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10 lchapter mcguffey verne c modeer jr victor a keith turner subsurface and a exploration 1 introduction angle of all geologic deposits pore pressures in water bearing strata depth to ...

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                                    10 
        LChapter 
                                                                                                                                      MCGUFFEY, 
                                                                                                                      VERNE C. 
                                                                                                                                    MODEER, JR., 
                                                                                                                  VICTOR A. 
                                                                                                                                 KEITh TURNER 
         SUBSURFACE                                                                                                  AND A. 
         EXPLORATION 
         1. INTRODUCTION                                              angle of all geologic deposits, pore pressures in 
                                                                      water-bearing strata, depth to controlling features, 
                lope instability reflects soil, rock, and ground-     and probable vertical and lateral limits of sliding. 
          Swater conditions that are hidden beneath the               Interpretation of such data identifies and quantifies 
                ground surface. Although geologic structures          potential solutions for landslide movements. 
         and strength properties of earth materials can often 
         be inferred from surface investigations, subsurface          1.1Classification of Subsurface 
         investigations are also required to obtain definitive        Exploration Methods 
         data and samples. Subsurface explorations exhibit a          Subsurface exploration methods may be classed as 
         wide range in cost. In order to save time and money,         direct methods  and indirect methods  (Hunt 1984). 
         subsurface exploration programs should be under-             Direct methods, such as test borings and the exca-
          taken following site reconnaissance and surficial           vation of test pits, allow the examination of mate-
          investigation programs (see Chapters 7 and 9) and           rials, usually with the removal of samples. Indirect 
          before the selection of instrumentation (see Chap-          methods, such as geophysical surveys and use of the 
          ter 11). Subsurface investigation follows an itera-         cone penetrometer, provide a measure of mate-
          tive process that incorporates new procedures and           rial properties that, by correlation with other data, 
          adjustments as information is discovered and tested         allows the estimation of material type. Exploration 
          against multiple working hypotheses and proposed            methods may be further classified into the follow-
          mitigation strategies.                                      ing key categories: 
             Selection of exploration methods and develop-
          ment of a plan for the subsurface exploration                  Reconnaissance methods, 
          program are based on considerations of study                   Surface-based geophysical methods, 
          objectives, size of the landslide area, geologic               Test and core borings, 
          conditions, surface conditions, access to the area,            Borehole logging, and 
          and limitations of budget and time. Available in-              Field testing, including specialized sampling 
          formation concerning the site, including any plans             from test pits, adits, and shafts. 
          for construction or remedial treatment, should be 
          used to support this selection and planning process.         1.2 Definition of Appropriate Exploration 
          A subsurface exploration program should provide              Program 
          information that allows for qualification and quan-          Decisions regarding the type and location of sub-
          tification of pertinent material properties. The 
          exploration program should provide values for the            surface explorations are dependent on the infor-
          undisturbed and residual shear strength or friction          mation needed to quantify the various working                                231 
            	
        232                     Landslides: Investigation and Mitigation 
                                hypotheses. Some rules of thumb that may be               still be active and thus may pose a constant risk to 
                                helpful in deciding on a reasonable approach to a         the workers and equipment. The possibility of hav-
                                subsurface exploration program are as follows:            ing loose or unstable material upslope of the explo-
                                                                                          ration crew should be considered, and precautions, 
                                   Reconnaissance methods involve low-cost                such as building protective cages or setting up man-
                                   techniques requiring a minimum of equipment.           ual or automated movement-warning devices, 
                                   They provide both direct and indirect data.            should be taken. In some situations, it may be ad-
                                   Surface geophysical explorations provide only          visable for crews to work in shifts around the clock 
                                   indirect data but are relatively inexpensive and       to reduce the duration of such safety measures. 
                                   can cover a large area in a very short time. 
