Environmental Engineering Reference
In-Depth Information
The preliminary determination of geologic conditions through literature search, landform
analysis, and field reconnaissance is included. Landform analysis is stressed throughout this
volume because of its high value in establishing geologic conditions, in particular in identi-
fying those that are hazardous and in classifying geologic materials based on their origin and
mode of occurrence. When these latter factors are established, it is possible to reach prelimi-
nary conclusions on the engineering characteristics of the various formations.
Landform analysis is based on the interpretation of geologic and topographic maps,
such as the USGS quad sheets, and remotely sensed imagery, be it in the form of satellite,
SLAR, or aerial photographs. For moderate to large land areas, the preparation of an engi-
neering geology map, which depicts surface and shallow subsurface conditions of rock,
soil, water, and geologic hazards, is useful for preliminary site planning and the thorough
programming of the field investigation.
Subsurface sectioning is accomplished by geophysical methods, test borings, and vari-
ous reconnaissance methods involving test pits, augers, and so on. Information over large
areas and areas that are difficult to access is obtained efficiently by means of geophysical
techniques, which can include refraction seismology and electrical methods (on land), and
refraction and reflection profiling (in water). Other techniques employed occasionally are
surveys with gravimeters, magnetometers, and ground-probing radar. There are a number
of simple and economical methods of exploring shallow depths, and a number of proce-
dures for performing test borings on land and in water. The static cone penetrometer, long
used in Europe, is finding increasing application in the United States. Various types of
remote-sensing equipment such as cameras and geophysical methods are available for
providing a continuous log of borehole conditions, particularly in rock masses. Nuclear
probes provide data on
in situ
water contents and densities. Determining groundwater
conditions is an important aspect of investigation, and reliable data are obtained only by
using proper procedures.
The recovery of samples of the geologic materials for identification and laboratory test-
ing is a major objective of the exploration program. A wide variety of tools are available,
their selection depending on the type of material to be sampled and the use to which the
sample will be put. Sample quality depends on extraction techniques, particularly when
one is sampling soils or coring rock.
Measurement of Properties
(Chapter 3
)
The basic and index properties, used for the identification and correlation of engineering
properties, such as permeability, strength, and deformation, are obtained for soil and rock
both in the field and in the laboratory. When representative undisturbed samples can be
obtained, such as from soft to stiff intact clays, their properties are measured in the labo-
ratory. However, there are many materials from which undisturbed samples are difficult
or impossible to obtain, such as clean cohesionless sands, residual soils, glacial till, and
soft or heavily jointed rock masses. For these materials, properties are best measured
in
situ
. Soft to firm clays and hard-fissured clays are also tested
in situ
.
Permeability measurements in soils and rock masses are best made by field tests
because of the mass effects of stratification and joints, but numerous correlative data exist,
particularly for soils, to permit estimates that are adequate for many studies.
Shear strength and deformation characteristics are measured by
in situ
testing with
either full-scale load tests on the surface, load tests in pits or tunnels, or load tests with
instruments lowered into boreholes. Geophysical methods are used to obtain measures of
dynamic properties, and the field data are correlated with laboratory dynamic testing data
to obtain deformation moduli for design. Dynamic testing is also important for evaluating
rock-mass quality.
