Effective stress paths and pore-pressure responses during undrained shear along the bedding planes of varved Fort William Clay

Previous studies of the varved sensitive clays from the Thunder Bay area indicated that the geotechnical properties were influenced by their layered structure. When sheared along the bedding planes, double strength envelopes were apparent, with strength parameters depending on the stress levels applied. Thus, a series of consolidated isotropical undrained triaxial tests with pore-pressure measurements was carried out on specimens with the varves inclined to the vertical axis. A number of unusual results were observed: (i) low B-values for presumably saturated samples; (ii) failure of the specimens along the clay seams at stress levels above precompression load, exhibiting highly compressive behaviour and considerable cohesion; (iii) failure of the specimens in the silt seams at stress levels below the precompression load, exhibiting dilatancy and a lower cohesion than in the normally consolidated range; (iv) slightly anisotropic elastic deformation behaviour almost up to failure for testing in the precompression range; however, strongly anisotropic, largely nonelastic behaviour in the normally consolidated stress range, indicating larger stiffnesses vertically than horizontally. The stress paths in the normally consolidated stress range suggested also that consolidation of the clay seams occurred during undrained shear owing to internal dissipation of pore pressures into dilating silt seams. Key words: varved sensitive cemented clay, pore-pressure response, undrained shear, internal pore-pressure dissipation, anisotropic elastic behaviour, critical-state conditions.