INTERNATIONAL JOURNAL OF GEOMATICS AND GEOSCIENCES Volume 2, No 3, 2012

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1 INTERNATIONAL JOURNAL OF GEOMATICS AND GEOSCIENCES Volume 2, No 3, 2012 Copyright 2010 All rights reserved Integrated Publishing services Research article ISSN Micro-Hazard Evaluation and validation of landslide in a part of North Western Garhwal Lesser Himalaya, Kishor Kumar 1, Rahul Devrani 2, Anil Kathait 1, Neha Aggarwal 2 1- Geotechnical Engineering Division, Central Road Research Institute, Mathura Road, New Delhi, 2- Department Of Geology, University of Delhi, rahuldevrani18@gmail.com ABSTRACT A large number of natural factors control the occurrence of landslide phenomenon. However, anthropogenic activities, added with natural factors contribute handsomely to destabilize the hill slopes. The Himalaya being unique in its physiographic, tectonic and climatic characteristics coupled with many natural and man-made factors is inherently prone to landslides. Present article includes a study of certain parameters (as factors), selected as per their role in inducing landslides in one of the hazard prone regions of Northwestern Himalaya. A base map on 1:500 scale was prepared using high precision Total Station and GPS that was further divided into equal sized grids measuring 45m X 45m. The parameters such as lithology, structures, slope gradients, landuse and landcover and groundwater condition have been quantified and statistically analyzed for evaluating the landslide hazard potential of each of the grid. Based on the integration of qualitative and quantified weightages given to each factor of each grid, a micro-hazard zonation map of the area has been developed for guiding the remedial actions required for preventing the landslides. The results were compared with results of geomechanical classification.i.e. RMR and SMR; of same site to support modified methodology.the map has been physically verified and was found to be mostly accurate. Keywords: Landslide, Zonation, Scale, Hazard, Parameters and Weightage 1. Introduction Landslide Hazard Zonation (LHZ) is one of the commonly used methodologies for ranking different parts of an area according to the degree of potential hazard from landslide (Varnes, 1980 & 1984). It has been argued well and accepted that geomorphologic and geological data apart from the historical data on landslides is an important source parameter for landslide hazard zonation (Hutchinson, 1995). The landslide hazard zonation (Anbalagan, 1992) is a method to divide the landscape of into different zones characterizing by its degree of instability and potential. Many researchers have proposed this method Brabb(1984), Varnes (1984), Carrara (1983,1988), Gupta and Joshi (1990), Carrara et al(1990,1991), Anbalagan (1992), Carrara et al(1992), Mehrotra et al(1992),pachauri and Pant(1992), Gupta et al(1993),sarkar et al (1995),Anbalagan and Singh (1996), Panikkar and Subramanyan (1996), Gupta et al(1999), Uromeihy and Madavifar (1999), Saha et al(2002), Carro et al(2003). The zonation of most geological hazards and prediction of their risks remain largely unsolved problems (Carrara et al 1999) but the large scale hazard maps can provide us more details (Westen et al 1997) which can decipher this problem. Submitted on February 2012 published on February

2 The Kaliasaur Landslide (Figure 1) is taken as case study where the new ratings have been specified to the parameters, data is been analysed statically where the values is been obtained from the data itself or is defined to it. As defined works-scale is 1:500, where earlier workers scale were too small to be fit in a model of site specific; also scale varies with different factors ( Aleotti and Chowdhry,1998) in our study chosen scale is site specific. Aim of study is to go into a large scale zonation map, where the weightage of parameters were unfitted as convolution gets increase with large scale (Sarkar et al, 1995). 2. Study Area The Kaliasaur Landslide has been a threat since more than five decades on the strategic Highway of the nation (NH-58) (Figure 1). The history of Kaliasaur landslide dates back to 1920 (Kishor Kumar et. al., 2005), the presence of fault zones, shear zones, extensive mass are major causative factors of the sliding activity in this region. Also there are many other significant landslides, both active and dormant, occurring in the Srinagar Rudraprayag region (Sinha et al 2002). The landslides along the Rishikesh Badrinath National Highway are mostly debris slides, debris flows, rock slides and rock falls (S. Sarkar et al., 2006). The study area being tectonically active is a sensitive region consisting of complex topographic, geological and geomorphic setup (Kishor Kumar et. al. 2008a). Figure 1: Location map of study area (Kaliasaur Landslide) These conditions coupled with extrinsic factors such as high precipitation and seismic vibrations, rapid weathering and erosional processes, make the area susceptible to variety of landslides of different magnitudes such as landslides, rock fall, debris fall, debris slide, slumps and rockslides etc. (Kishor Kumar et. al. 2008b).These soil creeps could easily be identified as slowly moving unconsolidated soils and overburden manifested in typical bending of tree trunk along the movement direction and associated with tension cracks on the road pavements or a down slope tilting of stone walls.. Since then, the slide has been experiencing frequent recurrences. Heavy landslides have occurred in 1952, 1963, and

