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 A L L UNCONSOLIDATED ROCK FRAGMENTS SAND OR SILT MIXED MOSTLY PLASTIC ROCI\FRAGMENT DRY SAND LOESS 'FLOW RUN FLOW FLOWS ' RAFD DEBRIS SLOW EARTHFLOW AVALANCHE EARTHFLOW SAND OR SILT WET DEBRIS FLOW FLOW MUDFLOW COMPLEX COMBINATIONS OF MATERIALS OR TYPE OF MOVEMENT Figure 5. Classification of landslides, abbreviated version (see plate 1 for complete chart with drawings and explanatory text). 22 LANDSLIDES .J— Limestone, sandstone, - lava or other bed resistant to erosion. Shale, tuff, volcanic ash or I other easily weathered bed A. Differential weathering B. Frost wedging in jointed homogeneous rock Water—filled joint '1 I L_) C. Jointed homogeneous rock. D. Homogeneous jointed rock. Hydrostatic pressure acting Blocks left unsupported or on loosened blocks. loosened by overbreakage and blast fracture. E. Either homogeneous jointed F. Either homogeneous jointed rock or resistant bed rock or resistant bed underlain by easily eroded underlain by easily eroded rock. Wave Cut cliff, rock. Stream Cut cliff. Figure 6. Examples of rockfalls. LANDSLIDE TYPES AND PROCESSES 23 Figure 7. Rockfall due to undercutting along shore of Las Vegas Bay, Lake Mead. Nev. The rock is the Muddy Creek formation (Pliocene?) consisting here of siltstone overlain by indurated breccia. The move- ment is straight down by gravity, in contrast to rockslide. which slides on a sloping surface. (Photograph by U. S. Hureau of Reclamation. February 24, 1949) Materials are classed, for falls and TYPE I - FALLS slides, into bedrock and soils. The term "soils" is used in the engineering sense Falls are very common. In rockfall and and includes elastic material, rock frag- soilfall, the moving mass travels mostly ments, sheared or weathered bedrock, through the air by free fall, leaping, and organic matter. Falls and slides in- bounding, or rolling, with little or no volving bedrock are shown in the upper interaction between one moving unit and left part of the chart; those involving another (P1. 1-a, b). Movements are very soils are shown in the upper right part rapid to extremely rapid (see rate of of the chart. The materials of flows are movement scale, P1. 1-u) and may or may grouped into various categories. Material not be preceded by minor movements. is classified according to its state prior Several varieties of rockfall are illus- to initial movement, or, if the type of trated in Figures 6 and 7. movement changes, according to its state at the time of the change to the new TYPE II - SLIDES type of movement. Types of movement are divided into In true slides, the movement results three principal groups - falls, slides, from shear failure along one or several and flows. A fourth group, complex surfaces, which are either visible or may slides, is a combination of any or all of reasonably be inferred. Two subgroups the other three types of movement. There of slides may be distinguished according may be, of course, variations in the type to the mechanics of movement - those of movement and in the materials from in which the moving mass is not great- place to place, or from time to time, in ly deformed (Type hA), and those an actual landslide, so that a rigid classi- which are greatly deformed or consist of fication is neither practical nor desir- many small units (Type JIB). The Type able. hA group includes the familiar slump 24 LANDSLIDES or rotational shear types of slides; it in- slope (see Fig. 9). In slumps the move- cludes also undeformed slides along more ment is more or less rotary about an axis or less planar surfaces, for which the that is parallel to the slope. The top sur- term "block glides" is here proposed. Type JIB slides include most rockslides, debris slides, and failures by lateral spreading. A - Relatively Uncle formed Material Type IIA slides are made up of one or a few moving units. The maximum di- mension of the units is greater than the relative displacement between units and is comparable to or greater than the dis- placement of the center of gravity of the whole mass. Movement may be structural- Figure 9. Rotational shear on cylindrical surface. ly controlled by surfaces of weakness, such as faults, bedding planes, or joints. Slumps. - The commonest examples of face of each unit tilts backward toward Type hA or undeformed glides are the slope (see Figs. 9, 10, 11, and 12, slumps. Slumps, and slumps combined and P1. 1-c and 1-h). with other types of movement, make up Figure 10 illustrates some of the com- a high proportion of the landslide prob- moner varieties of slump failure in va- lem facing the highway engineer. The rious kinds of material. Figure 12 shows movement in slumps takes place only th'e backward tilting of strata exposed along internal slip surfaces. The exposed in a longitudinal section through a small cracks are concentric and concave to- slump in lake beds. Although the sur- ward the direction of movement. In many face of rupture of slumps is a concave- slumps the underlying surface of rup- upward curve, it is seldom a circular arc ture, together with the exposed scarps, of uniform curvature.