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Recognition and Identification of Landslides

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Recognition and Identification of Landslides Chapter Four Recognition and Identification of Landslides Arthur M Ritchie The problems of recognition of land- movements have actually taken place or slides and identification of landslide are likely to do so in the future, and the types are as complex as are the ma- means of identifying the various types terials and processes that cause them. of landslides and their constituent parts. As treated more fully in Chapter Three, The application of these methods and the basic conditions that favor slides criteria to the solution of landslide prob- depend to a large extent on the char- lems appears in succeeding chapters, par- acter, stratigraphy and structure of the ticularly in Chapter Six. underlying rocks and soils, on the topog- .raphy, climate and vegetation, and on Evidence for Actual or Potential surface and underground waters. All of Landslides these factors vary widely from place to place; their variations are reflected in ENVIRONMENTAL FACTORS differences in the rate and kind of land- slide movements that result from their A knowledge of the general setting is interaction. essential in the recognition of either po- Because so many variables are in- tential or actual landslides. By setting is volved in the production of landslides, meant all the factors that make up the it is natural that many tools, theoretical physical environment - geology, soils, and practical, must be used in recogniz- topography, climate - for a difference in ing and classifying them. The selection any single factor, such as climate, can of methods and criteria for use in solu- have pronounced effect on the other fac- tion of a given problem is made with tors, hence on the probability of slides. appreciation of the local conditions. Ob- Similar geologic and soil conditions viously, not all the available techniques tend to give rise to similar landslides - are used on every job. One single factor and recognition features - but only un- may call for entirely different methods der constant climatic cnditions. For ex- in determining the danger of landslides. ample, in an arid region a mature topog- Nevertheless, it is important to have all raphy developed on old inactive slumps the methods in mind in case they are is characterized by smooth, low rounded needed. surfaces, known collectively as hum- It is the purpose of this chapter to mocky ground (see Fig. 33). Even describe some of the techniques used in though modified, the slump blocks are recognition and classification and to in- still discernible; the depressions between dicate their possible applications. One them are disconnected and there is no important method - photointerpretation well-defined drainage system. In regions - is treated separately in Chapter Five. of heavy rainfall, however, and in the The other approaches can be logically same amount of elapsed time, the topo- treated in two groups -the means of graphic expression of identical geologic determining whether or not landslide conditions is entirely different. The 48 RECOUNITION AND IDENTIFICATION 49 - At p h. _r,.?.-._ -.-. •---m %(. y I* Fts I—WK__ iO - - . — Figure 33. Ilunirniok> ground, 'inc of the most easily a1,plicd criteria for recogflh,.iflg landslides. In this slide, one mile south of Springdale. Utah, the hummocks are larger and more irregular than they are on many slides. (Photograph by It. E. Gregory, I. S. Geological Survey) slopes of the hummocky ground are flat- ent kinds of rocks and soils that cover ter and the depressions are filled with them. swamp mud and water, with some semb- Only rarely do geologic or soil maps lance of an integrated drainage system. show landslides as such or describe their This difference is, of course, due to the causes, nor (10 most of them show fea- fact that landslide topography reaches tures in the detail that is necessary to maturity more rapidly in humid regions answer specific problems. Such maps, than it does in arid ones. however, as well as air photos, are of Knowledge of the general setting is great assistance in giving background best had by means of thorough personal knowledge that is needed as a setting acquaintance with the area and long- for detailed studies. Some of the useful continued careful observations and anal- facts that can be gleaned from maps ysis. Failing this method, and even ac- of one kind or another, or from air companying it, much of the necessary photos, are as follows: background knowledge can be gained Rock and soil units and their char- through study of available aerial photo- acteristics. graphs (see Chapter Five) and of topo- Areal distribution of rock and graphic, geologic and soil maps. With soil units. these facilities the trained observei' can Sequence of rock and soil units. obtain a great deal of information on For example, a weak unit that could the character of the slopes, of surface cause failure may not be exposed at the and subsurface drainage, and on the surface but may be plainly shown on a character and distribution of the differ- geologic (ross-section or on a soil profile. 50 LANDSLIDES Character and distribution of slides that grade downward into flows. folds, faults and joints in bedrock, all The practical difference to the engineer of which may seriously affect its sus- is that the fine-grained slump-forming ceptibility to sliding. materials will not stand up in fills, thus Location of volcanic cinder cones requiring road construction to be made and similar features that offer special entirely within cuts. On the other hand, problems. the coarser materials that form debris Drainage pattern - streams, lakes slides farther east can be used success- and swamps, all of which give indica- fully in fills if special precautions are tions of relative permeability of under- taken in beginning them. lying materials. The example just given illustrates the Bowl-shaped headwater regions of fact that generalized geologic maps and creeks, which suggest landslide origins. descriptions have definite limitations, but Terraces, slopes, and depressions. this does not imply any lack of faith in Abnormally steep slopes, with their value as a tool for coping with mounds of possible landslide origin at landslide problems. Indeed, as the boun- their bases. daries of the Astoria formation - and Scalloped escarpments that sug- of other geologic units that present com- gest landslide origins. parable problems - become b e t t e r Anomalous constrictions in can- known, and the facies characteristics are yons, quite possibly caused by landslides. mapped in greater detail, it is increas- ingly feasible to predict the kind and Aerial photographs and geologic maps character of landslides that are likely to must be used with caution; they can be occur in any part of the formation. of great assistance in developing the The study of the environment of the setting for more detailed studies, but area should give at least a general answer they seldom contain the detail that is to a fundamental question: "Do land- required to answer specific questions. slides already exist along the proposed Many geologic formations throughout location?" If the answer is no, it may be the country are labeled as troublemak- taken as. an indication that there are no ers because they are susceptible to land- regional factors that are themselves con- sliding, yet no formation forms slides ducive toward landslides. If the answer throughout the entire extent of its out- is yes, it is probable that there are two crop area. It is, thus, incorrect to give or more regional factors which, acting the entire formation a bad name. Slight together, may lead to landslides. For ex- differences in environmental factors or ample, in the north-central part of Wash- in facies within the formation can and ington there is a great and widespread do lead to stability as well as to in- deposit of silt, called the Nespelem stability. Moreover, slight differences formation. When dry, as in small iso- within a formation may give rise to en- lated hills or terrace remnants, this silt tirely different types of landslide. The stands well in natural or artificial cuts. Astoria formation of Oregon and Wash- When wet by spring waters, however, the ington is a good case in point. It is com- silt is very likely to slide. The conjunc- posed of sandstone and siltstone that tion of the two factors - silts and spring grade imperceptibly eastward from finer horizons - should be a danger signal to to coarser grain sizes. In the western the engineer who is planning a highway portion of its area of outcrop the As- toria produces many deep-seated slump or other structure in the area. landslides; many of the higher hills are Studies of all existing landslides in nothing more than remnants of slump the region are thus warranted in order blocks. Eastward, however, where the to determine their geologic settings and materials are somewhat coarser grained, their causes. If similar conditions are the formation is characterized by debris present along the proposed location it RECOGNITION AND IDENTIFICATION 51 can be assumed that slides will occur evide rice of stretching consists of small there too. cracks that surround or touch some rig- id body, such as a root or boulder, in POTENTIAL SLIDES otherwise homogeneous material; these cracks form because the tensional forces Even if the preliminary examination tend to concentrate at or near the rigid of the general environment has indicated bodies. that no landslide movements have yet For recognition of a potential land- taken place, it is still incumbent on the slide condition where bedrock is hidden,' investigator to determine whether the a preliminary but adequate field investi- ground to be disturbed by the proposed gation of the soil, coupled with shear construction will prove reasonably stable.
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