Compaction of granular soils by uniform vibration equivalent to vibrodriving of piles

Summary

A programme of laboratory tests was undertaken to identify the volume reduction of a range of granular soils in response to uniform vibration. The purpose of the tests was to quantify the potential compaction settlement of granular soils due to vibrations typical of those experienced when sheet or bearing piles are installed or extracted by vibrodriver. The vibrations in soils within 2–20 m of a vibrodriven pile are well defined as sinusoidal, at the frequency of the vibrodriver, and typically in the range of 1.0g down to 0.02g (whereg=gravitational acceleration). Thein-situ conditions were replicated in the laboratory by vibration of a Rowe cell containing a soil sample. This allowed control of stresses before and during vibration. Tests were conducted on nine granular soils, ranging from uniform Leighton Buzzard sands to a sandy fine-to-medium gravel. Soil samples were prepared at minimum relative density, generally in the saturated state, but also dry and partly saturated. Each sample was consolidated at a selected effective stress of between 10 and 100 kPa. It was then vibrated vertically at increments of controlled acceleration, under conditions of free drainage and maintained confining stress. Reduction in sample thickness was measured continuously. Results showed that even in a small, drained, sample the full volume reduction was reached only after many minutes, although the early response was more rapid. The trends of results showed that: increased surcharge pressure reduced the subsequent vibratory compaction; well-graded soils showed greater compaction than more uniform sands; compaction increased markedly when acceleration exceeded 1 g; saturated soils showed larger compaction than dry and partially saturated soils; compaction was influenced little by frequency, although rate of compaction reduced with frequency increase.