a u se s , a l t h o u g h p e r h a p s o n l y t oo n e t r i g ge r m e c h a n i s m ( S o w e r s a n d S o w e r s , 1 9 6 1 , p . 2 2 8 ) .

S k e m p t o n a n d H u t c h i n s o n ( 1 9 6 9 ) a n d H u t c h i n s o n ( 1 9 6 8 ) s u g g es t t h a t m a s s m o v e -m en t s exh ib i t su ch g r ea t va r i e ty t ha t r i go rous c l a s si f ic a t i on is ha rd ly pos s ib l e . The gene ra lc l a s s if i c a ti ons , such a s t hose o f Sha rpe and Va rne s , have r ece ived t he m os t a t t en t i o n i ng e o m o r p h i c l i t e r a tu r e . H o w e v e r , m a n y f i e ld w o r k e r s h a v e e x p e r i e n c e d d i f f i c u l t y in f i tt in gpa r t i cu l a r m ove m en t s i n to t he i nd iv idua l c a t ego r i e s o f the se c l a s s i fi c a t ions . Ward (194 5 ,p . 1 7 2 ) , I r w i n - H u n t ( 1 9 6 0 , p . 3 6 ) , B a i le y a n d R i c e (1 9 6 9 , p . 1 7 2 ) , a n d R i c e e t a l. ( 1 9 6 9 ,p . 6 4 7 ) r e p o r t l a c k o f a g r e e m e n t o n t e r m i n o l o g y a n d p r o b l e m s w i t h c l as s if ic a ti o n .C u m b e r l a n d ( 1 9 4 4 , p . 8 3 ) f o u n d d i f f ic u l t y in r el a ti n g t h e m a n y c o m b i n a t i o n s o f s li p pa g ea n d f lo w a g e f o r m s f o u n d i n in l a n d T a r a n a k i , N e w Z e a l a n d , t o t h e c a t e g o r ie s e l a b o r a t e dby S ha rpe . Ya t su (196 6) i s h igh ly c r i t i c a l o f S ha rpe s c l a ss i f ic a t i on , cons ide r ing V a rnes sg roup ing t o be supe r io r .TABLE IThe 92 landslides grouped according toVarnes (1958)Classification NumberSlump 1D6 bris slide 11D~bris avalanche 2Complex 78

Us ing Va rn es s c l a s s i fi c a t ion , 78 o f t he 92 l ands l ide s i nves t i ga t ed i n t he p r e sen t s t udya re g roup ed a s com plex s lope f a i lu r e s (Tab l e I ) . M os t o f t he se 78 l ands li de s i nvo lve com -b ina t i ons o f two o f the t h r ee t yp es o f s l ope f a i lu r e de f ined b y V a rnes a s d~b r is sl ide ,d~br is f l ow , d~b r i s ava l anche . How eve r , f i ve o f t he 78 l ands l i de s do no t c l ea r ly f i t i n to t hec o m p l e x c a t e g o r y , a s t h e y d o n o t i n v ol v e c o m b i n a t i o n s o f m a t e r i a ls o r c o m b i n a t i o n s o f


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NUMERICALCLASSIFICATIONOF SELECTED LANDSLIDES 101TABLE IIThe 92 landslides grouped according o a modifiedVarnes's classificationClassification NumberSlump 1D~bris slide 11D~bris avalanche 2D~bris slide-d~bris avalanche 24D~bris avalanche-d~bris flow 49Underthrust slide 3Incipient underthrust slide 2

types o f movement. Three of these five landslides have been called un erthrust sli es(Blong, 1971). Two other movements exhibit only incipient underthrusting. The name

underthrus t slide is given to a type o f mass movement characterized by a series ofunderthrust shear surfaces in coeval buried soils, subparallel transverse ridges, a markedbulging toe, and overlapping erosional and depositional zones (Blong, 1971).

In Table II, the transitional types of failure have been removed from the complexlandslide category. This modified Varnes's classification was used in the field identific-ation of landslide type. The transitional types d~bris slide-d6bris avalanche and d~brisavalanche-d6bris flow were recognized in the field mainly on the basis of the degree ofcohesion retained in the moving mass. Where rafted blocks occur on the shear plane, butwhere much of the moving mass has lost cohesion, a d~bris slide-avalanche is defined.Where the landslide has characteristics o f a d~bris avalanche, but where incoherent flowdeposits reach the base o f the hillslope or beyond, the transitional type d~bris avalanche-d~bris flow was recognized. Categorization of individual occurrences proved somewhatsubjective; continual reference was made to type examples.

With Varnes's original classification, the major flaw is that most landslides fall into thecomplex category, a grouping that provides little information about the characteristics ofan individual slope failure. With the modified classification, the problem becomes one ofaccurately placing a landslide in the correct category when so many categories are closelyrelated and transitional with one another. Both classificationsseem to be unsatisfactory.

