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 Furthermore, the maps in soil survey reports are that will be important in controlling slope stabil- on an aerial-photographic base. These photo- ity. Regional maps generally are made at scales of graphs, although not stereoscopic in the published 1:100,000 or less and are usually published by gov- soil surveys, are helpful in several ways, as dis- ernment agencies. cussed in Section 2.3. Soil surveys usually include Local geologic and tectonic information pro- some limited data on the engineering properties of vides a useful basis for an understanding of the selected soils. This information can serve as the general nature of the site geology. The rock types, basis for estimating initial values for preliminary surficial deposits, ages, stratigraphic relationships, slope stability calculations (Hasan 1994). and structural features are better portrayed on Areas of slope instability often have recurring maps at scales larger than 1:100,000. (In the problems. A report may have been prepared before United States, common scales for local geologic construction of a highway project, but a slope maps are 1:62,500 and 1:24,000; in many other movement may occur after the project has been countries, common scales are 1:50,000 and completed. In such cases, a report on the specific 1:10,000.) Unpublished theses from local univer- geology or geotechnology, or both, prepared by an sities may include larger-scale geologic maps to agency or consultant may be available for the sub- supplement published maps. ject area. Such reports are excellent sources of per- Groundwater conditions are particularly im- tinent information and should be utilized to the portant in slope stability evaluations. Preliminary greatest extent possible. information often can be collected from regional and local geologic maps. Recharge and discharge 2.3 Aerial Photographs areas may be discernible from a knowledge of the regional climate and preliminary analysis of the Aerial photographs, which are discussed in Chapter terrain, including the distribution of rock types, 8, may be available from government agencies or faults and fractures, and springs and marshes. commercial aerial-photography companies, usually Isohyetal maps and other maps representing cli- for specific projects or purposes, possibly for making matic information are available from government topographic maps. Although older photographs are agencies. Some geologic maps provide reasonably commonly in black and white, since the late 1960s detailed information on surficial deposits, includ- they have usually been in natural color. The older ing landslide deposits. Other geologic maps are black-and-white photographs were produced at made specifically to portray bedrock relationships; scales that are of marginal usefulness for detailed Surface Observation and Geologic Mapping slope mapping. Scales of 1:15,000 or larger are re- quired before the photographs havc the dctail needed for mapping. Some features are lost even at a scale of 1:5,000 in natural color photographs (see Chapter 8). Aerial photographs at scales between 1:24,000 and 1:12,000 are available in the United States for areas covered by the Natural Resources Conservation Service soil surveys and for federal land administered by the Forest Service, Bi.ireai.i Land Management, and Narional Park Service. For most modem highway projects, including some low, volume roads, project specific aerial pho- tographs arc used in planning and dcsign. Oftcn the photographs are used to prepare topographic strip maps of the right of-way. Such photographs are particularly valuable as a base for geologic map- ping. Items used in working with aerial pho- tographs are shown in Figure 9-1. Acetate sheets (such as those used for overhead projector trans- parencies) may be taped over the photographs and special extra-fine-point permanent- ink pens (over- and shadowed scarps visible on one of the pho- FIGURE 9-1 head projector pens) used to make notations in the tographs is shown in Figure 9-3. Items useful in working with aerial field. To help keep the photographs clean and dry photographs: A, in the field, suitably sized plastic food storage bags 2.4 Topographic Maps contact prints of with tightly fitting closures (self-sealing bags) are stereoscopic aerial particularly useful and inexpensive. Nonpre- Existing topographic maps provide information for photographs; B, scription reading glasses are an alternative to a assessing site access and general conditions. How- acetate sheets pocket stereoscope; the glasses may be worn like ever, these maps usually are not at a scale suitable (overhead projector regular glasses, even over prescription lenses. for the level of detail needed in most landslide in- films) to be taped to When current aerial photography is available, vestigations. Furthermore, existing maps most aerial photographs; C, extra-fine-point it is often possible to obtain photographs taken for likely were made before recent slope movement, permanent-ink pens the same area 5, 10, or even 50 years earlier. Older and thus the features of active landslides may not for writing on photographs may show evidence of past landslide be visible. acetate sheets movement at or near the area of interest. In areas Project-specific topographic maps can be tai- (overhead projector with extensive urban development, the original lored to suit the needs of the project. These maps pens); D, eraser with topography will be visible on aerial photographs usually are produced by photogrammetric methods emulsion for erasing taken before urbanization. These photographs can from stereoscopic aerial photographs. Therefore, permanent ink from acetate sheets; F, the photographs and maps are useful in surface provide valuable insight into conditions affecting pocket stereoscope; slope stability in these areas. field- investigation activities. Current technology F, nonprescription Landslide features, particularly scarps and fis- allows for production of orthogonally rectified reading glasses, sures, can be accentuated by low-sun-angle illumi- aerial photographs. The best base map can be real- which can be nation. Data and procedures described in a current ized by superimposing the topographic contours on used as pocket solar ephemeris permit calculation of the opti- an orrhophotographic base. stereoscope; G, mum time of day and day of year for morning and self-sealing plastic food storage afternoon low-sun-angle illumination at any loca- 3. ON-SITE ENGINEERING-GEOLOGIC bags to keep tion where the latitude and longitude are known. INVESTIGATIONS photographs clean Figure 9-2 shows a plot and data table for the po- and dry in field. sition of the sun referenced to the Thousand Peaks On-site engineering-geologic investigations of areas landslide area in northern Utah on August 23, of active and potential slope movements consist of 1990, the date when photographs were taken for a reconnaissance observations, engineering-geologic pipeline investigation. A detail of the highlighted mapping, and reconnaissance instrumentation.
182 Landslides: Investigation and Mitigation
The results of the earlier office-based investiga- N tions should be well understood before this field investigation is begun.
3.1 Reconnaissance Observations 300 450 Aerial reconnaissance is the preferred type of ini- + r"~- tial surface investigation. The perspective ob- tained from the air is valuable in understanding ar~, + the relationships among landslide features, surfi- cial and bedrock materials, landforms, vegetation, and surface water features. In addition, some as- pects of logistics, such as roads and trails, may be observed best from the air at a close distance. General geologic features and landforms can be noted on topographic maps, aerial photographs, or both during the aerial reconnaissance. Features such as bedrock exposures, vigorous vegetation THESE SUNANGLE DATA ARE FOR: marking shallow groundwater or springs, breaks in
THOUSAND PEAKS LANDSLIDE AREA - UTAH slope angle, terraces, ground cracks, tilted trees, NORTH LATITUDE: 40.98 DEGREES ( 400 59 0°) and areas of eroded bare slopes should be noted for WEST LONGITUDE: 111.08 DEGREES (111° 5' 00) ON 23 AUGUST 1990 subsequent examination on the ground. A thor- SUNRISE AT 6:47 AT AZIMUTH 74.9 ough understanding of these types of features often SUNSET AT 8:06 AT AZIMUTH 284.8 leads to an interpretation of the causes of or COORDINATES OF factors contributing to slope movement. LOWER HEMISPHERE MOUNTAIN ANGLE OF EQUAL ANGLE Reconnaissance observations also can be of DAYLIGHT SUN SUN ABOVE PROJECTION value in assessing physical access to the site. The TIME AZIMUTH HORIZON (UNIT RADIUS) location of roads and trails is important, not only (HR) (DEG) (DEG) X Y for the geologist, but also as access for subsurface
6 67.1 -8.4 - investigation equipment. In some locations, utili- SUNRISE 74.9 0.0 0.966 0.260 ties may cross the site. Even in relatively remote 7 77.1 2.4 0.935 0.214 8 86.8 13.6 0.786 0.044 areas, buried communications cables and pipes car- 9 96.8 24.9 0.634 -0.076 rying water for stock could be present. To the ex- 10 108.1 35.9 0.485 -0.159 tent possible, these utilities should be identified 11 122.2 46.1 0.341 -0.215 12 141.2 54.6 0.200 -0.249 and located during the reconnaissance or subse- 1 166.9 59.6 0.061 -0.265 quent engineering-geologic mapping. Overhead 2 196.1 59.3 -0.076 -0.264 electric-power lines can represent a constraint to 3 221.1 53.8 -0.215 -0.246 drilling equipment. A nearby canal or pond for wa- 4 239.4 45.0 -0.356 -0.210 5 253.1 34.7 -0.502 -0.152 tering stock can be a source of water for the drilling 6 264.2 23.6 -0.651 -0.066 operation. 7 274.1 12.2 -0.804 0.058 Property ownership and land use should be in- 8 283.8 1.1 -0.953 0.234 SUNSET 284.8 0.0 -0.967 0.255 vestigated. Permission may be needed to cross 9 293.9 -9.7
FIGURE 9-2 Position of sun calculated on hourly basis from sunrise to sunset on August 23, 1990, at Thousand Peaks landslide area in northern Utah. Sun position is calculated using solar ephemeris with reference to local horizon at point of interest. Input data required to calculate sun position are latitude and longitude, number of hours of difference between local time and Greenwich mean time, and Greenwich hour angle and declination from ephemeris for day of interest (day after for areas west of Greenwich and day before for areas east of Greenwich). Tabulated data of sun azimuth and sun angle are converted to coordinates of lower hemisphere equal angle projection (WuIff net) for plotting on stereographic projection. Surface Observation and Geologic Mapping 183 property for access to a specific site. In addition, project, such as a pipeline, can have a rigIa-uf-wa much narrower than a specific landslide or poten- tial slope movement, requiring permission from adjacent property owners for access to conduct in- vestigations of the pertinent slopes. Adjacent land use can influence the performance of a slope. For example, agricultural irrigation can transmit water , to places that would oiherwise be diy. Nearby buried water pipelines could be threatened by slope movement, or they could leak and contri- bute to the potential for slope movement.
3.2 Engineering-Geologic Mapping
The purpose of engineering-geologic mapping is to document surface conditions to provide a basis for projecting subsurface conditions and to assist 1 -
the design engineer in understanding key factors \d 1 ..'. that must be accommodated in construction. A geologic map is an artistic depiction of the geol- ogy of a site. An engineering-geologic map must not only be interpretative, it should also be docu- interpreting causes of landslides and potential for FIGURE 9-3 (above) mentary. For example, an area mapped as sand- slope movement. An interpretive result of engi- Part of aerial stone should be exposed sandstone, not colluvial neering-geologic mapping will be classification of photograph of Thousand Peaks deposits containing abundant fragments of sand- slopes in terms of stability, as shown in Figure 9-5. landslide area in stone. Notes should be made regarding uncertain- northern Utah taken ties, with an attempt to quantify them if possible. 3.2.1 Landslide Surface Features on August 23, lithe uncertainties cannot be quantified, notes 1990, at about 8:00 should be made that outline the reasons. Features on the ground surface provide the key to am. local time. Engineering-geologic mapping should charac- understanding the details of landslide processes Nominal negative terize a site or region in terms meaningful for and and causes. Landslide types and processes are dis- scale of photograph useful to the design engineer. Areas of similar geo- cussed in Chapter 3 and common landslide trig- is 1:12,000. Scalps and ground cracks logic characteristics are designated structural do- gers are described in Chapter 4. Landslide deposits can be classified by age and type of movement ac- caused by mains for rock slope engineering (Piteau and movement of Peckover 1978). Boundaries of structural domains cording to the features observed on the ground landslides in 1983 usually coincide with major geologic features, such surface. Therefore, particular attention must be are enhanced by as faults and bedrock formation contacts. Boun- paid to these features, and detailed notes and highlights or daries of straight construction slope segments that sketches should be made. shadows caused by have similar orientations are determined by the de- The boundaries of landslide deposits may ap- low angle of sun. COURTESY OF KERN RIVER sign engineer and superimposed on an engineering- pear gradational, but a boundary may actually be a GAS TRANSMISSION COMPANY geologic map that shows structural domain zone of suhparallel cracks and bulges that mark a boundaries to form design sectors (Piteau and shear zone. With continuing displacement, the Peckover 1978). Within each design sector, repre- cracks and bulges may give way to a single contin- sentative rock defect data are selected for use in de- uous crack along which lateral shearing occurs. sign calculations (see Chapter 15). Figure 9-6 illustrates this evolution at the Aspen Engineering-geologic mapping for landslide in- Grove landslide in central Utah, showing condi- vestigations should focus on landslide features, as tions (a) in August 1983 and (b) in August 1984. shown in Figure 9-4. In addition, surficial deposits An active landslide may affect existing struc- and bedrock exposures must be carefully docu- tures, utilities, and other artificial features in ways mented. Surface-water features are important in that provide insight into the processes and cause of 184 Landslides: Investigation and Mitigation
/ HESF /
HES YESO HESF ( YESF HES YESE F-,0 HESF MESF YESF ,HESF .YESF HESF jp 91 HESF YES YE?' '
MESF
4L. HES _ MEF 0 1000 2000 ft HESFs4L I I ! 0 300 MEF / HEF / - EXPLANATION- HEF Gemorphology Aft : l • Element Active Landslides Young Landslides HESF F1 Mature Landslides YES Colluvial [] Slopes Q5 HEF F Floodplsins Abandoned Proposed D Pipeline Alignment 1" Pipeline Alignment Q. Quadrilaterals
FIGURE 9-4 Engineering-geologic map of Thousand Peaks FIGURE 9-5 Stability classification of Thousand Peaks landslide landslide area in northern Utah (modified from Keaton et al. area in northern Utah (modified from Keaton et al. 1992). See 1992). See Table 9-2 for definition of landslide abbreviations Table 9-6 for definition of stability classes. USED WITH PERMISSION OF AMERICAN SOCIETY OF CIVIL ENGINEERS USED WITH PERMISSION OFAMERICAN SOCIETY OF CIVIL ENGINEERS
the feature. Cracks in pavement, foundations, and the slip surface (Carter and Bentley 1985). A se- other brittle materials can support inferences ries of lines is projected through stations used to about the stress produced by movement of the construct a topographic cross section from the landslide. The timing of breakage of water lines, main scarp to the toe of the landslide. By graphical electrical cables, and similar utilities can suggest representation, the lines define the probable slip the sequence of deformation before field observa- surface. Hutchinson (1983) noted several other tions or supplement observations of continuing techniques using surface observations to infer the movement. Measuring the tilt of structures as- slip surface and related subsurface movement. sumed to be vertical or horizontal before move- Seismic-refraction and electrical-resistivity tech- ment can give an idea of the amount of niques (Carroll et al. 1968; Miller et al. 1980; displacement on certain parts of the landslide. Cummings and Clark 1988; Palmer and Weis- Internal features, such as those described in garber 1988) and acoustic-emission and electro- Chapters 3 and 8, should be documented by the magnetic methods (McCann and Forster 1990) geologist when and where observed so they may be have been used in landslide investigations. It prob- used to interpret the subsurface conditions. The ably will be necessary to interpret the results ob- geometry and nature of the sliding surface are tained along a number of geophysical lines and to among the most important of the subsurface con- incorporate surface observations of ground cracks ditions in landslide evaluations. Surface measure- - and bedrock exposures. Landslide deposits com- ments can be employed to estimate the shape of monly are extremely variable, resulting in severe
Surface Observation and Geologic Mapping 185
FIGURE 9-6 Plane-table maps of (a) August 1983 (b) August 1984 160 part of right flank of Aspen Grove / //. z EXPLANATION 12.6,. landslide, Utah, in 1983 and Fa,jttraae 13 Narrow 0,00k / in. 1984 (Fleming V \ tZZ7 Open crook - I' \ . onoou and iohnsQn 1989). .4f \.t (width 009e00tOd) 11 1j,, v.011001 dlaptacn.noflt Scale is for HarIOXntaI diopt0000,nflt / reference to specific 170 'l...... P' Thrunt butt - landslide features. :onks /i77.on. Mapping was done :aid 14. f *13 Survoyatatian - in field at scale of ç\, / 1:200. Note evolution of en echelon cracks into through-going slip planes. REPRINTED WITH - PERMISSION OF ELSE VIER SCIENCE PUBLISHERS
/ SCALE ,11
C.I..o.5n1 ci.. ton, 1 o- / 2 N Arbitrary Datum
• ,n
176 /t71t__ 200
free
210
2200.
energy attenuation and complex arrival times in for tens of thousands of years have features that are seismic-refraction surveys and complicated pat- subdued and poorly defined. The changes of land- terns in electrical-resistivity soundings. slide features from sharp and well-defined to sub- Landslide features become modified with age. dued and poorly defined were incorporated into an Active landslides have sharp, well-defined surface age classification by McCalpiñ (1984), as shown in features, whereas landslides that have been stable Table 9-1, for the Rocky Mountains of western 186 Landslides: Investigation and Mitigation
North America. The key features are the main The reader is cautioned that these classifica- scarp, lateral flanks, internal morphology, vegeta- tions are different from the system proposed by the tion, and toe relationships. The rate of change of UNESCO Working Party on the World Landslide landslide features in climates other than that of the Inventory (WP/'(fLI 1990, 1991, 1993a, b) that is Rocky Mountains has not been documented, and described at length in Chapter 3 of this report. A the estimated age of most recent movement shown major source of confusion may result because all in Table 9-1 may not be valid for other climates. classification systems use similar, even identical, However, by intuition, the general sequence of terminology with different meanings. The basis of changes must occur in all climates. all terms used must always be clearly defined. A classification system based on activity, degree of certainty of identification of the slide boundaries, 3.2.2 Surficial Deposits and the dominant type of slide movement was de- veloped by Wieczorek (1984). McCalpin's age clas- Surficial deposits must be mapped in terms that will sification and Wieczorek's certainty and type of be meaningful to the design engineer. The movement classification were combined for this Genesis-Lithology-Qualifier (GLQ) System of en- chapter into the Unified Landslide Classification gineering-geologic mapping symbols (Galster 1977; System, shown in Table 9-2. Changes in typical Keaton 1984; Compton 1985) provides a useful features for different ages of landslides are shown in method for accomplishing this; it is proposed here Figure 9-7. An example of a map made with this that the name of this system be changed to the system is presented in Figure 9-8. Unified Engineering Geology Mapping System.