3 and in subsequent years as well. Besides the major events, minor failures kept on taking place in this zone which houses rocks of very weak condition. During 1970, 1971 and 1972 occurrence, Kaliasaur landslide damaged the highway severely. The traffic was restored by having new formation cut of length 215 m, 100 m, and 125 m respectively (Kishor Kumar et. al. 2008c). Again in 1984, due to heavy precipitation, comparatively major slide took place and damaged road considerably. The crown portion of the slide has extended more than 20 m since Two-third landslides initiated by human activities are mostly because of removal of slope toes at road cuts. The study conducted required selection of known parameters that had contributed as causes of the slope failure in the area (Kishor Kumar et. al., 1996). The selected parameters of Landslide Hazard Evaluation Factor (LHEF) rating scheme have been defined for further analysis described in following section. A total estimated hazard (TEHD) value for each grid is obtained which is then classified into Landslide Hazard Zones. 2.1 Geology and Tectonics of study area The study area belongs to one of the Lesser Himalayan divisions, i.e. Rudraprayag Formation of Garhwal Group. Rudraprayag Formation has five litho-types (members) viz. Uttyasu Quartzite, Lameri Member, Haryali Quartzite, Thalasu Schistose Grit, and Karanprayag Metavolcanics. Kaliasaur area comprises of Uttyasu Quartzite (the oldest stratigraphic unit exposed). This member consists of white, pink, purple, medium to coarse grained, profusely ripple marked quartzite and subordinate purple/greenish gray slates with Basic Metavolcanics. Kumar and Agrawal (1975) have referred this unit as Rudraprayag Quartzite of Rudraprayag Formation. As the Formation and the Member unit cannot be designated by the single term, R.N. Srivastava and A. Ahmad (1979) have replaced the term Rudraprayag Quartzite with Uttyasu Quartzite. 3. Methodology and Procedure The objective of this study basically was to prepare a micro-hazard zonation map having risk/ hazard prone areas which could guide the road maintenance agencies so as to take appropriate safety measures for risk free unhindered flow of traffic particularly in rainy seasons. The process of accomplishment of the above said objectives began with a detailed field study of the selected site that included structural and lithological mapping of the area and collection of the necessary geological and geotechnical data. Prior to detailed field investigation a large scale topographic map on 1:500 scale was prepared. All subsequent field mapping was carried out in the same scale. Thereafter the map was divided into grids measuring 45 x 45 m (Figure 2) to individually study each grid for a realistic and accurate evaluation of stability of the slope and inferring the type of possible failure for each of the grids. Following the above mentioned methodology each grid has been classified into different zones of potential landslide hazard. These grids with their landslide hazard potential have been utilized to develop a landslide hazard map of the whole area. Stability has also been assessed for rock slopes of the area by quantifying the various parameters viz. lithology, structural relationships, slope gradient, landuse/ landcover, and water conditions that define the susceptibility of slopes to failure, for each of the grids of the study area. The Landslide Hazard Evaluation Factor (LHEF) rating scheme used in this study, is a field investigation based technique and is being effectively used for Landslide Hazard Zonation in (BIS Code 14496) [5] i.e. the classification of an area into small zones on the basis of their relative susceptibility to landslides. The parameters of investigation usually consist of information regarding geological and geotechnical has been modified to some extent as per the site condition (Table 1). 880

4 Figure 2: Grid map of the study area. Table 1: Proposed LHEF Rating Scheme Parameter Rating Lithology 2 Structural Relationships 3 Slope Gradient 2 Landuse/ Landcover (including Road Construction)) 2 Groundwater Condition 1 TOTAL 10 The ratings have been assigned to the selected parameters for each of the grids. The obtained ratings for all the parameters in each of the grids were then added to get a resultant total estimated hazard (TEHD) value for each of the grids. This TEHD value is then used to define the micro hazard zonation class (Table 2). For each of the five selected factors, based on the gathered information, separate layers have been created. These five layers have been superimposed to obtain the TEHD value and the final micro-hazard zonation map. Table 2: Micro Hazard Zonation on the basis of Total Estimated Hazard Zone TEHD Value Description of Zones I <2 Very Low Hazard VLH) Zone II 2 4 Low Hazard (LH) Zone III 4 6. Moderate Hazard (MH) Zone IV 6 8 High Hazard (HH) Zone V >8 Very High Hazard (VHH) Zone 4. Rating Scheme of Parameters 4.1 Lithology Lithology of an area combined with various conditions of the rocks, i.e. compaction, deformation, fracturing, intrusions, alteration etc. holds good significance as a factor responsible for causing landslides. The stability of a site, thus, can be inferred from the lithology/ rock type it inhibits and its conditions ( Brabb, 1972). The study done by Gupta and Joshi (1990) in Ramganga catchment area shows that the landslide is more prone in 881

