Environmental Engineering Reference
In-Depth Information
2.4.2
Test Boring Soil Sampling
Types Commonly Used
The split-barrel sampler (SS) is used in all soil types for
representative samples
used for iden-
tification and index tests.
Thin-wall tubes
are used in soft to firm clays and
coring samplers
are used in stiff to hard
cohesive soils for undisturbed samples used for engineering properties tests.
Required Boring Diameters
Wash or exploratory borings for split-barrel sampling are normally of 2½ in. diameter (cas-
ing I.D.). UC borings are normally of 4 in. diameter, but may be larger to improve sample
quality. Core borings vary from 2 in. to larger, normally with NX core taken in a hole
started with 4 in. diameter casing.
Sampling Interval
Samples are normally prescribed for 5 ft (or 1 m) intervals and a change in strata. Samples
should also be taken at the surface to record the topsoil thickness, and continuously from
the surface to below the depth of shallow foundations to assure information at footing
depth (5 ft intervals often do not provide information at shallow footing elevations).
Continuous sampling is also important through miscellaneous fills that vary widely in
materials and that often overlie a layer of organic soil, which may be thin but significant,
and through formations with highly variable strata. In deep borings, sampling depths are
often changed to 10 or 20 ft intervals after several normally sampled borings are com-
pleted and general subsurface conditions defined.
Factors Affecting Sample Quality
Sampler Wall Thickness
A large outside diameter relative to the inside diameter causes deformation by material
displacement.
Sampler Conditions
Dull, bent, or otherwise deformed cutting edges on the sampler cause sample
deformation.
●
Inside friction, increased by rust, dirt, or, in the case of tubes, omission of lacquer,
causes distortions which are evidenced by a turning downward of layers, result-
ing in conical shapes under extreme cases. A very slight edge disturbance is illus-
trated in
Figure 7.92.
●
Boring Operations
Dynamic forces caused by driving casing can loosen dense granular soils or den-
sify loose granular soils.
●
Sands may rise in the casing when below the GWL (
Figure 2.60)
.
●
Overwashing, jetting, and high fluid pressures also loosen granular soils or
soften cohesive materials (Figure 2.60).
●
Coarse materials often remain in the hole after washing, particularly in cased
borings (Figure 2.60). These “cuttings” should be removed by driving a split bar-
rel sampler, by pushing a Shelby tube, or with a cleanout auger. Contamination
is common after boring through gravel layers or miscellaneous fills containing
cinders, etc.
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