Table 9-1 Age Classification of Most Recent Activity for Landslides in Rocky Mountain—Type Climate (modified from McCalpin 1984)
Acrivrn MAIN LATERAL INTERNAL TOE ESTIMATED STATE SCARP FLANKS MORPHOLOGY VEGETATION RELATIONSHIPS AGE (YRs) Active, Sharp; Sharp; Undrained Absent or Main valley <100 (historic) reactivated, or unvegetated unvegetated; depressions; sparse on stream pushed suspended; streams at edge hummocky lateral and by landslide; dormant- topography; internal scarps; floodplain historic angular blocks trees tilted covered by separated and/or bent debris; lake by scarps may be present Dormant-young Sharp; partly Sharp; partly Undrained and Younger or Same as for 100 to 5,000 vegetated vegetated; drained different type active class (Late Holocene) small tributaries depressions; or density but toe may be to lateral streams hummocky than adjacent modified by topography; terrain; older modem stream internal cracks tree trunks vegetated may be bent Dormant-mature Smooth; Smooth; Smooth, rolling Different type Terraces covered 5,000 to 10,000 vegetated vegetated; topography; or density by slide debris; (Early Holocene) tributaries extend disturbed internal than adjacent modem stream onto body of slide drainage network terrain but not constricted same age but wider upstream floodplain Dormant-old Dissected; Vague lateral Smooth, Same age, Terraces cut > 10,000 or relict vegetated margins; no undulating type, and into slide (Late Pleistocene) lateral drainage topography; density as debris; uniform normal stream adjacent modem pattern terrain floodplain Nom: See Chapter 3 for definitions of terms. Activity states dormant-stabilized and dormant.abandoned may have features of any age classification; the stabilized and abandoned states must be interpreted from other conditions. Surface Observation and Geologic Mapping 187
Table 9-2 Unified Landslide Classification System (modified from Wieczorek 1984)
AGE OF MOST DOMINANT DOMINANT TYPE OF RECENT AcllvITYa MATERIAL" SLOPE MOVEMENT"
SYMBOL DEFINITION SYMBOL DEFINITION SYMBOL DEFINITION A Active R Rock L Fall R Reactivated S Soil T Topple S Suspended E Earth S Slide H Dormant-historic D Debris P Spread Y Dormant-young F Flow M Dormant-mature 0 Dormant-old T Stabilized B Abandoned L Relict Nom: See Chapter 3 for further definitions of terms. Landslides classified using this system are designated by one symbol from each group in the sequence activity-material-type. For example, MDS signifies a mature debris slide, HEF signifies a historic earth flow, and ARLS signifies an active rock fall that translated into a slide. Based on activity state in Table 3-2 and age classification in Table 9-1. Based on material and type in Table 3-2.
This system consists of a series of letters to indicate do not reveal staining under hand lens examina- how the material was deposited (genesis) and its tion are considered to be fresh (State 1). Rocks basic grain size (lithology). If additional informa- that do not appear stained to the unaided eye but tion is needed to improve the understanding of the do reveal stained areas under examination with a geology, qualifying features may be indicated by hand lens are considered to be slightly weathered additional letters. The basic elements of the (State 2). Rocks that are stained but cannot be Unified Engineering Geology Mapping System are broken by hand are considered to be moderately shown in Table 9-3. weathered (State 3). Rocks that can be broken by hand into gravel and larger fragments of rock in a soil matrix are considered to be severely weathered 3.2.3 Bedrock (State 4). Rocks that can be completely disaggre- Bedrock consists of the rock material itself and gated into mineral grains are considered to be the discontinuities that cut through it. Bedrock completely weathered (State 5). must be mapped in terms that will be meaningful In the Unified Rock Classification System, to the design engineer. The Unified Engineering strength is also designated as one of five states. Geology Mapping System, introduced in Section Strength is estimated with the aid of a ball-peen or 3.2.2 and described in Table 9-3, and the Unified geological hammer. A rock from which the ball- Rock Classification System (Williamson 1984), peen hammer rebounds without leaving a mark is shown in Table 9-4, provide useful methods for much stronger than concrete and is considered to accomplishing this purpose. The Unified Engi- have very high strength (State 1). A rock that re- neering Geology Mapping System uses a version acts elastically and on which a ragged pit is pro- of the conventional geologic shorthand consisting duced is stronger than concrete and is considered of two capital letters to denote bedrock type. The to have high strength (State 2). A rock that can be elements of the Unified Rock Classification Sys- dented by a ball-peen hammer has an unconIined tem are degree of weathering, estimated strength, cbmpressive strength approximately the same as and estimated density. concrete and is considered moderately strong In the Unified Rock Classification System, de- (State 3). A rock that reacts plastically and on grees of weathering are designated States 1 which a dent is produced surrounded by a sheared through 5. The degree of weathering is determined crater is not as strong as concrete and is considered by examining intact rock fragments with the un- to have low strength (State 4). A rock that can be aided eye and by simple strength tests. Rocks that broken by hand has very low strength (State 5).
(a) Bedrock is freshly exposed in main scarp or flank; bedding and structure - are discernible. Many cracks exist above and subparallel to the main - — --...O scarp; cracks extend across the slide; radial cracks occur within the - ,'toe portion. Original vegetation has been disrupted; the present orientation of vegetation indicates direction of principal movement and rotation. Water is ponded in closed depres- sions caused by rotational movement or by blockage or original runoff paths. Gh-
Although bedrock is still visible in many places, weathering has obscured the original structure. Cracks are no longer visible within or adjacent to the slide mass. Hydrophilic vegetation has established itself in the ponded areas. Minor scarps and transverse ridges have been modified to the point where the ground has a distinctive hunmocky appearance. T& /
The main scarp and flanks have been considerably modified by both erosion and revegetation. Erosion has reduced the slopes of the scarp, flank and toe regions. Erosion has resulted in gullies and establisliment of new drainage paths within and adjacent to the landslide. likewise the original huninocky surface has been somewhat subdued.
FIGURE 9-7 (facing pages) Block diagrams of morphologic changes with time of idealized landslide (a) in humid climate (Wieczorek 1984) and fb) in and or semiarid climate (modified from McCalpin 1984): A. active or recently active (dormant-historic) landslide features are sharply (b)
Sharply defined components.
A
Slopewash and shallow mass movements modify sharp edges, but drainage lines are not established.
Drainage tollo mass, internal I dissected, mat mass.
Slide mass is almost completely removed, drainage network shows weak structural control, valley drainage re-establishesits pre-slide profile. [I:
defined and distinct; B, dormant-young landslide features remain clear but are not sharply defined owing to slope wash and shallow mass movements on steep scarps; C.. dormant-mature landslide features are modified by surface drainage, internal erosion and deposition, and vegetation; D, dormant-old landslide features are weak and often subtle. 190 Landslides: Investigation and Mitigation
Table 9-3 Basic Elements of Unified Engineering Geology Mapping System (modified from Keaton 1984 and Compton 1985)
SYMBOL DEFINITION SYMBOL DEFINITION SYMBOL DEFINITION
SURFICIAL DEPOSITS Genetic A Alluvial C Colluvial E Eolian F Fill 0 Glacial L Lacustrine M Marine R Residual V Volcanic Lithologic c Clay m Silt s Sand g Gravel k Cobbles b Boulders r Rock rubble t Trash or debris e Erratic blocks p Peat o Organic material d Diatomaceous earth Qualifier (deposits) Alluvial (f) Fan morphology (fp) Floodplain (te) Terrace (p) Pediment (dO Debris fan Colluvial (sw) Slope wash (ta) Talus (cr) Creep deposits Eolian (d) Dune morphology (1) Loess Fill (u) Uncompacted (e) Engineered Glacial (t) Till (m) Moraine (o) Outwash (es) Esker (k) Kame (ic) Ice contact Lacustrme and (b) Beach (de) Delta (ma) Marsh marine (tc) Tide channel Residual (sa) Saprolite (bh) B horizon (kh) Calcic horizon Volcanic (af) Air fall (pf) Pyroclastic flow (s) Surge (pc) Pyroclastic cone (I) Lahar
BEDROCK MATERIALS Sedimentary SS Sandstone ST Siltstone CS Claystone co Conglomerate LS Limestone SH Shale Igneous GR Granite AN Andesite BA Basalt SY Syenite RH Rhyolite DI Diorite Metamorphic QT Quartzite Sc Schist GN Gneiss SL Slate MA Marble SE Serpentine NOTE: Surficial deposits are designated by a composite symbol: Ab(c), where A is a genetic symbol, b is a lithologic symbol, and (c) is a qualifier symbol; for example, Csmg(sw) signifies colluvial slope wash composed of silty sand and gravel. This system was formerly called the Genesis-Lithology-Qualifier (GLQ) System because of the symbol for surficial deposits. Bedrock materials are designated by two capital letters; for example, LS signifies limestone bedrock. See Table 9-4 for additional bedrock classifications. Rocks in strength States 4 and 5 should be treated as soil rather than rock in the engineering sense. ---- I II II • The estimates of rock density utilized by the ' , Unified Rock Classification System can be deter- mined rapidly for rock samples using Archimedes' if principle, a spring-loaded "fish" scale, and a bucket of water. A rock sample is suspended on a string from the scale and weighed in air; the weight of the string is neglected. The same sample is then sub-
FIGURE 9-8 (left) Detail of map of part of Thousand Peaks landslide area in northern Utah (scale 1 :4,800). Landslides defined by Unified Landslide Classification System (Table 9-2); materials defined using Unified Engineering Geology Mapping System (Table 9-3). Ccms stands for colluvial deposits composed of silty and sandy clay; ST-CS stands for siltstone-claystone bedrock. Surface Observation and Geologic Mapping 191
Table 9-4 Unified Rock Classification System (modified from Williamson 1984; Geological Society Engineering Working Party 1977; and Hoek and Bray 1977)
STATE DEFINITIoN DEscRIFrIoN * DEGREE OF WEAThERING 1 None No visible sign of rock material weathering 2 Slight Discoloration on major discontinuity surfaces; rock material may be discolored and somewhat weaker than fresh rock 3 Moderate Less than half of the rock material is present either as a continuous framework or as corestones 4 Severe Most of rock material is decomposed, disintegrated to a soil, or both; original mass structure is largely intact 5 Complete All rock material is converted to a soil; mass structure and material fabric are destroyed; a large change in volume has occurred, but soil has not been transported significantly
ESTIMATED STRENGTh
1 Very high Geological hammer rebounds; can be chipped with heavy hammer blows; unconfined compressive strength: q> 100 MPa 2 High Geological hammer makes pits; cannot be scratched with knife blade; unconfined compressive strength: 50 < q < 100 MPa 3 Moderate Geological hammer makes dents; can be scratched with knife blade; unconfined compressive strength: 20 < 50 MPa (range of concrete) 4 Low Geological hammer makes craters; can be cut with knife blade; unconfined compressive strength: 5 < q < 20 MPa 5 Very low Moldable by hand; can be gouged with knife blade; unconfined compressive strength: q < 5 MPa (behaves like soil)
ESTIMATED DENSITY 1 Very high D > 25 kN/m3 2 High 23.5 < D < 25 kN/m3 3 Moderate 22 < D < 23.5 kN/m3 (range of concrete) 4 Low 20.5 dip are measured with a Brunton compass, and dip direction and dip magnitude are measured with a Clar compass. These compasses are shown in Figure 9-9 and the measurement techniques are shown in Figure 9-10. More complete discussion of : the Brunton compass was given by Compton : (1985), and Hoek and Bray (1977) described the use of the Clar compass. The use of rock-structure data in rock-slope engineering is discussed in Chapter 14. Continuity of rock defects is an ex- pression of the length or persistence of a joint or other feature across a rock exposure. A single long or through-going joint can contribute more to in- stability than a number of parallel but short joints. Table 9-5 Rock Defect Data (modified from Bieniawski 1989) FIGURE 9-9 SYMBOL DEHNITI0N Geologic compasses: Right, Brunton compass, also Defect type J joint known as pocket transit; left, Clar compass, also F Fault known as Breithupt-Kassel rock structure compass. B Bedding See Figure 9-10 for use of compasses; refer to T Fracture Chapter 14 for analysis of rock-structure data. S Schistosity/foliation Length 1 Very short: L < 1 m 2 Short:l Aperture of rock defects is an expression of the nearly closed topographic depressions. Streams on openness or lack of contact of rock surfaces across some landslides may make abrupt changes in di- the defects. Roughness is a measure of the irregu- rection or gradient. Springs and seeps near the larities on the defect surfaces. Roughness is impor- crest of a slope can supply recharge zones that pro- tant because of its contribution to the coefficient of vide groundwater to the unstable or potentially friction and resistance to sliding. Joint roughness unstable slope. Springs and seeps near the base of can be estimated using a fractal-dimension proce- a slope indicate discharge zones that can be help- dure (Carr and Warriner 1987) and shadow pro- ful in projecting piezometric surfaces in the slope. filometry (Maerz et al. 1990). Infllling materials, Localized closed depressions on slopes usually are such as clay, can contribute significantly to insta- zones of groundwater recharge, particularly if bility, as can the abundance of water along the ground cracks arepresent in or adjacent to them. defects. Culverts and other artificial features diverting Harp and Noble (1993) developed an engi- surface water or affecting subsurface flow should be neering rock classification to evaluate seismic identified. Culverts may divert water from a rock-fall susceptibility using a modification of the greater area than would normally contribute sur- rock mass quality designation (Q system) of face flow to a landslide. Poorly constructed ditches Barton et al. (1974). The rock mass quality classi- provide depressions for water to pond and recharge fication for rock-fall susceptibility is groundwater. The compacted material forming the = 115 —3.3 iv Jr road prism or related retaining structures may act (9.2) in Ja AF as barriers to downslope movement of ground- water, which may be closer to the ground surface in the slope immediately above these features. where iv = total number of joints per cubic meter; Jn = joint set number, ranging from 0.5 for no 3.2.5 Field-Developed Cross Sections or few joints to 20 for crushed rock; The topographic profile of a geologic cross section Jr = joint roughness number, ranging from 0.5 can be obtained directly from a topographic base for slickensided planar joints to 4 for dis- map or it can be measured in the field during geo- continuous joints; logic data collection. The field-developed profile Ja = joint alteration number, ranging from 0.75 for tightly healed, hard, nonsoften- can be measured using several methods. The con- ing joints to 4 for low friction clay filling ventional method is to use surveying instruments the joints; and and a two-person crew to collect topographic data along the desired line of section. This method AF = aperture factor, modified from the stress reduction factor of Barton et al. (1974), does not produce a profile in the field for plotting ranging from 1 for all tight joints to 15 for geologic data. many joints open more than 20 cm. Alternative single- or two-person methods uti- lize a 50-rn-long tape measure, 2-rn-long folding Harp and Noble (1993) described a seismic rock- rulers, a hand level, and a Brunton compass fall susceptibility rating as follows: (Williamson et al. 1981; Koler and Neal 1989). Rock Mass The tape measure is stretched on the ground sur- Quality Category Rating face in the line of profile, and its orientation is measured with the compass. The folding ruler is !~ 1.41 A Highly susceptible Q used to position the hand level at a known height 1.41 194 Landslides: Investigation and Mitigation FIGURE 9-1 1 Landslide Classification System (Tables 9-1 and Methods of Hand level Tape on 9-2) would be mapped as Ia, Ib, or Ic in the stabil- surveying for ity classification in Table 9-6. Dormant-historic, developing cross -young, -mature, and -old landslides would be sections in the field: groufld (a) single-person mapped as ha, lib, lic, and lid, respectively. Slopes method using hand O~Folclinag ruler that do not show evidence of prior slope move- level, tape, and ment but appear potentially unstable would be folding ruler; (b) (b) Hand level mapped as III and those that appear stable would two-person method be mapped as IV. using hand level Other classifications of landslide hazards exist and folding rulers; and have been summarized by Vames (1984) and and (c) incremental measurements of Folding rulers Hansen (1984). Such classifications are general- slope profile using a ized and best suited for regional application rather circle level, or than for specific landslide sites. Many classifica- Slope-a-Scope tion schemes are based on multivariate regression method (Lips and of numerous attributes, including such factors as Keaton 1988). (See rock type, which must be assigned an ordinal rank text for details.) value for inclusion in regression analyses (Jade and Sarkar 1993; Anbalagan 1992; Pachauri and Pant .IvIJ -a- IJJuIII u 1992). 