Bedding-parallel shear zones as landslide mechanisms in horizontal sedimentary rocks

The occurrence of large translational paleolandslides in horizontally bedde d sediments can not be completely explained by the presence of "weak" clay rocks and oversteepened natural slopes. When the shear strength of a landsl ide's basal rupture surface is back-calculated, residual shear strengths ar e usually required for failure, This is because peak shear strengths are to o high to allow failure, even assuming the most conservative estimate of gr oundwater levels. Data obtained during geologic mapping and downhole loggin g of large-diameter borings suggest that the principal factor leading to tr anslational landsliding within horizontally bedded sediments is the presenc e of a pre-existing shear zone. A new term, bedding-parallel shear zone (BP S), is proposed for these features. When shearing parallel to bedding resul ts from folding or thrust faulting, it is tectonic in origin. When similar shearing is found in horizontally bedded sediments that have not been tecto nically deformed, it is often misinterpreted as conclusive evidence of land sliding. Mechanisms that produce BPS are: 1.Elastic rebound. 2.Progressive failure of overconsolidated claystone, 3.Differential consolidation. 4.Gravitational creep. It is important for engineering geologists to recognize BPS and to have an understanding of the mechanisms responsible for their formation and relatio nship to translational landsliding, Knowledge of where and how BPS occur al lows an understanding of why landslides have occurred in the past as well a s allowing prediction of where large landslides are likely to occur in the future. Their misinterpretation as landslide slip surfaces has obvious effe cts on the accuracy of engineering geology studies and stability analyses. For example, a stability analysis for a typical landslide yielded a factor- of-safety of 1.2. An analysis of the same slope configuration representing a condition where a BPS is present, but not the entire landslide failure su rface, yielded a factor-of-safety of 1.9.