Landslides can also be grouped according to the dominant mode o f movement. Sowersand Sowers (1961) recognized three types: rotational slides, linear shear slides, and flowslides. For the sample o f 92 landslides, 55 were classed as predominantly flowage pheno-mena, 15 as translational movements, 2 exhibited rotation, 1 was classed as flow-fall, andthe remaining 20 slope failures were considered to be transitional between flowage andtranslational phenomena.


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102 R~ J. BLONGIn a further attempt at grouping the landslides according to their field characteristics,

50 of the 92 slope failures were considered to be simple in that only one failure hadoccurred and only one type of movement was involved (although that one type could beflow-translation). Compound landslides exhibit more than one slope failure, tile haterfailure probably resulting from loss of support when tire first failure occurred, and closelyfollowing the first in time. Compound failures also exhibit only one movement mode (i.e..rotation, translation, flow, flow-translation). Twenty-four landslides were distinguished inthe sample of 92. Complex landslides exhibit more than one slope failure and more thanone movement mode. Eighteen landslides of this type were identified in the upperMangawhara catchment following careful examination of shear-plane configuration anddeposition morphology.

Landslides can also be grouped according to the number of phases of movement. Thedistribution and nature of deposits on the shear plane and the shape of the shear planeitself frequent ly provide evidence of more than one phase of failure. On the basis of suchevidence, 51 of the 92 landslides suffered only one phase of movement, 34 had failedtwice in rapid succession, and 7 landslides were considered to exhibit 3 phases o1 failure.

However, these last-described groupings of landslides are no less subjective than thedecision to place a landslide in one of the three categories d6bris slide, d6bris avalanche,d6bris slide--d6bris avalanche. They may also be less informative.

The landslide depth/length ratio defined by Skempton (1953) has been used success-fully by several authors to characterize landslide form. A single criterion can be used toseparate various types of slope failure which are related to conditions broadly classed asrotation, translation, or flow. However, as the length measurement is made from thelandslide crown to the depositional toe, where the moving mass is truncated by a stream,accurate ratios cannot be established. For the present sample of 92 landslides, depth/length ratios could be accurately measured in only 47 cases.

It is evident that the simple methods, used to differentiate between the closely relatedlandslide types investigated here, are inadequate. Very simple classifications, with onlythree or four divisions, fail to distinguish among landslides that have different morpho-logical features. As noted by previous authors, more detailed classifications can only beapplied subjectively; while individual landslides share some features in common, morpho-logical diversity is still either apparent or subsumed under complex groupings. Thepresent study investigates, therefore, numerous aspects of landslide morphology in anattempt to identify criteria that usefully distinguish between d6bris flows, d6bris slides,and d6bris avalanches.DEFINITION OF LANDSLIDEFORM ATTRIBUTES

All types of landslide observed in the upper Mangawhara catchment are included in thesample. Within small catchments, selected non-randomly, all landslides unaffected byfarm roads and/or other obvious human activity were included in the sample. Eighteenout o f 110 landslides sampled were subsequently rejected for the above reasons or because


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NUMERICALCLASSIFICATIONOF SELECTED LANDSLIDES 103data were incomplete. Field measurements were made using a simple slope surveyingdevice Blong, 1972), with a 26-inch 66-cm) diameter bicycle wheel calibrated to recorddistances, or with percentage visual estimation charts Folk, 1951, p.33).

In the absence of prior information concerning the value of individual morphologicalproperties, a wide range of attributes was estimated. A total of 19 numerical attributes,concerned mainly with size, shape, gradient, and locational properties of the erosionalzone and 43 disordered multistate attributes assessing general morphological character-istics 1 of erosional, transportational, and depositional zones of the 92 landslides weremeasured.

Although several computational procedures were tested as detailed below), only theresults of one classification Class II) are reported here in detail. Consequently, only theseven numerical and the nine multistate attributes used are defined here. Details of theattributes used in Class I are available from the author.)ELEN erosional slope length, the groundsurface length from the landslide headwallto the foot of the shear plane measured in the direction of maximum slope.

HEAD - the height of the landslide headwall mean of 3 measurements).WID - erosional zone width measured at the foot of the shear plane.EROS - the slope of the straight line joining the landslide crown top of the headwall)

to the foot of the shear plane.DO - a visual estimate of the degree of overlap between erosional and depositional

zone s

PECO - the groundsurface distance from the hillslope crest to the base of the shearplane expressed as a percentage of total hillslope groundsurface length.

CR - the ratio of landslide erosional zone area to the area of a circle having the sameperimeter as the landslide cf. Miller, 1953, p.8).

The nine multistate attributes are listed in Table III.

COMPUTATIONAL PROCEDURES

The numerical attributes were first subjected to simple and multiple correlation andregression analyses. These analyses enabled the interrelationships among attributes to beidentified see Blong, 1973). In order to isolate those morphological attributes most valu-able for landslide classification purposes, an agglomerative polythetic grouping wasperformed using all 62 attributes. The general principles of numerical taxonomy havebeen outlined by Sokal and Sneath 1963). More specific information concerning theselection of proven methods for specific problems is contained in Lance and Williams1967a, b). Following the advice of Mr. P. W. Milne, Division of Computing Research,

CSIRO, Canberra, an agglomerative polythetic grouping procedure using the CSIRO pro-These attributes express characteristics such as relationship of the landslide to topography, surfaceroughness, parent material, cross-sectional shape, symmetry, type and location of depositionalmaterial, and the number of phases of movement.