3.3 Reconnaissance Instrumentation and bedrock exposures, is recorded along with the to- Surveying pographic data. An optical range finder can be used in lieu of a tape measure. Reconnaissance instrumentation and surveying A third single-person method uses a rigid two- are intended to provide early quantitative infor- dimensional frame equipped with a circle level— mation regarding landslide movement and piezo- the Slope-a-Scope (Lips and Keaton 1988). The metric level. Formal field instrumentation is slope is measured in inclined increments equal to described in Chapter 11. Reconnaissance instru- the length of the crossbar of the frame (commonly mentation must be simple and easy to install. The 1 or 2 m), as shown in Figure 9-11(c). The incli- instrumentation discussed in this section is re- nation and increment length are noted and can be stricted to open-standpipe piezometers. Other re- plotted in the field. Geologic information is connaissance techniques, such as reference bench recorded while the profile data are being collected. marks, aerial and terrestrial photography, and quadrilaterals, are directly related to surveying, which is discussed in Section 4. 3.2.6 Classes of Slope Stability Preliminary information on shallow piezometric Slopes range from apparently stable segments to surfaces can be collected at the reconnaissance actively moving landslides. Surface observation level if the soil is relatively soft. The device used is and geologic mapping provide the basis for inter- an open-standpipe piezometer constructed of con- preting degree of slope stability. A stability classifi- ventional 1.25-cm-diameter galvanized or black cation for slopes developed on the basis of ideas by iron pipe. It is convenient to use three or four Crozier (1984) is presented in Table 9-6. This clas- 1-rn-long sections that can be connected by sification is based on recurrence of movement, threaded couplings (Figure 9-12). A bolt is placed analogy to stable or unstable slopes, and results of in the bottom end of the first pipe section and stress analyses. The reader is once again cautioned driven into the ground using a capped, 30-cm-long that this classification differs from the classification section of larger-diameter pipe that will slide over system of the UNESCO Working Party on the the piezometer pipe. A coupling or cap small World Landslide Inventory (WP/WLI 1990, 1991, enough to fit inside the driver pipe should be used 1993a, b) defined in Chapter 3. Active, reacti- on the piezometer pipe to prevent damage to the vated, or suspended landslides in the Unified threads. Subsequent sections of piezometer pipe are Surface Observation and Geologic Mapping 195 Table 9-6 Stability Classification of Slopes and Landslides (modified from Crozier 1984) CLASS DEscRIPTIoN I UNSTABLE SLOPES Ia Active landslides; material is currently moving, and landslide features are fresh and well defined lb Reactivated landslides; material is currently moving and represents renewed landslide activity; some landslide features are fresh and well defined; others may appear older Ic Suspended landslides; slopes with evidence of landslide activity within the past year; landslide fea- tures are fresh and well defined II SLOPES WITH INACTIVE LANDSLIDES ha Dormant-historic landslides; slopes with evidence of previous landslide activity that have undergone most recent movement within the preceding 100 years (approximately historic time) hlb Dormant-young landslides; slopes with evidence of previous landslide activity that have undergone most recent movement during an estimated period of 100 to 5,000 years before present (Late Holocene) lic Dormant-mature landslides; slopes with evidence of previous landslide activity that have undergone most recent movement during an estimated period of 5,000 to 10,000 years before present (Early Holocene) lid Dormant-old landslides; slopes with evidence of previous landslide activity that have undergone most recent movement more than 10,000 years before present (Late Pleistocene) III POTENTIALLY UNSTABLE SLOPES Slopes that show no evidence of previous landslide activity but that are considered likely to develop landslides in the future; landslide potential is indicated by analysis or comparison with other slopes IV APPARENTLY STABLE SLOPES IVa Stabilized landslides; slopes with evidence of previous landslide activity but that have been modified by artificial means to a stable state I\lb Abandoned landslides; slopes with evidence of previous landslide activity but that are stable because external forces causing movement are no longer active IVc Relict landslides; slopes with evidence of previous landslide activity that clearly occurred under geomorphological or climatic conditions not currently present IVd Stable slopes; slopes that show no evidence of previous landslide activity and that by analysis or comparison with other slopes are considered stable NOTE: See Chapter 3 for definition of terms. connected to the initial section as it is driven into activity. As discussed in Chapters 7 and 8, initial the ground. The piezometer may be driven to a reconnaissance studies frequently utilize existing depth of 2.5 to 3.5 m with 0.5 m of pipe protruding maps or aerial photography to provide information above the ground surface. The piezometer is then concerning topography. In the case of larger land- pulled upward about 2 cm, creating a gap between slides, these existing sources can be supplemented the bolt and the bottom of the piezometer pipe to by larger-scale aerial photographs and topographic maps produced from them by photogrammetric allow free entry of groundwater. methods to provide an overall view of the site con- ditions. However, because considerable topo- graphic detail is required to locate many critical 4. SURVEYS OF LANDSLIDE SITES landslide elements, which in many environments The topography at a landslide site often provides may be masked by vegetation, detailed ground sur- the first indications of potential instability and the veys generally must be included as a major compo- degree to which the area has undergone landslide nent of landslide investigations. iw- Landslides: Investigation and Mitigation Coupling In this section, information provided in Chap- be affected by any movements. Ultimately, the Oversize pipe ters 4 and 5 of a previous landslide report (Sowers bench marks should be related to a geographic ref- for driving and Royster 1978; Wilson and Mikkelsen 1978) is erence, such as control monuments of federal and piezometer into ground expanded. Basic survey methods have been well state survey systems or latitude and longitude. The pe described in numerous textbooks (e.g., Moffitt and Global Positioning System can be useful for locat- Sectic Bouchard 1975); however, information on recent ing bench marks, particularly in remote areas. advances in survey techniques was obtained from However, for convenience, a subsystem of local appropriate experts (D. Little, personal communi- bench marks should be established close enough to Coup! cation, 1994, Virginia Department of Transporta- the zone of movement so that they can be used as tion, Richmond). ready references for continuing surveys. At least two monuments of position and elevation should 4.1 Purposes of Ground Surveys be established on each side of the zone of suspected pe movement. As indicated in Figure 9-13, these Sectic In an active landslide area, surface movements are monuments should be as close as possible to the normally monitored to determine the extent of movement zone but not influenced by future en- landslide activity and the rate of movement largement of the landslide. Experience suggests (Merriam 1960; Franklin and Denton 1973). Such that the distance from a bench mark to the closest Coup! monitoring requires the establishment of an accu- point of known movement should be at least 25 rate three-dimensional reference system, includ- percent of the width of the landslide zone. In areas ing identifiable reference stations, which can be with previous landslides, the minimum distance established only by ground-based surveying. may be greater. In mountainous areas, adequate pe Sectio Therefore, ground surveys are required to outcrops of bedrock sometimes can be found uphill or downhill from the landslide; in areas of thick Establish the ground control for photogram- soil, deep-seated bench marks may be necessary. metric mapping and instrumentation, The bench marks should be tied together by tri- Obtain topographic details where the ground angulation and precise leveling traverses. With suf- Bolt surface is obscured by vegetation (these details ficient bench marks, movement by any one are particularly important because of the accu- can be detected by changes in the control network. FIGURE 9-12 racy required in mapping landslides), and Intermediate or temporary bench marks are some- Reconnaissance- Establish a frame of reference against which level open-standpipe movements of the ground surface can be com- piezometer. Sections pared. of galvanized steel or black iron pipe Terzaghi stated: connected with couplings are driven If a landslide comes as a surprise to the eyewit- into ground with aid ness, it would be more accurate to say that the of oversized pipe observers failed to detect the phenomena section and cap. which preceded the slide. (Terzaghi 1950, 110) Open bottom of piezometer pipe is protected from The implication is that the smallest possible clogging with soil movements should be measured at the earliest pos- by insertion of sible time. In the following sections each of these loose-fitting bolt. three requirements is discussed in turn. After piezometer is driven to desired 4.1.1 Ground Control depth (usually about 3 m), it is pulled The first requirement of ground surveys is a system upward about 2 cm of local bench marks that will remain stable during to allow water to the course of the investigation and as far into the enter freely around future as movements will be observed. These bolt. FIGURE 9-13 bench marks must be located far enough outside Bench marks and triangulation leveling network the suspected zone of movement that they will not (Sowers and Royster 1978). Surface Observation and Geologic Mapping 197 times established closer to the zone of movement 4.1.2 Topographic Details for use in the more frequent surveys of the land- slide area. However, the locations of these bench Aerial photographs may not provide sufficiently marks should be determined relative to the perma- accurate or detailed topographic information for nent monument grid each time they are used. landslide studies because vegetation obscures the The direct surface measurements described ground surface or because the important landslide above supplement and serve as a basis for the more features cannot be identified. Therefore, detailed sophistiated instrument systems that are used to on-site mapping is necessary to define major fea- determine the at-depth movements (see Chapter tures, such as scarps, cracks, bulges, and areas of 11). Surveying must be used to accurately define disrupted topography (Figure 9-14). the surface location and elevation of the reference Because of the changing nature of landslides, points of these instrumentation systems each time surface surveys conducted after aerial photographs observations are made. have been taken may not correspond directly to PRINCIPAL SCARP UPSLOPE ECONDARY SCARPS ZONE OF DEPRESSION OR SUBSIDENCE DIAGONAL TENSION-SHE AR/ _ CR ACK I NG: EN ECHELON CRACKS 1' ZONE OF BULGING DOWNSLOPE TOE TENSION CRACK: POSSIBLY SHEAR DISPLACEMENT DOWNHILL MUD WAVE OR TOE RIPPLE TENSION CRACK ON TOE BULGE FIGURE 9-14 Cracks, bulges, scarps, and springs SPRINGS. SEEPS (Sowers and Royster 1978). 198 Landslides: Investigation and Mitigation the features shown on the photographs. It may not 4.1.3 Movement Grids and Traverses be possible to obtain a precise correlation between the surface topography determined on the ground The continuing movement of a landslide can be over a period of days or weeks and that obtained at measured by a system of grids or traverses across a single instant from aerial surveys. Differences be- the landslide area (Figures 9-15 and 9-16). tween ground surveys and aerial surveys should be Typically, a series of lines more or less perpendic- expected, and these differences are particularly ular to the axis of the landslide and spaced 15 to useful in understanding landslide deformation. 30 m apart with stakes at intervals of 15 to 30 m Topographic maps should include the accurate should be maintained and referenced to the con- representation of landslide features belonging to trol bench marks. Grids should be laid out so that two classes: (a) cracks and bulges and (b) springs the reference points are aligned with trajectories and seeps. of maximum slope or apparent continuing move- ment. In addition, where soil and rock weaknesses 4.1.2.1 Cracks and Bulges cause secondary movements that are skewed to Although many cracks and bulges, as well as other the major landslide, intermediate points should be minor topographic details, can be identified in established. For small landslides or widely spaced aerial photographs, their full extent seldom can areas of suspected movements, single traverse lines be determined unless the photographs are taken of reference often are used (Figure 9-16). with an unusually high degree of resolution in Line-of-sight monuments can be established vegetation-free areas (see Chapter 8). Therefore, with end monuments on stable ground and inter- independent surveys of cracks and bulges should mediate points on the moving mass in areas where be made by surface methods. Developing cracks, vegetation does not obscure visibility. The monu- particularly the ends, often are obscured by grass, ments are established in a line and smooth or pol- leaves, and root mats; these cracks should be care- ished metal plates are fixed on top of the fully uncovered so that their total extent can be intermediate points. A line perpendicular to the mapped. Hidden cracks can be identified by subtle line of sight is scribed onto each plate. A transit is changes in leaf mold patterns, torn shrubs, and located on or over one of the stable monuments distorted trees and tree-root systems. Boulder and the other stable monument is used to set the alignments or sliding trajectories should be noted. horizontal angle of the instrument. Each of the in- Cracks should be staked on both sides, and all termediate points is sighted, and a line is scribed. stakes should be referenced to the movement on its plate. Movement from one measurement to monitoring system because the entire crack system the next is simply measured at each plate with the shifts with continuing landslide movement. aid of a machinist's rule. Scribed lines are visible in Figure 9-17. 4.1.2.2 Springs and Seeps Appropriate location flags or markers should be Springs and seeps are the ultimate areas of dis- placed nearby so that the staked points can be charge for water-bearing strata and cracks and found despite severe movement. The elevation and thus are indicators of the water flow paths that in- coordinates of each point should be determined on fluence soil and rock stability. Because seeps often the traverse or reference grids by periodic surveys. follow cracks that have been opened by soil or In areas where highly irregular topography suggests rock movement, they can sometimes be traced to rapid differences in movement from one point to sources uphill. The points of disappearance of sur- another, reference points should be spaced more face runoff into cracks and fissures should be closely regardless of any predetermined grid pattern. mapped also. Seeps, springs, and points of water Such closely spaced stakes help to define the lateral loss change with rainfall, snowmelt, and ground limits of the landslide as well as the direction of movement. Thus, meaningful data on their loca- movement of localized tongues within the land- tion and shifts cannot be obtained by a single sur- slide. This is particularly important in the later vey or at regular intervals of observation. Instead, stages of movement because secondary movements they should be located during and shortly after pe- often develop as a result of weakening of the dis- riods of intense rainfall or snowmelt and after placing materials. Depending on the rate of move- episodes of significant movement. ment, these grid points should be checked at Surface Observation and Geologic Mapping 199 FIGURE 9-15 +80 E10+00 +20 +40 +60 E11+00+20 +40 +60 +80 E12+00 +20 E12+60 Observation grid (Sowers and Royster 1978). - 0 N 13+00 - - 0 +80 ::: A —4 Ni 2+00 A I +60 / II \ +20 • Nil::: 0 +60 +40 1 +20 A NlI0OA A BENCH MARK Grid: 20 to 40-rn spacing o POINT FOR MEASUREMENT Note: 1 m = 3.3 ft. intervals ranging from a few days to several months. conducting an accurate survey should not be al- In addition, they should be observed after periods lowed to obstruct general observations of topo- of unusual environmental conditions, such as snow- graphic features. As discussed in the following melt, high rainfall, or marked temperature changes. section, general observations often provide impor- In this way any relation between landslide move- tant information and should be incorporated into ment and climatic changes can be established. the surveyor's notes and reports. Conventional surveying methods commonly are adapted to the needs of landslide investigators. 