Recognition and Identif Ication

Chapter 3 Recognition and Identif ication Harold T. Rib and Ta Liang The adage that half the solution to a problem is the recogni- The use of aerial techniques for evaluating landslides is tion that a problem exists is especially appropriate to land- emphasized in this chapter because of their proven value slides. The recognition of the presence or the potential de- and the unique advantages they offer. No other technique velopment of slope movement and the identification of the can provide a three-dimensional overview of the terrain type and causes of the movement are important in the de- from which the interrelations existing among slope, drain- velopment of procedures for-the prevention or correction age, surface cover, rock type and sequence, and human ac- of a slide. tivities on the landscape can be viewed and evaluated. In Several basic guidelines developed through years of ex- addition, the availability of new types of aerial imagery, perience in investigating landslides form the basis for the including satellite, infrared, radar, and microwave radiom- present-day approach to landslide investigations. etry, extend the advantages of this technique. (In this chapter, the term imagery is used to describe .those data Most landslides or potential failures can be predicted collected from sensor systems other than cameras.) if proper investigations are performed in time. The cost of preventing landslides is less than the cost TERRAIN EVALUATION FOR of correcting them except for small slides that can be LANDSLIDE INVESTIGATIONS handled by normal maintenance procedures. Massive slides that may cost many times the cost of Basic Factors the original facility should be avoided in the first place. The occurrence of the initial slope movement can Recognition and identification of landslides are as complex lead to additional unstable conditions and movements. as are materials and processes that cause them. The basic causes of sliding movements are given in Chapter 2. Be- This chapter discusses techniques for recognizing the cause, as Chapter 2 states,a landslide can seldom be attrib- presence or potential development of landslides and the ob- uted to a single definite cause, the overall terrain must be servable features that aid in identifying the types of slope analyzed and the individual factors and the interrelations movements and their probable causes. The techniques dis- among those factors distinguished before a potential slope cussed include (a) a review of topographic maps and geo- movement can be recognized and identified. For this pur- logic, pedologic, and engineering reports and maps; (b) the pose, the factors contributing to slides are more conveniently analysis of aerial photography and other forms of aerial grouped into categories relating to features that can be de- images; and (c) preliminary field reconnaissance surveys. lineated on maps and photographsor that can be measured These three techniques complement one another and to- and quantified, rather than into the mechanisms of failure getherforni the, basis of the preliminary analysis in a land- listed in Chapter 2. The basic factors considered in evaluat- slide investigation. The follow-up detailed field investiga- ing the terrain and the major elements included within each tions required to accurately delineate the landslides or are given in Table 1 and discussed below. landslide-prone areas, to identify the causative factors, and to determine the physical and chemical properties required Geologic Factors for the design of corrective measures are discussed in subsequent chapters. The present-day landscape—including its topography, geo- 34 logic structure, and composition—is the result of millions of susceptible to slope movement because of the nature of years of development and modification. The topographic their development and the stage of their evolution. features exposed on the surface are relatively young in age, but the geologic structures and composition from which Environmental Factors the features were carved can be quite old. For example, the structural features that characterize the Rocky Moun- tains culminated at the close of the Cretaceous period (ap- The development of landforms and the occurrence of slope proximately 60 million to 70 million years ago), but little movements are greatly affected by environmental factors. of the topography in that area dates beyond the Pliocene Significantly different landscapes develop from the same (several million years), and the present canyons and details geologic materials in different climatic zones. For example, of relief are of Pleistocene or Recent age (less than 1 mil- a limestone bedrock will usually be found as a cliff or ridge lion years old) (137). Thus, to properly evaluate the in an and area, but will commonly form a low undulating present-day landscape requires an appreciation of the geo- plain in a humid area. The former situation would be much logic and climatic changes that occurred during recent geo- more susceptible to landslides than the latter. Similarly, logic periods (a determinant of topography) as well as an landslides occurring in arid regions are usually distinct and understanding of the development of the geologic structure easily recognized because of lack of cover, while those oc- and composition. The science that deals with landscape de- curing in wet tropical climates are weathered, more subdued, velopment is geomorphology. covered by vegetation, and difficult to discern. The basic unit of the landscape distinguished in geomor- Variations in microclimate, such as differences in altitude, phology is the landform. The distinctive development of exposure to moisture-bearing winds, and exposure to sun- landforms depends on three factors: (a) initial composition light, can cause significant differences in the geomorphic and structure; (b) processes that act to modify the initial processes. Various observers have reported that some composition and structure; and (c) stage of development. south-facing slopes of east-west valleys in the northern A change in any one of these factors will produce a uniquely hemisphere are less steep than adjacent north-facing slopes. different landform. North-facing slopes have snow cover longer, experience To evaluate landforms, one must understand their geo- fewer days of freeze and thaw, retain their soil moisture logic composition and structure including mineralogy and longer, and probably have a better vegetative cover, all of lithology; physical hardness of the constituent materials which result in less active erosion and steeper slopes (3.37). and their susceptibility to weathering; mode of deposition Numerous investigators have demonstrated that there is and subsequent stress history; structural attitude and pres- a relation between slope movement and precipitation. ence of structural discontinuities and weaknesses such as Záruba and Mend (3.40) evaluated rainfall records for sev- joints, bedding planes, faults, and folds; and permeability eral different sites in Czechoslovakia covering periods of of constituent materials or layers. 50 to more than 75 years. They were able to correlate the The many physical and chemical ways by which the orig- years of heaviest precipitation with the most active periods inal composition and structure are modified are called geo- of slope movements. morphic processes. The most important of these processes Sudden changes in the landscape, often represented by are associated with changes due to (a) actions of water, slope movement, have resulted from catastrophic occur- wind, or ice; (b) weathering, mass wasting, and erosion; and rences, such as earthquakes, hurricanes, and floods. Figure (c) diastrophism and vulcanism. 3.1 shows numerous slides that occurred in western Virginia The modification and eventual destruction of the land- along the path of hurricane Camille in 1969. These slides forms are considered to occur in stages that geomorpholo- were rapidly documented from the analysis of aerial photo- gists generally designate as youth, maturity, and old age. graphs taken after the storm (3.39). Other case histories Qualifying adjectives such as early and late are often used documenting the effect of catastrophic occurrences are to designate substages. Chronological age is not inferred, referenced in Chapters 1 and 2. Undercutting by stream but rather a relative stage of development. and wave action and the erosion and mass movement of The concept of classifying distinct landforms is of prime slopes by seepage, wetting and drying, and frost action are importance in recognizing and identifying landslides. Ex- also well-known environmental phenomena resulting in perience has demonstrated that certain landforms are more slope movement. Table 3.1. Basic factors considered in evaluating terrain. Factor Element Examples Geologic Landform Geomorphic history; stage of development Composition Lithology; stratigraphy; weathering products Structure Spacing and attitude of faults, ioints, foliation, and bedding surfaces Environmental Climate and hydrology Rainfall; stream, current, and wave actions; groundwater flow; slope exposure; wetting and drying; frost action Catastrophes Earthquakes; volcanic eruptions; hurricanes, typhoons, and tsunamis; flooding; subsidence Human Human activity Construction; quarrying and mining; stripping of surface cover; over loading, vibrations Tempora a aCommon to all categories and factors 35 Human Factors way to initiate preliminary landslide investigations. Geo- morphologists have divided

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