COMPACTION OF SOILS.

Abstract

Synopsis. Soil compaction is the process whereby soil is mechanically compressed through a reduction in the air voids. It is measured in terms of the dry density or amount of solid matter in unit volume. Soils range in dry density from about 140 lb. per cubic foot for coarse-grained gravels and sands to about 90 lb. per cubic foot for heavy clays. For a constant amount of compaction, all soils have an optimum moisture content at which a maximum dry density is obtained. The leboratory tests used to determine the compaction characteristics are described in this Paper and factors affecting their results are discussed. In the field, compaction is effected by different types of roller, rammer, or vibrator. The results of full-scale investigations which have been made at the Road Research and sheepfoot rollers, a frog rammer, and vibrating plant, are discussed. The main Laboratory of the compaction of five British soils by smooth-wheel, pneumatic-tired, conclusions are that:— (l) There is an optimum moisture content for each type of plant which may differ hm that indicated by laboratory tests. (2) Granular soils are best compacted by smooth-wheel rollers, wet clay soils by pneumatic-tired rollers, and dry clay soils by sheepsfoot rollers. (3) Provided that the thickness of loose layer does not exceed 9 inches, the decrease of dry density with depth below the surface was about the same for all the rollers tested. A suggested procedure is outlined for carrying out full-scale compaction trials on a job, and an example of its use is given.