5 quartzite, where the study area is also rich in the quartzite. The grids of 45 X 45 m were placed over the geological map of the area which was prepared over the surveyed area (Figure 3). Different types of quartzite, i.e. pink, white and brown (Figure 4. a,b,c) having varying degree of weathering and other conditions are intruded by scattered patches of metavolcanics (Figure 4. d). The rock type falling in each of the grids was noted and given a rating. The rating was then corrected for various weathering condition (Table 3) of the rocks. If within a grid, two or more rock types are encountered, the one holding higher percentage was considered for the whole grid, but the grid was further subdivided in cases where the occurring rock types are in approximately equal proportion.on the basis of the obtained ratings and lithological rating map have been prepared (Figure 6.a). Figure 3: Geological and Structure map of study area a b c d Figure 4: (a) Pink Quartzite (b) White Quartzite (c) Brown Quartzite (d) Metavolcanics 882

6 Table 3: Proposed LHEF Rating Scheme for Lithology Factor Category Rating Remarks Lithology Correction factor for weathering: (a) Highly weathered rock discolored, Rock Type Type I Quartzite (white, pink & brown) 0.2 joints open with weathering products, rock fabric altered to a large extent; Correction factor C 1. (b) Moderately weathered rock discolored with fresh rock patches, weathering more around joint planes, but rock intact in nature; Correction factor C Structure Type II Metavolcanics 0.4 (c) Slightly weathered rock slightly discolored along joint planes, which may be moderately tight to open, intact rock; Correction factor C 3. The correction factor for weathering should be multiplied with the fresh rock rating to get corrected rating. For rock type I C 1 = 4, C 2 = 3, C 3 = 2 For rock type II C 1 = 1.5, C 2 = 1.25, C 3 = 1.0 Structure of rocks is studied based on the attitude of beds, slope gradient, discontinuities present, and their inter-relationships (Hoek & Bray, 1981). The discontinuities include joint sets, faults, foliation, fractures etc. The interaction of structural parameters helps in classifying the weak and stable zones in a site and produces a clear picture of the area s potential landslide zones. Structural analysis in itself is a very important parameter but becomes indispensable in landslide recognition when related to slope gradient. On the basis of slope and joint orientations, two types of possible failures were identified using stereoplots as planar (Figure 5a) and wedge (Figure 5b) failures for each of the grids. Figure 5: Stereoplots of the study area; (a) Planar failure (b) Wedge failure A structural map of the area (Figure 3) has been prepared and the grids distributed on it. Attitude of beds and major discontinuities in form of sets of joints and foliation in each grid have been identified. Structural entities have been defined for both planar and wedge failure conditions (Table 4). First the type of probable failure is assessed from the grid s stereoplot and then the appropriate mathematical formula, as per the structural rating scheme (Table 4) is applied. The obtained results for the type of failure in a grid almost match with what had 883

7 been observed in the field. On the basis of the obtained ratings, and structural rating map have been prepared (Figure 6.b). Table 4: Proposed LHEF Rating Scheme for Structural Factor Category Rating Remarks Structure Relationship of structural discontinuity with slope. Relationship of parallelism between the slope and the discontinuity.* PLANAR (α j α S ) WEDGE (α i α S ) Relationship of dip of discontinuity and inclination. PLANAR (β j β S ) WEDGE (β i β S ) Dip of discontinuity. PLANAR (β j ) WEDGE (β i ) I > 30 0 II ( ) III ( ) IV ( ) V 5 0 I > 10 0 II ( ) III = 0 0 IV (0 0 (-10 0 )) V < (-10 0 ) I > 15 0 II ( ) III ( ) IV ( ) V α j = dip direction of joint α i = dip direction of line of intersection of two discontinuities α s = direction of slope inclination β j = dip of joint β i = plunge of line of intersection of two discontinuities β s = inclination of slope. Category I = Very favourable, II = Favourable, III= Fair, IV = Unfavourable, V = Very unfavourable *Discontinuity refers to the planar discontinuity or the line of intersection of two planar discontinuities, whichever is important concerning instabilities. 4.3 Slope Gradient The slope gradient has been assessed for each grid using topographic map of the area. This angle is then classified into five categories as very gentle, gentle, moderate, steep and very steep slope. Table 5: Proposed LHEF Rating Scheme for Slope Gradient Category Gradient Rating Escarpments/ Cliff > Steep slope Moderately steel slope Gentle slope Very gentle slope <

