Determining the Density and Unit Weight of Soil in Place by Nuclear Methods (Shallow Depth) - Soil Properties: Testing, Measurement, and Evaluation - page 232

density becomes suspect during the day's use, perform another
standardization check.

As indicated previously, the general procedure for determining (wet)
density/unit weight of soil in place by nuclear methods is carried out by
placing a nuclear apparatus on the ground or compacted fill and causing
it to emit gamma rays through the soil. Some of the rays will be
absorbed; others will reach a detector. Through proper calibration, nuclear
count rates received at the detector can be translated into values of
(wet) density/unit weight.
The actual step-by-step procedure is as follows (ASTM D 2922-05 [1]):

PROCEDURE

(1) Standardize the gage. (See the “Standardization and Reference
Check” section.)

(2) Select a test location. If the gage will be closer than 250 mm
(10 in.) to any vertical mass that might influence the result, such
as in a trench or alongside a pipe, follow the manufacturer's
correction procedure.

(3) Remove all loose and disturbed material. Remove additional
material as necessary to reach the material that represents a valid
sample of the zone or stratum to be tested. Surface drying and spa-
tial bias should be considered in determining the depth of material
to be removed.

(4) Plane or scrape a smooth horizontal surface so as to obtain
maximum contact between the gage and the material being tested.
The placement of the gage on the surface of the material to be
tested is always important, but is especially critical to the success-
ful determination of density when using the backscatter method.
The optimum condition in all cases is total contact between the bot-
tom surface of the gage and the surface of the material being tested.
To correct for surface irregularities, use of native fines or fine sand
as a filler may be necessary. The depth of the filler should not ex-
ceed approximately 3 mm ( 1 ⁄ 8 in.), and the total area filled should
not exceed 10% of the bottom area of the instrument. The maxi-
mum depth of any void beneath the gage that can be tolerated with-
out filling shall not exceed approximately 3 mm ( 1 ⁄ 8 in.). Several trial
seatings may be required to achieve these conditions.

(5) Proceed with the test in the following manner:

(5.1) Backscatter Procedure:

(5.1.1) Seat the gage firmly on the prepared test site.

(5.1.2) Keep all other radioactive sources away from the gage to
avoid affecting the measurement so as not to affect the readings.

(5.1.3) Secure and record one or more readings for the normal
measurement period in the backscatter position.