The occurrence of large translational paleolandslides in horizontally bedded sediments can not be completely explained by the presence of "weak" clay rocks and oversteepened natural slopes. When the shear strength of a landslide's basal rupture surface is back-calculated, residual shear strengths are usually required for failure. This is because peak shear strengths are too high to allow failure, even assuming the most conservative estimate of ground-water levels. Data obtained during geologic mapping and downhole logging of large-diameter borings suggest that the principal factor leading to translational landsliding within horizontally bedded sediments is the presence of a pre-existing shear zone. A new term, bedding-parallel shear zone (BPS), is proposed for these features. When shearing parallel to bedding results from folding or thrust faulting, it is tectonic in origin. When similar shearing is found in horizontally bedded sediments that have not been tectonically deformed, it is often misinterpreted as conclusive evidence of landsliding. 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 relationship to translational landsliding. Knowledge of where and how BPS occur allows an understanding of why landslides have occurred in the past as well as allowing prediction of where large landslides are likely to occur in the future. Their misinterpretation as landslide slip surfaces has obvious effects 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 surface, yielded a factor-of-safety of 1.9.

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