Civil Engineering Reference
In-Depth Information
FIGURE 5.9
Close-up view of trench excavation.
Cohesive Soils.
Although undisturbed cohesive soil samples can often be obtained dur-
ing the subsurface exploration, the usual approach in practice is to obtain the soil engi-
neering properties from standard laboratory tests. In terms of the undrained shear strength
of the soil, the unconfined compression test (ASTM D 2166) or the consolidated undrained
triaxial compression test (ASTM D 4767) is usually performed. Typically standard soil
sampling practices, such as the use of thin-walled Shelby tubes, are used to obtain undis-
turbed cohesive soil specimens (see Day 1999). Section 5.5.1 describes the interpretation
of this data for use in geotechnical earthquake engineering analyses.
Granular Soils.
There are three different methods that can be used to obtain undisturbed
soil specimens of granular soil (Poulos et al. 1985, Ishihara 1985, Hofmann et al. 2000):
1.
Tube sampling:
Highly sophisticated techniques can be employed to obtain undis-
turbed soil specimens from tube samplers. For example, a fixed-piston sampler consists of
a piston that is fixed at the bottom of the borehole by a rod that extends to the ground sur-
face. A thin-walled tube is then pushed into the ground past the piston, while the piston rod
is held fixed.
Another approach is to temporarily lower the groundwater table in the borehole and
allow the water to drain from the soil. The partially saturated soil will then be held
together by capillarity, which will enable the soil strata to be sampled. When brought to
the ground surface, the partially saturated soil specimen is frozen. Because the soil is
only partially saturated, the volume increase of water as it freezes should not signifi-
cantly disturb the soil structure. The frozen soil specimen is then transported to the lab-
oratory for testing.
