Determining the Density and Unit Weight of Soil in Place by Nuclear Methods (Shallow Depth) - Soil Properties: Testing, Measurement, and Evaluation - page 234

(6) If the volume tested as defined in (4) in the “Interferences” section
has excess oversize material with respect to the limitations in the ap-
propriate Test Methods D 698, D 1557, or D 4253, then a correction
for wet density (unit weight) and water content must be applied. This
correction will be done in accordance with Practice D 4718. This test
method requires sampling from the actual test volume.

(6.1) If samples of the measured material are to be taken for pur-
poses of correlation with other test methods or rock correction, the
volume measured can be approximated by a 200-mm (8-in.) diame-
ter cylinder located directly under the center line of the radioactive
source and detector(s). The height of the cylinder to be excavated will
be the depth setting of the source rod when using the direct trans-
mission method or approximately 75 mm (3 in.) when using the
backscatter method.

(6.2) An alternative to the correction for oversize particles that can
be used with mass density methods or minimal oversize situations
involves multiple tests. Tests may be taken at adjacent locations
and the results averaged to get a representative value. Comparisons
need to be made to evaluate whether the presence of a single large
rock or void in the soil is producing unrepresentative values of den-
sity. Whenever values obtained are questionable, the test volume
site should be dug up and visually examined.

The density/unit weight determined by nuclear methods is the in-place,
wet density/unit weight. If dry unit weight is needed, as is usually the
case, the soil's moisture content can be determined by the conventional
oven method (Chapter 4), the microwave oven method (Chapter 5), or a
calcium carbide gas pressure tester (Chapter 13); the dry unit weight
can then be found using Eq. (16-2):

CALCULATIONS

g wet

w + 100 * 100

(16-2)

g d =

where:

d dry unit weight

wet wet unit weight

w water content, as a percent of the dry mass.

Alternatively, a nuclear instrument that determines moisture content
by neutron thermalization may be used to find the soil's moisture con-
tent. If this method (ASTM Test Method D 3017) is used, dry unit weight
is computed simply by subtracting the lb/ft 3 of moisture found from the
lb/ft 3 of wet unit weight.
Some models of the nuclear density device give readouts not only of
wet density/unit weight but also of moisture content and dry
density/unit weight, in which case a separate moisture content deter-
mination is not needed.