TM 5-818-4/AFM 88-5, Chap. 5
even a slight adjustment is not usually justified. A
D 2922 are Method A-Backscatter, Method B-Direct
Transmission, and Method C-Air Gap. Of the three
more satisfactory procedure is to record the field
measurement in terms of percent of this standard
methods, Method B-Direct Transmission is recom-
count rate, which should be within a reasonable per-
mended over Method A and Method C because it elim-
centage ( 5) of the given reference count. Use of the
inates the effect of vertical density variations.
(a) Modern nuclear-moisture density equipment
percent of standard count, rather than simply the
counts per minute, is recommended for increased accu-
incorporates a radioactive source emitting neutrons
racy. Use of this procedure largely cancels out the
and gamma rays and measuring elements (geiger
effects of such variables as reduction in source
tubes) or "scalers" into a single, self-contained unit.
strength, background count, and changes in sensitivity
The determination of moisture by the nuclear method
of the detector tubes.
is dependent on the modifying of high energy or "fast"
(d) The calibration curve for the soil being
neutrons into low energy or "slow" neutrons (ASTM,
tested is entered with the value of the density meter
STP No. 293). Any material containing hydrogen will
count rate (taking into consideration the variation
moderate fast neutrons. Since hydrogen is present pri-
from the standard count) to obtain the wet unit weight
marily in the molecules of free water, the degree of
of the soil. Similarly, the moisture meter yields the
weight of water per cubic foot of soil. The unit dry
atoms represents a measure of the water content of the
weight of the soil is simply the wet unit weight ob-
soil, Density measurements are based on the scattering
tained by the density meter minus the weight of water
of gamma rays by the orbital electrons on the atoms
obtained by the moisture meter. By dividing the water
comprising the soil. Since the scattering is a function
of the electron density, which in turn is approximately
measurement by the dry density, the water content
can be expressed in the more familiar terms of percent-
proportional to the density of the soil, it is possible to
age of dry weight.
correlate the backscatter of the gamma rays with the
(e) Anyone working with nuclear meters must
soil density.
(b) To obtain a water content or density meas-
recognize that a possibility of exposure to radiation ex-
ists if the safety rules listed by the manufacturer are
urement, the appropriate meter is set in place and the
not followed. When proper procedures and safety rules
voltage setting is adjusted to the correct operating
are followed, the radiation hazard is negligible. For
voltage. After the scaler is turned on, a short warmup
certain instruments, operating personnel must wear a
period (not exceeding 1 minute) is allowed before the
body radiation film badge and carry a pocket dosim-
test count is started. Intimate contact at the interface
eter. These instruments must be ready weekly to en-
between meter and soil is necessary for Method A-
sure that the maximum permissible weekly dosage is
Backscatter because the scattering of the gamma rays
for the density measurement is quite sensitive to even
less than 100 milliroentgen. Other safety rules deal
minute air gaps. The normal counting period is 1
with handling the devices and being aware of the built-
minute, with one or two repeat counts taken as a
in safety devices. The safety precautions mentioned
check. Calibration curves for both moisture and densi-
above may vary or not be applicable for some of the
ty determinations, once the count rates have been es-
newer devices being manufactured. Therefore, the
tablished, are furnished by the manufacturers for each
manufacturer's literature should be carefully studied to
individual unit. In general, the calibration curve for
determine appropriate safety requirements.
moisture determination is more reliable than the curve
(f) It is possible, using nuclear-moisture density
for density determination. However, it is advisable to
apparatus, for one inspector to conduct perhaps 30 wa-
correlate both calibration curves on each type of soil
ter content and 30 density tests per 8-hour working
with which the instrument is to be used. Such a corre-
day. The time required per test is only 20 or 25 percent
lation should be accomplished by using current stand-
of that required in direct sampling methods. A large
ard methods for moisture and density determinations
number of tests with the nuclear meter correlated with
or by calibrating on blocks of material of known mois-
a much smaller number of direct sampling determina-
ture and density. Examples of calibration for shale
tions can be of great benefit in ensuring that adequate
materials are given in Appendix A of FHWA-
compaction of the backfill is being obtained. A simple
RD-78-141.
statistical analysis of the data can be made, such as a
(c) For all nuclear-moisture density devices,
plot of dry density versus number of tests (ASTM STP
separate standards are provided so that the count rate
No. 293). The resulting bell-shaped curve is a very use-
can be determined on each instrument at any time in
ful tool since each day's results can easily be added to
the plot of previous test results. This procedure can
the field. A standard count should be taken three or
four times during a day's operation. Although adjust-
provide an up-to-date picture of the fill densities being
ments can generally be made on the instruments so
obtained and can show the effect of changes made in
that the count will coincide with the standard count;
field compaction procedures.
B-9
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