Page 309 - Geotechnical Engineering Soil and Foundation Principles and Practice

Compaction
304 Geotechnical Engineering

encounter hydrogen ions in the soil they are slowed, and those returning to the
unit are counted. The count includes all hydrogen ions including those that are

structurally held as OH ions in mica and the clay minerals, plus hygroscopic
water that is not driven off by normal oven-drying, plus hydrocarbons in organic
matter. Calibration therefore is required by counting similar soils with moisture
contents that are determined by oven-drying (ASTM Designation D-3017).

13.11.5 Accuracy of Nuclear Determinations

With modern instruments and accurate calibrations, density determinations are
accurate to within about 1 percent, and moisture contents to within about 2.5
3
3
to 10 kg/m (0.2 to 0.6 lb/ft ), depending on the counting time. The upper part of
the tested layer is emphasized, as about 50 percent of the counts come from the
upper 25 percent of the soil layer tested by the transmission method.

13.11.6 Operating Nuclear Devices
The U.S. Nuclear Regulatory Commission requires that nuclear density testers
have radiation warnings and be kept under lock and key when not in use.
Operators must be specially trained and certified, and wear film badges to ensure
that dosage does not exceed prescribed levels. Training normally is available from
instrument manufacturers. Units are periodically checked for radioactivity
leakage, and require periodic replacement of nuclear source materials. With
proper safety precautions the radiation danger is negligible, particularly when
compared to hazards from construction traffic.

13.12 FIELD COMPACTION EQUIPMENT

A broad catalog of compaction equipment is available and is continually being
improved as manufacturers seek a competitive edge. Most common are heavy
rollers, vibratory rollers, and tamping rollers. In close quarters such as trench
backfills, hydraulic hammers can be mounted on backhoes. Hand-operated
vibratory or ‘‘jumping jack’’ equipment may be used for moderate needs close to
buildings. Applicability of different equipment for different soil types is indicated
in Table 13.2.

13.13 CHEMICAL COMPACTION AIDS

Many different chemical compounds have been marketed as ‘‘compaction aids.’’
These generally are detergents that are added in small mounts to water to decrease
capillary tension, which reduces intergranular friction so that a specification
density can be achieved with less effort. However, reducing capillary attraction

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