Location: Chungthang Village near Mangan, North Sikkim
Brief of the Problem: Unstable Massive Sliding Material in the Vicinity of Dam Spillway
The Site geology of the dam site area lies in the rock formation of central crystalline; subdivided into Chungthang series, Darjeeling gneiss & Rongli series of Pre-Cambrian age. Chungthang series comprises of quartz, biotite gneiss, biotite schist, calc-silicate and thinner bands of argentiferous – sillimanite heavily micaceous gneissose. Rock units are highly folded; asymmetric, isoclinal in nature with NE dips.
Landslide issue was observed due to excessive stripping and cutting of the left abutment site near the spillway chute. About 1 lac m3 debris slid down the hill after the cutting work at the left abutment of the dam site.
Engineered Solution: Mitigation Measure for Slope Stabilization
How to cut the area vertically 30 m*400 m horizontally in this critical geology was the biggest question and that’s when Mr. SK Aggarwal (MD, Teesta Urja Limited) came up with the concept of cladding wall with permanent ground anchors.
Energy Infratech calculated various designs of ground anchors like Rock bolt, Self-Drilling Anchor and finally they came up with a design of 1-meter thick cladding wall with permanent active cable anchor. The anchors of a 100-ton capacity of 20-50 meter length at the inclination of 150 with the spacing of 3(H) × 3(V) meters was purposed and was tested for the design loads as per the Standard Codal provisions.
The mechanism behind the stabilization is due to an increase in the shear strength along the failure surface, as a prestressed cable anchor increases the active resistance resulting from the stage of mobilization of sheared mass along the slip line.
Difficulties faced during execution:
Presence of variable strata such 30-35 m thick overburden, boulders created damage to driving operation for the casing. A greater amount of boulder strata created damage to casing shoe. GEO SPAR import some specialized tools to overcome challenges so as to operate and drill through loose and thick overburden, underlying profiles.
The technology used for Execution:
GEO SPAR used robust machines of Atlas Copco – Mustang-5 for site execution operations to tackle the restriction in drilling (15 – 45 m) and driving casing (3-30m).
Stabilization of poor class rock in seismic and region of high pore pressure is very vital. The use of a prestressed cable anchor provides an effective and long term solution.
The current status of the project site is that the 30m vertical cladding wall having prestressed cable anchor with cladding wall survived the 2011 earthquake of 7 Richter magnitude scale with no signs of distress, whose epicenter was near the site. The earthquake caused significant damage to the dam body as well as the spillway.
This incident shows our solution accuracy and our perfect execution of slope stability work.
Geo Spar’s mission to provide fast and efficient services with lasting solutions was achieved.