Advancements in Shear Strength Interpretation, Testing, and Use for Landslide Analysis
Authors: Tiwari Binod , Ajmera Beena 
Published: December 2023

Landslides are devastating natural disasters that result in loss of life, property damage, and community disruption. They have global impacts, causing fatalities and economic losses, particularly in mountainous regions near densely populated areas. Landslides can be caused by natural factors, including water saturation from heavy rainfall, snowmelt, and changes in groundwater levels, as well as seismic activity such as earthquakes and volcanic eruptions. Human activities, such as altering drainage patterns, destabilizing slopes, and removing vegetation, also contribute to landslides. Construction and development on slopes, over-steepening, and improper land management practices can further increase the risk of landslides. A key component in understanding the stability of slopes will be knowledge of the shear strength of the soils involved. However, to do so, it will be necessary to understand the various measuring methods of shear strength, loading conditions, and other parameters. Different methods and tests are employed to determine the shear strength of soil, depending on the specific conditions and objectives. Direct shear tests are often utilized to measure peak and fully softened shear strengths. Triaxial tests, on the other hand, are suitable for assessing both peak and fully softened shear strengths under drained or undrained conditions. Generally, the ring shear device is preferred for measurements of the residual shear strengths. However, multiple reversal direct shear tests and specifically modified direct shear tests as well as triaxial tests have also been utilized for this purpose. The cyclic simple shear test is recommended as an effective technique for replicating in-situ conditions to investigate the cyclic resistance and post-cyclic shear strengths of soils. Several correlations have been developed in the literature to estimate various shear strengths, including the fully softened and residual shear strengths of soil, as summarized in this paper. These correlations utilize parameters such as the liquid limit, plasticity index, mineralogy, clay fraction, and effective normal stress. The undrained shear strength of over-consolidated soils can be captured with the use of the Stress History and Normalized Soil Engineering Properties (SHANSEP) method. Extending this approach with the use of the normalized undrained strength ratio can result in two correlations that can capture the undrained shear strength. The paper also presents correlations for the true and base friction angles to estimate the shear strength using Hvorslev’s theory. This allows for a departure from the use of the cohesion intercept and friction angle in the Mohr-Coulomb failure envelope, both of which are dependent on the over-consolidation ratio. The power function effectively represents the cyclic strength curves in soils with the curve fitting parameters a and b defining their shape and position. A correlation between the normalized undrained strength ratio and post-cyclic effective stress ratio to assess the undrained shear strength after cyclic loading was also introduced. This correlation was shown to also capture the effects of excess pore pressure dissipation and reductions in shear strength induced by a second cyclic load.