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Surface Observation and Geologic Mapping 6,Chapter 9 JEFFREY R. KEATON AND JEROME V. DEGRAFF SURFACE OBSERVATION AND GEOLOGIC MAPPING 1. INTRODUCTION Basic tools and techniques needed in surface observation and geologic mapping include access y assessing causes of and factors contributing to existing information, use of topographic maps B to slope movement, surface observation and and aerial photographs, use of aircraft for aerial re- geologic mapping of slopes provide the basis for connaissance, access to the field site, use of limited subsurface investigations and engineering analyses hardware and simple instruments, and the ability that follow. Accurate interpretation of the surface to observe and interpret geologic features caused features of a landslide can be used to evaluate the by and related to slope instability. Some of the mode of movement, judge the direction and rate of techniques that are described in this chapter are movement, and estimate the geometry of the slip relatively new and have not received widespread surface. Surface observation and geologic mapping exposure. Those techniques that are well known should be done on slopes with active or inactive will be mentioned in the context of the chapter landslides and on slopes with no evidence of past but not discussed in detail. A review of more so- landslides to provide a basis for evaluation of the phisticated survey technology applicable to land- likelihood of new or renewed slope movement. slide evaluations is also included in this chapter. Information obtained through surface observation and geologic mapping of a particular site extends 1.1 Duties of the Geologist and utilizes knowledge of landslide types and proc- Surface observation and geologic mapping are use- esses discussed in Chapter 3 and the recognition ful for an existing project that has developed an and identification procedures described in Chapter unstable slope and for a proposed project that has 8. The results of geologic mapping provide the basis the potential for slope movement. The usual se- for planning the subsurface investigations described quence of events is similar for both projects; the in this chapter and for locating the instrumenta- sense of urgency and the time available to. respond tion discussed in Chapter 11. The geologist should are different. remain involved in the project during the subsur- The geologist is notified that the project has face investigation to aid in the correlation of sur- been authorized and the area is described as well as face and subsurface data. During the design of slope possible by a knowledgeable person, perhaps some- stabilization measures, discussed in Chapters 17 one who witnessed or discovered a slope move- and 18, the geologist should be available to answer ment. Geologic and topographic maps and aerial questions about the geology, and during construc- photographs are examined if available to provide tion the geologist should be on site to compare an initial understanding of the general character of conditions encountered with those predicted. the site. 178 Surface Observation and Geologic Mapping 179 An aerial reconnaissance is made if possible, formal geologic map is prepared from field notes, providing a very useful perspective of the geology; and a verbal report consisting of pertinent findings, the nature of the slope movement if it has occurred; observations, and recommendations for locations the general configuration of the landscape, includ- and numbers of borings and test pits may be given ing vegetation, geomorphology, and surface water to the design engineer. A written report is pre- features; and access to the site. Proximity to utili- sented to those responsible for planning the sub- ties and other nongeologic features of importance surface investigation, and the geologist assists in also can be observed readily from the air. The most coordinating the subsurface investigation results practical aircraft for geologic reconnaissance is a with the surface observations and geologic maps. high-wing, single-engine airplane. Helicopters are excellent reconnaissance aircraft; however, they are 1.2 Active Slides Versus Stable Slopes much more expensive than fixed-wing aircraft and Investigations of stable slopes, even those with in- generally are less available. active landslide deposits, may be more methodical The geologist begins the actual investigation than investigations of active slide areas because with a ground-based geologic reconnaissance. The such analyses are not performed under conditions relationship of the topographic map and aerial of urgency. On active slides the investigative tools photographs to the actual landscape is recognized and techniques used for stable slope areas are sup- and geologic mapping begins. If slope movement plemented with other techniques. The immediate has occurred, particular attention is paid to fea- information needed by a design team investigating tures such as ground cracks, closed topographic de- an active landslide includes the boundaries of the pressions, tilted trees, and seeps and springs. Other slope movement, the rate and direction of move- geologic features, such as bedrock exposures, sur- ment, and the probable causes of movement. The face water drainage patterns, surficial deposits, and engineering geologist is well equipped to collect geologic structure, are also mapped and recorded. and interpret this kind of information rapidly. A special effort should be made to photograph im- Reconnaissance instrumentation for monitoring portant features. Photographs are a visual supple- deformations and pore pressures should be de- ment to the geologic map and the field notes ployed in the early stages of an investigation of an produced during the reconnaissance. Photographs active slide to provide an early and long record. can record information that may become less evi- Experience on the part of the geologist is needed dent over time and can facilitate communication because of the lack of time for methodical investi- with specialists unfamiliar with technical landslide gation and because of possible hazards such as terminology. open ground cracks, falling rock, or debris flows. Early in the surface observation and geologic mapping effort at sites with active slope move- 2. WORK REQUIRED BEFORE FIELD ments, reconnaissance instrumentation should be VISITATION deployed across selected ground cracks and within the body of the landslide. Instrumentation mea- Efficient surface observation and geologic map- surements may be repeated several times during the ping must be planned in the office before the site geologic mapping to provide early information on is visited. The area of interest must be identified, the rate and nature of slope movement. Shallow and available geologic and geotechnical informa- groundwater information acquired by simple instru- tion, aerial photographs, and topographic maps mentation is especially valuable. must be collected and reviewed. Topographic profile and geologic information should be collected to produce geologic cross sec- 2.1Area of Interest tions at important locations. Topographic maps usually provide adequate detail for cross sections in The area of interest includes the slope with the ac- less important locations. Conceptualization of geo- tive landslide or the potential for slope movement logic conditions follows field data collection; how- as well as adjacent regions that could be contribut- ever, much of the conceptualization is developed ing to causes of movement. Adjacent land uses, as part of the "multiple working hypothesis" ap- such as agricultural irrigation, may be important proach used in geology (Chamberlin 1965). The factors. Regional geologic conditions could be di- WE Landslides: Investigation and Mitigation recting groundwater from adjacent recharge areas these maps often disregard the surficial deposits into the area subject to slope movement. Some and show bedrock relationships as if surficial landslide types are capable of traveling relatively materials were not present. far from their sites of origination. When such land- Some countries have published soil survey re- slide types are anticipated, it is prudent to consider ports that may be helpful in understanding the the adjacent areas upsiope from the project site weathering products of some geologic formations. where such landslides might originate as well as the The U.S. Department of Agriculture (USDA), for adjacent areas downslope that might be affected by example, includes the Natural Resources Conser- landslides generated at the project site. vation Service (formerly the Soil Conservation The area of interest must be defined to permit Service), an agency responsible for mapping soils searching for available geologic and geotechnical in agricultural areas. The USDA Forest Service information, aerial photographs, and topographic maps soils on national forests, which cover exten- maps and for planning aerial and ground-based sive areas in the western United States and geologic reconnaissance. Alaska. Soil surveys usually are restricted to the upper 1.5 m of the soil profile. Nonetheless, this 2.2 Geologic and Geotechnical information can be particularly useful because of information the level of detail (Hasan 1994). Soil moisture, seeps, springs, and marshy areas are important in Regional geologic and tectonic information pro- agricultural soil surveys; therefore, these features vides an understanding of the geologic context, are well documented in the published surveys. which will be helpful in anticipating those factors
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