Overview of Rainfall Induced Landslide Events and Importance of Geotechnical Investigations in Nilgiris District of Tamil Nadu, India

Landslide is a major natural hazard, which possesses serious threat to human population and various other infrastructures such as road, rail routes and other structures in the hilly terrain. Nilgiris district, located in southern state of Tamil Nadu in India, is basically a hilly region, lying at an elevation of 1000–2600 m above Mean Sea Level (MSL). The district regularly receives rainfall during both the southwest and the northeast monsoons. The average rainfall in this region is between 1500 and 3000 mm. Due to the steep slopes and heavy rainfall during monsoons, the district is highly prone to landslides. In recent times, occurrence of landslides in Nilgiris is increased due to sudden burst of development activities in the region. The present study gives an overview of landslide events and remedial options practiced so far in Nilgiris and emphasises the importance of geotechnical investigation in that region. The history of landslide events has been collected and soil samples have been taken from the places where, major landslides have occurred in the past to characterise them. The rainfall data have been collected to understand the influence of rainfall on landslide occurrence. From this study, it is observed that the main triggering factor for landslide in Nilgiris is heavy intense rainfall. Though the rainfall is main triggering factor, many causal factors are also involved in triggering of landslides. It is also observed that, the remedial measures practiced in this area was not site specific and mainly concentrated on construction of retaining walls and small gabion walls. To understand the failure mechanism of landslides, various effects of pore water pressure and to suggest suitable site specific remedial measure to prevent the recurrence of such failures in the future, detailed geotechnical investigations are required.