LANDSLIDE HAZARD ZONATION AND SLOPE STABILITY ASSESSMENT TECHNIQUES : APPLICATIONS TO SRINAGAR-RUDRAPRAYAG, GARHWAL HIMALAYA

Abstract

Landslide is one of the most destructive geological processes which not only causes extensive damage to roads, bridges and houses but may also lead to loss of lives. Recent anthropogenic activities, particularly the road construction on vulnerable slopes, have greatly deteriorated slope stability and triggered landslides. This implies that the existing slope stability conditions must be systematically studied before implementing any developmental project. Such a study calls for slope stability assessment through spatial prediction of landslide potential zones. The present study, carried out in Srinagar-Rudraprayag area of Garhwal Himalaya, is an attempt towards development and evaluation of techniques for landslide hazard zonation and slope stability assessment. The work has been initiated with the selection ofimportant terrain factors which govern the slope stability. The terrain factors considered are lithology, distance from major thrust, slope, relative relief, drainage density and landuse. To determine the spatial distribution of various categories of these factors and of the existing landslides in the area, various terrain factor maps and alandslide map of the area have been prepared. Then the binary relationships between these factors and landslides have been identified on the basis of landslide density for each category of the factors. This study has revealed the degree of susceptibility of the factor categories to the landslide occurrence. The study has also highlighted the complex dependence of landslide occurrence on various factors. The two techniques developed for the regional landslide hazard zonation study are the Subjective Regional Zonation (SRZ) and the Objective Regional Zonation (ORZ). Although the SRZ technique employs the inferred relationships between the landslide occurrence and the terrain factors to assign ratings to the factor categories, yet the rating assignment is subjective. The ORZ technique reduces the impact of subjectivity in rating assignment, by deriving these from the frequency distribution of landslides. In this technique, an attempt has been also made for ajudicious classification of different hazard classes. Following these two techniques, the SRZ and ORZ maps ofthe study area have been prepared. Next, to obtain finer details of the hazard zones in a small sub-area, a third zonation technique, the Detailed Regional Zonation

(DRZ), has been employed. DRZ technique requires detailed field investigations to acquire additional field data pertaining to the weathering condition of rocks, the relation of discontinuities with reference to slope and the surface water conditions. Using this technique, the DRZ map of the sub-area has been prepared. The quality of hazard assessment for all the three zonation maps has been determined and it is found that these maps broadly show hazard zones in accordance with the existing landslide distribution. Acomparative study of the results of the three zonation maps shows that considerable amount of information, available from the DRZ map, is present in the SRZ and ORZ maps. Further, the general trends of zonation in the three maps are found to be in broad agreement, particularly for the very high and high hazard zones. This study has demarcated the applicability of all the three techniques depending on the nature of available data and the purpose of study. When sufficient apriori information about the terrain is available so as to enable judicious choice of ratings, the SRZ technique should be used. Otherwise, the ORZ technique should be used, particularly when large data set are available. In case of a need for detailed regional zonation, the DRZ technique should be used provided the necessary detailed field investigation is possible. The assessment of stability of an individual slope is always not possible from the regional zonation maps. For this purpose the Slope Mass Rating (SMR) technique has been used. This technique is based onthe Rock Mass Rating (RMR), coupled with adjustment factor ratings for joint orientation and method of excavation of slopes. The technique has been used for thirty slopes along the Srinagar-Rudraprayag road. With the help of SMR technique, the slopes have been classified into different classes of stability conditions. The results arrived at are consistent with the existing field conditions and therefore point at the potentiality of the technique. The study has also revealed the importance of the relation between joint and slope orientations for a better understanding of various modes of slope failure. A conservative stability estimate has been obtained by anticipating the worst stability condition under water saturation. The present work provides a systematic approach to the comprehensive slope stability assessment of a large region.