Compressibility of Clay

Experiments on artificially sedimented clays at loading rates as low as 10 -4 offer direct evidence that the compressibility of sedimented clays is much smaller during deposition than that measured in the usual oedometer test. Normally consolidated clays subjected to long periods of secondary compression will not compress along the virgin curve until a substantial pressure increment (as large as 0.4 of the effective overburden pressure) has been added. High quality sampling techniques should include minimizing the time duration between stress release in sampling and stress reapplication in the oedometer. Tests on freeze-dried kaolin clay indicate no measurable viscous retardation to compression over a wide range of load-increment ratios. Subsequent creep strains were approximately linear with log time. A marked reduction in compressibility was observed with decreasing load-increment ratio, but creep strains were found to decrease moderately with increasing effective stress. A hypothesis for the mechanism of compression is proposed and its validity is examined in the light of available data.