Bedding-Parallel Shear Zones as Landslide Mechanisms in Horizontal Sedimentary Rocks

摘要

The occurrence of large translational paleoland-slides in horizontally bedded sediments can not be completelyexplained by the presence of "weak" clay rocks andoversteepened natural slopes. When the shear strength of alandslide's basal rupture surface is back-calculated, residualshear strengths are usually required for failure. This is becausepeak shear strengths are too high to allow failure, even assumingthe most conservative estimate of groundwater levels. Dataobtained during geologic mapping and downhole logging oflarge-diameter borings suggest that the principal factor leadingto translational landsliding within horizontally bedded sedimentsis the presence of a pre-existing shear zone. A new term,bedding-parallel shear zone (BPS), is proposed for thesefeatures. When shearing parallel to bedding results from foldingor thrust faulting, it is tectonic in origin. When similar shearingis found in horizontally bedded sediments that have not beentectonically deformed, it is often misinterpreted as conclusiveevidence 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 andto have an understanding of the mechanisms responsible fortheir formation and relationship to translational landsliding.Knowledge of where and how BPS occur allows anunderstanding of why landslides have occurred in the past aswell as allowing prediction of where large landslides are likelyto occur in the future. Their misinterpretation as landslide slipsurfaces has obvious effects on the accuracy of engineeringgeology studies and stability analyses. For example, a stabilityanalysis for a typical landslide yielded a factor-of-safety of 1.2.An analysis of the same slope configuration representing acondition where a BPS is present, but not the entire landslidefailure surface, yielded a factor-of-safety