Page 307 - Geotechnical Engineering Soil and Foundation Principles and Practice

Compaction
302 Geotechnical Engineering

sand. Details of these tests will be found in laboratory testing manuals or standard
methods of the ASTM.

Another method involves driving a thin-walled steel tube into the soil and
measuring the weight and volume of soil extracted. This is considered less accurate
than the methods described above because of sample disturbance and compaction
during sampling.

A thin-walled steel tube called a Shelby tube can be pushed instead of driven, by
use of a drilling machine. The sample then is extruded in the laboratory, and the
weight, volume, and moisture content of representative samples measured.
Strengths also can be determined. This is the same procedure that is used in
exploration drilling for foundation soils, and has the advantage that it also is used
for sampling and testing soil underneath a compacted layer. Routine samples for
purposes of soil identification are obtained by driving a thick-walled Standard
Penetration Test (SPT) device, but the soil is so disturbed and compacted that it
is not acceptable for density determinations. Details of these tests are found in
Chapter 26.

13.11.3 Nuclear Density Measurements
The most common method for control of field compaction operations involves
measuring both the soil density and moisture using a nuclear density-moisture
meter. Two modes of operation are available, backscatter, which is as the name
implies, and transmission. For transmission measurements a probe containing a
radioactive source is pushed down into a hole in the soil and radiation
is transmitted upward at an angle to detectors in the base of the device. In the
backscatter mode, both the source and counter are at the ground surface, which
gives lower penetration and is less accurate. The backscatter mode is particularly
useful for measuring densities of pavements.

For the transmission mode a probe is driven into the soil to the required depth
and withdrawn, or a hole is made with an auger. The nuclear unit is placed over
the hole and a rod that is slightly smaller than the probe hole is lowered from
within its shield down into the hole, as shown in Fig. 13.18. The rod contains a
radioactive source that emits gamma rays. Best accuracy is obtained if the source
is against the side of the hole closest to the detectors. Gamma radiation
transmitted through the soil up to the bottom of the device is counted for
15 seconds to 4 minutes, depending on the accuracy desired and the transmission
characteristics of the soil.

3
Although nuclear devices measure density in kg/m , they also are calibrated to
3
3
read unit weights in lb/ft or kN/m . Test units require daily calibration, which
can be done by testing in concrete blocks of known density or other materials such
as limestone, granite, or aluminum (ASTM Designation D-2922). Operation
requires trained and certified personnel.
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