International Journal of Research p-ISSN: 2348-6848 e-ISSN: 2348-795X Available at http://internationaljournalofresearch.org/ Volume 02 Issue 02 February 2015 Study of Nilgiri Hills - A Landslide Prone Area and Its Seepage Analysis Mohit Chugh *, Shraddha Satish Thumsi ** , Vivek Keshri *** & Dr. Chandrasekaran S.S *** * *School of Mechanical and Building Sciences, VIT University, Vellore, Tamil Nadu, India [email protected] ** School of Computer science and engineering, VIT University, Vellore, Tamil Nadu, India *** School of Mechanical and Building Sciences,VIT University, Vellore, Tamil Nadu, India **** Associate Professor, School School of Mechanical and Building Sciences, VIT University, Vellore, Tamil Nadu, India Email Id [email protected] ABSTRACT analyses of all the three case histories were carried out using PLAXFLOW India is among the top ten countries with software to understand the failure the highest percentage of landslide mechanism and contributing factors to fatalities for the past few years. Intense determine the critical slip surface and rainfall during the monsoon in 2009 in the factor of safety. Safety analysis using flow hilly district of Nilgiris, in the state of field technique is carried out for the slope Tamilnadu in India, triggered landslides at at Aravankadu..Degree of saturation and more than three hundred locations which pore pressure of Coonoor site revealed affected road and rail traffic and that the zone of intense shearing behind destroyed number of buildings that left the retaining walls due to combined effect more than forty people dead and hundreds of surcharge loading of building and homeless. In this report three case generated pore pressure. histories are investigated: failure of slope in a railway track at Aravankadu , failure Keywords : Landslide; rainfall; slope; of retaining walls supporting buildings at retaining wall; finite element Coonoor, failure of slope and retaining INTRODUCTION wall along national highway at Chinnabikatty. Laboratory investigations Landslides are one of the natural hazards are carried out on soil samples collected that affect fifteen per cent of the land area at the sites. Soils at all the three locations of India. India is among the top ten countries with the highest percentage of have high fine content and low values of landslide fatalities for the years 2003, 2007 coefficient of permeability. Finite element and 2008 (Kirschbaum et al. 2010). Available online: http://internationaljournalofresearch.org/ P a g e | 139 International Journal of Research p-ISSN: 2348-6848 e-ISSN: 2348-795X Available at http://internationaljournalofresearch.org/ Volume 02 Issue 02 February 2015 Landslides of different types are frequent in geodynamically active regions in the Slope stability is analyzed by the limit Himalayan and the north eastern parts as equilibrium methods, boundary element well as in the relatively stable regions of methods (Jiang 1990), finite element the Meghalaya Plateau, Western Ghats and methods (Matsui and San 1992, Kellezi et Nilgiri Hills of the country (NDMG 2009). al. 2005), neural network methods (Cho The phenomenon of landslides is 2009), reliability methods (Dodagoudar pronounced during the monsoon period and Venkatachalam 2000, Zhang et al. (NDMG 2009). Rainfall induced 2011), distinct element method (Bobet et landslides in India in the recent past al. 2009) and hybrid continuum- include slides in Sikkim, Uttrakhand and discontinuous methods (Stead et al. 2006). Western ghats (Anbarasu et al. Limit equilibrium methods of slices are the 2010;Sengupta et al. 2010; Venkatachalam most commonly used methods among et al. 2002). Landslide disasters have both others since simplicity and ease of use are short-term and long-term impact on the main advantages (Alkasawneh et al. society and the environment. In the year 2008, Cheng et al. 2007). 2005 alone, more than five hundred human lives were lost due to landslides in India (NDMG 2009). In the recent past, many The finite element method offers an studies have been reported on shallow alternative that is more rigorous, free of landslides, debris flow and rainfall- excessively unrealistic assumptions, and induced landslides (e.g. Ozdemir and able to provide stress and deformation Delikanli 2009, Teoman et al. 2004, information. Among the methods for slope Jotisankasa and Vathananukij 2008). It is stability analysis using finite elements, generally recognized that the rainfall- gravity increase method (Swan and Seo induced landslides are caused by excess 1992) and strength reduction method pore pressures and seepage forces during (Matsui and San 1992) are most widely periods of intense rainfall. It is the excess used methods (Alkasawneh et al. 2008). pore water pressure that decreases