Deepak Raj Bhat

Residual-state creep behavior of typical clayey soils

Few researchers have studied the creeping displacement behavior of clayey soils using a triaxial compression cell and oedometer; however, in most cases, they have concentrated on the pre-peak state of shear. Clayey soil from a landslide is assumed to have already reached the residual-state, necessitating a study on residual strength to understand the creeping displacement behavior of clayey soils from landslides. In this work, an existing torsional ring shear apparatus was modified to understand the creeping displacement behaviors of typical clayey soil. The newly developed creep test apparatus is capable of measuring the displacement with respect to time under the application of a constant creep stress. This paper focuses mainly on residual-state creep behaviors of typical clayey soils. Residual-state creep failure prediction curves are also proposed, which may be used to predict failure time and displacement of creeping landslides in the future.

Effect of Shearing Rate on Residual Strength of Landslide Soils

The change in shearing rate due to the earthquake or volcanic activities may affect the residual strength of the slip surface soils of a reactivated landslide. Few studies have been performed to address the effect of shearing rate on residual strength of soils. However, the shearing rate effect and their mechanism on residual strength are not understood yet. In this study, the slow shearing rate effect on residual strength of landslide soils are studied using a ring shear machine. The shearing rate in the ring shear tests are varied in a range of 0.073 mm/min–0.586 mm/min (as 0.073 mm/min, 0.162 mm/min, 0.233 mm/min, 0.313 mm/min, 0.398 mm/min, and 0.586 mm/min) at an effective normal stress of 98.10 kN/m2. Test results show that a slight increased in residual strength after the shearing rate of 0.233 mm/min. Based on the test results, the shear-rate effect mechanism on residual strength of landslide soils are discussed.

Method of Residual-State Creep Test to Understand the Creeping Behaviour of Landslide Soils

Many researchers have studied creeping behaviour of landslide soils in the laboratory using both odometer and triaxial tests; however in most cases, they have only concentrated on the pre-peak creep behaviour of soil, which does not adequately explain the creep movement of landslides that undergo large displacements at close-to-residual state of shear. This necessitates the importance of further studying creep behaviour of clayey soils in residual-state of shear. In order to investigate creep behaviour of clayey soils, a method of residual-state creep test in a modified torsional ring shear machine is developed in laboratory, which can simulate the creeping behaviour of a large-scale landslide. This paper discusses on the typical results obtained from four representative landslide soils, i.e., commercially available Kaolin clay and three other samples from the landslide area in Japan and Nepal which have higher percentage of smectite, chlorite, and mica. Finally, possibilities towards displacement prediction are discussed.

A new concept of Residual-State Creep test to understand the creeping behavior of Clayey soils

Many researchers have studied creeping behaviour of clayey soils in the laboratory using both oedometer and triaxial tests; however in most cases, they have only concentrated on the pre-peak creep behaviour of soil, which does not adequately explain the creep movement that undergo large displacements at close-to-residual state of shear. This necessitates the importance of further studying creep behaviour of clayey soils in residual-state of shear. In order to investigate creep behaviour of clayey soils, a new concept of residual-state creep test in a modified torsional ring shear machine is developed in laboratory, which can simulate the creeping displacement behaviour of clayey soils. Representative clayey soils, which have higher percentage of Smectite, Chlorite, Mica and commercially available Kaolin clay, are taken in this study. A series of residual-state creep test (i.e. seven tests) with varying applied constant shear stress for each sample were conducted, the results thus obtained are interpreted in terms of Residual-State Creep Stress Ratio (RCSR), at which the soil samples fail at their residual-state of shear. The term RCSR is the ratio of applied constant shear stress with residual strength. The test results show that when RCSR ≤1, the clayey soil does not show creeping behavior where as the soil undergo creeping behavior when RCSR>1. This paper mainly focus on the methods of residual-state creep test, its implications for the study of creeping displacement behavior of clayey soils.

Shear Strength Recovery of Clayey Soils Following Discontinuation of Shear at a Residual State

Residual shear strength is generally used for design and repairs on slopes containing pre-existing shear surfaces in large-scale landslides. Some recent research works suggest that the pre-existing shear surface of a large-scale landslide can regain strength with the passage of time, which should be considered in designing the slope stability measures. In this study, three landslide soils were tested in a ring shear apparatus with rest periods between shear of 1, 3, 7, 15, and 30 days, with the following main objectives (1) to understand the strength recovery behavior of landslide soils in residual state of shear after as long as 30 days of rest between shearing, (2) to understand the comparative pattern of strength recovery in highly plastic and less plastic soils, and (3) to understand the mechanism involved in strength recovery at residual state of shear. The obtained experimental results indicate that the recovery of shear strength in the residual state started to appear slightly after shear was discontinued for 3 days, and was lost immediately after a very small shear displacement. On the other hand, as understood from the experimental work in this study, the trend of strength recovery, is somewhat in increasing order with prolongation of the period that shear is discontinued.

Evaluation of factor of safety for vegetated and barren soil slopes with limit equilibrium computations

Limit equilibrium method (LEM) is the most conventional and popular technique in slope stability analysis. Location of the unique failure surface and the determination of corresponding critical factor of safety (critical-FOS) of the soil slope requires a lot of trial and research. With this amount of trial and research results, we evaluate the field of factor of safety (FOS-field), which represents the critical-FOS at a point within the domain of all possible failure surfaces. Such a field can be very important for the precise judgment of the most critical-FOS of the slope and its perturbation. This paper also presents the evaluation of the FOS-field of vegetated slopes, thus providing an analytical way to examine the effect of vegetation on the soil slope stability. We evaluated the stability of vegetated and barren soil slopes under dry and fully saturated conditions. With dry and fully saturated conditions, the behaviour of slopes in most favourable and worst conditions can be simulated. Evaluation of FOS-field for various slope geometries and conditions show that the more unstable the slope becomes, the more difficult it will be to pinpoint the location of the critical failure surface with the unique least FOS. We verified the LEM codes with finite element method (FEM) and spectral element method (SEM) codes considering a sample problem from Smith and Griffiths’ book (Smith and Griffiths 2003).

New numerical scheme in finite element method for the effective evaluation of the vegetation effects on slope stability modeling

New numerical scheme in finite element method (FEM) is used to simulate both the progressive nature of failure and interaction of the soil-root matrix continuum. Computations suggest that a certain range of Root area ratio (effective RAR-range, i.e. ERAR-range) should be identified for the given slope geometry, vegetation type and maturity period. The ERAR-range represents all the RARs which impart a factor of safety (FOS) greater or equal to a particular FOS (benchmark FOS, i.e. BMFOS), BMFOS being the anticipated FOS depending on various factors (demographic distribution, proximity to utilities etc.). Beyond the ERAR-range vegetation does not impart to the stability of the slope. Theoretically, higher range of RAR beyond the ERAR-range (ineffective RAR-range, i.e. IERAR-range) even causes the slope failure. Hence, the simulation can be useful tool for the effective evaluation of vegetation effect on slope stability modeling.

Strength Recovery of Landslide Soils from the Residual State of Shear

A pre-existing shear surface of a reactivated landslide was subjected to repeated sliding and recession. During the sliding, the shear strength of the shear zone was reduced to the residual state. However, the strength may be recovered from the residual state, to some extent, during a rest period. In this study, three landslide soils collected from the different large-scale landslide sites in Nepal and Japan are tested in a ring shear apparatus for the discontinued shear rest periods of 1, 3, 7, 15, and 30 days. Test results show that recovered strength measured in the laboratory is slightly noticeable after a rest period of 3 days; but, recovered strength is lost after a very small shear displacement. This paper focuses mainly on the strength recovery behavior in highly plastic and less plastic soils from the residual state of shear. The probable causes of the strength recovery are also discussed.