3. TEST TRENCHES. Test trenches are particularly useful for exploration in
very heterogeneous deposits such as rubble fills, where borings are either
meaningless or not feasible. They are also useful for detection of fault
traces in seismicity investigations.
1. APPLICATION. Disturbed samples are primarily used for classification
tests and must contain all of the constituents of the soil even though the
structure is disturbed. Undisturbed samples are taken primarily for
laboratory strength and compressibility tests and in those cases where the
in-place properties of the soil must be studied. Many offshore samplers
fall in a special category and are treated separately in this section.
2. GENERAL REQUIREMENTS FOR SAMPLING PROGRAM. The number and type of
samples to be taken depend on the stratification and material encountered.
a. Representative Disturbed Samples. Take representative disturbed
samples at vertical intervals of no less than 5 feet and at every change in
strata. Table 9 lists common types of samples for recovery of
representative disturbed soil samples. Recommended procedures for obtaining
disturbed samples are contained in ASTM Standard D1586, Penetration Test and
Split Barrel Sampling of Soils.
b. Undisturbed Samples. The number and spacing of undisturbed samples
depend on the anticipated design problems and the necessary testing program.
Undisturbed samples should comply with the following criteria: they
should contain no visible distortion of strata, or opening or softening of
materials; specific recovery ratio (length of undisturbed sample recovered
divided by length of sampling push) should exceed 95 percent; and they
should be taken with a sampler with an area ratio (annular cross-sectional
area of sampling tube divided by full area of outside diameter of sampler)
less than 15 percent. Table 10 lists common types of samplers used for
recovery of representative undisturbed samples.
Obtain undisturbed samples in cohesive soil strata, so that there is
at least one representative sample in each boring for each 10 feet depth.
Recommended procedures for obtaining undisturbed samples are described in
ASTM Standard D1587, Thin-Walled Tube Sampling of Soils. Additional
cautions include the following:
(1) Caving. Use casing or viscous drilling fluid to advance
borehole if there is danger of caving. If groundwater measurements are
planned, drilling fluid should be of the revert type.
(2) Above Groundwater Table. When sampling above groundwater table,
maintain borehole dry whenever possible.