Analysis and behavior of preexisting landslides
The selection of shear strength parameters for the design and repair of landslide is important and difficult. Skempton (1964) concludes that if a failure has already occurred in clayey soils, any subsequent movement along the preexisting slip surface will be controlled by the drained residual strength. Skempton (1985) suggests that the strength of a clay also will be at or close to the residual value on slip surfaces in soliflucted slopes, bedding shears in folded strata, sheared joints or faults, and after an embankment failure. Therefore, the drained residual shear strength has been and still is being used for analysis of slopes that contain a preexisting shear surface. Some recent research suggests that preexisting shear surfaces may exhibit a shear strength that is greater than the drained residual value after a period of time in which the slope remains stable, i.e., does not experience shear displacement. If so, strength recovery could impact landslide mitigation and remedial measures because the increased strength could result in savings to insurance companies and/or landslide mitigating agencies. Thus, it is important to determine if the shear strength after a long rest period during which no movement occurred still corresponds to the residual strength or has attained a strength that is greater than the residual value. The main objectives of this research are to study the shear strength and long term behavior of landslides and in particular preexisting shear surfaces. The research involved laboratory testing to determine the strength recovery, if any, of cohesive soils with varying plasticity, effective normal stress, and the applicability of the recovered strength to remedial measures and the back-analysis of landslides. Some of the issues addressed include (a) if the shear strength increases from the residual value with time, (b) what is the maximum recovered strength, (c) how long does it take to reach the maximum recovered strength, (d) if the strength increases with time, does the strength return to the residual value with additional shear displacement and if so how much shear displacement is required to reduce the strength back to the residual value, and (d) what is the maximum shear strength that can be obtained from strength recovery and used for design purposes. Back-analysis of landslides is important for evaluating the mobilized recovered strength and thus back-analysis procedures were reviewed and augmented. Empirical correlations for drained residual and fully softened friction angles are important in the back-analysis of landslides because they provide estimates for use in preliminary design and serve as a check for laboratory test and back-analysis results. The empirical correlations for drained residual and fully softened friction angles proposed by Stark et al. (2005a) in graphical form are widely used in geotechnical practice. To capture the stress dependent nature of the residual and fully softened failure envelopes, a trend line for an effective normal stress of 50 kPa was developed to better describe the stress dependent nature of the drained residual strength and data for seven additional soils were added to the database. In addition, mathematical equations were developed for each trend line and which can be used to estimate stress dependent failure envelopes for use in stability analyses. These mathematical expressions were incorporated in a spreadsheet that can be used to estimate the shear strength parameters of a soil using only two index properties, i.e., liquid limit and clay-size fraction. These mathematical expressions were also coded in Microsoft Visual Basic (VB 6.0) that may facilitate slope stability software developers incorporating the expressions in software to facilitate the use of stress dependent strength parameters in stability analyses.
Soil mechanics, Landslides, Overconsolidated clays, Shear strength, Strength recovery, Slope stability, Ring shear test
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign