TM 5-818-7
stored in the sampling tube cannot be scraped to re-
completely immersed and covered with the molten
move soil contaminated by water that may have pene-
wax, and then allowed to cool before moving.
trated into the perimeter of the sample during sam-
(e) When the samples are being transported,
pling. The sample may also later adhere to the tube
they should be protected from rough rides and bumps
wall because of rust. If samples are stored in tubes, the
to minimize further sample disturbance.
tubes should be brass or lacquered inside to inhibit cor-
f. Inspection. A competent inspector or engineer
rosion. An expanding packer with a rubber O-ring in
should accurately and visually classify materials as
both ends of the tube should be used to minimize mois-
they are recovered from the boring. Adequate classifi-
ture loss. The following procedures should be followed
cation ensures the proper selection of samples for lab-
in the care and storage of extruded samples.
oratory tests. A qualified engineering geologist or
(a) Expansive soil samples that are to be ex-
foundation engineer should closely monitor the drill
trubed and stored should be removed from the sam-
crew so that timely adjustments can be made during
pling tubes immediately after sampling and thorough-
drilling to obtain the best and most representative
ly sealed to minimize further stress relief and moisture
samples. The inspector should also see that all open
loss. The sample should be extruded from the sampling
borehoes are filled and sealed with a proper grout,
tube in the same direction when sampled to minimize
such as a mixture of 12 percent bentonite and 88 per-
further sample disturbance.
cent cement, to minimize penetration of surface water
(b) Samples extruded from tubes that were ob-
or water from a perched water table into deeper strata
tained with slurry drilling techniques should be wiped
that might include moisture deficient expansive clays.
clean to remove drilling fluid adhering to the surface
of the sample prior to sealing in the storage con-
3-4. Groundwater
tainers. An outer layer of 1/8 to 1/4 inch should be
Meaningful groundwater conditions and engineering
trimmed from the cylindrical surface of the samples so
properties of subsurface materials can often best be
that moisture from the slurry will not penetrate into
determined from in situ tests. In situ tests, however,
the sample and alter the soil swelling potential and
are not always amenable to simple interpretation. The
strength. Trimming will also remove some disturbance
pore water conditions at the time of the test may differ
at the perimeter due to sidewall friction. The outer
appreciably from those existing at the time of con-
perimeter of the soil sample should also be trimmed
struction. A knowledge of groundwater and the nega-
away during preparation of specimens for laboratory
tive pore water pressure are important in evaluating
tests.
the behavior of a foundation, particularly in expansive
(c) Containers for storage of extruded samples
soil. Every effort should be made to determine the po-
may be either cardboard or metal and should be
sition of the groundwater level, its seasonal variation,
approximately 1 inch greater in diameter and 1.5 to 2
and the effect of tides, adjacent rivers, or canals on it.
inches greater in length than the sample to be encased.
Three-ply, wax-coated cardboard tubes with metal bot-
liable and frequently the only satisfactory method for
toms are available in various diameters and lengths
determining groundwater levels and positive pore
and may be cut to desired lengths.
water pressures is by piezometers with tips installed at
(d) Soil samples preserved in cardboard tubes
different depths. Ceramic porous tube piezometers
should be completely sealed in wax. The wax and card-
with small diameters (3/8-inch) risers are usually ade-
board containers provide an excellent seal against
quate, and they are relatively simple, inexpensive, and
moisture loss and give sufficient confinement to mini-
sufficient for soils of low permeability.
mize stress relief and particle reorientation. A good
wax for sealing expansive soils consists of a 1 to 1 mix-
b. Measurement of in situ negative pore water pres-
ture of paraffin and microcrystalline wax or 100 per-
sure, Successful in situ measurements of negative pore
cent beeswax. These mixtures adequately seal the sam-
water pressure and soil suction have been performed
ple and do not become brittle when cold. The temper-
by such devices as tensiometers, negative pore pres-
ature of the wax should be approximately 20 degrees
sure piezometers, gypsum blocks, and thermocouple
Fahrenheit above the melting point when applied to
psychrometer. However, each of these devices has
the soil sample, since wax that is too hot will penetrate
certain limitations, The range of tensiometers and
pores and cracks in the sample and render it useless, as
negative pore pressure piezometers has been limited to
well as dry the sample. Aluminum foil or plastic wrap
the cavitation stress of water under normal conditions,
may be placed around the sample to prevent penetra-
which is near one atmosphere of negative pressure.
tion of molten wax into open fissures. A small amount
The fluid-filled tensiometer is restricted to shallow
of wax (about 0.5-inch thickness) should be placed in
soils less than 6 feet in depth. The useable range of the
the bottom of the tube and allowed to partly congeal.
tensiometer is reduced in proportion to the pressure
The sample should subsequently be placed in the tube,
exerted by the column of fluid in the tensiometer. Gyp-
3-5
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