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Piezoelectric quartz pressure gages were used to measure the dynamic stresses generated in soil by compaction equipment including a smooth-wheel roller, a tractor, two types of power rammer, and a vibratory roller. Vibrating wire pressure gages were also used in some of the tests with the slower machines. During compaction, the peak pressure at a given initial depth increased with the number of passes of the machine until it attained a limiting value which was related to the limiting value of the soil density attained at that depth and which decreased rapidly with depth. Tests with some of the machines on a few different soils and at various moisture contents indicated that the relation between soil density and the limiting value of peak pressure depended on soil type and moisture content. Tests on silty clay at a moisture content of 15 to 16 per cent showed that the relation between density and pressure was independent of the type of compacting machine, that is, it did not depend on the duration of the impulse generated by the machines which ranged from 20 to 25 milliseconds for the rammer to 2 to 5 sec for the roller and tractor. It appears that roller compacting machines might be operated at higher speeds than those at present commonly employed. Although the measurement of the pressures generated in the soil could expedite the work of determining the minimum number of passes giving maximum compaction, the gradient of density in the soil beneath the machine can be inferred only when information is available concerning the relation between density and the limiting values of recorded peak pressure. Such information has been obtained for only one soil and moisture content, and the determination of a precise relation between pressure and density may be costly in that it involves a number of experiments. It was concluded that the measurement of the pressures generated would not contribute more to developing improved forms of compaction equipment than the measurements of soil density, and its future employment in this work must depend on the relative cost of the various methods available for studying the performance of compacting machines.
Whiffin, A. C.
Road Research Laboratory,