Natural deposits in the Athabasca Oil Sands area contain many shear-induced structures such as shear zones or discontinuities. Locations and features of such structures are unknown in advance, and their internal fractures and geometries are complex, so these structures pose a challenge to tailings dam construction. Design or evaluation of a tailings dam demands an understanding of the shear behavior and pore pressure of shear zones. The principal objective of this study is to provide an approach for measuring the pore pressure of a shear zone and its response to different displacement rates.

An innovative large triaxial testing system and related transducers, laboratory techniques, and procedures were developed to measure the pore pressure of a shear zone. The results reveal that pore pressures on the shear plane and on the base of a sample for three materials (compacted Athabasca clay, over-consolidated Highvale mudstone, and Fort McMurray weak rocks) are identical at a shear displacement rate of less than 14.4 mm/day. This implies that at slow movement the pore pressure obtained from in situ instrumentation, which may not be set exactly on the shear plane, can be used as the shear zone pore pressure. The results also reveal that the pore-pressure responses differ for different materials with different stress histories (over consolidation ratios).

Detailed geological field mapping by the author has revealed that shear zones in the Athabasca Oil Sands area often develop in clay beds with high clay content such as clay shale, basal clay, Paleosol, pond mud, and highly weathered limestone. The water content and Atterberg limits of the material in the shear zone are markedly higher than those of the adjacent unsheared material, while the grain size of the material in the shear zone is much smaller than that of the adjacent unsheared material.

The shear behavior of shear zones, especially their post-peak characteristics, was investigated by using a large triaxial cell. The results show that the stress-displacement curve of a shiny planar shear surface in the highly weathered limestone with relatively large prior shear displacement shows no peak, and after a small displacement, reaches the residual strength. Conversely, a rough shear surface in the same material with small prior shear displacement has a significant peak followed by a decrease in the strength with further shear displacement. A large displacement is needed to reach residual strength. When the shearing was imposed in different minor shear combinations in Paleosol, different stress-displacement curves, post-peak characteristics, and shear strength parameters resulted.