Mechanical properties of heavily overconsolidated clays : laboratory investigation of the compressibility of lower lias clay

Abstract

A laboratory investigation was carried out to study the compressibility characteristic of heavily over-consolidated clay, and in particluar the Lower Lias Clay from a site at Blockley Gloucestershire. It has been estimated that the sampling site has been subjected to an overburdenin excess of 520m. This is tentatively confirmed by high pressure odometer tests on undisturbed and remoulded samples. The in-situ value of the coefficient of earth pressure at rest, Ko is determined by measurement of the pore suction in undisturbed samples in both triaxial and odometer apparatus: a value of 3 is obtained. The deformation of cylindrical undisturbed samples are studied under various axi-symmetric stress paths in the triaxial apparatus and under one-dimensional strain conditions in the odometer. A new triaxial cell is developed in which axial and radial stresses are applied, in order that samples can be returned to in-situ stress state and loaded under stress or strain controlled conditions. Results obtained from the series of triaxial tests demonstrates that the Lias clay exhibits a high degree of anisotropy. It is found that under drained conditions the strain ratio- incremental effective stress ratio relationship can be predicted by an orthotropic elastic theory. The vertical "Young's" module, is found to be reasonably linear at low stress range, although dependent on the incremental sttress ratio K. Results from block, core and tube samples demonstrate that the mechanical disturbance has little effect on the Young's module, but significantly reduces the volumetric strains.