Clay Shear Strength Rate Dependency and the Available Strength Along Earth Slides Rupture Surfaces

ABSTRACT: Earth slide movements are affected by the shear behavior of the material in their shear zone. In many cases, the earth slide shear zones are made of weak clay layers. Traditional geotechnical laboratory testing utilizes standard loading rates for determining the shear behavior of clay. When an earth slide movement creates field shear rates that are faster or slower than the typical laboratory test ranges, the available shear strength along the rupture surfaces can differ from the laboratory predicted strength. The effect of the clay viscosity on observed behavior of earth slides founded on clay beds is examined in this paper. This paper demonstrates how some of the earth slides characteristics can be explained by considering the rate dependency of the shear strength of clay. This paper provides a review of available clay shear strength within an earth slide shear zone from pre-failure to failure conditions. The pre-failure peak and residual shear strength rate are reviewed. The residual strength during failure and its dependency on the clay content, pore fluid chemistry, and stress history is discussed. In general, the lower shear rate results in smaller available shear strength along a rupture surface. Both peak and residual shear strength reduce with slower shear rate prior to the failure. After full development of the rupture surface, the shear strength increases nonlinearly with the shear rate increase. The non-linear increase in the clay strength during failure is due to the clay viscosity shear thinning characteristics. The clay viscosity depends on the clay mineralogy, pore fluid chemistry, and liquidity index. This paper discusses that by correlating the clay viscosity with the stress history, clay mineralogy, and clay content; an estimation of the available shear strength of clay beds and order of magnitude of rate of movement are possible.