Shantanu Sarkar

STATISTICAL MODELS FOR SLOPE INSTABILITY CLASSIFICATION

Abstract Statistical models for reliable hazard assessment of landslides in a given area are presented. The models are based on information theory and regression analysis. Information theory is a branch of probability in which the most significant feature is unpredictability and regression analysis is based on simple statistical theory. Both methods of statistical prediction have been applied to slope instability to provide cross validation of the results. A FORTRAN 77 program for computer-aided assessment of the landslide hazard has been developed. The program considers the factors affecting the slope instability and history of past landslides of the area as its input. The program also calculates the information and regression value in a given area and classifies the area into different grades of instability. A case study of landslide hazard assessment of a 66 km2 area in parts of the Alkananda valley, the Gharwal Himalaya is presented. The factors considered in the analysis are angle and height of the slope, rock type and geological structures such as faults and thrusts. The results obtained are illustrated in the form of landslide hazard maps. The predicted results are compared by the above methods.

Approaches for comparative evaluation of raster GIS-based landslide susceptibility zonation maps

Abstract Evaluation of maps generated from different conceptual models or data processing approaches at spatial level has importance in many geoenvironmental applications. This paper addresses the spatial comparison of different landslide susceptibility zonation (LSZ) raster maps of the same area derived from various procedures. In hilly regions such as the Himalayas, occurrence of landslides is frequent, which necessitates the study of landslides in the region for future developmental planning. A critical aspect in landslide studies is the procedure for assignment of weights to various causative factors affecting the occurrence of landslides. A detailed study on conventional, artificial neural network (ANN) black box, fuzzy set based and combined neural and fuzzy weighting procedures for LSZ mapping in the Himalayas has recently been published by the authors in [Kanungo, D.P., Arora, M.K., Sarkar, S., Gupta, R.P., 2006. A comparative study of conventional, ANN black box, fuzzy and combined neural and fuzzy weighting procedures for landslide susceptibility zonation in Darjeeling Himalayas. Engineering Geology 85, 347–366]. The evaluation of various maps in that study was however based only on comparison of areal extents of various landslide susceptibility zones. In this paper, we present a spatial level comparative evaluation of those maps to get a detailed insight into the performance of each of the weighting procedures for landslide susceptibility zonation. The evaluation has been done through three approaches, viz., landslide density analysis, error matrix analysis and difference image analysis. Based on the landslide density values, it is inferred that the combined neural and fuzzy procedure for LSZ mapping appears to be significantly better than other procedures. The error matrix analysis highlights the significant difference between the conventional subjective weight assignment procedure and the objective combined neural and fuzzy procedure. Finally, the significant influence of a causative factor has been revealed by difference image analysis. The use of these spatial evaluation approaches in tandem may be highly beneficial to quantitatively assess the landslide susceptibility zonation or any other such maps.

A fuzzy set based approach for integration of thematic maps for landslide susceptibility zonation

Spatial prediction of landslides is termed landslide susceptibility zonation (LSZ). In this study, an objective weighting approach based on fuzzy concepts is used for LSZ in a part of the Darjeeling Himalayas. Relevant thematic layers pertaining to landslide causative factors have been generated using remote sensing and geographic information system (GIS) techniques. The membership values for each category of thematic layers have been determined using the cosine amplitude fuzzy similarity method and are used as ratings. The integration of these ratings led to the generation of LSZ map. The integration of different ratings to generate an LSZ map has been performed using a fuzzy gamma operator apart from the arithmetic overlay approach. The process is based on determination of combined rating known as the landslide susceptibility index (LSI) for all the pixels using the fuzzy gamma operator and classification using the success rate curve method to prepare the LSZ map. The results indicate that as the gamma value increases, the accuracy of the LSZ map also increases. It is observed that the LSZ map produced by the fuzzy algebraic sum has reflected a more real situation in terms of landslides in the study area.

Rock slope stability assessment using finite element based modelling – examples from the Indian Himalayas

Numerical modelling of rock slides is a versatile approach to understand the failure mechanism and the dynamics of rock slopes. Finite element slope stability analysis of three rock slopes in Garhwal Himalaya, India has been carried out using a two dimensional plane strain approach. Two different modelling techniques have been attempted for this study. Firstly, the slope is represented as a continuum in which the effect of discontinuities is considered by reducing the properties and strength of intact rock to those of rock mass. The equivalent Mohr-Coulomb shear strength parameters of generalised Hoek-Brown (GHB) criterion and modified Mohr-Coulomb (MMC) criterion has been used for this continuum approach. Secondly, a combined continuum-interface numerical method has been attempted in which the discontinuities are represented as interface elements in between the rock walls. Two different joint shear strength models such as Barton-Bandis and Patton’s model are used for the interface elements. Shear strength reduction (SSR) analysis has been carried out using a finite element formulation provided in the PHASE2. For blocky or very blocky rock mass structure combined continuum-interface model is found to be the most suitable one, as this model is capable of simulating the actual field scenario.

