TM 5-818-4/AFM 88-5, Chap. 5
same sand occupying an in-place volume of sampled
tion curves may also be induced by differences in the
material. Changes in effective gradation between or
seating, background count, sample heterogeneity, and
within batches of sand may significantly affect the
surface texture of the material being tested.
test results. This error can be minimized by frequent
B-8. Rapid field water content control
calibration of the sand's bulk density.
procedures. In many cases, particularly in con-
(b) Loose sand increases in density when sub-
fined zones, it is important to rapidly determine the
jected to vibrations. Care must be taken not to jar the
dry density of a given part of the backfill in order to
sand container while calibrating bulk density in the
prevent the possibility of costly tear out and rebuild
laboratory or during in-place volume measurements in
operations. The test procedures for determining dry
the field. A common error is to use the sand cone meth-
densities using the sand-cone and rubber-balloon
od for in-place volume measurements adjacent to the
methods sometimes require extensive drying times
operation of heavy equipment. Heavy equipment can
(depend on material type up to 16 hours) to determine
generate vibrations that densify the sand and result in
water content. Alternate techniques for rapidly deter-
erroneously high-volume measurements and low in-
mining water content are discussed below.
place densities.
(c) Appreciable time intervals between bulk
a. Microwave ovens. Microwave energy may be used
density determination of the sand and its use in the
to dry soil rapidly and thus enable quick determination
field may result in change in the bulk density caused
of water content (ASTM STP No. 599). However, in
by a change in the moisture content of the sand.
drying soils with microwaves, the only control on the
(2) Rubber-balloon method. The major sources of
amount of energy absorbed by the soil is exposure
possible error are as follows:
time; consequently, if soils are left in the oven too
(a) New rubber-balloon volumeters should be
long, severe overheating can occur. This overheating
calibrated against several known volumes of different
of the soil can cause bound water, a part of the soil
sizes covering the volume range of in-place measure-
structure, to be driven off and thus result in signifi-
ments.
cant errors in water content measurements. In addi-
(b) For stiff soils such as clay, it is possible to
tion, continuous heating can result in excessive heat
trap air between the sides of the sample hole and bal-
being generated; certain soils have been observed to
loon. This error can be minimized by placing lengths of
fuse or explode and thereby create hazards to both
small-diameter string over the edge of the hole and
equipment and personnel.
down the inside wall slightly beyond the bottom cen-
(1) Times required for drying in a microwave oven
ter.
are primarily governed by the mass of water present
(c) The application of the 2- to 3-pounds-per-
and the power-load output of the oven, as expressed by
square-inch pressure to extend the balloon into exist-
ing irregularities in the hole will cause a noticeable up-
ward force on the volumeter. Care must be taken to en-
sure that the volumeter remains in intimate contact
where
with the base plate.
T
= time in the microwave oven, seconds
MW = mass of water present in the soil-water
checked for leaks.
mixture, grams
(3) Nuclear moisture-density method. The major
w = water content of the specimen
sources of possible error are as follows:
= initial temperature of the soil-water mass,
(a) The single consistent source of error is re-
degree Centigrade
lated to the accuracy of the system. The overall system
P
= power output of the oven, watts
accuracy in determining densities is statistical in na-
This governing equation indicates that in order to pre-
ture and appears to vary with the equipment used, test
dict accurately the drying times required, an estimate
conditions, materials tested, and operators. If proper
of the specimen water content must be made and the
procedures are followed, the standard deviations in
oven power versus load relationship must be estab-
terms of accuracy will vary on the order of 3 to 5
lished by calibration.
pounds per cubic foot for density tests and 0.5 to 1.0
(2) The limitation of having to estimate the initial
pound of water per cubic foot of material for water
water content of the specimen is not insurmountable.
content tests.
Test results indicate that slight overestimations of the
actual water content, i.e., longer drying times, general-
each piece of equipment. Due to the effects of differing
ly result in small differences between conventional
chemical compositions, calibration curves may not be
oven and microwave oven water contents. Conversely,
applicable to materials not represented in establishing
underestimations of water content result in more seri-
the calibration curve. Apparent variations in calibra-
ous errors. If an accurate estimate of water content
B-11
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