Environmental Engineering Reference
In-Depth Information
rapid. If precipitation rates exceed infiltration rates,
the
exist, in coarse sediments, in areas with extensive roots,
below the water table, or in areas known to have under-
ground utilities. For these reasons, a hand auger should be
used with caution.
water table can rise to intersect land surface.
Infiltration tests can be conducted readily in the field.
For example, single- or double-ring infiltrometers can be
installed; water is added to the infiltrometer, and the rate of
water-level drop is measured to determine the rate of infil-
tration. Alternatively, a jug or carboy filled with water and
turned on end in a collar of PVC three-way pipe can be used.
A hole typically is dug 2-3 ft (0.6-0.9 m) deep above the
water table, and 1-3 in. (2.5-7.6 cm) of gravel is added at
the bottom to reduce turbidity when the water is added, and
the rate of water decrease can be measured with a stopwatch.
A quick assessment of infiltration also can be accom-
plished by conducting a percolation test. A shallow hole is
dug and the soil removed. A volume of water, such as 1 gal
(3.7 L), is added to the hole and allowed to drain. The time to
completely drain is recorded. Drainage is faster for sands
and gravels relative to clays and silts. This also provides
evidence of the ability for air to mix in different soil
types, determined by the presence of bubbles in the water-
filled hole.
6.5.4 Hollow-Stem Auger Method
A hollow-stem auger is essentially a large hand-auger bucket
that is advanced into the soil by the power of an engine rather
than manually and is used for deeper drilling than possible
with a hand auger. The hollow-stem auger method also is
called the rotary drilling method. Typical auger lengths are
4-8 ft (1.2-2.4 m); holes can be drilled deeper by attaching
separate augers together. Similar to the hand-auger method,
soil samples are compromised by substantial disturbance of
the retrieved sample. This can be overcome, however, by
attaching a hollow metal tube called a Shelby tube (after its
inventor), or by attaching a metal tube that can be split into
two sections, called a split-spoon sampler, to the end of the
first auger. A benefit of using the hollow-stem auger method
is that a monitoring well can be constructed and installed
within the hollow stem of the auger after the sediment has
been removed from the borehole and the auger sections then
removed.
Boreholes also can be created in sediments by using air
under high pressure. Called air-rotary drilling, high-pressure
air from the end of a pipe is used to blow unconsolidated soil
particles out of an advancing borehole and to bring them to
land surface. This method does not permit the collection of
intact soil samples, may cause volatile compounds in the
unsaturated zone to be transported greater distances by the
air pressure near the borehole, and presents an air-quality
risk to drilling personnel.
6.5.3 Hand-Auger Method
A rapid method used to collect sediment samples, delineate
the extent of soil contamination, and to describe the
hydrogeologic properties of sediments at a site is the hand-
auger method as described in Chapelle (1993). Frequently
overlooked or undervalued because of its simplicity, a hand
auger should be considered an essential component of any
site investigator's toolbox. A hand auger consists of a metal
cylinder that either is completely enclosed or contains
openings that can be used when clayey soil is encountered.
Attached at one end of the cylinder, or bucket, are two
cutting blades that help the bucket to advance into the soil.
After advancing the bucket to a desired depth, the auger is
removed from the hole and emptied of its contents. Interest-
ingly, perhaps this design was first employed by Bernard
Palissy, whom we met in Chap. 2, as he examined the soils in
France in the sixteenth century (Darcy 1856).
The hand-auger method also can be used to reveal the
thickness of the capillary fringe, even if wells are present,
because the sediment samples removed from the bucket can
be examined for moisture content. Moreover, the approxi-
mate extent and thickness of the capillary fringe can be
denoted by an increase in difficulty in removing the auger
from the borehole as a result of water held under tension in
the pore spaces.
The hand-auger method has some limitations for site
assessment, however. One is that the sediment sample is
disturbed during collection. Also, the method is not applica-
ble in paved areas, in areas where subsurface obstructions
6.5.5 Sonic Technology
Sonic drilling methods advance the drill rod into the subsur-
face by agitation, or mechanical vibration, rather than with
mechanical rotary action. The hollow drill rods are vibrated
at 50-150 Hz (hertz), or cycles per second. The hollow rods
will retain the undisturbed sediment, which can be analyzed
for aquifer properties or contaminant concentrations. Sonic,
or vibracore, methods are suitable to unconsolidated aquifers
although the total drilling depth may be less than with other
methods.
6.5.6 Direct-Push Technology
Direct-push technology is based on using an engine-driven
hydraulic pump to hammer a hollow rod with a solid tip into
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