4.2 Surveying Methods Table 9-7 is a summary of the characteristics, ad- Surveying is an integral part of the broader surface vantages, and limitations of several survey methods. observations and geologic mapping activities dur- Conventional survey methods, which are discussed ing landslide investigations. The mechanics of further in Section 4.2.2, are sometimes supple- 200 Landslides: Investigation and Mitigation FIGURE 9-16 Observation C) c3+55 UPSLOPE traverses for rough topography or less c3+40 important landslides 63+00 (Sowers and Royster c 3+25 1978). c3+10 B2+40 B 2+10 61+90 cz+ao 61+00 B1+30 81+60 c2+50 A3+00 A 2+ 7 0 c2+20 A 2+40 A2+ 10 cl+75 A 1+90 C 1+45 A 1+60 cl+30 Al+30 FIGURE 9-17 (below) A A1+OO Smooth metal plate ci+oo on line-of-sight monument with DOWNSLOPE scribed lines showing movement Q POINT FOR MEASUREMENT STATIONS Cm) between successive Note: 1 m = 3.3 ft. observations. mented by other, more specialized survey methods, which are discussed firrher in Section 4.2.3. 4.2.1 Genera! Observations When surface cracking is not apparent, detection of small surface movements requires a trained ob- server. Horizontal stretching results in localized in- stability of individual rocks that can be detected by I, small gaps between the soil and the rocks or cxpo- sure of different coloring or surface weathering pat- terns. Trees whose trunks are inclined at the base I1UL change to vertical orientation a meter or SC) above the ground (J-shaped trees) typically mdi- care snow creep, which affects only small-diameter Table 9-7 Comparison of Ground Survey Methods (modified from Wilson and Mikkelsen 1978 and Cording et al. 1975) RELIABILITY METHOD RANGE AccuicY ADVANTAGES LIMITATIONS Compass Variable Low; provides Rapid; requires no Reasonable initial Varies with experience and pace only approximate equipment; may be estimates on uniform of personnel and values useful in establishing terrain without roughness of terrain overall dimensions of obstacles landslide Varies with experience Compass, Variable - Moderate; provides Rapid; moderate Reasonable estimates hand level approximate accuracy; produces on rough terrain of personnel and and tape values a map or section roughness of terrain Moderate; errors may Plane table 5 to 500 m 1:100 Relatively rapid; Awkward in rough and alidade moderate accuracy; or steep terrain be due to instru- produces a map ment, drafting, and instability of plane table Transit and 5 to 500 m 1:200 to 1:500 Relatively rapid; Complex corrections needed Moderate to goo4; stadia moderate accuracy for measurements along may be lower in steeply inclined vegetated and directions rough areas - Transit and 50 to 500 in 1:800 at 500 m; Relatively rapid; most Careful orientation of Good subtense bar 1:8,000 at 50 In accurate of optical subtense bar required; may methods; can measure be difficult to use in inclined distances rough terrain Direct measurement by tape or chain Ordinary Variable 1:5,000 to Simple and Requires clear, relatively flat Excellent survey 1:10,000 inexpensive; provides surface between measured direct observation points and stable reference monuments Precise Variable 1:20,000 to Relatively simple Corrections for temperature Excellent survey 1:20,0000 and inexpensive; and slope must be applied provides direct and standard chain observation tension used Electronic 20 to 3000 m 1:50,000 to Precise, long-range, - Accuracy is influenced by Good distance 1:30,0000 and rapid; usable atmospheric conditions; measurement over rough terrain accuracy over shorter (EDM) distances (30 to 90 m) is less for most instruments Total station 1.5 to 3000 in 5 mm minimum Precise and rapid; Accuracy may be Good to excellent error; 1:300,000 reduces computa- influenced by over longer tional effort and atmospheric distances errors; provides conditions digital data; usable over rough terrain Global 1.5 in to 100 m with a single Precise absolute Accurate measurements Excellent Positioning 40 km receiver; 1:300,000 horizontal position; require 45 to 60 mm System (GPS) with two or more possible precise continuous operation; receivers recording vertical position elevations require receiver four or more dedicated to bench mark; satellites; 0.3-cm tree limbs over antenna minimum error interfere with reception; possible backscatter continued on next page 202 Landslides: Investigation and Mitigation Table 9-7 (continued) METHOD RANGE ACCURACY ADVANTAGES LIMITATIONS RELIABILITY Optical leveling Ordinary Variable 3 to 6 mm (vertical) Simple and fast,espe- Limited precision; requires Excellent (second or cially with modem good bench marks third order) self-leveling instruments Precise Variable 0.6 to 1.2 mm More precise than' Requires good bench marks Excellent (first order) (vertical) , ordinary leveling and reference points; careful adherence to standard procedures Offsets from baseline Theodolite 0 to 1.5 m 0.6 to 1.5 mm Simple; provides direct Requires baseline Excellent and scale observation unaffected by ground movements and good monuments; accuracy can be improved by using a target with a vernier and repeating sighting from opposite end of baseline Laser and Otol.Smm 1.5 mm More rapid than Serioissly affected by Good photocell theodolite-and-scale atmospheric conditions detector method Triangulation Variable ' 0.6 to 12 mm Usable when direct Requires precise Good measurements are measurement of base impossible; useful in distance and angles; tying into points requires good reference outside immediate monuments area Terrestrial Variable 1:5,000 to 1:50,000 Records hundreds of Limited by Good photo- potential movements weather conditions grarnmetry at one time for determining overall displacement pattern trees, rather than old landslide movement. Trees ment. Overly taut or excessively sagging utility with straight trunks that are inclined in different lines, misalignment of fence posts or utility poles, directions within an area ("jackstrawed" trees or or distress to pavement are excellent indicators of "drunken" forests) indicate recent movement. ground movements. Such movements, when accu- Trees with completely curved trunks (C-shaped rately monitored, serve as important tools in as- trees) are found on slopes with long histories of sessing the potential hazard to transportation continuing minor movement. Observations of facilities, nearby structures, and the public. trees should not be used as the sole basis for iden- tifying landslide movement because other phe- 4.2.2 Conventional Surveying nomena may produce similar effects (DeGraff and Agard 1984). The principles of surveying have been described Cracks covered with leaves, surface litter, or extensively in numerous textbooks (e.g., Moffitt duff can be detected by an experienced observer and Bouchard 1975) and thus will not be discussed walking over the area and noting firmness of foot here. One of the basic operations of surveying is support. Livestock and other animals may avoid the determination of the distance between two grazing or browsing in an active landslide area be- points on the surface of the earth. In surveys of cause of uncertain support or hidden fissures. Small areas of limited extent, which is the case for most openings on the downhill sides of structures or landslides, the distance between two points at dif- next to tree trunks may indicate incipient move- ferent elevations is reduced to an equivalent hori- Surface Observation and Geologic Mapping 203 zontal distance either by the procedure used to surveying methods. These approaches are dis- make the measurement or by computation of the cussed further in Sections 4.2.2.3 and 4.2.2.4. horizontal distance from the measured slope dis- tance and inclination. Vertical distances are inde- 4.2.2.2 Transit and Tape Methods pendently computed and recorded. In the case of Optical instrument surveys and tape measurements landslides, both vertical and horizontal distances, are commonly used to determine lateral and verti- and rates of change in these distances, provide im- cal positions of points accurately. Bench marks and portant information needed to evaluate landslide transit stations located on stable ground provide mechanisms. the basis from which subsequent movements of As noted in Table 9-7, many surveying tech- monuments can be determined optically and by niques may be used to determine distances and in- tape measurement. As shown in Figure 9-18, tran- clinations for computing horizontal and vertical sit lines can be established so that the vertical and distances. Some simple methods, such as pacing to horizontal displacements at the center and toe of determine approximate distances, are of limited the landslide can be observed. Lateral motions can use in most landslide investigations. be detected by transit and tape measurements from each monument. When a tension crack has opened 4.2.2.1 Tacheometry above the top of a landslide, simple daily measure- A series of methods has been developed for indi- ments across the crack can be made between two rectly measuring distances using optical surveying markers, such as stakes or pieces of concrete- instruments in conj unction with measuring bars reinforcing steel driven into the ground. In many or rods. These methods may be referred to collec- cases the outer limit of the ground movements is tively as tacheometry (Moffltt and Bouchard 1975, not known, and establishing instrument setups on 14). They include plane table and alidade surveys, stable ground may be a problem. transit and stadia surveys, and transit and subtense Various techniques and accuracies achieved in bar surveys (see Table 9-7). optical leveling, offset measurements from transit These measurements are performed rapidly and lines, chaining distances, and triangulation have comparatively well over rough, uneven ground. been discussed extensively in the literature (Gould They may be sufficiently accurate for many land- and Dunnicliff 1971; British Geotechnical Society slide investigations, but their accuracy is consider- 1974), particularly for dams, embankments, and ably less than that of other techniques involving buildings. Although conventional surveys, partic- direct distance measurement by tapes or chains or ularly higher-order surveys, can define the area of the determination of distances by electronic movement, more accurate measurements may be means. Thus, tacheometry is often used to provide required in many cases. rapid supplementary data concerning the locations of intermediate points of landslide features, partic- 4.2.2.3 Electronic Distance Measurement ularly when field-developed maps are needed for Equipment geologic descriptions. A somewhat more accurate Electronic distance measurement (EDM) devices tacheometry measurement involves the use of a have proved particularly suitable for rugged ter- transit and a short horizontal baseline, referred to rain; they are more accurate and much faster than as a subtense bar. Distances are determined by ordinary surveying techniques and require fewer using a precise transit to measure the small hori- personnel (Dallaire 1974). Lightweight EDM zontal angle subtended by the bar. instruments can be used efficiently under ideal Tacheometric techniques remain a viable and conditions for distances as short as 20 m and as economical surveying solution for landslide inves- long as 3 km; errors are as small as 3 mm (St. John tigations in many locations, and the required and Thomas 1970; Kern and Company Ltd. surveying equipment is commonly available. 1974). Larger instruments that employ light However, whenever possible, tacheometric meth- waves or microwaves can be used at much longer ods should be replaced with the more accurate distances. The accuracy of EDMs is influenced by and rapid distance measurements provided by weather and atmospheric conditions; comparative electronic distance measurement (EDM) equip- readings with three different instruments were de- ment or by the even more modern total station scribed by Penman and Charles (1974). 204 Landslides: Investigation and Mitigation MARKER BOARD - CRACK MONITORING POINTS TRANSIT STATION -.,." ------__- 77- pgj.ER - -. . ...... -,. ,. (SJEUKND). , OFfSET of J - TO BENCH MARK ,1 / i•• / - 7LH_5 0 7 Ott / ' fi t IV ON & PHOTOGRANMETRY -._ TILTNETER TABtE GROUND) POINTS FIGURE 9-18 EDM equipment can be used to monitor large tal movements. Figure 9-19(b) shows recorded Movement landslides with large movements and provide a changes for two points believed to be on stable measurements rapid way to survey many points on the mass from ground. The variation of monthly readings is seen in typical a single, readily accessible location. An example of to be 60 mm and the variation is no greater for a landslide area (Wilson and such an installation involves a reactivated ancient 4765-rn distance than for a 1844-rn distance. Mikkelsen 1978). landslide in the state of Washington along the Columbia River where the active landslide is more 4.2.2.4 Total Station Equipment than 0.6 km wide and 5 km long. Movements vary Total station survey systems have improved sur- from 1 to 9 m per year, and the rates of movement veying practices to a great extent. These devices depend on the time of year and rainfall intensity can measure vertical and horizontal positions and duration. A permanent station, readily acces- within a three-dimensional coordinate framework sible all year, has been set up on the side of the having x-, y-, and z-axes (east, north, and eleva- river opposite the landslide and distance readings tion). They have proved particularly suitable for are taken monthly to 14 points located on the rugged terrain, and building on the experiences landslide and 2 points located on stable ground gained with the earlier EDM systems, they per- outside the landslide boundaries. The distances in- form even more accurately and faster than these volved vary from about 1.5 to 6km. Figure 9-19(a) earlier systems and require fewer personnel. shows the movements (changes in distance) dur- Current total station survey systems allow the ing a one-year period (1972-1973) for two se- surveyor the flexibility not only of measuring the lected points at this Columbia River landslide horizontal and vertical positiotoany point with based on monthly readings recorded by an EDM a high degree of accuracy, but also of recording all Instrument; At the end of the year, the points were readings into hand-held or instrument-located resurveyed by triangulation. The discrepancy is data collection devices. Field data can be down- about 10 cm, which, although larger than antici- loaded from these devices to computers, where pated, is quite satisfactory considering the to- the data can be processed, printed in formal Surface Observation and Geologic Mapping 205 FIGURE 9-19 POINTS LOCATED ON SLIDE Microwave 1912- 1913 -I measurements of I- landslide J A S 0 N D J F H A H M J 0 0 movements along Columbia River, 104,1 Washington State * (Wilson and \ — — - Mikkelsen 1978). -0.6 -2 RIANGU LATION 1 ---b -1.2; (D z z I I 0 0 w 0 0 z I— -6 —'oF-.Cn Cn 0 -MIcR AVE -2.4 TRIAN GULN -3.0 -10 POINTS LOCATED ON STABLE GROUND 0.3 LU do LU records, and plotted as maps. In some cases data determines the position of the points, and another reduction and èhecking can be performed in the person plots the points on the plane table. field while the survey continues, allowing maccu- Portable radios facilitate communication of geo- racies to be identified and corrected immediately logic information to be recorded on the map. and saving remobilization costs. Extreme care must be taken concerning the Field-developed maps can be produced with calibration and adjustment of all equipment in- total station equipment, a plane table, and a three- volved in total station surveys. The following person crew. A geologist positions the retroprism equipment must be checked carefully to confirm pole on features of significance. The surveyor calibration or proper adjustment: 206 Landslides: Investigation and Mitigation Total station, Point A shows consistent direction of movement Tripods, and uniform, incremental increase in magnitude Retroprisms, of the displacement vector. Point B shows random Prism poles, and direction and small incremental magnitude of cu- Optical plummet tribrachs. mulative movement; it is a stable point and docu- ments the survey's accuracy. As with EDM equipment, the accuracy of total FIGURE 9-20 station surveys is influenced by weather and atmo- 4.2.3 Other Surveying Techniques Plots of direction spheric conditions. Lightweight total stations can and cumulative be used efficiently for distances as short as 1.5 m magnitude of In some landslide investigations the conventional monitor point and as long as 3 km. Errors may be as small as 30 surveying approaches discussed in the previous movement. Point mm over a 3-m distance; however, these instru- sections are supplemented by other, more special- A: moving point ments generally have a minimum resolvable dis- ized surveying techniques. These techniques in- with consistent tance accuracy on the order of 5 mm. Thus for clude the use of lasers, methods involving aerial direction and short distances, the possible error in the distance and terrestrial photogrammetry, techniques for uniform trend of measurement will always be at least 5 mm. For monitoring internal deformation and the growth magnitude. many landslide investigations, this accuracy is and movement of surface cracks, and the use of Point B: stable much better than the many potential sources of point with random specialized equipment such as tiltmeters. In addi- error in locating and reestablishing reference direction and tion, videography and digital image analysis and small incremental points. the current Global Positioning System (GPS) are magnitude that is Results of repeated surveys can be plotted ef- discussed in the following subsections. The use of caused by error of fectively as direction and cumulative magnitude of specialized slope movement monitoring devices is surveying technique movement, as shown in Figure 9-20. The plot for discussed in Chapter 11. These supplementary surveying methods con- Monitor Point A tinue to evolve rapidly. Some already are in fairly 360 .1.1,1. .1.1.1.1. 