Determining the Density and Unit Weight of Soil in Place by Nuclear Methods (Shallow Depth) - Soil Properties: Testing, Measurement, and Evaluation - page 227

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CHAPTER SIXTEEN

Determining the
Density and Unit
Weight of Soil in Place
by Nuclear Methods
(Shallow Depth)

(Referenced Document: ASTM D 2922)

As previously discussed in Chapter 14, after a fill layer of soil has been
placed by the contractor, the compacted soil's in-place dry unit weight
must be determined to ascertain whether the maximum laboratory dry
unit weight has been attained. If the maximum dry unit weight (or
an acceptable percentage thereof ) has not been attained, additional
compaction is required.
The sand-cone and rubber-balloon methods for determining density
and unit weight of soil in place were presented in Chapters 14 and 15,
respectively. Although widely used, these are destructive testing meth-
ods, in that a sizable hole must be dug in the ground or compacted fill.
They are also fairly time consuming, a significant factor when numer-
ous tests must be performed as quickly as possible at a construction site.
A nondestructive and relatively quick method for determining density and
unit weight of soil in place utilizes a nuclear apparatus (see Figure 16-1).
This apparatus contains a radioactive source and a radiation detector.
The detailed test procedure for this method is described later in this
chapter, but the basic premise of the test is that the nuclear apparatus,
when placed on the ground or compacted fill, emits gamma rays through
the soil. Some of the rays are absorbed; others reach the detector. The
amount of radiation reaching the detector varies inversely with soil unit
weight; thus, through proper calibration, nuclear count rates received at
the detector can be translated into values of soil (wet) density/unit weight.

INTRODUCTION

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