Environmental Engineering Reference
In-Depth Information
Field Reconnaissance
The region or site location is visited and notations are made regarding seepage points,
vegetation, creep indications, tension cracks, failure scars, hummocky ground, natural
slope inclinations, and exposed geology. The data collected during landform analysis pro-
vide a guide as to the more significant areas to be examined.
Preliminary Evaluations
From the data collected, preliminary evaluations are made regarding slope conditions in
the region or project study area, the preliminary engineering geology and hazard maps are
modified, and an exploration program is planned for areas of particular interest.
Explorations
Geophysical Surveys
Seismic refraction profiling
is performed to determine the depth to sound rock and the prob-
able groundwater table, and is most useful in differentiating between colluvial or residual
soils and the fractured-rock zone. Typical seismic velocities from the weathering profile
that develops in igneous and metamorphic rocks in warm, humid climates are given in
Table 2.10
and
Figure 9.100.
Surveys are made both longitudinal and transverse to the
slope. They are particularly valuable on steep slopes with a deep weathering profile where
test borings are time-consuming and costly.
Resistivity profiling
is performed to determine the depth to groundwater and to rock.
Profiling is generally only applicable to depths of about 15-30 ft (5-10 m), but very useful
in areas of difficult access. In the soft, sensitive clays of Sweden, the failure surface or
potential failure surface is often located by resistivity measurements since the salt content,
and therefore the resistivity, often changes suddenly at the slip surface (Broms, 1975).
Test Boring Program
Test borings are made to confirm the stratigraphy determined by the geophysical explo-
rations, to recover samples of the various materials, and to provide holes for the installa-
tion of instrumentation. The depth and number of borings depend on the stratigraphy and
uniformity of conditions, but where the slope consists of colluvial or residual soils, borings
should penetrate to rock. In other conditions, the borings should extend below the depth of
any potential failure surface, and always below the depth of cut for an adequate distance.
Sampling should be continuous through the potential or existing rupture zone, and in
residual soils and rock masses care should be taken to identify slickensided surfaces.
Groundwater conditions must be defined carefully, although the conditions existing at the
time of investigation are not likely to be those during failure.
In Situ Measurements
Piezometers
yield particularly useful information if in place during the wet season. In
clayey residual profiles, confined water-table conditions can be expected in the weathered
or fractured rock zone near the interface with the residual soils, or beneath colluvium. A
piezometer set into fractured rock under these conditions may disclose artesian pressures
exceeding the hydraulic head given by piezometers set into the overlying soils, even when
they are saturated
(Figure 9.130).
Instrumentation
is installed to monitor surface deformations, to measure movement
rates, and to detect the rupture zone if the slope is considered to be potentially unstable or
is undergoing movement.