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104 R. J. BLONGTABLE Ii1Disordered multistate landslide att ributes

Qualitativeattribute number State description* Number oflandslidesin state

M9 Shear-plane shape

M10 Cross-sectio n shapeof shear plane

M16 Shear-plane shape isformed on

M26 Depositi onal outline

M28 Depositionalmaterial consists of

M33 Deposit ional areahas a toe which is

1. rectilinear2. convex3. concave4. sigmoid5. multiconcave6. multirectilinear7. convex-rectilinear8. concave--rectilinear1. concave2. slightly concave3. rectilinear4. slightly convex5. convex6. rectilinear-convex8. sigmoid1. ill situ red weathe red greywack e2. colluvial grey wacke3. Hamil ton ash beds6. colluvial mixt ure7. yellow weath ered and shattered

greywacke in situ1. semicircular2. lobate3. elonga ted lobate4. regular other than 1, 2, 3 above)5. irregular6. very irregular7. obliterated1. rafted blocks2. flow material3. veneer deposi tion4. combina tion 1 and 25. comb ina tion 2 and 38. no evidence9. one large block1. absent2. 0-20 cm3. 2 0- 50 cm4. 50-100 cm5. 100 -20 0 cm6. 200-300 cm7. > 300 cm

46t318

1lI84

412522

l11

38355

4110233511

12

106

36235

274

561

1211

921


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NUMERICAL CLA SSIFICATION OF SELECTED LANDSLIDESTABLE I I I con t inued)Disordered mu l t i s ta te landsl ide a t t r ibute s

105

Qual i ta t ive State descr ip t ion*a t t r ibu te number N u m b e r o flandslidesin stateM34 M ajor i ty of 1 . on hi l l s lope 52dep osit io n occurs 2. on valley f loo r 313. bey ond valley f loor a t foot ofhil lslope 9M38 Failu re 1. simple 502. com pound 243. comp lex 18M40 Dom inant movem ent 1 . ro ta t ion 12. transla tion 153. f low 556 . f l ow - t r ans l a t io n 207 . f l ow - fa l l 1*Where no landslides exist in a particula r state, the state is no t l isted but the original numb ering isre ta ined.

g ra m s M U L T B E T , G R O U P E R , M A X G O W E R , a n d G O W E C O R w e re s e l ec t ed fo r us e int h e p r e s e n t s t u d y . A c e n t r o i d s o r t i n g s t r a t e g y w a s e m p l o y e d t o g e t h e r w i t h a S h a n n o n -t y p e i n f o r m a t i o n s t a t i s t ic L a n c e a n d W i ll ia m s , 1 9 6 7 a ) . T h e f o u r p r o g r a m s p r o v i d e : a nh i e r a r c h i c a l s t r u c t u r i n g o f t h e i n d i v i d u a l l a n d li d e s , a n a n a l y s is o f t h e s i g n if i ca n c e o f e a c ha t t r i b u t e i n f o r c in g e a c h m a j o r g r o u p f o r m a t i o n , a p r i n c i p l e c o - o r d i n a t e a x e s a n a l y s is

G o w e r , 1 9 6 6 ) , a n d a s u m m a r y o f th e c o r r e la t i o n b e tw e e n e a c h l a t e n t ro o t a n d e a c ha t t r i b u t e L a n c e a n d W i l l ia m s , 1 9 6 7 a , b ; L a n c e e t a l. , 1 9 6 8 ). T h e s e r e s u lt s a l l o w d e c i si o n st o b e m a d e a b o u t t h e v a l u e o f e a c h a t t r i b u t e i n t h e c l a s s i f ic a t i o n .

RESULTS

T h e f ir s t c l a s s i f ic a t i o n C l a s s I ) e m p l o y e d a l l 19 n u m e r i c a l a n d 4 3 d i s o r d e r e d m u l t i -s t a t e a t t r i b u t e s . F i g . 1 i l l u s t r a t e s t h e h i e r a r c h i c a l s t r u c t u r e o f t h e c l a s s i f i c a t i o n . O n l y t h el a s t t e n f u s i o n s a re s h o w n . T h e h i g h v a lu e o f t h e E u c l i d e a n m e t r i c 4 5 2 . 8 2 ) f o r th e f i n a lf u s i o n i n d i c a t e s th e e s s e n t i a l d i s s i m i l a r it y b e t w e e n t h e t w o m a j o r g r o u p s , A a n d B .