Landslide hazard assessment in the upper Alaknanda valley of Indian Himalayas

Landslides are one of the major natural disasters that are frequently occurring in the Indian Himalayas causing considerable loss of lives and property every year. A proper landslide hazard assessment is imperative to minimize such losses. In the upper reaches of Alaknanda valley of Garhwal Himalaya, there are several landslide potential zones along the Pipalkoti–Badrinath National Highway (NH-58). In the present study, landslide hazard assessment has been carried out in the above said area by delineating a few landslide potential zones. An attempt has been made to define landslide intensity to assess the degree of hazard in these potential zones of landslide. A landslide intensity scale was defined for this part of Garhwal Himalaya. So far the criterion for landslide intensity has not been defined in the Indian context. The approach used can be used in other potential landslide areas of the Himalayas.

Stability analysis and remedial measures of a landslip at Keifang, Mizoram – a case study

Landslips are often triggered due to non-engineered excavation of potential unstable slopes. Such slips can be stabilized by implementing suitable remedial measures.A landslip occurred at a drilling site of Oil India Limited in Mizoram State due to slope excavation. There was an immediate concern to protect the slope as the drilling platform and the highway at the top of the slope are at risk if further landslide occurs in future. Slope stability analysis of the failed slope was carried out to design suitable control measures for the protection of the slope from further sliding. Slope stability using various methods indicated that the slope is marginally stable. To improve the stability of the slope, suitable retaining structure at the toe of the slope was suggested and designed. Stability analysis performed with inclusion of retaining wall showed a significant increase in factor of safety of the slope. The suggested remedial measure has been implemented at the site and there is no landslip reported since then. The paper presents the results of the slope stability analysis and the design details of the retaining structure prescribed as the protection measure.

Slope Stability Assessment and Monitoring of a Vulnerable Site on Rishikesh-Uttarkashi Highway, India

Landslides are frequently occurring phenomenon in the Himalayas, in the Northern part of India. There were many landslide disasters in the recent past which have taken lives and caused extensive damage to property and public utility services. It is an imperative task to assess the slope instability and evaluate the risk where lives and property are at danger. A vulnerable slope on Rishikesh-Uttarkashi road having few houses, which are under distress, was studied for slope stability assessment. The slope instability was evident by road subsidence and development of cracks in few houses. A comprehensive geological and geotechnical investigation was carried out along with slope movement monitoring. The paper describes the findings of the study.

Soil Nailing: An Effective Slope Stabilization Technique

The present chapter discusses the soil nailing technique as an effective stabilization measure for slopes, excavations, rail or road embankments, tunnels and retaining walls. Different aspects of the technique such as favorable ground conditions, advantages and limitations over other methods have been reported. Further, different installation process, failure modes of soil nailed structures, design philosophies, effects of various construction parameters on the design method has been discussed in detail. The pullout response of the soil nail is the critical parameter for the soil nail design. Analytical, numerical, field and lab testing procedures are usually used to determine the pullout capacity of the soil nail. A chronological literature review examines the influence of various parameters such as grouting pressure, overburden pressure, soil dilation, degree of saturation, roughness of the nail surface and borehole on pullout capacity of soil nail. A comparative study based on different types of experimental setup reported in the literature along with the innovative pullout system developed at CSIR-CBRI for determination of pullout capacity of soil nail has also been summarized. The last section briefly describes the recent advancements in the soil nail technique and its beneficial effects over the conventional soil nailing system.

Optimization Techniques in Slope Stability Analysis Methods

The estimation of factor of safety (FoS) or design reliability of slopes is a pre-requisite for an efficient and safe application of landslide mitigation measures for ensuring long-term slope stability. The evaluation of stability of slopes, especially in a hilly region with wide variations in its geological formation is an upfront challenging task for geologists as well as for geotechnical engineers and till date, has been tackled using several optimization algorithms and slope stability analysis methods. The purpose of this book chapter is to present an up-to-date along with an overall review of the slope stability analysis methods which have used different optimization algorithms for deterministic and probabilistic or stochastic evaluation of FoS or reliability index, respectively, including some case studies from published literatures. This review shows that the FoS or reliability of slopes obtained by applying the commonly established analysis methods coupled with optimization algorithms, using both the deterministic and the probabilistic approaches, may vary in their values as well as in their computational effort and errors encountered.

Bioengineering as an Effective and Ecofriendly Soil Slope Stabilization Method: A Review

Soil-bioengineering is a cost-effective and eco-friendly alternative to the conventional methods of soil slope stabilization and erosion control. Numerous techniques such as fascines, bush layering, vegetated gabions etc. have been developed to enhance the soil slope stability, arrest soil erosion and improve the aesthetic aspect of a project, using plants as well as inert materials. Nevertheless, a limited control on the properties of the plants and the complex interaction of plant roots with the soil and other materials poses a challenge for the accurate design of soil-bioengineering techniques. The design of bioengineering techniques involves accurate evaluation of the root and root-soil properties. Different methods have been developed for the analysis of root and soil-root system that can aid in a better understanding of the complex phenomenon. The Present study provides a review on different aspects of bioengineering techniques for soil slope stabilization measures, especially, the existing techniques of physical modeling, laboratory scale testing and numerical techniques for evaluating the effect of root system on the strength properties of soil-root matrix. The different failure modes of the soil-root system i.e. adhesion failure, tension failure and progressive failure are briefly discussed. The present review will be useful for the design of bioengineering measures for soil slope stabilization or erosion control.