Chapter Three Landslide Types and Processes
David J. Varnes3
It is the purpose of this chapter to re- geology, and hence lie outside the prov- view the whole range of earth move- ince of the committee. ments that may properly be regarded as landslides and to classify these move- Types of Landslides ments according to factors that have some bearing on prevention or control. CLASSIFICATION As defined for use in this volume the term "landslide" denotes downward and Many classifications have already been outward movement of slope-forming ma- proposed for earth movements, based terials composed of natural rock, soils, variously on the kind of . material, type artificial fills, -or combinations of these of movement, causes, and many other materials. The moving mass may pro- factors. There are, in. fact, so many such ceed by any one of three principal types schemes embedded with varying degrees of movement: falling, sliding, or flowing, of firmness in geological and engineering or by their combinations. Parts of a literature that the committee has ap- landslide may move upward while other proached the question of a "new" classi- parts move downward. The lower limit fication with considerable misgivings. As of the rate of movement of landslide ma- Terzaghi has stated (1950, p. 88), "A terial is restricted in this book by the phenomenon involving such a multitude economic aspect to that actual or po- of combinations between materials and tential rate of movement which provokes disturbing agents opens unlimited vistas correction or maintenance. Normal sur- for the classification enthusiast. The re- ficial creep is excluded. Also, most types sult of the classification depends quite of movement due to freezing and thaw- obviously on the classifier's opinion re- ing (solifluction), together with ava- garding the relative importance of the lanches that are composed mostly of many different aspects of the classified snow and ice, are not considered as land- phenomenon." Each classification, in- slides in the sense here intended, al- cluding the one proposed in this volume, though they often pose serious problems is best adapted to a particular mode of to the highway engineer. Such move- investigation, and each has its inherent ments are not discussed because they advantages and disadvantages. However, appear to depend on factors of weather, as pointed out by Ward (1945, p. 172), ice physics, and thermodynamics, rather "A classification of the types of failure is necessary to the engineer to enable than on principles of soil mechanics or him to distinguish and recognize the '' Publication authoricecl by the Director, U. S. different phenomena for purposes of de- Geological Survey. sign and also to enable him to take the 20 LANDSLIDE TYPES AND PROCESSES 21 appropriate remedial or safety measures (b) the type of movement, which usu- where necessary. The geographer, and ally may be determined by a short period geologist need a classification so that of observation or by the shape of the they may interpret the past and predict slide and arrangement of debris. In its the present trends of topography as re- emphasis on type of movement the classi- vealed by their observations." fication resembles, more than any other, The classification adopted there is that proposed by Sharpe (1938) for shown in Plate 1 and is further described landslides 'and other related movements. in the following paragraphs. Definitions The chart (P1. 1) shows examples of of the parts of a landslide appear in slides by small drawings. The type of Plate 1-t. An abbreviated version, with- material involved is indicated by the out diagrams and explanatory txt, is horizontal position of the drawing with- shown in Figure 5. In prepaiing' this in the chart; the type of movement is classification of landslides, a deliberate indicated by the vertical position of the effort has been made to set it up ac- drawing. Water content of flow-type cording to features that may be observed landslides is indicated by the relative at once or with a minimum of investi- vertical position of the drawing within gation, and without reference to the the flow group. Each drawing also has a causes of the slides. Two main variables note giving the general range of velocity are considered: ('a) the type of material of movement of the landslide type, ac- involved, which usually is apparent oii cording to the scale of velocities at the inspection or preliminary boring; and bottom of the chart (P1. 1-u).
TYPE OF TYPE OF MATERIAL MOVEMENT BEDROCK SOILS
FALLS ROCKFALL SOILFALL
ROTATIONAL PLANAR PLANAR ROTATIONAL FEW UNITS SLUMP BLOCK GLIDE BLOCK GLIDE BLOCK SLUMP SLIDES I ' DEBRIS FAILURE BY, MANY UNITS ROCKSLIDE SLIDE LATERAL SPREADING