2.00 extensive use (e.g., EDM equipment and lasers), 315 - -1.75 whereas others are in limited use or are in the de- 270 - -1.50 velopmental or experimental stage (e.g., terrestrial 1225 ------1.25 0____0 0 photogrammetry and videography). They will un- 180 --o°0-0--o-°'------1.00 doubtedly find increasing use in field measure- 135 --1------0.75 ments in the future. 1 90 0.501 45 ------0.25 4.2.3.1 Lasers 0 0.00 0 4/1 4/15 4/29 5/13 5/27 6/10 6/24 7/8 7122 8/5 8/19 Laser instruments already are widely used for set- Date ting alignments, and they are well suited for set- 0. MjiitJ •• Displacement ting a reference line for offset measurements to surface monuments. Laser beams are also used with some EDM instruments. It should be possible Monitor Point B to measure offsets with errors no greater than 3 to 360 .1,1,1,1. .1, .1.1. 2.00 6 mm (Gould and Dunnicliff 1970. 315 ------1.75 Laser total station instruments are available in '-270 ------1.50 hand-held, monopod, and tripod formats (Laser 225 ------1.25 ZS 180 _O%..Oç 0O - - - - - Technology, Inc. 1992). These instruments are par- _/ ------0.75 ticularly well suited for rapid topographic profiling 135 / 90 - 0.50 in locations too steep or dangerous for other meth- 45 - 0.25 ods. The laser total station is equipped with a flux- 0 _I I 10.00 gate compass and tilt-angle sensor, in addition to a 4/1 4/15 4/29 5/13 5127 6/10 6/24 7/8 7/22 8/5 8/19 serial port for communicating data to other com- Date puter applications. The range of the laser is up to .0. Azimuth Displacement 1 about 460 m to a target that is 20 percent reflective; to a retroprism the range is 12 200 m. Accuracies of Surface Observation and Geologic Mapping 207 0.3 degree azimuth, 0.2 degree vertical angle, and FIGURE 9-21 Simple reference 9.4 cm range can be obtained from units that weigh =1m targets for -i less than 3 kg and operate up to 8 hr on a Nicad -1 h monitoring landslide battery. displacements. Targets can be 4.2.3.2 Aerial and Terrestrial Photography made of plywood or and Photogrammetry sheet metal and Important features of active landslides cannot be painted with bright documented on aerial photographs taken before colors such as red and white. Targets the slope movement; therefore, photographs taken can be deployed at the beginning of an investigation of an active rapidly for landslide permit documentation of current condi- site. The feature should be identified in the initial photographic tions for navigation as well as geologic interpreta- photograph for targeting in later photographs to documentation. It is tion. Photographs taken with a small-format ensure comparable views. Showing some object or wise to place two or (35-mm) hand-held camera from a fixed-wing air- measure, such as a folding ruler, for scale in the three targets outside craft during aerial reconnaissance can provide an photograph is recommended. landslide boundary excellent base for use on the ground (Wracker For greater precision the location of the photo- to document graphic bench mark can be established for later use absolute landslide 1973). Stereoscopic photographs can be taken in displacements. this way in vertical or near-vertical and oblique by driving a metal or plastic stake into the ground. orientations. Commercial rapid processing of color The height of the camera above the stake should be film is available in many cities. Thus, an aerial re- noted along with the azimuth and inclination of connaissance can be made and prints received for the view. A tripod can be used to simplify estab- use in the field within a single day. An alternative lishing these measurements and setting up for later to hand-held photography from a fixed-wing air- photographs. More precision is obtained by using craft would be to contract with an aerial photogra- the same camera and lens combinations. Terrestrial phy company to produce stereoscopic photographs photographs are more usable when later views are taken at a similar illumination (time of day) as the for subsequent use on the project. Prominent targets visible from the air or from a initial ones. This ensures similar contrast and visi- bility of the features among different photographs. distance on the ground can be used as references Malde (1973) offered additional ideas for using ter- for aerial or terrestrial photographs taken at some restrial photography for geologic bench marks. interval of days, weeks, or months. Suitable targets Photogrammetric measurements of ground ge- are circles or squares 30 cm to 1 m across with op- ometry can be made from oblique photographs ob- posite quadrants painted red and white, as shown tained at the ground surface. For example, two or in Figure 9-21. If such targets are to be deployed more permanent photography sites that overlook a from aircraft onto landslide surfaces, both sides of landslide area can be used to document landslide the targets should be painted so that it will not movement through successive sets of stereoscopic matter which side faces up. If a single photograph photographs. Phototheodolites are used to take suc- includes a stable bench mark and one or more cessive stereophotographs from these. fixed stations; landslide bench marks, the direction and magni- movements are identified in a stereocomparator, tude of deformation can be estimated with the ac- and accuracies of 6 to 9 mm have been reported curacy of the photographs. Oblique photographs, (Moore 1973). Although the data reduction may of course, show more complicated distortion than be more complex than that for conventional aerial do vertical aerial photographs. photogrammetric mapping, the technique is useful Terrestrial photographs can serve as bench for determining movement of any selected points marks for qualitative changes in landslides. provided that they can be seen in the photograph. Several examples of photographs documenting Terrestrial photogrammetry has been used in some crack propagation in a large landslide may be cases to measure changing dam deflections (Moore found in a paper by Fleming and Johnson (1989). 1973) and to monitor rock slopes in open-pit mines Simple photographic bench marks involve taking (Ross-Brown and Atkinson 1972), but no landslide a photograph next to some prominent feature that measurements using this mehod have been found can be found readily on subsequent visits to the in the literature. Landslides: Investigation and Mitigation 4.2.3.3 QuadriateraIs Quadrilaterals provide a convenient method for measuring slope deformation (Baum et al. 1988). A quadrilateral consists of an array of four stakes FIGURE 9-22 initially in a nearly square configuration. Quadri- 4. (below) laterals that span the lateral flank of a landslide or .. Quadrilateral are located within the body of a slide deform with , configuration for landslide movement. Subsequent measurements of determining quadrilaterals are compared with initial and previ- All displacement, ous measurements; they are used to compute dis- strain,and tilt placements across landslide boundaries and strains across landslide and tilts within the body of the landslide. The ge- flank. Initially deployed at ometry of a quadrilateral straddling a landslide positions A, B, C, boundary is illustrated in Figure 9-22. and D, quadrilateral The measurements required for quadrilateral deforms to positions deformation analyses consist of the relative eleva- f. A, B', C', and D at tions of the four stakes, the six distances between second observation. pairs of stakes, and the azimuth of one leg of the -. - -. 4•'•4 Quadrilateral quadrilateral. Computer programs for this purpose defines four triangles; ABC, were written in BASIC by Johnson and Baum ACD, ABD, and (1987). The measurements of the quadrilaterals BCD, which deform are used to define four triangles. The shape and tilt toAB'C', AC'D, of each triangle are computed, and changes are AB'D, and B'C'D. used to define landslide deformation. FIGURE 9-23 Each triangle is The accuracy of displacement, strain, and tilt es- analyzed as Machined pin on steel rod used in quadrilateral. timates is limited by measurement error. Quad- conventional three-point problem rilaterals measured by tape and hand level require for orientation. large movements before the differences are signifi- Changes in position cant. Laser geodimeters or tape extensometers pro- of B, C, and D with vide the most precise measurements but may respect to A are exceed the intentions of reconnaissance instru- used to calculate mentation. Tops of steel rods can be machined to displacements. allow repeated positioning of tape measures (Figure Changes in leg 9-23), and a cap can be machined to fit over the length and triangle shape and area are steel rod with a groove so that the tape can be posi- used to calculate tioned for repeated measurements (Figure 9-24). A strains and tilts. special sleeve-type anvil is required to allow the FIGURE 9-24 Machined cap to fit over quadrilateral rods for repeated-measurement accuracy. Surface Observation and Geologic Mapping 209 steel rods to be driven into the ground without damage to the machined pins. A tensioned steel Graduated Water-filled tape corrected for temperature (Moffitt and stick and container Bouchard 1975) provides a suitable level of preci- transparent ,on tripod sion in distance measurements. Elevation differ- manometer ences can be determined precisely with a water tube level or manometer (Figure 9-25). Care must be ex- I ercised in selecting quadrilateral locations so that LI the relief within the quadrilateral does not prevent subsequent measurements. \ Quadrilateral 4.2.3.4 Crack Monitoring [Ti-' -IT rods Monitoring the system of cracks found on the sur- face of a moving mass often is of critical impor- tance for landslide investigation programs. Crack monitoring includes crack mapping and crack FIGURE 9-25 to provide accurate and continuing indications of measurement or gauging. Details of crack movement. Figure 9-26 shows one such de- manometer for vice, which consists of two vertical reinforcing steel use in determining Crack Mapping: Most earth movements are rods and a heavy-duty elastic rubber strap or band elevation of accompanied by cracking and bulging of the stretched between the two rods (Nasser 1986). quadrilateral rods. ground (Figure 9-14). Survey points, in addition to Initial measurements of length, bearing, and incli- the predefined grid or traverse points (Figures 9-15 nation of the band provide a basis for comparison and 9-16), should be set on the more prominent of with subsequent measurements. Reduction of the these features and in areas beyond them. Repeated, field measurements provides values of lateral and FIGURE 9-26 detailed mapping of areas of cracking serves to doc- vertical movements along the crack. Vertical offsets Crack measurement ument the evolution of the cracks, which provides on cracks and scarps also may be obtained from di- with rubber-band the basis for interpretation of the stresses responsi- rect measurement. If total station surveying equip- extensometer ble for them. An example of repeated, detailed ment is available, similar measurements can be (modified from mapping of an area of landslide cracks by plane made readily and referenced to stable bench marks. Nasser 1986). table and alidade is shown in Figure 9-6. Crack Measurement: Instrumentation of landslides is discussed in detail in Chapter 11. Rubber Band However, certain reconnaissance and supplemen- (initiafly set tary measurements should be a part of the survey approx. horizonta program. When geologic mapping is considered adequate to describe the area affected by a land- slide, simple qualitative measurements can pro- vide knowledge of the activity. Movements on Rebar Stake cracks, particularly those uphill and downhill from well-defined zones of movement, indicate possible Rebar Stake )j increasing size associated with many landslides. Therefore, it is desirable to monitor the change in width as well as the change in elevation across the D cracks. This can be done easily by direct measure- II ment from quadrilaterals straddling the cracks or pairs of markers set on opposite sides of the cracks. Quadrilateral measurement techniques were de- scribed in the previous section. Crack width changes also may be measured directly by taping be- Scarp tween stakes set on opposite sides of the crack. Crude, simple gauges can be constructed in the field 210 Landslides: Investigation and Mitigation However, such sophisticated surveying methods are Monitoring of cracks usually provides informa- not always justified. tion on movement within the upper half to two- In areas of cracked pavement or jointed rock, thirds of the landslide mass. The lower half to the change in crack or joint width can be deter- one-third of the landslide may exhibit little crack- mined by scribing marks or bonding washers onto ing because of compression and overriding of ma- the pavement or rock surface on opposite sides of terial within this portion. Movement in this area the crack or joint and simply measuring between must be detected in reference to fixed objects ad- the pairs of marks. Small-scale quadrilaterals 5 to 10 jacent to or within the path of this overriding. cm across may be installed to monitor displace- Photographic points that show the toe of the land- ments, strains, and tilts, as described in the previ- slide in relation to fixed features such as trees, rock ous section. On pavement or rock surfaces the outcrops, buildings, utility poles, or roads are one quadrilaterals can consist of stainless steel pins with means for this monitoring. The photographic washers fixed by epoxy into small-diameter drilled point must be established in a location that per- holes. Another simple device is a small hardwood mits a clear view of the lower part of the landslide wedge lightly forced into an open crack and marked and the reference features. Establishing the pho- at the level of the pavement or rock surface. If the tographic point should follow the procedures crack opens, the wedge will fall deeper into the described for terrestrial photography in Section crack. Such a wedge indicator may not be helpful if 4.2.3.2. the crack closes or is subject to shear movement. Another way to monitor movement at the toe of the landslide is to establish one or more rows of Cracks often are visible in the early stages of targets or stakes parallel to and within the expected landslide deformation. In places where the cracks path of the landslide. The first target or stake are not likely to be destroyed rapidly, such as on should be placed as near as possible to the toe. rock faces or paved surfaces in urban areas, a sim- Succeeding targets should be placed a set distance ple technique for monitoring crack propagation apart. For example, a slow movement feature might rate is to mark the end of the crack and observe be monitored by placing five targets or stakes at the crack over a period of time, such as days or 1 -m intervals. It is important to mark the individ- weeks. The end may be marked by placing a ual stakes with paint or some other indelible sub- FIGURE 9-27 straight-edged piece of cardboard or similar mate- stance. As the landslide movement overrides or Paint at end of rial across it so that it is just covered and spraying covers the first stakes, determination of the amount crack to monitor paint so that a straight-edged mark is left on the of movement depends on knowing the number of propagation. rock face or paved surface (Figure 9-27). The stakes buried and where the end stake is located in Straight-edged monitoring process can be repeated using different the line. The number of parallel lines of targets or material such as cardboard is colors of paint and the distance between succes- stakes depends on how broad the toe of the land- positioned at end of sive marks can be measured. In active traffic areas slide is and the configuration of the slope below it. crack on landslide where repaving may destroy monitoring locations, mass, small paint attempts to monitor crack propagation may not 4.2.3.5 Tiltmeters mark is made, and provide acceptable results. Tiltmeters can be used to detect tilt (rotation) of a date is recorded. surface point, but such devices have had fairly lim- Marks made in subsequent ited use in landslide investigations. They have been observations allow used mostly to monitor slope movements in open- crack propagation pit mines and highway and railway cuts, but they to be documented. may be used in any area where the failure mode of a Paint marks have Propagation of crack mass of soil or rock can be expected to contain a ro- greatest utility if since February 17 Cardboard tational component. One type of tiltmeter is shown made on paved in Figure 9-28, and sample tiltmeter data from a surfaces or bedrock Paint at end of crack exposures. mine slope are shown in Figure 9-29. Tiltmeters use Broad-tipped pens the same types of servo-accelerometers as those or markers could be used with the more sensitive inclinometers de- used to mark cracks scribed in Chapter 11. The prime advantages of tilt- in lieu of paint. meters are their light weight, simple operation, Surface Observation and Geologic Mapping 211 capable of measuring orientation along the line FIGURE 9-28 provides the initial measurement of its inclination. Portable tiltmeter By comparing the initial measurement with later (Wilson and Mikkelsen 1978). oneS along the same line, any tilting related to dis- placement on the landslide can be detected. Trees and poles should be measured parallel to their lengths at points 0, 90, 180, and 270 degrees around their circumferences. This measurement provides better control of the direction of tilting when the tree or pole could tip in any direction. The uneven surface of the tree can be made smoother for more accurate measurement by nailing wood strips paral- lel to its length (Figure 9-30). These strips also serve to mark the alignment for later remeasure- ment. The orientation of the wood strips can also compactness, and relatively low cost. Disadvantages of portable tiltmeters in reconnaissance instrumen- be measured with a Clar compass. tation are their power requirements and their digi- tal data output. 