Environmental Engineering Reference
In-Depth Information
Evaluations and Analyses
Possible failure forms are predicted and existing failures are delineated as falls, slides, ava-
lanches, or flows, and the degree of the hazard is judged. Depending upon the degree of
risk, the decision is made to avoid the hazard or to eliminate or reduce it. For the cases of
falls, avalanches, flows, and failures by lateral spreading, the decision is based on experi-
ence and judgment. Slides may be evaluated by mathematical analysis, but in recognition
that movements may develop progressively.
Preliminary analysis of existing or potential failures by sliding includes the selection of
potential failure surfaces by geometry in the case of planar slides, or analytically in the case
of rotational slides, or by observation in the case of an existing slide. An evaluation is made
of the safety factor against total failure on the basis of existing topographic conditions, then
under conditions of the imposed cut or fill. For preliminary studies, shear strengths may be
estimated from published data, or measured by laboratory or
in situ
testing. In the selection
of the strength parameters, consideration is given to field conditions
(Table 9.6)
as well as
to changes that may occur with time (reduction from weathering, leaching, solution). Other
transient conditions also require consideration, especially if the safety factor for the entire
slope is low and could go below unity with some environmental change.
9.5.3
Detailed Study of Cut, Fill, or Failure Area
General
Detailed study of the area of the proposed cut or fill, or of the failure, is undertaken after
the stability of the entire slope is assessed. The entire slope is often erroneously neglected
in studies of cuts and side-hill fills, and is particularly important in mountainous terrain.
Explorations
Seismic refraction surveys
are most useful if rock is anticipated within the cut, and there are
boulders in the soils that make the delineation of bedrock difficult with test and core borings.
Test and core borings
, and
test pits
are made to recover samples, including undisturbed
samples, for laboratory testing. In colluvium, residuum, and saprolite, the best samples
are often recovered from test pits, but these are usually limited to depths of 10 to 15 ft (3 -
5 m) because of practical excavation considerations.
In situ testing
is performed in materials from which undisturbed samples are difficult or
impossible to procure.
Laboratory Testing
Laboratory strength testing should duplicate the field conditions of pore-water pressures,
drainage, load duration, and strain rate that are likely to exist as a consequence of con-
struction operations, and samples should usually be tested in a saturated condition. It
must be considered that conditions during and at the end of construction (short-term) will
be different than long-term stability conditions. In this regard, the natural ability of the
slope to drain during cutting plays a significant role.
Evaluation and Analysis
Sections illustrating the proposed cut, fill, or failure imposed on the slope are prepared at
a 1:1 scale. The selection of cut slope inclination is based on the engineer's judgment of sta-
bility and is shown on the section together with the stratigraphy, groundwater conditions
measured, and the soil properties as shown in
Figure 9.130.