T h e f i rs t c l a s s i fi c a t io n i s t o o l e n g t h y a n d c o m p l e x t o b e s u m m a r i z e d i n t a b l e f o r m .P e r u s al o f t h e r e s u l t s in d i c a t e s t h a t t h e r e i s n o c r i t e r i o n m u t u a l t o a l l g r o u p s i n a n h i e r -a r c h i c a l le v e l . N o s i ng l e l a n d s l i d e f o r m a t t r i b u t e c o n s i d e r e d h e r e c a n b e u s e d a s a c l a s si -f i c a t o r y c r i t e r i o n fo r t h e s a m p l e o f 9 2 l a n d s l id e s . F u r t h e r m o r e , f r o m t h e r e s u l ts , i t se e m sd o u b t f u l t h a t t h e u s e o f s e v e ra l a t t r i b u t e s t o g e t h e r p r o v i d e s a m o r e d e c is i ve c l a s s i fi c a t io n .I n v i e w o f t h e d i f f ic u l t i e s e x p r e s s e d b y v a r i o u s a u t h o r s i n c a t e g o r iz i n g in d i v i d u a l l a n d -s l i d e s , t h e s e c o n c l u s i o n s a r e n o t s u r p r i s i n g .


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106 R.J. BLONGS O

A3 8

G

2 3 l SM8 5

C2

E

K L1 1 1 1

4

N u r n d e r s r e f e r t o n u m b e r o f l a n d s l id e s i n e a c l l g r o u p

I

300 0

200.0

Fig.1. Landslide classification structure for the last 10 fusions using 19 numerical attributes and 43disordered multistate attributes.

However, the agglomerative polythetic classification illustrated in Fig.1 can also beused to examine the validity of the classifications presented earlier. The simple classifi-cations, such as the modified Varnes s classification and the Sowers and Sowers division oflandslide phenomena, are based on the general appearance of slope failures. A wide varietyof individual aspects of landslides is considered and assumptions are made regarding thegenesis of the landslides. Similarly, the grouping of landslides according to the number ofphases of movement or by the recognition of the landslide as simple, compound, orcomplex, concentrates on shear-plane phenomena; a variety of criteria is examined andjudgements (albeit subjective) are made concerning the nature of the failure. The com-puter classification considers a wide variety of landslide features, some requiring a generalimpression of the landslide, some relating to specific morphological aspects of the failurezone, the depositional zone, and the landslide environment. The classification proceduresubsequently selects those landslide attributes that promote the distinctiveness of groups.Within these limitations, it seems reasonable to expect some correspondence between thecomputer-produced groupings and those intuitively divined in other classifications.Table IV illustrates the relationships between the agglomerative grouping achieved hereand four of the classifications discussed earlier. The computer classification can also be


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NUMERICAL CLASSIFICATION OF SELECTED LANDSLIDES

TABLE IVComparison o f various classifications

107

Group name: Total A B C D E F G H I J K L M NNumber ingr oup: 92 38 54 20 34 24 10 23 15 14 6 11 13 8 15M37Modified Varnes s classification:

1. Slump 12. D6bris slide 115. D6bris avalanche 2

11. D~bris slide-avalanche 249. D~bris avalanche-flow 493. Under thrust slide 3

12. Incipient underthrust slide 2M40Dominant movement is:

1. Rotation2. Translation3. Flow6. Flow-translation7. Flow-fall

M38Failure is:1. Simple2. Compound3. Complex

1 1 13 8 6 2 2 - 2 1 5 1 - 2 2 -1 1 1 1 11 2 3 8 1 5 5 1 0 1 8 3 2 1

32 17 - 17 19 - 19 13 8 9 5 143 3 32 2 2 . . . .

1 1 1 11 5 2 13 10 3 3 - 1 1 4 6 - 3 1 -55 33 22 1 21 19 2 20 13 1 - 10 9 6 I420 1 19 9 10 2 8 1 - 9 - 1 1 - 1

1 1 1

50 25 25 16 9 9 - 17 12 10 6 8 1 7 624 12 12 - 12 11 1 5 3 2 9 1 818 1 17 4 13 4 9 1 4 1 3 1

M39No. of phases of movement:1. 512. 343. 7

25 26 16 10 10 - 17 12 10 6 8 2 7 611 23 4 19 12 7 4 3 4 3 9 1 7

2 5 - 5 2 3 2 2 - 2

comp ared with an unmod if ied Varnes s c lassi f icat ion of the 92 landsl ides by regroupingd6bris s l ide-avala nche, d6bris ava lanc he- f low, un der thrus t s l ide, and incipient under -thrust slides as co mpl ex landslides.

I t is evident that at no level in the hierarchy is there a close correspondence betweenthe c lass if ications l is ted and the comp uter grouping presented. The lack of cor res ponden cedoes less to invalidate the com put er classif ication than it does to em phasiz e the failings ofmore t radi t ional ly-or ien ted groupings , even where these c lass if icat ions have been modif iedby exper ience re levant to the present sample .

To tes t the hypo thes i s that there is no super ior way of dis t inguishing unequivocal lybetween landslides broadly c lassed as d6bris s l ide s-d6br is f l ows- d6b r is avalanches , asecon d com pu te r classif ication was per for med (Class II) . In Class I too m any attr i bute sare emplo yed, man y o f which do l i tt le except confuse . Some of the numer ical a t t r ibutes


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108 R..I. BLO NG1 6 0 0 -

el

0

41

U V

2

INum bers r l fer to number o f l andsl ides In eac h g roup

R

/ 0

T

t 0 _o 120 0

X

d,.=,8 0 - 0

Fig.2. Landslide classification structure fo r the last ten fusions us ing seven num erical and ninedisordered multistate attributes.a r e l o g i c a ll y a n d m a t h e m a t i c a l l y c o r r e l a t e d , t h u s r e i n f o r c i n g t h e a g g l o m e r a t i o n o f p a r ti -c u l a r g r o u p s w i t h i n t h e c l a s s i fi c a ti o n . F u r t h e r m o r e , s o m e o f t h e d i s o r d e r e d m u l t i s t a tea t t r i b u t e s a r e e x t r e m e l y s u b j e c t i v e .

F o l l o w i n g e x a m i n a t i o n o f t h e re l a t i o n s h ip s a m o n g t h e n u m e r i c a l a t t ri b u t e s ( B l o n g ,1 9 7 3 ) , t h e s e v e n a t t r i b u t e s d e f i n e d a b o v e w e r e s e l e c t e d a s s a t i s fa c t o r i ly r e p r e s e n t i n gq u a n t i t a t i v e a s p e c t s o f l a n d sl i de m o r p h o l o g y . T h e o t h e r t w e l v e n u m e r i c a l a t t r i b u t e s a r ee i t h e r m o d e r a t e l y h i g h l y c o r r e l a t e d w i t h t h o s e l i s t e d a b o v e o r c o n t r i b u t e d l i t t l e t o t h ep r e c e d i n g a n a l y s is . I n a n a t t e m p t t o r e d u c e t h e c o m p l e x i t i e s o f t h e e a r li e r c l a ss i f ic a t i on ,3 4 o f t h e 4 3 d i s o r d e r e d m u l t i s t a t e a t t r i b u t e s h a v e b e e n e li m i n a t e d . T h e r e m a i n i n g n i n ea t t r i b u t e s ( T a b l e I l l ) w e r e s e l e c t e d a s c o v e r i n g th e m a j o r c h a r a c t e r i s t i c s o f t h e s a m p l elands l ides .

T h e c l a s s i f i c a t i o n s t r u c tu r e f o r t h e l a s t t e n f u s io n s i s s h o w n in F ig .2 . T h r e e m a jo rg r o u p s o f la n d s l id e s a r e e v i d e n t f r o m t h e c l a s s i fi c a ti o n s t r u c t u r e . A g r o u p o f e l ev e n l a nd -s l id e s , l a b e l l e d O in F ig .2 , is q u i t e d i s t i n c t i v e , b e in g f o r m e d a t a l o w l e v e l o f f u s io n a n dr e m a i n i n g d i s ti n c ti v e u n t i l a l l l a n d sl i d es a r e j o i n e d t o g e t h e r i n t h e o n e g r o u p . T e n o f t h ee l e ve n m o v e m e n t s a r e t ra n s l a t i o n a l ; m o s t o f t h e d e p o s i t e d m a t e r i a l c o n s is t s o f r a f t e db l o c k s o r h a s r e m a i n e d a s a si ng le b l o c k . D e p o s i t i o n a l o u t l i n e s a r e s e m i c i r c u l a r o r l o b a t e .T h e d e p o s i t i o n a l o v e r l a p ( D O ) f o r t h e g r o u p o f e l e v e n a v e ra g e s 7 4 . A l l t h e l a n d sl i d e s int h i s g r o u p h a v e a d e p o s i t i o n a l t o e a n d a ll a r e s i m p l e f a il u r es i n t h a t o n l y o n e p h a s e o fm o v e m e n t h a s b e e n i d e n t i f i e d . E r o s i o n a l s l o p e s a r e r el a t iv e l y g e n tl e w i t h a m e a n g r a d i e n to f 2 4 . T h e f a il u r e z o n e s h a v e a n a v e r ag e w i d t h o f 1 7 m .

O n t h e o t h e r h a n d , G r o u p P w i t h 8 1 l a n d sl id e s is c h a r a c t e r i z e d b y f l o w (6 8 ) a n df l o w - t r a n s l a t i o n ( 2 5 ) m o v e m e n t s . T h e d e p o s i t i o n a l z o n e s a r e e l o n g a t e d l o b a t e o r l o b a t e ,


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NUMERICALCLASSIFICATIONOF SELECTED LANDSLIDES 109and the average depositional overlap is only 22 . Of these failures 69 have no obviousdepositional toe, and the mean erosional gradient is 34 . Shear planes average only 11 min width. More than half of these landslides are either compound or complex slopefailures.In terms of the modified Varnes's classification (Table II), the group with eleven land-slides includes all the underthrust and incipient underthrust slides, the single slump, andfive of the twelve d~bris slides. The taxonomic classification, then, confirms the subjec-tive grouping to some extent, but does not accurately duplicate the field classification.

