Slope Protection and Drainage Improvement at Tindharia West Bengal, India

• Inclement Weather: Tindharia receives approximately 185 cm of average rainfall annually. Monsoon is active from end of May to beginning of October. Work got either suspended or very limited activities could be carried out during this period. With the heavy rainfall and surface water, the risk of damage to the structure was also very high during construction. Suitable measures had to be taken to cater this challenge.
• Unfavourable Geology: The geological strata of this area are susceptible to weathering and fracture. The available bond strength within the soil strata was also not very high. The drilled holes were found to be connected with each other in many occasions. This resulted in collapse of the drilled hole also. Consolidation grouting was adopted to solve this problem.
• Presence of Ground Water: The strata was almost saturated due to continuous percolation of water during monsoon or presence of natural aquifer. Therefore, while drilling the holes for nail, significant water flow was encountered. Long semi-perforated pipes were installed within the slope to cater to this challenge.
• Safety: Working in such a height was also a key challenge from safety point of view for men and machineries.
• Timely Completion: A major challenge was to complete the project within time considering the impact of high rainfall during monsoon and a long strike imposed by the local legislation.
• Impact of Covid-19: In 2020, the project schedule was badly affected by Covid-19. There was serious scarcity of workforce, the lockdown forced to stop the work for more than months. Utmost safety and precautionary actions were taken collectively by all stake holders in this project to overcome this challenge.

After going through the details of this project, Geoquest India promptly made a site visit and prepared a detailed feasibility report keeping in mind the technical and practical challenges of this location and client’s requirement. Out of various possibilities, Geoquest India chose to propose TerraLink® as the solution in this project.

TerraLink® is a shored Reinforced Earth® structure. In this project, the soil reinforcing High Adherence (HA) GeoStrap® 5 of the Reinforced Earth® structure is mechanically connected with the GeoTrel® facing on front and with TerraNail® and FreyssiAnchors® on the rear end. Galvanized steel GeoTrel® facing has been used in this project having mechanical connection with HA GeoStrap® 5 soil reinforcements. The choice of TerraLink® was made in a scientific manner considering the following facts in the project:

• Stability of the Slope: The construction was required to be done on valley side of a collapsed hill slope. Heavier load imposed on the slope by conventional structures such as RCC retaining wall, gabions etc. would reduce the stability. On the other hand, TerraLink® is constructed after stabilizing the failed slope using TerraNail® and FreyssiAnchor®. Therefore, TerraLink increases the stability of the collapsed slope and at the same time widens the road bench without increasing the dead load on the slope significantly.
• Construction within Limited Space: Innovative TerraLink® technology makes it possible to build very tall retaining structures (>35m) with limited available space at base (<2m), using direct and mechanical connection systems.
• Reduction of Backfill Quantity: Almost 650,000 cum backfill was required with conventional reinforced soil structure. TerraLink® can reduce the backfill quantity to 1/10th i.e. 60,000 cum. This would reduce the number of transportation loads on the narrow hill road, cut down on the use of machinery, reduce carbon foot print and eventually reduce the cost.

In the detailed design stage, lidar survey along with geological and geotechnical investigation were done at the project site. Based on the inputs of these investigation, Geoquest India conducted internal, external and global stability analysis to define the design of the structure. Geoquest India validated the design using finite element model. The maximum length of TerraNail® and FreyssiAnchor® drilled and installed in this project were 25m and 32m respectively. FreyssiAnchors® are completely re-stressable cable ground anchors used in this project to stitch the new construction with the existing hill profile.

Tindharia is located in a high rainfall zone. Drainage is very crucial for this kind of structure and improper drainage is often considered to be one of the major root causes of landslides in the hills. Special care was taken to improve and facilitate the surface and subsurface drainage systems in this project. Semi-perforated drainpipes wrapped with nonwoven geotextile were drilled and installed within the collapsed slope to drain out the entrapped water and release the pore water pressure. The location at S2 had a depth of 102.8m, hence Geoquest India designed the benches in slope for channelizing the surface drainage. Drops and guide drains were also designed and constructed on both sides of the structures.