                                   Borings constitute the most common subsurface          1.4 Supervision by Geologist or 
                                   explorations. They include a wide variety of           Geotechnical Engineer 
                                   techniques and can vary from relatively routine 
                                   and low-cost approaches to highly specialized          On-site supervision by a knowledgeable experi-
                                   and expensive methods. Because borings gener-          enced geologist or geotechnical engineer is critical 
                                   ally are used to provide samples, they provide         to the success of most subsurface investigation pro-
                                   direct data. Samples obtained by different tech-       grams to ensure that the intent of all specifications 
                                   niques vary considerably in their utility; in many     is preserved and that the field activities are properly 
                                   cases samples obtained from borings produce            executed so that the desired results can be achieved. 
                                   inaccurate values for material properties because      The chief functions of the supervision are to 
                                   of their relatively small volumes. 
                                   Field tests range from relatively inexpensive             Enforce all technical and legal contract specifi-
                                   penetration tests that can be performed as part           cations; 
                                   of exploratory boring programs to expensive               Maintain liaison with the designer of the ex-
                                   specialized test pits. Results obtained from field        ploration program; 
                                   tests provide confirmation of strength property           Select and approve modifications to the pro-
                                   estimates obtained in laboratory tests.                   gram specifications as new or unanticipated 
                                   Test pits provide direct data and the potential           conditions are revealed (such as the addition or 
                                   for collecting large samples or performing in situ        deletion of borings, changes in depths of bor-
                                   field tests to obtain landslide information not           ings, changes in the types, depths, or intervals 
                                   available from other sources. These pits can              of sampling, etc.); 
                                   usually reach only shallow depths; they become            Ensure that complete and reliable field reports 
                                   extremely costly as the depth increases.                  are developed; and 
                                   Geophysical or other methods for logging test             Identify all geologic conditions accurately. 
                                   borings often provide valuable information at a 
                                   modest additional cost.                                Lack of such a knowledgeable on-site decision 
                                   Specialized sampling and investigations requir-        maker during the exploration program can lead to 
                                   ing the construction of adits and shafts are           large additional expenses if site revisitation be-
                                   extremely expensive and time consuming. Adits          comes necessary to obtain additional required in-
                                   and shafts may be hazardous in landslide areas         formation. In some instances, without such 
                                   because of the nature and inherent instability of      expertise available, serious mistakes can be made 
                                   earth materials; accordingly, they are used only       during the exploration program that will aggravate 
                                   rarely during landslide investigations.                the landslide conditions. 
                                 1.3 Safety Considerations                                1.5 Sources of Information 
                                 The safety of the subsurface exploration team            Numerous sources of information are available for 
                                should be evaluated before the site is occupied.          more specific guidance concerning the importance 
                                                                                 diffi-   of subsurface investigation or proper procedures 
                                 Explorations of landslides are often located in 
                                 cult terrain, and excavations may require temporary      for planning and conducting subsurface explo-
                                 falsework to protect personnel. The landslide may        rations on landslides. Several basic engineering 
                                                                                                                       	 233 
                                                                                                Subsurface Exploration 
        geology and geotechnical engineering textbooks            hypotheses. A successful subsurface exploration 
        contain chapters on subsurface exploration tech-          program will identify the controlling subsurface 
        niques (Schultz and Cleaves 1955; Krynine and             deposits and quantify all variables that might con-
        Judd 1957; Hunt 1984; Johnson and DeGraff                 trol landslide activity according to the various 
        1988). Such sources may be supplemented by gov-           alternative hypotheses using an iterative process 
        ernment manuals prepared by agencies for training         that must be continuously modified to answer the 
        and guidance of their personnel (USBR 1974;               critical design questions. The subsurface explo-
        NAVFAC 1982). In addition, numerous guides to             ration program must define the spatial relation-
        suitable sampling and exploration practices have          ships and provide quantitative information on the 
        been prepared by professional societies, standards-       density, shear strength, and perrreability of each 
        setting organizations, manufacturers of explo-            of the subsurface layers. The necessary parameters 
        ration equipment, and commercial publishers               required by design solutions for the landslide 
        (Hvorslev 1949; ASTM 1951; Mohr 1962; Merritt             should be quantified, including definition of prop-
        1974; Lowe and Zaccheo 1975; Broms 1980; Hunt             erties for the very strong as well as for the very weak 
        1984).                                                    materials. Instrumentation may be needed to 
           Research literature concerning subsurface explo-       quantify the ranges of water pressure that can be ex-
        ration procedures and the advantages and disad-           pected in each of the important geologic deposits. 
