Five years before the February 22 collapse of the SLBC Tunnel in Telangana, which trapped eight men, a report commissioned by Jaiprakash Associates Ltd. alerted the public to a potential "fault zone" in the tunnel—areas where there is a significant risk of collapse, The Indian Express reported.
According to the Tunnel Seismic Prediction (TSP) - 303 Plus report, prepared by Amberg Tech AG, a tunnelling surveying business in January 2020, the fault zone was located between 13.88 km (13,882 m) and 13.91 km (13,914 m) from the tunnel's mouth. The research also noted that the stretch was a water-bearing zone and discovered a decline in rock strength in this area.
The tunnelling contract was given to Jaiprakash Associates Ltd, a Noida-based business, in 2005. The company calculated the risk by transmitting seismic waves into the rock mass and documenting and analysing the areas with anomalies. "Decrease in rock stiffness," "possible jointed sheared rock mass," and "possible water-bearing zone" were among the notes in the study.
Whether the report submitted to Jaiprakash Associates Ltd in 2020, was shared with the Irrigation Department of the state government is not known,” Indian Express quoted a source.
According to rescuers in the area, the collapse occurred close to this section. According to a senior official involved in the rescue efforts, "the roof collapsed for three meters around the fault zone."
The rescue operations have had to be intermittently stopped because the tunnel keeps flooding, and search and rescue operations are going on after 13.5 km into the tunnel, a rescuer said. Here, the tunnel boring machine is stuck at a dead-end, and the workers are nowhere in sight, the official said.
"In four years, the ground condition would remain the same — that is, the rocks would remain the same," a geologist responded when asked if the ground conditions may alter. However, there can be greater opportunities for water seepage.
In the meantime, a separate abnormality was discovered in 2020 by another examination on the tunnel. According to a study by Rituraj Deshmukh, an associate geologist at Jaiprakash, and Mandapalli Raju, the former director general of the Geological Survey of India, the tunnelling began with inadequate subsurface examination.
The paper, Geotechnical aspects of a long tunnel, driven by TBM at SLBC of AMR Project, Telangana State, India, reads, “As the entire tunnel alignment falls in Tiger Reserve Forest, subsurface investigations such as drilling boreholes or excavation of drifts along the tunnel alignment were not permitted. Further, most of the tunnel alignment is inaccessible for physical examination. As a result, excavation of the tunnel was commenced without any firm and confirmed geotechnical data.”
Tunnelling started with a “broad preliminary assessment of the terrain” based on “the data available and experience gained in tunnels of Srisailam Left Bank Underground Power Station present in the vicinity, and taking help of aerial photo study and using remote sensing techniques”, the paper read.
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