SLBC: More unstable zones were ahead of the collapsed tunnel section
The February 22 collapse in the inlet section of the Srisailam left bank canal (SLBC) tunnel is being viewed as a precursor to what might happen if the digging continues, using the second tunnel boring machine (TBM) in a bid to complete the work.
Hyderabad: The February 22 collapse in the inlet section of the Srisailam left bank canal (SLBC) tunnel is being viewed as a precursor to what might happen if the digging continues, using the second tunnel boring machine (TBM) in a bid to complete the work.
Officials said there were more fault zones and softer aggregate formations, such as the one that caused the collapse. Unless these locations are studied fully, proceeding with the work was not advisable.
Recent reinterpretation using modelling software — that was not available in the past — of geological data from previous tunnel seismic prediction (TSP) studies, according to sources, showed more clearly that the work was progressing through a weakened section of the rocks. The collapse occurred when the disaster-hit TBM was making progress through a fault zone.
There were two more such zones just ahead of this location in less than 500 metres. The TSP studies used artificially generated seismic waves to predict geological conditions during tunnel construction.
It was a result of this reinterpretation and 3-D modelling of past data, and the fact that the overburden of the Nallamala hills may not be as stable as previously believed, prompted a proposal to consider bypassing the likely unstable zones along the existing alignment.
A workable and safer solution to the problem, officials said, was to realign the tunnel, but not with the same existing 9.2 metres width but as two smaller 5-metre wide ones. “These can avoid the fault and shear zones, and the moisture laden overburden of the hills above. This is a proposal that merits pursuit,” an official involved with the ongoing rescue operations said.
This proposal came up for discussion during a review with all rescue team heads that Chief Minister A. Revanth Reddy held at the tunnel’s inlet site on Sunday. The tunnel is being dug under a protected forest area – the Amrabad tiger reserve – in which any overground activity, such as a survey requiring drilling of boreholes, that disturbs the surface is not allowed unless environment, forest, and wildlife clearances are obtained from the Centre.
This being the case, one of the ideas mooted was that the National Geophysical Research Institute (NGRI) be asked to conduct an aerial geophysical survey along the tunnel alignment and possible areas for the bypass tunnels before any decision is taken.
It may be recalled that Revanth Reddy, on Sunday, had called on the rescue agencies to prepare an action plan for the way forward, based on the lessons learnt from the accident. Such a document, and a plan, can be a blueprint for the future for anywhere is any such incident occurs, he had said.
