In September 2011, Tindharia, a picturesque town located on the Siliguri-Darjeeling highway NH-110 (previously NH-55) in West Bengal, India experienced massive landslides following an earthquake and heavy rainfall. The landslide significantly affected three locations – S1 (Height 34.9m), S2 (Height 102.8m) and S3 (Height 38.7m) – resulting in the collapse of a section of roadway on the valley side of the mountain. Part of the Darjeeling Himalayan Railway (DHR), a UNESCO World Heritage Site that runs through Tindharia, was also cut-off due to this landslide. There was severe socio-economic impact on the residents along the NH-110 (previously NH-55) due to stoppage of vehicular movements and shortage of key supplies. The Ministry of Road Transport & Highways, Government of India in collaboration with the Public Works Department (N.H. Circle-III), West Bengal took the initiative to restore the landslide location and the historic road.
Terre Armée India (Reinforced Earth India Pvt. Ltd.) was selected as the technology provider for this project. Terre Armée India designed a permanent and sustainable solution for restoration of this landslide. This complex engineering project required a mix of various innovative techniques to be considered at design and construction phase. Very limited space was available at the base of the valley for constructing such a tall retaining structure, to widen the road, and stabilize the existing unstable slope.
1. A challenging landslide protection and road widening project was successfully completed on NH-110 (previously NH-55) near Darjeeling in West Bengal, India.
2. Innovative TerraLink® technology was used by Terre Armée India to build this engineering marvel with limited available space at base (2m to 5m), using high adherence GeoStrap® directly and mechanically connected with GeoTrel® facing with TerraNail® and further strengthened using pre-stressed FreyssiAnchor® (completely re-stressable cable ground anchors) in this project to stitch the new construction with the existing hill profile.
3. The mechanical connection system adopted for this project as per codal provisions was an essential requirement to ensure transfer of loads between facing, soil reinforcement and anchors/ nails in high seismic zones and the overall stability and greater performance of the reinforced filled structure.
4. Highly compressible and durable (with heavy galvanisation of 500 gsm) fusion welded steel grid mesh (made of minimum 8mm diameter bars) was used to withstand higher deformations to meet any internal settlements.
5. Safety and quality were top priorities for the project. Terre Armée India adopted the safest construction method for this unique project.
6. Reduction of backfill quantity (up to 10x) led to cost optimisation and minimised environmental impact.
7. The improvement of surface and sub-surface drainage systems ensured the structure’s long-term performance.
8. The structure has performed well under seismic load when recently subjected to an Earthquake of 5.1 magnitude.
Terre Armée India offered their innovative TerraLink® technology (shored or composite Reinforced Earth® structure) for this project after conducting a detailed global and compound stability check of the collapsed slope. TerraLink® technology comprised of High Adherence GeoStrap® (geosynthetic strips made of polyester yarns encapsulated in LLDPE) as soil reinforcement, TerraNail® (soil nails) as in-situ passive soil anchoring elements, and FreyssiAnchors® (Prestressed Ground Anchor System) as in-situ active soil anchoring system with GeoTrel® as facia of the structure. All the soil reinforcement, facia and soil anchors are mechanically connected to provide a robustly and engineered structure which cannot be displaced from differential settlement or under impact from dynamic loads. The use of mechanical connection system between the soil reinforcement and soil nails ensures greater stability of the shored Reinforced Earth® structure and superior performance under extreme loading conditions. TerraLink® also optimized the cut and fill volume in this project. Terre Armée India, the designer of the solution, supplier of materials and project management along with their construction partners (GPT Infraprojects Ltd., Shiva Structures Pvt. Ltd. and Pioneer Foundation Engineers Pvt. Ltd.), DPR , project consultants (LEA Associates South Asia Pvt. Ltd.) and Authority Engineer (TASPL – Xplorer JV) successfully completed the three structures of the project.
The S2 Structure, which stands supreme at 102.8 meters, makes Project Tindharia the world’s tallest Reinforced Earth® structure and an acme of tall, steep slope soil reinforcement engineering. The innovative and sustainable TerraLink® solution used in this project has resulted in the widening of existing roadway, slope stabilisation, drainage improvement and rehabilitation of the massive landslide at Tindharia. The project is now complete, the highway and rail connectivity are fully open to commuters. Residents are now happy, and they look forward to socio-economic development of the area and see normal road and rail traffic back on this small mountain town leading to the hill station Darjeeling.
This project’s landmark success will surely instil greater confidence amongst the Engineering Fraternity in India as well as in various parts of the world and encourage them to take up more complex and challenging projects in the future.