tions are known to be uniform. Additional borings
Personnel should be well trained to expedite proper
should be made as required by the extent of the area,
sampling, sealing, and storage in sample containers.
the location of deep foundations such as drilled shafts,
(1) Disturbed sampling. Disturbed auger, pit, or
and the encountered soil conditions.
split spoon samplers may be useful to roughly identify
(2) The depth of sampling should be at least as
the soil for qualitative estimates of the potential for
deep as the probable depth to which moisture changes
soil volume change (para 4-1). The water content of
and heave may occur. This depth is called the depth of
these samples should not be altered artificially during
boring, for example, by pouring water down the hole
down about 10 to 20 feet below the base of the founda-
during augering.
tion or to the depth of shallow water tables, but it may
(2) Undisturbed sampling. Minimization of sam-
be deeper (para 5-4c). A shallow water table is defined
ple disturbance during and after drilling is important
as less than 20 feet below the ground surface or below
to the usefulness of undisturbed samples. This fact is
the base of the proposed foundation. The entire thick-
particularly true for expansive soils since small
ness of intensely jointed or fissured clays and shales
changes in water content or soil structure will signifi-
should be sampled until the groundwater level is en-
cantly affect the measured swelling properties.
countered because the entire zone could swell, provid-
(a) The sample should be taken as soon as pos-
ed swelling pressures are sufficiently high, when given
sible, after advancing the hole to the proper depth and
access to moisture. Continuous sampling is required
cleaning out the hole, to minimize swelling or plastic
for the depth range within the active zone for heave.
deformation of the soil to be sampled.
(3) Sampling should extend well below the antici-
(b) The samples should be obtained using a push
pated base of the foundation and into strata of ade-
tube sampler without drilling fluid, if possible, to
minimize changes in the sample water content. Drill-
quate bearing capacity. In general, sampling should
continue down to depths of 1.5 times the minimum
ing fluids tend to increase the natural water content
width of slab foundations to a maximum of 100 feet
near the perimeter of the soil sample, particularly for
and a minimum of three base diameters beneath the
fissured soil.
base of shaft foundations. The presence of a weak,
(c) A piston Denisen or other sampler with a
compressible, or expansive stratum within the stress
cutting edge that precedes the rotating outer tube into
field exerted by the entire foundation should be de-
the formation is preferred, if drilling fluid is neces-
tected and analyzed to avoid unexpected differential
sary, to minimize contamination of the soil sample by
movement caused by long-term volume changes in this
the fluid.
stratum. Sampling should continue at least 20 feet be-
e. Storage of samples. Samples should be immedi-
neath the base of the proposed foundation. Determi-
ately processed and sealed following removal from the
nation of the shear strength and stress/strain behavior
boring hole to minimize changes in water content.
of each soil stratum down to depths of approximately
Each container should be clearly labeled and stored un-
100 feet below the foundation is useful if numerical
der conditions that minimize large temperature and
analysis by the finite element method is considered.
humidity variations. A humid room with relative
humidity greater than 95 percent is recommended for
c. Time of sampling. Sampling may be done when
storage since the relative humidity of most natural
soil moisture is expected to be similar to that during
soils exceeds 95 percent.
construction. However, a design that must be adequate
(1) Disturbed samples. Auger, pit, or other dis-
for severe changes in climate, such as exposure to peri-
turbed samples should be thoroughly sealed in water-
ods of drought and heavy rainfall, should be based on
proof containers so that the natural water content can
maximum levels of potential soil heave. Maximum po-
be accurately measured.
tential heaves are determined from swell tests using
(2) Undisturbed samples. Undisturbed samples
soils sampled near the end of the dry season, which of-
may be stored in the sampling tubes or extruded and
ten occurs toward the end of summer or early fall.
preserved, then stored. Storage in the sampling tube is
Heave of the foundation soil tends to be less if samples
not recommended for swelling soils even though stress
are taken or the foundation is placed following the wet
relief may be minimal, The influence of rust and pene-
season, which often occurs during spring.
tration of drilling fluid or free water into the sample
during sampling may adversely influence the labora-
d. Sampling techniques. The disturbed samples and
tory test results and reduce the indicated potential
the relatively undisturbed samples that provide mini-
heave. Iron diffusing from steel tubes into the soil
mal disturbance suitable for certain laboratory soil
sample will combine with oxygen and water to form
tests may be obtained by the methods described in ta-
rust. Slight changes in Atterberg limits, erosion resist-
ble 3-2. Drilling equipment should be well maintained
ance, water content, and other physical properties may
during sampling to avoid equipment failures, which
occur. In addition, the outer perimeter of a soil sample
cause delays and can contribute to sample disturbance.
3-3
Previous Page