8 The highest frequency of landslides occurs in the slope class followed by and (S. Sarkar et al., 1995) Each of the grids is then assigned an appropriate rating on the basis of proposed LHEF rating scheme (Table 5 and Figure 6.c). 4.4 Landuse / Landcover Land when utilized efficiently can be saved from erosion and weathering and hence, stabilized. Landcover/ Landuse, thus, need to be studied for potential landslide assessment. Landcover can be inferred as dense forest, barren land, agricultural land and industrial or commercial land. The slope stability generally increases with better forestation and vegetation. Barren land is most susceptible to landslides whereas a dense forest or highly vegetated landcover is least susceptible. It has been observed that the slopes which are barren have been utilized for the purpose of road construction and in current study is considered one of the most influencing anthropogenic factors on slope stability.based on the landuse and vegetation cover, the rating system has been given as indicated in Table 6 and Figure 6(d) Table 7: Proposed LHEF Rating Scheme for Landuse/ Landcover Landuse and Landcover Rating Agricultural land/ Populated flat land 0.40 Thickly vegetated area 0.80 Moderately vegetated area 1.20 Sparsely vegetated area with lesser ground cover 1.60 Barren land/ road construction 2.00 Table 8: Proposed LHEF Rating Scheme for Groundwater conditions Groundwater Condition Dripping Flowing Wet Damp Dry Rating Water Conditions Hydrology of an area plays vital role in determining the susceptibility of rock/ soil to failure. Hydrological properties of an area include presence of streams, rivers, underground water conditions, saturation state of rock/soil, and drainage pattern of the area. Dislocated water bodies due to presence of discontinuities and shallow water-table conditions in hilly terrains along with heavy rainfall make the slopes prone to instability.during the prolonged monsoon phases, increased pore-water pressure creates favorable conditions for deep-seated landslides (Y.P. Sundriyal et al., 2007).The hydrological conditions at the chosen site do not pose much of the challenge for slopes as the investigations reveal, the area is almost dry. Only near the toe of the slope along the bank of river Alaknanda, the slopes are found damp. Drainage 885

9 density in the area is also low. Thus, ratings for dry condition have been assigned to the study area (Table 7).Included in landuse is also the road construction factor. Toe cutting for road construction in hilly terrains like this is one of the most powerful triggering factors of the landslide, as after cutting the slope face is left barren that gets easily degraded. Thus, the areas along the highway, where slope has been cut have been assigned rating two. Figure 6: (a) Lithological rating map of the study area (b) Structural rating map of the study area (c) Slope gradient rating map of the study area (d) Landcover / Landuse rating map of the area 5. Conclusion Kaliasaur landslide, five decades old problem is still persisting on NH-58 at km 147. The Micro Landslide Hazard Zonation map has been developed following LHEF rating scheme in a large scale of 1:500. The area divided into equal-sized grids of 45 x 45 sq. m provides opportunity to assess the susceptibility to failure for each grid individually. The Micro LHZ map developed (Figure 7) indicates three levels of hazard, low, moderate and high (II, III, & IV). Low hazard zones represent currently stable slopes whereas the maximum area is covered under the moderate degree of hazard representing the slopes that are not stable and may fail as soon as any of the triggering factors come into play. Several zones are marked by high degree hazard indicating the slopes that are not stable and have already failed. Total estimated hazard value, obtained by adding the individual ratings of all the parameters, varies 886

10 between 3 (medium) and 8 (high) which are descriptive zones of moderate to high hazard. The Micro Hazard Zonation Map (MHZM) has clearly indicated the presently affected areas under high hazard. The results have been justified in the field and found worth recommendation. Figure 7: Final Micro Hazard Zonation Map Even comparing MHZM with Rock Mass Rating (RMR) (Figure 8) and Slope Mass Rating (SMR) (Figure 9) of the study area, area coming within high hazard zone is also falling in poor stable conditions describe by above mentioned geomechanical classifications. Figure 8: RMR of the study area Figure 9: SMR of the study area So approach to new methodology defining new ratings to parameters.i.e. Litholgy, Structure, Slope, Landcover and Landuse and Water conditions; are coming approximately accurate for Kaliasaur landslide. Also defined methodology can be modified for site specific study. 887

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