As indicated in Fig.2, Group P is composed of two sub-groups of nearly equal size.Group Q, with 41 members, is compared with Group R (40 members) in Table V in orderof increasing similarity of attributes between groups. The biggest difference betweenthese groups is in terms of location on the slope. Where landslides occur very low on thehillslope (Group R), nearly one quarter of the depositional outlines have been obliterated.Otherwise, the differences between the two groups are not large, and do not form thebasis of a rational classification of landslides.TABLE VComparison of two major landslide groups

Group Q (41 members) Group R (40 members)PECO (mean) 55 80ELEN (mean) 17.0 m 16.7 mDO (mean) 26 18M26 depositional outline 24 elongated lobate 40 elongated lobate39 lobate 12.5 lobate0 obliterated 23 obliterated15 regular 12.5 regular0 lobate 12.5 obateM33 height of 61 no toe 78 no toedepositional toe 22 20-50 cm 0 0-50 cm0 100-200 cm 12.5 100-200 cm10 50-100 cm 0 50-100 cm

Group R is itself composed of two distinct groups (Fig.2). Group T, with fifteenmembers, has a mean value of PECO of 95 . Consequently, all of the landslides in thisgroup have had the majority of tile depositional material removed from the hillslope.Group S, with 25 members, has a mean value of PECO of 70 .

Group Q is also composed of two smaller groups, although these are less distinct thanin the case of Group R (Fig.2). The sub-group with eighteen members (Group U) isdominated by both flow and translational movements, while Group V is dominated byflow-translational and flow movements. Other differences are relatively minor.

In general, at the higher levels of classification, no actual landslide characteristics seemcapable of distinguishing adequately between the types of landslides recognized in thefield. However, it should be remembered tha t the computer classification was undertaken


12/16

110 R.J . BLONG

because of the dif f icul t ies of dif ferent i a t ing object ively betwe en c losely re la ted s lope-failure types. PECO, a landslide locat ion att r ibut e, is the most effect ive measure used herein grouping landslides. Shear planes located near the base of hillslopes, with hi gh values ofPECO, ten d to have the deposi t ional phase of the mov eme nt obl i tera ted, dep osi t io n pre-domi nant ly be yo nd the fo ot of the hi l ls lope, and shear planes forme d in colluvia l mater ia lor in-situ yel low- bro wn wea the red grey wacke. All these latter character is tics are, m fact,the result of shear-plane location near the base of the hillslope.

Table VI indicates the po or cor re spon denc e bet wee n the second comp ute r c lassi f icationand the four t radi t ional groupings of the 92 landsl ides. Although the under th rus t s lidesremain tog eth er as a group with some of the d6bris slides, as they did in the f irst com-put er classif ication (Table IV), it is evident tha t the co mpu te r groupings cut across theother c lassif icat ions . However , the cor resp onden ce achieved betwe en the c lassif ications is

TABLE VIComparison of classifications with compute r Classification 2Group Name: Total O P Q R S T U VNumber in group: 92 11 81 41 40 25 15 25 15M37Modified Varnes s classification:

1. Slump 1 1 . . . . . . . . . . .2. D~bris slide 11 5 6 5 1 1 - 3 25. Dgbris avalanche 2 - 2 1 1 - 1 1 -

11. D~bris slide-avalanche 24 - 24 18 6 3 3 1 179. Ddbris avalanche-flow 49 - 49 17 32 21 11 13 43. Underthrust slide 3 3 . . . . . . . . . .

12. Incipient underthrust slide 2 2 . . . . . . .M40Dominant m ovemen t is:

1. Rotation 1 1 . . . . . . . . . .2. Trans lation 15 10 5 4 1 1 2 23. Flow 55 55 21 34 23 11 16 56. Flow-translation 20 20 16 4 1 3 167. Flow-fall 1 - 1 - 1 - 1 -

M38Failure is:1. Simple 50 11 39 17 22 12 10 11 6

2. Compound 24 - 24 12 12 10 2 7 53. Complex 18 - 18 12 6 3 3 -- 12

M39Number ofp hases o f movement:1. 51 11 40 18 22 12 10 12 62. 34 - 34 20 14 11 3 4 163. 7 - 7 3 4 2 2 2 1


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NUMERICAL CLASSIFICATIONOF SELECT ED LAND SLIDES 111grea t e r t han t ha t r e su l t i ng f rom the f i r s t compu te r g roup ing . Neve r the l e s s , t he poo re s tco r r e spo nden ce occu r s i n t he ca se o f t hose l ands li de s b roa d ly c l a s sed a s d~br is s l i d e s -d6b r i s ava l anches -d6b r i s f l ows .

