Restoration Measures of Landslide Affected Valley Slope—A Case Study of Tindharia Area Along NH 55 in West Bengal

Abstract

The existing road width including the narrow gauge railway line of “NF Railway” were affected partially to fully due to the occurrences of landslides on the valley slopes in total of three numbers of locations within the chainage from km 50 + 155 to 53 + 485 in and around location of “Tindharia” area along NH 55, i.e., Darjeeling–Siliguri road in the state of West Bengal on September 18, 2011. The landslides happened during the rainy season of the said year subsequent to the occurrence of earthquake in the adjoining area of Sikkim. Total height of the landslide affected valley slope in “Tindharia” area was in between 30 m and little more than 100 m. The probable reasons of landslide in the area were anticipated due to frequent occurrences of earthquakes in addition to the heavy rainfall, poor drainage, and also due to the fragile geological conditions of the existing slope. A detailed investigation program covering the topographical, geological cum geotechnical aspects of the affected area and also on the proposed construction materials were undertaken to formulate the suitable rehabilitations measures of the failed valley slopes. The detailed engineering design by analyzing the stability of proposed rebuilding of valley slopes corresponding to widening of the affected road bench was done as per the available standards and guidelines for ensuring the adequate safety under static and seismic conditions. The proposed restoration measures consisted of composite reinforced earth technology by strengthening the existing exposed valley slope surface with adequate steel nailing at first and then rebuilding of the road bench by newly added steep reinforced soil slope using layers of designed polymeric reinforcement in multi-tiered manner covering the entire affected heights. The nailing of the existing slope was proposed to connect suitably with the soil reinforcement of the newly added valley side steep slope. Adequate surface and subsurface drainage measures including the erosion protection schemes were provided for the restored valley slope. The surface cracks generated due to landslide at the existing road bench and also on the hill slope portion were sealed with suitable filling material. This paper presents the details of investigation, design analysis, and actual construction works done for the restoration of one of the landslide affected valley slopes of moderate height of 30–34 m in “Tindharia” area.KeywordsLandslideInvestigationsValley slopeEarthquakeRainfallReinforcement steel nailingReinforced soil slopeDrainage and erosion protection measures