        vantages of several techniques includes discussions       The subsurface exploration program must be coor-
        of the applicability of geophysical exploration           dinated and integrated with the instrumentation 
        methods to engineering investigations by Griffiths        program (see Chapter 11) so that the parameters 
        and King (1969), Saayman (1978), van ZijI (1978),         that cannot be quantified by using conventional 
         Hunt (1984), and Johnson and DeGraff (1988).             exploration techniques can be defined by the 
         Methods of logging boreholes have been described         instrumentation. 
        by Deere (1963), Myung and Baltosser (1972),                 Alternative exploration strategies and their 
        Underwood (1974), and Knill (1975), and in a re-          required equipment and techniques should be 
         port prepared by the Association of Engineering          identified on the basis of the initial site evalua-
        Geologists (AEG 1978). The use of penetrometers           tions, both those in the office and from initial field 
         and the evaluation of penetrometer data have             inspection. This information should clearly iden-
         been discussed by Krynine and Judd (1957),               tify the anticipated conditions, thereby allowing 
         Sanglerat (1972), Alperstein and Leifer (1976),          the investigator to select appropriate equipment, 
         and Schmertmann (1978). Additional references            such as a Christensen core barrel, a borehole 
         to specific applications of various subsurface explo-    camera, or undisturbed-sampling tools. Careful 
         ration techniques are given in subsequent sections       attention must be given to alternative methods for 
         of this chapter.                                         sealing high artesian water pressures if they are 
                                                                  encountered. 
         2. PLANNING SUBSURFACE                                      The preliminary layout, spacing, and depth of 
         INVESTIGATIONS                                           borings will depend on the prior site information. 
                                                                  As a minimum, there should be borings near the 
         The initial planning of a subsurface investigation       top, middle, and bottom of a potential landslide, 
         program incorporates information concerning ter-         with as many profiles of borings as appear to be 
         rain features, site accessibility, and anticipated geo-  required to define the subsurface conditions. 
         logic conditions to define the areal extent of the       Philbrick and Cleaves (1958) suggested that a pro-
         investigation; types of investigative procedures; test   file of borings be developed along the centerline of 
         boring locations, spacings, and depths; and required     the landslide and that the first boring be placed 
         types of samples and sampling frequencies.               between the midpoint and the scarp or head of the 
            Previously conducted surface investigations           landslide. The area outside the landslide perimeter 
         (see Chapter 9) will often sugg'est possible modes       should also be explored to provide comparative 
         of landsliding. The subsurface exploration pro-          data on the stable and unstable portions of the 
         gram must be designed to resolve the remaining           slope. Such information may also be needed to 
         uncertainties and to define the operative landslide       provide data on possible further expansion of the 
         mode (or modes) from among the various                   landslide or possible design of remedial measures. 
            	 Landslides: Investigation and Mitigation 
        234 
                                 2.1 Area of Investigation                                   uphill scarps can be sketched onto these cross sec-
                                 The area of the investigation is determined partly          tions; these surfaces may suggest possible maximum 
                                 by the size and type of an affected transportation          depths for movement. Continuous thick, hard 
                                 project and partly by the extent and type of topo-          strata within the slope may limit depths of 
                                 graphic and geologic features believed to affect            movement. However, at least one boring should 
                                  the landslide activity. At sites where there is            extend far below the suspected failure surface; 
                                 potential for future landslide movements, the area          deep, slow movements often are masked by more 
                                  to be investigated cannot be easily defined in ad-         rapid movements at shallower depths. 