A com par i son o f the two c om pu te r c l a s s i f ic a t ions i s d i f f icu l t because t he m a jo r l and -s li de cha r ac t e r i s ti c s r e spons ib l e f o r t he agg lo me ra t i on o f e ach g roup a r e d i f f e r en t i n e achcase. I n C l a s s if i c a ti on 1 s i x g roups l e t t e r ed E , F , G , H , I , and J occ u r a t t he s econd m a jo rh i e r a r ch ica l l eve l. S imi l a rl y , in t he s econd co m pu te r c l a s s if i c a ti on , g roups O , S , T , U , andV fo rm the s econd h i e r a r ch i ca l leve l . Tab l e VI I i nd i ca te s t he g roup i n to wh ich each l and -s lide fa l l s for eac h of the co m pu ter c lass i f ica t ions as wel l as for the fou r s imp le c lassi fi -c a t i ons ba sed on d i so rde r ed mu l t i s t a t e va r i ab l e s 37 , 38 , 39 , and 40 , and a s p r e sen t ed i nTab l e s I I I , IV , and V1 .

I t is a p p a r e n t f r o m T a b l e V I I t h a t t h e r e m o v a l o f a l ar ge n u m b e r o f v a r ia b l es f r o mClas s i fi c a t ion 1 ha s p ro duc ed m an y changes in t he g roup ings ach i eved in C la s s i f ic a t ion 2 .The c l a s s if i c a ti ons cann o t , t he r e fo r e , b e r ega rded a s s t ab le . The da t a p r e se n t ed i n Tab l eVI I a l so show th a t l ands li de s g roupe d t og e the r i n even fou r o u t o f s ix o f t he c l a s s i f ic a t ionscan be w ide ly s ep a ra t ed i n t he o the r tw o c l a s s i fi c a ti ons .SUMMARY AND C ONCLUSIONS

Ne i the r o f t he tw o c om pu te r c l a s s i f ic a t i ons o f l andsl i de s p r e sen t ed he r e r evea l s anys ing le l ands li de f o rm a t t r i bu t e o r g roup o f a t t r i bu t e s t ha t c an be u sed t o d i s ti ngu ish ,c lear ly , be tw een such c lose ly re la ted ty pe s of s lope fa i lures as d6br is sl ides, d6br i s f lows ,and d6b r is ava l anches . Fu r the r m ore , t he fo rm a t ion o f i nd iv idua l g roups w i th in t he com -pu t e r c l a s s if i c a ti ons i s depe nden t up on a w ide va r i e ty o f lands l ide cha r ac t e r is t i c s. Theg roups , once fo rmed , f r equen t l y bea r no r e semblance t o l ands l i de ca t ego r i e s de f i ned i nmore t r ad i t i ona l c l a s s i f i c a t i ons ; t h i s i s no t an i nd i c tmen t o f t he compu te r g roup ings . Thel ack o f ag r eemen t be tween t he compu te r c l a s s i f i c a t i ons t e s t ed i nd i ca t e s t ha t s t ab i l i t y ha sno t been ach i eved , and t ha t o the r cha r ac t e r i s ti c s w il l have t o be i nves t i ga ted i f a r a t i ona lde sc r i p ti ve c l a s s if i c a ti on o f l ands l ide t ypes i s t o b e cons t ruc t ed .

No c l ea r ba s is c an be i den t i f i ed fo r d i f f e r en t i a t i ng amo ng t he l ands l ide s g roup ed byVarnes (1958 ) a s com plex , bu t t o g rou p so man y l ands l ide s o f pos s ib ly d ive r se f o rm s a sc o m p l e x r e m a i n s u n s a t i s f a c t o r y . T h e r e m o v a l o f d e p o s it s b y s t r e a m a c t i o n p r e v e n t s t h eu se o f S k e m p t o n s ( 1 9 5 3 ) d e p t h / l e n g t h r a t io i n a l ar g e n u m b e r o f c as e s i n th e u p p e rMangawhara ca t chmen t . A l though l ands l i de l oca t i on on t he h i l l s l ope p roved a u se fu lc r i t e r ion i n t he s econd c om pu te r c l a s s i f ic a t i on , because som e o the r l oca l l ands li decha rac t e r i s ti c s a re pa r t l y con t ro l l ed by d i s t ance f ro m the ba se o f the h i l l, t h i s a t t r i bu t e i sha rd ly l i ke ly t o p rov ide t he ba s is o f a un ive r sa l ly app l i cab l e c l a s s i fi c a t ion .

As no one o f t he c l a s s i fi c a t ions o f lands li de s a t t em p te d he r e p rov ides any r a t i ona l ba s isfo r com par i son o f s l ope f a il u r e s f r om a r ea t o a r ea , un t i l a re a l is t ic s chem e is f o r t hco m ingi t s eems sens ib l e t o r e ly on a s imp le d iv is i on o f l ands li de s acco rd ing t o t he na tu r e o f t hed o m i n a n t m o v e m e n t . H o w e v e r , i t s e e m s t h a t t h e t h r ee b a si c t y p e s o f fa i lu r e - r o t a t i o n ,t r a n sl a ti o n , a n d f l o w a g e - m u s t b e s u p p l e m e n t e d b y a t ra n s i ti o n a l f o u r t h t y p e , t h a t


14/16

L Numb

M3 M3 M3 M4 Cla

Cla

L

d

Numb

M3 M3 M3 M4 Cla

Cla

L

d

Numb

M3 M3 M3 M4 Cla1

Cla

~ ~

~~. g a

Z


15/16

NUMERICAL CLASSIFICATION OF SELECTED LANDSLIDES 113

domi nate d both by flow and by translation al failure. Where possible, this four-folddivision should be supplemented by Skempton s depth/length ratio.

