Determining the Density and Unit Weight of Soil in Place by Nuclear Methods (Shallow Depth) - Soil Properties: Testing, Measurement, and Evaluation - page 230

This test method presents serious potential hazards. As noted previously,
the equipment used in the test contains radioactive material that may be
hazardous to the user's health. No one should approach this equipment or
attempt to use it without proper training. Users should be thoroughly
familiar with safety procedures and government regulations. Additionally,
safety procedures, such as proper storage of the equipment, testing for
leaks, and recording, evaluating, and monitoring personal radioactive
badge data, should be routinely and rigorously followed. (Many states
require certification of operators and periodic inspection of equipment.)

HAZARDS

Ordinarily, manufacturers of the nuclear apparatus used in this test will
provide applicable calibration curves or tables. These should be verified
every year or so and after any significant equipment repairs. Results of
rubber-balloon tests or sand-cone tests may be used as a basis for
comparison. More specific calibration instructions are presented in
ASTM D 2922 in its Annex.

CALIBRATION

(1) The chemical composition of the sample may affect the mea-
surement, and adjustments may be necessary.

INTERFERENCES
[1]

(2) The test methods exhibit spatial bias in that the instrument is
more sensitive to the density of the material in close proximity to
the surface (Backscatter Method only).

Note 1 —The nuclear gage density measurements are some-
what biased to the surface layers of the soil being tested. This
bias has largely been corrected out of the direct transmission
method, and any remaining bias is insignificant. The backscat-
ter method is still more sensitive to the material within the
first several inches from the surface.

(3) Oversize rocks or large voids in the source-detector path may
cause higher or lower density determination. Where lack of unifor-
mity in the soil due to layering, rock, or voids is suspected, the test
volume site should be dug up and visually examined to determine if
the test material is representative of the full material in general
and if rock correction [see (6) in the “Procedure” section] is required.
(4) The sample volume is approximately 0.0028 m 3 (0.10 ft 3 ) for the
Backscatter Method and 0.0057 m 3 (0.20 ft 3 ) for the direct trans-
mission method when the test depth is 15 cm (6 in.). The actual
sample volume is indeterminate and varies with the apparatus and
the density of the material. In general, the higher the density the
smaller the volume.

(1) Nuclear gages are subject to long-term aging of the radioactive
source, detectors, and electronic systems, which may change the
relationship between count rate and material density. To offset this
aging, the gage may be calibrated as the ratio of the measured

STANDARDIZATION
AND REFERENCE
CHECK [1]