Ram Chandra Tiwari

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.

3D SEM Approach to Evaluate the Stability of Large-Scale Landslides in Nepal Himalaya

Abstract This paper deals with the stability of large-scale landslides with the aim of exploring reliable safety factor in 3-dimensions (3D) for the effective implementation of landslide stability enhancement measures (LSEM) in the mountainous country like Nepal. The paper uses ‘Specfem3D_Slope’, an open-source spectral element method (SEM) based program that can effectively handle the major challenges of 3D modeling to evaluate the stability of large-scale landslides. The SEM prefers p-refinement techniques (increasing the degree of interpolation or polynomial degree or spectral degree) instead of h-refinement (refining the mesh related to elemental budgets) unlike to FEM. Then the safety factors, thus obtained seem to be reliable values as per the material properties and slope model supplied, and that can encouragingly be used in effective design and implementation of various LSEM projects. Moreover, a complete draw down effect after the LSEM gives a complete sense of the effectiveness of the measures itself. In addition, the paper emphasizes some experimental tests to know the displacement behavior of landslide’s soils (e.g., Krishnabhir landslide). A theoretical model is chosen to do some deeper exercises on 3D for the reliability of the results and later implements such criteria for ‘Laprak landslide’ of Nepal Himalaya. With this modeling method (elasto-plastic) the paper incorporates all possible instability conditions like pseudo-static earthquake loading, structural loading over the sliding mass, partially saturated ground water profile, etc., the same conditions as implemented by previous researchers for Laprak landslide. The study of two major landslides of Nepal ‘Krishnabhir landslides’ (relates to major highway) and ‘Laprak landslides’ (relates to a large number of settlements over the moving mass) reflects the major issues of large-scale landslide stability modeling of Nepal Himalaya.

3-D elasto-plastic spectral element application to evaluate the stability of large-scale landslides

This research deals with the computational accuracy impact factors of elasto-plastic spectral element applications to slope stability. It contains three categories of impact factors: 1) modelling domain related parameters, 2) spectral parameters, and 3)X elasto-plastic parameters. Each category has many accuracy impact factors. The modelling domain and elasto-plastic related impact factors can be equally applied to the elasto-plastic finite element method, however spectral corrections can only be applied for spectral element applications. This work presents a numerical evaluation of all three impact factors to the stability factor of a slope in three representative theoretical domains of slope angle, of 26.26°, 36.69°, and 45°. In addition, the study focuses on spectral element applications to slope stability evaluation, because it is a new and reliable approach in slope stability modelling. It has several benefits over the existing modelling methods to apply for slope stability modelling: 1) geometric flexibility of the finite elements, 2) high computational efficiency, 3) reliable spectral accuracy, and 4) high numerical stability.

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.