                                  vance. A grid of borings should be placed within              Borings or other direct investigative techniques 
                                  the suspected area to delineate the landslide              should extend deep enough (a) to identify materi-
                                  (Figure 104). Once a landslide has occurred, the           als that have not been subjected to movements in 
                                 area of investigation can be better defined (Figure         the past but that might be involved in future move-
                                  10-2). However, in either case the area studied            ments and (b) to clearly identify underlying stable 
                                  must be considerably larger than that comprising           materials. Boring depths are sometimes revised 
                                  the suspected activity or known movement for               repeatedly as field investigation proceeds. Later, 
                                  three reasons:                                             when field instrumentation has been installed and 
                                                                                             has begun to yield data, the existing or planned 
                                    The landslide or potential landslide must be             boring depths may be found to be insufficient, and 
                                    referenced to the stable area surrounding it,            increases in these depths may become necessary. 
                                    Most landslides enlarge with passage of time, and        The exploration program specifications should be 
                                    Many landslides are much larger than first sus-          flexible enough to allow for additional depths of 
                                    pected from the overt indications of activity.           investigation when the .data obtained suggest 
                                                                                             deeper movements. 
                                     As a rule of thumb, the area to be studied should       2.3 Duration of Investigation 
                                  be two to three times wider and longer than the 
                                  area suspected. In some mountainous areas, it is           Since most landslides are affected by climate 
                                  necessary to investigate to the top of the slope or to     changes, a minimum period for investigation 
                                  some major change in lithology or slope angle. The         should include one seasonal cycle of weather, 
                                  lateral area must encompass sources of groundwater         which is one year in most parts of the world. 
                                  and geologic structures that affect the landslide          Longer-term climate cycles, such as several years 
                                  stability.                                                 with periods of wetter and drier weather, are com-
                                                                                             mon, however; thus landslide investigations often 
                                  2.2 Depth of Investigation                                 require a monitoring phase lasting for many years 
                                  The depth of investigation is even more difficult to       or even several decades. In practical terms, such a 
                                  define in advance. Initial estimates of investigation      long-term assessment often is impossible because 
                                  depths can be made by applying various rules of            there is usually a need to draw conclusions and 
                                  thumb, including the following:                            make decisions concerning corrective action much 
                                                                                             more quickly. 
                                    The depth of movement at the center of the                  Experience has shown that false conclusions 
                                    slide is rarely greater than the width of the zone       have often been reached on the causes of landslides 
                                    of surface movement.                                     and the effectiveness of corrective measures 
                                    The maximum depth of the failure surface is              because the effects of severe climate conditions 
                                    often approximately equal to the distance from           were not adequately considered by the engineers 
                                    the break in the orginal ground surface slope to         and geologists. The worst climate conditions pos-
                                    the most uphill crack or scarp (McGuffey 1991).          sible during the life of a project are likely to control 
                                                                                             the risk to the project of landsliding. Investigations 
                                     Longitudinal cross sections drawn along the             made during climate conditions that are less severe 
                                  landslide centerline may also be helpful in defining       can prove to be too optimistic, and those made 
                                  initial investigation depths. Circular or elliptical       during a particularly severe climate cycle may be 
                                  failure surfaces connecting possible toe bulges and        too pessimistic. 
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...Lchapter mcguffey verne c modeer jr victor a keith turner subsurface and exploration introduction angle of all geologic deposits pore pressures in water bearing strata depth to controlling features lope instability reflects soil rock ground probable vertical lateral limits sliding swater conditions that are hidden beneath the interpretation such data identifies quantifies surface although structures potential solutions for landslide movements strength properties earth materials can often be inferred from investigations classification also required obtain definitive methods samples explorations exhibit may classed as wide range cost order save time money direct indirect hunt programs should under test borings exca taken following site reconnaissance surficial vation pits allow examination mate investigation see chapters rials usually with removal before selection instrumentation chap geophysical surveys use ter follows an itera cone penetrometer provide measure tive process incorporates...

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