Abstract
Series of undrained compression (UUC), undrained extension (UUE), consolidated undrained compression (CU), and uniaxial compression (UC) tests were carried out on samples of Chalk Marl from the Channel Tunnel. The analysis of the results of these tests has shown that the Chalk Marl is characterised by slightly non-linear stress-strain behaviour, very brittle failure, a substantial loss of post-peak strength and small strains to failure. Mohr failure envelopes in terms of effective stress for vertical and horizontal specimens were found to be straight lines within the range of stresses investigated. The stress-strain relationships are characterised by hysteresis of residual strain upon removal of stress. A series of isotropically consolidated drained tests on specimens of different orientations was carried out. The effect of anisotropic consolidation was studied by carrying out some anisotropically consolidated drained tests using the modified Hoek Cell. Strength and deformation characteristics measured lateral deformations, Poisson's ratio and the range of the elastic behaviour were studied in detail. Values of the elastic parameters required for the isotropic mathematical model were determined. The anisotropic behaviour of Chalk Marl in drained and undrained conditions was analysed and the five elastic parameters required to describe the complete anisotropic stress-strain relationships were determined. It was found that Chalk Marl exhibits a marked degree of deformation anisotropy, with a degree of anisotropy n = E'h/E'v = 1.5. The undrained and drained strengths were nearly isotropic. The effect of stress path on the undrained deformation behaviour was studied both under isotropic and anisotropic consolidation conditions. A series of multi-stage stress path tests on vertical and horizontal specimens was carried out.. The testing technique followed was to subject the same specimen to three stages of consolidation and after each stage, the specimen was subjected to three or more different stress paths, simulating the-stress changes experienced by different soil elements around the tunnel excavation during tunnelling operation. It was found that the stress path followed has a significant effect on the deformation behaviour and the pore pressure response. The long term time-dependent deformation behaviour was studied under different states of stress including onedimensional, uniaxial, drained triaxial and undrained triaxial conditions. It was shown that the relationships between creep strains and log time are generally non-linear with only a limited range of linearity during the early periods of creep. The behaviour is characterised by limited strain instabilities and the absence of secondary and tertiary creep phases, i.e. creep being of primary nature only.