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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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