4.2.3.6 Videography and Digital Image Alternative devices to detect tilt on landslides Analysis are quadrilaterals and existing linear features on the Videography and digital image analysis are emerg- slide, such as utility poles and trees. Quadrilaterals ing technologies with promise for application in were described in previous subsections of this chap- surface observation and geologic mapping of land- ter and provide excellent information that can be slides. Videography is a computer-aided procedure used to compute tilt. Trees or structures can consti- in which multiple videotape recordings of the same tute crude tiltmeter bench marks within landslides area taken with different spectral bands or lens fil- or on slopes expected to move. A line is marked ters are registered to each other or to a geographic along which the inclination of the structure is mea- reference system and processed to create a single in- sured. Placing a Brunton compass or similar device terpreted image. Bartz et al. (1994) successfully de- +2.0 + 30 +1.5 + 2 0 E E LONGITUDINAL - AXIS TRANSVERSE AXIS FIGURE 9-29 Ar- 11 Tilt at ground surface due to advance of longwall mining face —0,.5 — 100 —50 0 +50 +100 +150 +200 +.Z U (Wilson and FACE ADVANCE (m) Mikkelsen 1978). Note: 1 m = 3.3 It; 1 mm = 0.04 in. 212 Landslides: Investigation and Mitigation FIGURE 9-30 potential value is great for rapid classification of a Tree trunk with landslide area once features such as ground cracks wood strips tacked and seeps have been identified. at 90-degree intervals to provide Digital image analysis is similar to videography, reference lines except that the analysis is based on a single image for successive rather than on multiple superimposed images. measurement of tilt. Terrestrial digital image analysis is an emerging Trend and plunge of technology that is related in some respects to satel- strips are measured lite image analysis. Multispectral remote sensing of on successive field landslide -susceptibleareas was addressed by observations to document direction, Jadkowski (1987). Vandre (1993) applied digital amount, and rate of image analysis to determine grain size distribution tilting. of cobble- and boulder-size particles; he also has used this technology on landslide evaluations in Utah (B. Vandre, personal communication, 1994, AGRA Earth & Environmental, Inc., Salt Lake City, Utah). I he basis for digital image analysis is rapid estimates of size based on pixel dimensions and a suitable scale visible in the image. A conven- tional video camera is used to record images in the field. The videotape is replayed through a computer monitor using a card with a special video jack (RCA format). Selected frames of the video are grabbed for display and analysis. Quantitative esti- mates of landslide movement can be obtained from digital image analysis of repeated terrestrial photog- raphy if the photographs are digitized and the fea- lineated riparian vegetation types using multispec- tures displaced or deformed by landslide movement tral airborne videography. They collected three are visible in the repeated photographs. bands of data—green (centered at 0.55 pm), red (centered at 0.65 pm), and near-infrared (centered 4.2.3.7 Global Positioning System at 0.85 pm)—and digitized selected frames of the Surveying imagery using special electronic computer hardware Global Positioning System (GPS) surveying has known as a frame-grabbing and display board. developed rapidly in the past few years. This tech- Multispectral images were created by superimpos- nique appears to provide precise surveying techno- ing the three individual hands, and continuous sec- logy anywhere in the world, but it has some severe tions of their study area were prepared as mosaics of limitations. The basis for GPS surveying is a net- the individual three-band images. The computer is work of satellites (26 in 1994) deployed by the "trained" to interpret specific combinations of U.S. Department of Defense. These satellites make image characteristics as specific geographic features up a positioning system called the Navigation by having the computer operator classify areas of Satellite Timing and Ranging (NAVSTAR) GPS. the feature. Areas of ponded water, for example, The positioning system in each satellite transmits would have a distinctive set of characteristics on at two frequencies with three modulations (Reilly the different spectral bands. 1992). The atomic clocks in the satellites generate Bartz et al. (1994) concluded that multispectral a fundamental frequency that is multiplied by two airborne videography can be used to collect inex- different factors to generate two carrier frequen- pensive and timely baseline information. A similar cies. Modulated onto these carrier frequencies are procedure based on Multispectral aerial videogra- pseudo-random-noise codes to protect military use phy was used by Shoemaker et al. (1994) for juris- for precise navigation. Civilian users do not have dictional delineation of wetlands. Videography has access to the precise military navigation code. The not yet been applied to landslide mapping, but the satellites also transmit messages containing clock Surface Observation and Geologic Mapping 213 correction coefficients, satellite ephenieris param- landslide area. The first map encompases the eters, and other data, including almanac data, landslide (or suspected landslide) plus the sur- which give orbit information for all of the satellites rounding area, including the topography extend- in the NAVSTAR GPS "constellation." ing uphill and downhill beyond major changes in A single GPS receiver can be used to determine slope or lithology. Topography should be devél- the position of an unknown point with an accu- oped on each side for a distance of approximately racy of about 100 m. This position is in an Earth- twice the width of the moving area (or more when centered, Cartesian-coordinate system that can be the zone of potential movement is not well de- expressed in terms of latitude, longitude, and fined). Typical scales for such mapping of large height on Earth's reference ellipsoid. Much better landslides may be 1:2,500 to 1:10,000. A portion accuracy in horizontal position can be achieved of such a map developed for a landslide investiga- with at least two GPS receivers simultaneously tion near Vail, Colorado, is reproduced in Figure recording at least four satellites. Horizontal accu- 9-31 (Casals 1986). racies of 1:300,000 are routine (Reilly 1992). The second topographic map is more detailed GPS surveying is not well suited for determin- and encompasses the observed landslide area plus ing ground-surface elevation. The best way .to de- all of the uphill and downhill cracks and seeps as- termine elevation from a GPS survey is to have sociated with the landslides Typically,, the de- at least one GPS receiver dedicated to a bench tailed map exten& beyond the boundaries of he mark with accurate known elevation. The bench landslide uphill and downhill for a distance of half mark receivers are used to calibrate the satellite the length of the landslide or to significantly flat- signals for determining the elevation of the rov- ter slopes. The detailed topography should extend ing receivers. Most of the error in elevation is re- beyond the limits of the landslide laterally at least lated to setting up the receiver antenna, and an half the width of the landslide area. Contour in- accuracy,of 0.3 cm is thought to be reasonable for tervals in such detailed topography should be as careful GPS measurements (Reilly 1992). close as 0.5 m for landslides that do not have too great a degree of vertical relief. The horizontal Accurate measurements with GPS receivers re- scale is typically 1:2,500 or larger. A portion of quire positions to be operated continuously for 45 such a landslide map prepared as part of field in- to 60 mm; Tree limbs and other material blocking vestigations of a landslide in western Colorado the antenna can cause problems with reception of (Umstot 1988) is shown in Figure 9-32.. satellite transmissions. Troublesome backscatter of Topographic data commonly are collected in transmissions can be caused by reflective surfaces the field electronically and compiled in the Qffice such as buildings, pavement, and buried pipelines. digitally. Many topographic maps for landslide in- GPS surveying is a powerful tool, but for most vestigations are plotted by computer. Data frdm , landslide investigations, the exact geographic po successive surveys can be compared with the aid of sition of the landslide is not needed, only accurate the computer, and plots of changes in topographic relative positions of points on and adjacent to it. contour-line position can be produced. Such com- parative contour-line plots provide useful informa- 4.3 Representation of Topographic Data tion on changes in topography of landslide masses but require complete resurveying of landslide sites. Survey data must be displayed and analyzed in An example of a map showing changes in eleva- order to be useful to landslide investigators. The tion of the surface of the Thistle landslide four most common representation methods are to- (Duncan et al. 1985) is shown in Figure 9-33. pographic maps, profiles, displacement vectors and trajectories, and strain ellipses. 4.3.2 Profiles In addition to a topographic map, profiles of the 4.3.1 Topographic Maps landslide area are prepared (Figures 9-34 and 9-35). Generally, topographic information obtained by The most useful profiles are perpendicular to the photogrammetric methods is correlated with steepest slope of the landslide area. 'Where the ground survey controls and detailed topographic movement definitely is not perpendicular to the information to establish two or more maps of the steepest slope, two sets of profiles are necessary: one 214 Landslides: Investigation and Mitigation FIGURE 9-31 Portion of topographic map prepared by photogrammetric methods showing region around head of landslide near Vail, Colorado (Casals 1986). Original map scale was 1 :2,400. Elevations are in feet; major contours are at intervals of 25 ft (8 m), with supplementary contours at 5-ft (1.5-rn) intervals. set should be parallel to the direction of movement parison where possible. However, it is difficult to and the other parallel to the steepest ground-sur- reference old maps precisely to the more detailed face slope. For small landslides, three profiles may topography obtained during landslide investiga- be sufficient; these should be at the center and tions. Differences between existing topography quarter points of the landslide width (or somewhat and prelandslide topography may represent survey closer to the edge of the slide than the quarter mismatches as well as deformation of the site points). For very large landslides, the longitudinal ground surface. Adjustments of the prelandslide profiles should be obtained at spacings of 30 to profile from old maps to the profile from new sur- 60 m. It is particularly important that the profiles veys may in some cases be made by comparing old be selected so as to depict the worst and less critical and new topography beyond the limits of observed combinations of slope and movement within the landslide movements. landslide area. It is usually desirable to have at least one additional profile in the stable ground 15 to 4.3.3 Displacement Vectors and 30 m beyond the limits of the landslide area on Trajectories each side so that the effect on the movement of ground-surface slope alone can be determined. Survey grid points and other critical points are Each longitudinal profile of the landslide is usu- plotted on the more detailed topographic map of ally plotted separately. Successive profiles can be the landslide area. Both the topography and the de- shown on the same drawing to illustrate changing picted grid points should be referenced to the same site topography. The original topography should geographic reference system, whether it is an arbi- be estimated from old maps and shown for corn- trary reference convenient for the landslide site or Surface Observation and Geologic Mapping 215 FIGURE 9-32 Portion of detailed map showing topographic and landslide features developed during landslide investigation in western Colorado (Umstot 1988). Original map scale was 1 :2,400 and contour interval was 5ft(1.5 m). FIGURE 9-33 (below) Elevation changes within Thistle landslide, Utah (modified from Keaton 1989 from Duncan et al. 1985). true geographic position on the Earth's surface. From the consecutive readings on the survey grids and traverses, the horizontal and vertical displace- ments of the ground surface can be determined. If the movements are large, the subsequent positions of the reference points can be plotted on the topo- graphic map. However, if the movements are small, the successive positions of the monuments may be plotted separately to a larger scale depicting vectors of movement. The vector map may show refer- ence-point locations and displacement vectors on a map with the landslide outline (Figure 9-36), or a topographic base map may be used if the informa- tion can be shown clearly. Although the initial po- sitions of the points are shown in their proper scale relations, the displacement vectors may be plotted to a larger scale; this difference in scale should be noted. Elevations at successive dates can be entered beside the grid points. 216 Landslides: Investigation and Mitigation FIGURE 9-34 RIGHT CENTER LEFT Landslide contours 485 and profile locations - 485 (Sowers and Royster 1978). 480 480 475 470 475 465 470 —C 465 460 ______45 460 455 --____•____ - -.'- 455 450 450 445 445 ,------445 440 7 440 440 - / 435 430 LU I•7' -----j .------430 LU 425 425 - 5- - 420 -- 420 425 415 4 15 420 415 410 S_RGHT.O,. 410 - S - 0 50 100 ft 36+50 36+00 0 25m Because topography changes significantly with the grid points. An example of a plot of direction continuing movement, the dates of the surveys and cumulative magnitude of displacement is should be noted on the maps. Furthermore, if a shown in Figure 9-20. significant period of time has elapsed between the dates of the surveys that establish the topography 4.3.4 Strain Vectors and Ellipses and the surveys that establish the movement grid, the elevations of the points on the grids will not Quadrilateral monitoring (Section 4.2.3.3) pro- necessarily correspond to those on the topo- vides a means for calculating strain from changes graphic map. in the length of chords, called stretch (Baum et al. Displacement-vector data also can be displayed 1988), between points on each quadrilateral. The separately from maps showing the distribution of directions and magnitudes of maximum and mm- Surface Observation and Geologic Mapping 217 I I 25 m HORIZONTAL AND lOOft VERTICAL SCALES ARE EQUAL 475 I 450 ______lOOft 0 E z - 425 0 CENTER LINE OF SLIDE > OPPOSITE STA. 36+55 uJ 475 w 400 450 425 RIGHT OF CENTER LINE OF SLIDE OPPOSITE STA. 36+88 400 2+00 1+75 1+50 1+25 1+00 0+75 0+50 0+25 STATIONING (100 m from center line of right-of-way) FIGURE 9-35 imum principal stretches can be determined and geologic data presentation is the geologic map. Landslide ground- The map and explanation must be carefully plotted in the form of ellipses on the topographic surface profiles base map or landslide outline map. Similarly, the crafted to present relevant information accurately (Sowers and Royster directions and magnitudes of area strain and finite and clearly. Geologic sections are used to illus- 1978). shear strain (Baum et al. 1988) can be determined trate subsurface relationships interpreted from sur- and plotted on maps to represent internal defor- face observations. Sections can be connected to mation rather than displacement. portray three-dimensional relationships. Selected photographs of critical features enhance the re- port user's understanding. The geologic report 5. INTERPRETATION AND DATA discussing methods, findings, conclusions, and re- PRESENTATION commendations must be carefully written to com- The geologic data collected in the field must be municate relevant information without using interpreted and presented in a form that commu- unexplained geologic terminology. - nicates useful and relevant information to non- The concept of multiple working hypotheses is geologists, usually engineers. The primary form of fundamental to geologic interpretation and is de- 218 Landslides: Investigation and Mitigation native explanations for the data that have been col- SC A P P lected. As more and more observations are made, the hypotheses continue to be tested; those hy- -10.4 potheses that can no longer be supported by the data are rejected. Ultimately the goal is to explain all of the facts with a unique model. This ideal goal may be difficult to achieve, and after some investi- -4.5 -6.5 T5.1 gation, two or more viable alternative explanations may remain. This situation identifies the essence of the frequent difficulties in communication between geologists and engineers. Engineers try to base their Q1.1 analyses and design on the concept that a unique solution exists and can be identified. It is essential that uncertainties in geologic in- terpretation be explained in terms relevant to en- gineers. Often with several viable alternative 0.1 Q0.5 0.3 0.1 ?.o.i explanations, one appears most likely. This situa- tion should be explained clearly, and the likeli- hood should be quantified if possible. If the geologist does not offer this kind of assistance to +2.5 +3.0 Q\+2.8 +1.1 the engineer responsible for design, the engineer 7 may be forced to make geologic decisions or to ig- nore geology altogether (Keaton 1990). The engi- neer responsible for design, on the other. hand, +2.1 +4.6 should seek relevant information from the geolo- gist. The engineer and the geologist must work to- gether as a team to accurately characterize a site and formulate a design based on that characteri- TOE zation. GRID POINT Much geologic analysis is based on the concept . - SUBSIDENCE (ml RISE (ml that current processes can be used to interpret the ~ geologic record; in other words, the present is the key to the past. For example, a stream transporting VECTOR OF HORIZONTAL MOVEMENT SCALE sand and gravel can be interpreted to mean that ' gravelly sandstone bedrock was deposited in a flu- vial environment. The engineering-geologic corol- lary to this concept states that processes that were FIGURE 9-36 scribed briefly in the next section. The measure- active in the recent geologic past should be ex- Movement vectors ments obtained from reconnaissance instrumenta- pected to remain active in the near future; in other showing tion also are discussed, followed by elaboration on words, the recent past is the key to the near ftiture. displacements since the geologic maps, sections, and report. For example, subdued hummocky terrain may indi- beginning of measurement cate that landslides that .vere active in prehistoric (Sowers and Royster 5.1 Importance of Multiple Working time have the potential to become active again. 