the The strength reduction method was used effective stress in the soil and thus, reduces for slope stability analysis by various the shear strength of the soil, consequently researchers (Matsui and San 1992, Dawson resulting in slope failure (Anderson and et al. 1999, Griffiths and Lane 1999). The Sitar 1995, Cascini et al. 2010). Xu and main advantages of strength reduction Zhang (2010) observed that the rainfall methods are: the sliding surface does not infiltration and a failed drainage system need to be defined beforehand and is behind the retaining wall resulted in high automatically found from the shear strain saturation of the clay and finally led to an arising from the application of gravity undrained failure of slope above a railway loads and the reduction of shear strength, line in China. Cascini et al. (2010) therefore, a shear surface closer to the presented geomehanical modeling of natural sliding surface is defined (Berilgen failure and post failure stages of rainfall 2007, Cheng et al. 2007); and can give induced shallow landslides of the flow- information such as stresses, movements type. The effects of positive pore pressure and pore pressures (Cheng et al. 2007). generation in saturated slopes from intense Deformation analysis using two rainfalls have been analyzed by Johnson dimensional finite element plane strain and Sitar 1990. Kirschbaum et al. (2010) model has also been used to study the compiled a landslide catalog for rainfall behavior of slopes (Berilgen 2007), sheet triggered events for several years. pile wall (Tan and Paikowsky 2008, Available online: http://internationaljournalofresearch.org/ P a g e | 140 International Journal of Research p-ISSN: 2348-6848 e-ISSN: 2348-795X Available at http://internationaljournalofresearch.org/ Volume 02 Issue 02 February 2015 Georgiadis and Anagnostopoulos 1998) and retaining wall (Shamsabadi et al. 2010). This report presents three case histories of failures suffered by infrastructural facilities due to rainfall induced landslides in Nilgiris, India in 2009. Soil samples were collected and experimental investigations were carried. Finite element analyses of all the three case histories were carried out using PLAXFLOW. THE NILGIRIS GEOMORPHOLOGY OF The Nilgiris district is located in the southern state of Tamilnadu in India. The NILGIRIS location of Nilgiris is shown in Figure 1. The Nilgiri Hills rise aloft from the upland The Nilgiris is bounded on the north by the of Coimbatore which is a plateau sloping state of Karnataka, on the east by steeply into the Mysore Plateau towards Coimbatore and Erode districts, on the north and merging gradually with the south by Coimbatore district and on the western ghats in the north-west, west and west by the state of Kerala. The Nilgiris is south-west. The drainage is radial at many situated at an elevation of 900 to 2636 points due to dominant high points, and is meters above Mean Sea Level (MSL). The influenced mainly by the joint pattern and topography of the Nilgiris district is rolling foliation trends of the rocks (Rajakumar et and steep. Tea, carrot, potato, pepper, al. 2007). Two types of landforms have cabbage, coffee and spices are important been identified in the Nilgiris region. The agricultural products of the district. The Dodabetta landform has many high peaks Nagapattinam -Gudalur National Highway having steep slope and escarpments with (NH67) passes through the district. The or without soil cover, around which a Nilgiris is an important tourist center in radial drainage pattern occurs. The southern India. Nilgiri Mountain Railway Ootacamund landform has gentle mounds (NMR), Mettupalayam to Ooty line, was with a thick soil cover. The Nilgiris region declared as World Heritage site by falls in the tropical zone of weathering. UNESCO. The Nilgiris is in seismic zone Most part of the Nilgiris is deeply III (IS 1893 -2002). According to landslide weathered and at some places thick soil hazard zonation map prepared by cover of up to 40 m is found (Rajakumar et Geological Survey of India, the Nilgiris al. 2007). Intense precipitations followed has moderate hazard (NDMG 2009). by dry periods have helped in the Available online: http://internationaljournalofresearch.org/ P a g e | 141 International Journal of Research p-ISSN: 2348-6848 e-ISSN: 2348-795X Available at http://internationaljournalofresearch.org/ Volume 02 Issue 02 February 2015 formation of considerable depth of Daily rainfall data for two weeks before weathering (Seshagiri et al. 1982). The the occurrence of slides in the discussed Nilgiris mountain ranges comprise of case histories collected from nearby rain Archaean metamorphic rocks like gauge stations and presented in Table 1. charnockite, biotite gneiss, magnetic quartzite,