Role of lateral stress in slope stability of stiff overconsolidated clays and clayshales

Abstract

<p>Because of abnormal behavior compared to other soil and rock materials, slope stability of overconsolidated clays and clayshales is of particular interest to researchers. Slope failure often occurs as a result of natural phenomena or of man-made changes, and sometimes from no apparent cause. Under stress, the strength of overconsolidated clays and clayshales may decrease with time. In spite of the variability of clayshales, the engineering behavior and the shear strength parameters are similar, and case studies of London clay, Oxford clay, Lias clay, Upper Cretaceous shale, Pennsylvanian shale, and other clayshales can be applied and compared. Three different kinds of failure models related to the strength reducing mechanism are discussed in this thesis; they are delayed failure, progressive failure, and creep. Because of unloading by natural erosion or excavation, negative pore water pressure may play a dominant role in delayed failure, while in some exceptional cases, a softening process may be important. Delayed failure occurs mostly in first-time slides of homogeneous materials, while progressive failure occurs mostly in reactivated slides of nonhomogeneous or layered soils. The displacement of progressive failure is much larger than for delayed failure. Creep has different definitions and models. For the prediction of landslide, the rheological model is a promising approach. In natural landslides, delayed failure, progressive, and creep all may coexist at different stages. In this thesis, the relationships between these three are extensively evaluated focusing on the differences in driving force, deformation, deformation rate, sliding zone, initiation of cracks, slide type, geological soil conditions, and strength parameters;Several landslide sites were studied and tested using the K(,0) Stepped Blade Test and the Borehole Shear Test. The K(,0) Stepped Blade Test was used in both failed and nearby unfailed slopes to obtain comparative data. The ability to predict slope failure is exemplified from the measured K(,0) profiles and back-analysis using Borehole Shear Test results. Also, the possible slide mechanisms are analyzed and remedial methods are suggested. The lateral stresses in a slide, a potential slide, and a nonsliding part of a slope in overconsolidated or compacted clay are related to stress relief at different stage of sliding.</p>