1978). Hypotheses 5.2 Analysis of Reconnaissance The multiple-working-hypothesis method proposed Instrumentation by Chamberlin (1965) has become the conven- tional geologic approach to scientific investigation, Information developed from reconnaissance in- although some believe the name should no longer strumentation can be valuable in understanding be applied to the method (Johnson 1990). This the cause, rate, and direction of slope movement. method consists of continuous evaluation of alter- These factors in turn may be helpful in planning Surface Observation and Geologic Mapping 219 the subsurface investigation and interpreting the evaluation, or site investigation. In urban areas, results. Measurements made with the instrumen- Leighton (1976) called these levels regional, tract, tation should be summarized as basic data and and site. Areal landslide maps show the general then interpreted in a geologic context. distribution and shape of large landslide deposits. Good examples are a landslide map of Utah (Figure 9-38) by Harty (1991) at a scale of 5.3 Engineering-Geologic Maps and 1:500,000 and a preliminary map of landslide de- Explanations posits in San Mateo County, California, by Brabb Engineering-geologic information is most useful and Pampeyan (1972) at a scale of 1:62,500. when plotted on a topographic base map. An or- Areal landslide maps provide some basis for char- FIGURE 9-37 thophotograph with superimposed topographic acterizing large-area suitability; feasibility and de- Part of contours (Figure 9-37) provides a most useful sign decisions, however, require greater detail. orthotopographic method for rapidly communicating relevant Increased detail is achieved at the regional eval- map of Thousand Peaks landslide area, uation level. Information on the landslide types geologic data to nongeologists if the photograph northern Utah. documents current conditions. The scale of and their features would be documented on maps Photographs at the base map controls the detail that can be for regional evaluation. Baum and Fleming (1989) nominal negative shown. Engineering-geologic mapping should be mapped landslides in central Utah at a scale of scale of 1:12,000 completed at the scale that will be used for 1:12,000 (Figure 9-39). Wieczorek (1982) mapped were adjusted to design drawings if possible. Detailed geologic landslides near La Honda, California, at 1:4,800, a map scale of mapping, particularly of complex areas, can be and Baum and Fleming (1989) showed the Aspen 1:4,800. Topographic Grove landslide in Utah at about 1:4,000 (Figure accomplished efficiently with the aid of a plane contours are at 5-ft table and alidade, as was done by Fleming and 9-40); these are reasonable scales for site investi- (1.5-rn) intervals. Johnson (1989) for the Aspen Grove landslide in gations. Fleming and Johnson (1989) mapped the Compare with Utah (Figure 9-6). Aspen Grove landslide at 1:200 using a plane Figure 9-8. table; a portion of this map is shown in Figure 9-41 COURTESY OF KERN RIVER Three levels of detail may be warranted in GAS TRANSMISSION landslide assessments: areal assessment, regional at 1:833. A smaller part of the Aspen Grove land- COMPANY 220 Landslides: Investigation and Mitigation FIGURE 9-38 with contoured data such as elevation on a topo- Example of landslide graphic map or isohyetal lines on a precipitation map for areal mwi map. A landslide map can be transformed into assessment, scale contoured data by isopleth mapping (Wright et at. 1 :500,000 (Harty 1991). Original map 1974; DeGraff and Canuti 1988), as shown in is colored and Figure 9-42. The percentage of a standard area that distinguishes r is underlain by landslide deposits is determined on between J a grid superimposed on the landslide map. The per- deep-seated, r. centage values determined for all the points on the I • I shallow, and grid are contoured to show isopleths of landslide undifferentiated 1i' r 11 deposits. landslides compiled from different The isopleth landslide map provides a way to sources; dots represent landslide activity in an area and improve represent landslides the understanding of conditions leading to insta- smaller than 600 m bility (DeGraff 1985), as shown in Figure 9-43. It across, and patches facilitates comparison of large and small landslides depict landslides slide mapped in 1983 and 1984 is shown in Figure that may he present in different parts of the area. larger than 600 m 9-6 at 1:333. Isopleth maps for different landslide types in an across. Area shown Regional evaluation maps give sufficient detail includes Ephraim area can be constructed to assess whether their Canyon, central for establishing feasibility and some preliminary distribution is controlled by different sets of fac- Utah; increasing design. Areas of exposed bedrock should he tors. Locations with especially high percentages of detail is shown in mapped as bedrock; where surficial deposits are landslides may merit more detailed investigation. Figures 9-39, 9-40, present at the surface, even if bedrock is at a shal- 9-41, and 9-6. low depth, these deposits should he shown. The 5.4 Engineering-Geologic Sections Unified Engineering Geology Mapping System, Engineering-geologic sections should be located so described in Section 3.2.2, provides a method for as to provide information that will be needed for indicating the estimated thickness of surficial de- planning subsurface investigations and stability posits overlying bedrock. analyses. Commonly these positions are perpendic- The site investigation map is utilized for design- ular to topographic contours and aligned longitudi- ing a project and therefore the greatest possible nally down active or dormant landslide deposits. level of detail should be presented. Not only should The projected shape of sliding surfaces, geologic the information on the bedrock and surficial de- contacts, and piezometric surfaces should he shown posits contained in a regional evaluation map be re- along with the topographic profile (Figure 9-44). tained, but greater detail should also be included. Geologic sections should be constructed at the Geologic map explanations should contain same horizontal and vertical scales. Exaggerated brief but complete descriptions of the units vertical scales do not permit direct measurement of mapped and the symbols used. Map units may be angular relationships and can he misinterpreted. correlated with geologic formation names and Furthermore, they do not aid in communicating ages. The locations of sections and reconnaissance geologic information to nongeologists. instrumentation should also be indicated on the map and included in the explanation. 5.5 Three-Dimensional Representations Comparison of mapped landslides with other physical characteristics of the area may provide in- Communication of geologic information can be sight on conditions influencing stability. A land- enhanced by diagrams of interconnected geologic slide map is essentially a map on which points sections. These interconnected sections (called within an area have been classified as either related fence diagrams) provide three-dimensional or to a landslide or not. There is little difficulty in pseudo-three-dimensional representation of the comparing landslide maps with geologic maps surface and subsurface conditions (Figure 9-45). where points within an area have been classified Block diagrams are created by constructing perpen- according to what geologic material is present. It is dicular sections and sketching the surface geology more difficult to make a meaningful comparison in an isometric projection. (Figure 9-7 shows Surface Observation and Geologic Mapping 221 FIGURE 9-39 Example of landslide map for regional assessment, scale 1:12,000 (Baum and Fleming 1989). Aspen Grove landslide, Utah, is identified near center; increasing detail is shown in Figures 9-40, 9-41, and 9-6; less detail is shown in Figure 9-38. Abbreviated explanation: 0/1, pre-1983 landslide; 0/2, landslide active in 1983 to 1986; Qd2, debris flow active in 1983 to 1986; QaI, alluvial deposits; Qc, colluvial deposits; Tf, Eocene-Paleocene Flagstaff limestone; TKn, Paleocene— Upper Cretaceous North Horn Formation. schematic block diagrams of landslide deposits of information to nongeologists. They are most use- differing ages.) Scale models constructed from ful when data from subsurface investigations has thin sheets of styrofoam (foam-core boards) are been integrated into them. the equivalent of orthogonal block diagrams (Figure 9-46). Computer-generated sections, 5.6 Engineering-Geologic Reports fence diagrams, and block diagrams can be pro- duced routinely with the use of commercially Engineering-geologic reports most likely will be available computer programs. These diagrams can used by nongeologists; therefore, geologic termi- be particularly helpful in communicating geologic nology must be minimized and explained in non- 222 Landslides: Investigation and Mitigation FIGURE 9-40 Example of landslide 2275 map for site assessment (Aspen 0 100 200m — — — Grove landslide, 1 Utah), scale about 25-rn contour interval 1:4,000 (modified from Baum et al. 2300 1988). More detail of part of landslide is shown in Figures 9-41 and 9-6; less Forest Service Road detail is shown in North Figures 9-38 and ) 2325 9-39. 2350 2375 0 1 2400 oBoring I 0 Quadrilateral Forest Service Road - geologic terms. For example, "fossiliferous silty 6. GUIDANCE FOR SUBSURFACE limestone" should be used rather than "biomi- INVESTIGATIONS crite." Engineering geology essentially is geology that is relevant to an engineering project; geologic The geologist often is the first technical specialist at a site or the first to spend a substantial amount information that is not relevant to the project of time there. Thus, valuable information can be should be omitted or placed in an appendix to the collected regarding logistical issues, such as access report. to the site for drilling and other equipment, loca- Engineering-geologic descriptions must be tions and types of utilities in the vicinity, and quantitative to the greatest extent possible. Phrases existing land uses. such as "moderately weathered," "moderately The results of the surface observation and strong," and "moderately dense" are not meaning- engineering-geologic mapping should provide ful unless they are quantified as in the Unified Rock detailed and quantitative information on Classification System (Table 9-4). The basis for ge- ological opinions and recommendations should be Distribution, modes, and rates of slope move- clearly stated. As additional information becomes ment; available, such as the results of the subsurface in- Positions and geometries of slip surfaces; vestigation, the basis for opinions can be evaluated, Position of groundwater table and piezometric and the opinions can be verified or modified. surfaces; FIGURE 9-41 Example of landslide 2385 map for site 2385 assessment (part of i Aspen Grove landslide, Utah). Fleming and Johnson (1989) z originally mapped this landslide at -300m < scale of 1:200, but _I original illustration 23 ° was published at scale of about 1:833. More detail I of part of this landslide is shown 2.315 in Figure 9-6 at scale of 1:300; less detail is shown in Figures 9-38, 9-39, :\<2375 and 9-40. 1? )b0t 10' U / 2.; -400 — EXPLANATION SCALE 2356— Contour 2.350 0 5 10 20m ault trace I I Thrust fault 235 ...- C.l.=lm ul Narrow crack 0 Wide crack 2355 F old Rail fence - Survey station B;* Borehole - 2350 224 Landslides: Investigation and Mitigation FIGURE 9-42 Isopleth Map Steps in constructing Land8iide Map Grid Overlay isopleth map of landslide distribution EEEE= cicm (DeGraff and Canuti 1988). 3 10 • 30 FIGURE 9-43 (below) Example of isopleth map of landslide distribution '-2.5 cm— (DeGraff 1985). Potentially difficult drilling or excavation con- ditions, or both; Recommendations for locations of surface geo- physics, borings, and test pits; and Possible locations and schemes for stabilizing structures. Of particular importance are the geologist's rec- ommendations concerning the best locations for subsurface investigations. Engineers may plan an investigation program of regularly spaced borings, test pits, or both, hoping to collect sufficient data to adequately characterize the site. However, ge- ology is not random; therefore, borings and test pits at appropriately selected locations are more likely to provide information needed to test geo- logic-model hypotheses. It must be recognized that additional borings may be needed to collect sufficient samples for laboratory testing. 7. CORRELATION OF SURFACE AND SUBSURFACE DATA The geologist should observe and log subsurface exposures and samples to provide a basis for corre- lating surface mapping with subsurface informa- tion. Subsurface data generally permit more specific geologic interpretation than do surface data. The geologic interpretation of the site based Surface Observation and Geologic Mapping 225 A A' 100 C 75 75 50 50 25 25 FIGURE 9-44 on surface mapping must be evaluated in the con- the adjacent property. Some adjacent land uses, Example of text of the subsurface data. If appropriate, revisions such as agricultural irrigation, can contribute to engineering-geologic should be made to the engineering-geologic map stresses causing slope movement at the project site. section of landslide and sections. The reevaluation of the geologic in- Repeated surface observations and instrumenta- deposit (Jibson 1992). terpretation of the site may reveal the need for sup- tion can permit early detection of the beginning of plemental subsurface data at locations not initially a problem. Alternatively, such observations can FIGURE 9-45 selected for investigation. provide a legal basis for justifying compensation (below) The basic goal of the surface and subsurface in- from the adjacent land owner for part of the dam- Example of fence vestigations is accurate, quantitative characteriza- ages at the site. diagram showing tion of the site. Accurate site characterization The potential for excessive precipitation, strong subsurface conditions permits selection of the best geologic-geotechnical earthquake shaking, or both, to occur at a site in landslide area (Sowers and Royster model of the slope system for use in stability anal- 1978). yses. The best model permits the most appropriate design for the project. 8. ADDITIONAL FIELD INFORMATION Additional field information may be needed to permit prediction of the performance of the slope and the potential risk of damage to the project, ad- jacent facilities, or both. Long-term monitoring of slope deformation and piezometric levels may be appropriate. The reconnaissance instrumentation can be converted to permanent instrumentation by replacing the rapidly deployed devices with more stable ones. Adjacent land use strongly influences the consequences of damage. High-value or high-oc- cupancy property adjacent to a slope movement may require a more conservative design and more extensive field monitoring to provide a basis for legal defense in the event of litigation on behalf of Landslides: Investigation and Mitigation maps based on aerial photography taken by the Directorate of Overseas Surveys of the Govern- ment of the United Kingdom. This is commonly black-and-white photography at a scale of 1:20,000. A project-specific aerial-photograph mission may be warranted for some projects; the cost of such missions varies widely depending upon the size and location of the project, but generally costs at least several thousand dollars. The amount of time required for a geologist to complete surface observation and geologic map- ping depends on the size of the site, the ease of ac- cess, the ruggedness of the site, and the degree of geologic complexity. Under most circumstances, however, one to two weeks should be sufficient. The reconnaissance instrumentation usually con- sists of relatively inexpensive wooden or steel stakes and small-diameter iron pipe. The equip- ment needed to conduct the monitoring is simple and is probably owned by most geologists so that FIGURE 9-46 should be recognized early in the investigation. It rental or purchase would not be needed. Example of may be appropriate to consider installing precipi- The amount of involvement by the geologist styrofoam tation gauges, strong-motion accelerographs, or during the subsurface investigation can be exten- (foam-core) model of Thistle landslide in both, to collect information that may be of great sive or minor, depending on the organization of the central Utah made value in improving design of slope systems. If slope effort. Often the geologist is assigned to the drilling by printing copies of movement occurs in response to precipitation or or excavation equipment to make field observa- topographic map at an earthquake, the intensity or magnitude of the tions. In these cases the geologist can be involved scale of 1:2,400 and event at the slope will be known. Otherwise, the for several weeks or even months. If the geologist is fixing them onto intensity or magnitude of the event will have to be not directly involved in the subsurface-data collec- sheet of styrofoam projected from the nearest recording station, tion, approximately one to two weeks of effort may with spray adhesive. which may be a substantial distance from the site. be required to examine the samples and correlate Thickness of each sheet of styrofoam the subsurface and surface data. was equal to vertical It may be appropriate for the geologist to assist rise of 40 ft (12 m), 9. COST OF SURFACE INVESTIGATIONS in installing instrumentation and to be involved and model was in long-term monitoring efforts. The geologist can made with no The pre-field investigation involves identifying, make repeated surface observations at the time vertical exaggeration. collecting, and reviewing available geologic and the instruments are being read. Long-term moni- Equivalent sun topographic information and aerial photographs. toring can extend over a period of years, with illumination is early to mid-afternoon. Usually no more than a few days of a geologist's readings being taken at intervals of weeks or time is required for this task. Up to several hun- months. The amount of time required for each se- dred dollars may be needed to purchase maps and ries of readings varies with the number of instru- available aerial photographs, depending upon the ment locations and the difficulty of access. size of the project. It may require six to eight weeks Generally, a few days to one week is sufficient to to obtain aerial photographs from government read the instruments at a typical site. agencies in the United States and Canada; else- Correlation and analysis of surface and subsur- where it may take much longer. Where aerial pho- face geologic data are commonly done as the data tography is produced by the air force of a particular are being collected. It may be appropriate for the country, its availability requires overcoming major geologist to synthesize the data at the end of a obstacles. However, many smaller countries in the major effort. Typically, a few days to one week is British Commonwealth produce their topographic sufficient time for such correlation and analysis. Surface Observation and Geologic Mapping 227 REFERENCES Casals, J.F. 1986. Analysis of a Landslide Along Inter- state 70 Near Vail, Colorado. Master of Engi- ABBREVIATIONS neering Report ER-3 172. Colorado School of Mines, Golden, 121 pp. UNESCO United Nations Educational, Scientific, and Chamberlin, T.C. 1965. The Method of Multiple Cultural Organization Working Hypotheses. Science, Vol. 148, pp. WP/WLI Working Party on the World Landslide Inventory (International Geotechnical 754-759 (reprinted from Science, Feb. 7, 1890). Societies and UNESCO) Compton, R.R. 1985. Geology in the Field. John Wiley & Sons, New York, 398 pp. Anbalagan, R. 1992. Landslide Hazard Evaluation Cording, E.J., A.J. Hendron, Jr., W.H. Hansmire, J.W. and Zonation Mapping in Mountainous Terrain. Mahar, H.H. MacPherson, R.A. Jones, and T.D. Engineering Geology, Vol.32, No. 4, pp. 269-278. O'Rourke. 1975. Methods for Geotechnical Obser- Barton, N., R. Lien, and J. Lunde. 1974. Engineering vation and Instrumentation in Tunneling. Depart- Classification of Rock Masses for the Design of Tunnel ment of Civil Engineering, University of Illinois, Support. Norwegian Geotechnical Institute, Oslo, Urbana-Champaign, and National Science Foun- Norway, 48 pp. dation, Washington D.C., Vol. 2, pp. 293-566. Bartz, K.L., J.L. Kershner, R.D. Ramsey, and C.M.U. Crozier, M.J. 1984. Field Assessment of Slope Insta- Neale. 1994. Delineating Riparian Cover Types bility. In Slope Instability (D. Brunsen and D.B. Using Multispectral, Airborne Videography. In Prior, eds.), John Wiley & Sons, New York, pp. Proc., 14th Biennial Workshop on Color Photo- 103-142. graphy and Videography in Resource Monitoring, Cummings, D., and B.R. Clark. 1988. Use of Seismic May 25-28, 1993, Utah State University, Logan, Refraction and Electrical Resistivity Surveys in American Society of Photogrammetry and Landslide Investigations. Bulletin of the Association Remote Sensing, Bethesda, Md. of Engineering Geologists, Vol. 25, No. 4, pp. Baum, R.L., and R.W. Fleming. 1989. Landslides and 459-464. Debris Flows in Ephrairn Canyon, Central Utah. Dallaire, E.E.. 1974. Electronic Distance Measuring U.S. Geological Survey Bulletin 1842-C, pp. Revolution Well Under Way. Civil Engineering, Cl-C12 (scale 1:12,000). Vol. 44, No. 10, pp. 66-71. Baum, R.L., A.M. Johnson, and R.W. Fleming. 1988. DeGraff, J.V. 1985. Using Isopleth Maps of Measurement of Slope Deformation using Quadrilat- Landslide Deposits as a Tool in Timber Sale erals. U.S. Geological Survey Bulletin 1842, 23 pp. Planning. Bulletin of the Association of Engineering Bieniawski, Z.T. 1989. Engineering Rock Mass Classi- Geologists, Vol. 22, No. 4, pp. 445-453. fications. Wiley Interscience, New York, 251 pp. DeGraff, J.V., and S.S. Agard. 1984. Defining Brabb, E.E., and E.H. Pampeyan. 1972. Preliminary Geologic Hazards for Natural Resources Man- Map of Landslide in San Mateo County, California. agement Using Tree-Ring Analysis. Environmental U.S. Geological Survey Miscellaneous Field Geology and Water Sciences, Vol. 6, pp. 147-155. Studies Map MF-344 (scale 1:62,500). DeGraff, J.V., and P. Canuti. 1988. Using Isopleth British Geotechnical Society. 1974. Field Instrumen- Mapping to Evaluate Landslide Activity in tation in Geotechnical Engineering. John Wiley & Relation to Agricultural Practices. Bulletin of the Sons, New York, 720 pp. International Association of Engineering Geology, Carr, J.R., and J.B. Warriner. 1987. Rock Mass Vol.38, pp. 61-71. Classification Using Fractal Dimension. In Rock Duncan, J.M., R.W. Fleming, and F.D. Patton. 1985. Mechanics: Proceedings of the 28th U.S. Symposium Report of the Thistle Slide Committee to the State of (I.W. Farmer et al., eds.), A.A. Balkema, Utah Department of Natural Resources, Division of Rotterdam, Netherlands, pp. 73-80. Water Rights. Salt Lake City, Utah, 62 pp. Carroll, R.D., F.T. Lee,J.H. Scott, andC.S. Robinson. Fleming, R.W., and A.M. Johnson. 1989. Structures 1968. Seismic-Refraction Studies of the Loveland Basin Associated with Strike-Slip Faults That Bound Landslide, Colorado. U.S. Geological Survey Landslide Elements. Engineering Geology, Vol. 27 Professional Paper 600-B, pp. B83-B87. (Richard H. Jahns Memorial Volume), Nos. 1-4, Carter, M., and S.P. Bentley. 1985. The Geometry pp. 39-114. of Slip Sui{aces Beneath Landslides: Predictions Franklin, J.A., and P.E. Denton. 1973. The Mon- from Surface Measurements. Canadian Geo- itoring of Rock Slopes. Quarterly Journal of technical Journal, Vol. 22, pp. 234-238. Engineering Geology, Vol. 6, No.3, pp. 259-286. 228 Landslides: Investigation and Mitigation Galster, R.W. 1977. A System of Engineering Ge- Symbols. Bulletin of the Association of Engineering ology Mapping Symbols. Bulletin of the Association Geologists, Vol. 21, No. 3, pp. 355-364. of Engineering Geologists, Vol. 14, No. 1, pp. 39-47. Keaton, J.R. 1989. The Thistle Landslide, Utah Geological Society Engineering Working Party. County, Utah. In Engineering Geology of Western 1977. The Description of Rock Masses for United States Urban 'Centers (J.R. Keaton and Engineering Purposes. Quarterly Journal of R. Morris, eds.), Field Trip Guidebook T181, Engineering Geology, Vol. 10, No.4, pp. 355-388. American Geophysical Union, Washington, D.C., Gould, J.P., and C.J. Dunnicliff. 1971. Accuracy of pp. 59-63. Field Deformation Measurements. In Proc., Keaton, J.R. 1990. Construction Geology: As-built Fourth Pan-American Conference on Soil Mechanics Conditions. In The Heritage of Engineering Geol- and Foundation Engineering, San Juan, Puerto ogy; The First Hundred Years (G.A. Kiersch, ed.), Rico, American Society of Civil Engineers, New Centennial Special Volume 3, Geological Society York, Vol. 1, pp. 313-366. of America, Boulder, Cob., pp. 429-466. Hansen, A. 1984. Landslide Hazard Analysis. In Slope Keaton, J.R., R.M. Robison, and J.D.J. Bott. 1992. Instability (D. Brunsden and D.B. Prior, eds.), John Landslide Hazard 'Analysis for Pipeline' Design, Wiley & Sons, New York, pp. 523-602. Northeast Utah. In Stability and Performance of Harp, E.L., and M.A. Noble. 1993. An Engineering Slopes and Embankments (R.B. Seed and R.W. Rock Classification to Evaluate Seismic Rock-Fall Boulanger, eds.), Geotechnical Special Pub- Susceptibility and Its Application to the Wasatch lication 31, American Society of Civil Engineers, Front. Bulletin of the Association of Engineering New York, N.Y., Vol. 1, pp. 192-204. Geologists, Vol. 30, No. 3, pp. 293-319. Kern and Company, Ltd. 1974. The Mekometer, Kern Harty, K.M. 1991. Landslide Map of Utah. Utah Geo- ME3000, Electro-Optical Precision Distance Meter. logical Survey Map 133, 28 pp. (scale 1:500,000). Tech. Information 10. Kern and Co., Aarau, Hasan, S.E. 1994. Use of Soil Survey Reports in Switzerland, 4 pp. Geotechnical Projects. Bulletin of the Association of Koler, T.E., and K.G. Neal. 1989. Chestershire and Engineering Geologists, Vol.31, No. 3, pp.367-376. Backdrop Timber Sales: Case Histories of the Hoek, E., and J.W. Bray. 1977. Rock Slope Engi- Practice of Engineering Geology in the Olympic neering. Institute of Mining and Metallurgy, National Forest. In Engineering Geology in Wash- London, 402 pp. ington (R.W. Galster, ed.), Washingtbn Division Hutchinson,' J.N. 1983. Methods of Locating Slip of Geology and Earth Resources Bulletin 78, Vol. Surfaces in Landslides. Bulletin of the Association of 2, pp. 933-944. Engineering Geologists, Vol. 20, No. 3, pp. 235-252. Laser Technology, Inc. 1992. Criterion Series Jade, S., and S. Sarkar. 1993. Statistical Models for Specifications. Laser Technology, Inc., Englewood, Slope Instability Classification. Engineering Geo- Cob., 2 pp. logy, Vol. 36, No. 1/2, pp. 9 1-98. Leighton, F.B. 1976. Urban Landslides: Targets for Jadkowski, M.A. 1987. Multispectral Remote Sensing Land-Use Planning in California. In Urban Geo- of Landslide Susceptible Areas. Ph.D. thesis. Utah morphology (D.R. Coates, ed.), Geological Society State University, Department of Civil and of America Special Paper 174, pp. 37-60. Environmental Engineering, Logan, 260pp. Lips, E.W., and J.R. Keaton. 1988. Slope-a-Scope: Jibson, R.W. 1992. The 'Mameyes, Puerto' Rico, A Convenient Tool for Rapid Topographic Landslide Disaster of October 7, 1985. In Land- Profiling. In Abstracts and Program, Association of slides/Landslide Mitigatzort (JE. Slosson et a1, eds.), Engineering Geologists 31st Annual Meeting, Reviews in'Engineering Geology, Geological Soci- Kansas City, p. 51. ety of America, Boulder, Cob., Vol. 9, pp. 37-54. Maerz, N.H1, J.A. Franklin, and C.P. Bennet. 1990. Johnson, A.M., and 'R.'L. Baum. 1987. BASIC Joint Roughness Measurement Using Shadow Programs for Computing Displacements, Strains, and Profilornetry. International Journal of Rock Me- Tilts from Quadrilateral Measurements. U.S. Geo- chanics, Mineral Science, and Geonnechanics logical Survey Open-File Report 87-343. 19 pp. Abstracts, Vol. 27, No. 5, pp. 329-343. Johnson, J.Q. 1990. Method of Multiple Working Malde, H.E. 1973. Geologic Bench Marks by Ter- Hypotheses: A Chimera.'Geology, Vol. 18, No. 1, restrial Photography. Journal of Research, U.S. pp. 44-45. Geological Survey, Vol. 1, pp. 193-206. Keaton, J.R. 1984. Genesis-Lithobogy-Qualifier McCalpin, J. 1984. Preliminary Age Classification (GLQ) System of Engineering Geology Mapping of Landslides for Inventory Mapping. In Proc., Surface Observation and Geologic Mapping 229 21st Engineering Geology and Soils Engineering 1993, Utah State University, Logan, American Symposium, University of Idaho, Moscow, pp. Society of Photogrammetry and Remote Sensing, 99-120. Bethesda, Md. McCann, D.M., and A. Forster. 1990. Reconnaissance Sowers, G.F., and D.L. Royster. 1978. Field Investi- Geophysical Methods in Landslide Investigations. gation. In Special Report 176: Landslides: Analysis Engineering Geology, Vol. 29, No. 1, pp. 59-78. and Control (R.L. Schuster and R.J. Krizek, eds.), Merriam, R. 1960. Portuguese Bend Landslide, Palos TRB, National Research Council, Washington, Verdes Hills, California. Journal of Geology, Vol. D.C., Chap. 4, pp. 81-111. 68, No. 2, pp. 140-153. St. John, C.M., and TL. Thomas. 1970. The N.P.L. Miller, C.H., A.L. Ramirez, and T.F. Bullard. 1980. Mekometer and Its Application in Mine Surveying Seismic Properties Investigation of the Springer Ranch and Rock Mechanics. Transactions, Institute of Landslide, Powder River Basin, Wyoming. U.S. Mining and Metallurgy, London, Vol. 79, Sec. A, Geological Survey Professional Paper 1170-C, pp. A31—A36. 7 pp. Terzaghi, K. 1950. The Mechanism of Landslides. In Moffitt, F.H., and H. Bouchard. 1975. Surveying, 6th Application of Geology to Engineering Practice ed. Harper & Row, New York, 879 pp. (S. Paige, ed.), Geological Society of America, Moore, J.F.A. 1973. The Photogrammetric Measure- Boulder, Cob., Berkey Volume, pp. 83-1 23. ment of Constructional Displacements of a Rock- Umstot, D. 1988. Field Studies and Modeling Analysis fill Dam. Photogrammetric Record, Vol. 7, No. 42, of the Roan Creek Landslide, Gafield County, pp. 628-648. Colorado. Master of Engineering Report ER-3420. Nasser, K.H. 1986. Slope Stability Analysis of the Colorado School of Mines, Golden, 186 pp. Wolcott Landslide, Eagle County, Colorado. Master Vandre, B.C. 1993. What Is Coarse and Durable of Engineering Report ER-3 110. Colorado School Mine Waste Rock? In Proc., 29th Symposium on of Mines, Golden, 111 pp. Engineering Geology and Geotechnical Engineering Pachauri, A.K., and M. Pant. 1992. Landslide Ha- (G.M. Norris and L.E. Meeker, eds.), Reno, zard Mapping Based on Geological Attributes. Nevada, March 22-24, pp. 22-33. Engineering Geology, Vol. 32, No. 1/2, pp. 81- Vames, D.J. 1984. Landslide Hazard Zonation: A 100. Review of Principles and Practice. United Nations Palmer, D.F., and S.L. Weisgarber. 1988. Geophysical Educational, Scientific and Cultural Organiza- Survey of the Stumpy Basin Landslide, Ohio. tion, Paris, France, 63 pp. Association Engineering Geologists, Bulletin of the of Wieczorek, G.F. 1982. Map Showing Recently-Active 363-370. Vol. 25, No. 3, pp. and Dormant Landslides Near La Honda, Central Penman, A.D.M., and j.A. Charles. 1974. Measuring Santa Cruz Mountains, California. U.S. Geological Movements of Embankment Dams. In Field Survey Miscellaneous Field Studies Map MF-1422 Instrumentation in Geotechnical Engineering, British (scale 1:4,800). Geotechnical Society; John Wiley & Sons, New Wieczorek, G.F. 1984. Preparing a Detailed York, 341-358. pp. Landslide-Inventory Map for Hazard Evaluation Piteau, D.R., and R.J. Peckover. 1978. Engineering and Reduction. Bulletin of the Association of Engi- of Rock Slopes. In Special Report 176: Landslides: Analysis and Control (R.L. Schuster and R.J. neering Geologists, Vol. 21, No. 3, pp. 337-342. Krizek, eds.), TRB, National Research Council, Williamson, D.A. 1984. Unified Rock Classification System. Bulletin of the Association Engineering Washington, D.C., Chap. 9, pp. 192-228. of Reilly, J.P. 1992. Practical Surveying with GPS. P.O.B. Geologists, Vol. 21, No. 3, pp. 345-354. Publishing Company, Canton, Mich., 184 pp. Williamson, D.A., K.G. Neal, and D.A. Larson. Ross-Brown, D.M., and K.B. Atkinson. 1972. Terres- 1981. The Field-Developed Cross-Section—A Syste- trial Photography in Open Pits: Description and matic Method of Presenting Three-Dimensional Use of the Phototheodolite in Mine Surveying. Subsurface Information. Olympic National Forest Transactions, Institute of Mining and Metallurgy, Geotechnical Section, USDA Forest Service, London, Vol. 81, pp. 205-2 13. Ft. Lewis, Wash., 35 pp. Shoemaker, J.A., T.B. Hardy, and C.M.U. Neale. Wilson, S.D., and P.E. Mikkelsen. 1978. Field 1994. Jurisdictional Delineation of Wetlands with Instrumentation. In Special Report 176: Landslides: Multispectral Aerial Videography. In Proc., 14th Analysis and Control (R.L. Schuster and R.J. Krizek, Biennial Workshop on Color Photography and eds.), TRB, National Research Council, Wash- Videography, in Resource Monitoring, May 25-28, ington, D.C., Chap. 5, pp. 112-138. 230 Landslides: Investigation and Mitigation WPIWLI. 1990. A Suggested Method for Reporting WP/WLI. 1993b. Multilingual Landslide Glossarj. Bi- a Landslide. Bulletin of the International Association Tech Publishers, Richmond, British Columbia, of Engineering Geology, No. 41, pp. 5-12. Canada, 59 pp. WPIWLI. 1991. A Suggested Method for a Landslide Wracker, D.A. 1973. Small Format Aerial Photo- Summary. Bulletin of the International Association of graphy. Mining Engineering, Vol. 25, No. 11, pp. Engineering Geology, No.43, pp. 101-110. 47-48. WPIWLI. 1993a. A Suggested Method for Describing Wright, R.H., R.H. Campbell, and T.H. Nilsen. the Activity of a Landslide. Bulletin of the Interna- 1974. Preparation and Use of Isopleth Maps of tional Association of Engineering Geology, No. 47, Landslide Deposits. Geology, Vol. 2, No. 10, pp. pp. 53-57. 483-485.