ACKNOWLEDGEMENTSThe au thor is inde bted to Mr. C. F. Pain (Australian National University) for his con-

siderable help and critical comme nt during field work. Mr. P. W. Milne (Division of Com-puting Research, CSIRO, Canberra) provided invaluable advice and assistance withcomp utin g procedures. Professor G. H. Dury (The University of Wisconsin), R. J. Wasson,M. F. Clarke and M. A. J. Williams (Macquarie Universit y) all made valuable criticisms ofa draft o f the ma nuscri pt. Financ ial assistance during field work and for comp utin g wasprovided by The University of Sydn ey and Macquarie University.

REFERENCESBailey, R. G. and Rice, R. M., 1969. Soil slippage, an indicator of slope instability on chaparral water-sheds of southern California.Prof. Geogr., 21(3): 172-177.Blong, R. J., 1971. The underthrust slide - an unusual type of mass movement. Geogr. Ann., 53A:52-58.Blong, R. J., 1972. Methods of slope profile measurement in the field. Aust. Geogr. Stud., 10: 182-192.Blong, R. J., 1973. Relationships between morphometric attributes of landslides.Z. GeomorphoL, inpress.Cumberland, K. B., 1944. Contrasting regional morphology of soil erosion in New Zealand. Geogr.Rev., 34: 77-95.Folk, R. L., 1951. A comparison chart for visual percentage estimation. J. Sediment. Petrol., 21 : 32-33.Gower, J. C., 1966. Some distance properties of latent root and vector methods used in multivariateanalysis.Biometrika, 53: 325-338.Hutchinson, J. N., 1968. Mass movement. In: R. W. Fairbridge Editor),Encyclopedia of Geomor-phology. Reinhold, New York, N.Y., pp.688-695.Irwin-Hunt, J. R., 1960. Some Quantitative Aspects o f Mass Movement. Thesis, Kings College,University of London, London, 76 pp.Ladd, G. E., 1935. Landslides, subsidences and rock-falls.Am. Railw. Eng. Assoc. BulL, 6:1092-

1162.Lance, G. N., Milne, P. W. and Williams,W. T., 1968. Mixed data classificatory programs. III. Diag-nostic systems.Aust. Comput. J., 1(3): 178-181.Lance, G. N. and Williams,W. T., 1967a. Mixed-data classificatory programs. I. Agglomerative systems.Aust. Comput. J., 1(1): 15-20.Lance, G. N. and Williams,W. T., 1967b. A general theory of classificatory sorting strategies. I. Hier-archical systems. Comput. J., 9(4): 373-380.Miller, V. C., 1953. A quantitative geomorphic study of drainage basin characteristics in the ClinchMountains areas, Virginia and Tennessee.Dept. Geol., Columbia Univ., ONR Proj. NR 389-420Tech. Rep., 3: 30pp.Rice, R. M., Corbett, E. S. and Bailey, R. G., 1969. Soil slips related to vegetation, topography, andsoil in southern California. WaterResour. Res., 5(3): 647-59.Sharpe, C. F. S., 1938. Landslides and Related Phenomena. Pageant, N.J., 125 pp.Skempton, A. W., 1953. Soil mechanics in relation to geology.Proc. Yorks. Geol. Soc., 28: 33-62.Skempton, A. W. and Hutchinson, J. N., 1969. Stability of natural slopes and embankment found-ations, lnt. Soil Mech. Conf.. Mexico, State-of-the-Art Rep., 291-340.


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114 R. J. BLONG

Sokal R. R. and Sneath P. H. A. 1963. Principles ojNumerical Taxonomy. Freeman San [:ranciscoCalif. 359 pp.

Sowers G. B. and Sowers G. F. 1961. Introductory Soil Mechanics and Foundations. Macmillan NewYork N.Y. 386 pp.

Terzaghi K. 1950. Mechanism of landslides. Geol. Soc. Am., Geol. Berkeley) Vol. : 83 - 123.Varnes D. J. 1958. Landslide types and processes. Highw. Res. Board. Spec. Rep., 29: 20-47.Ward W. H. 1945. The stabili ty of natural slopes. Geogr. J., 105: 170-197.Yatsu E. 1966. Rock Control in Geomorphology. Sozosha Tokyo 135 pp.Yatsu E. 1967. Some problems on mass movements. Geogr. Ann., 49A: 396-401.Zaruba Q. and Mencl V. 1969. Landslides and their Control. Elsevier Amsterdam 202 pp.

a numerical classification of selected landslides of the debris slide blong

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