Heliborne Survey to Map Rock Structures Next Week

Hyderabad: A helicopter-borne geomagnetic survey over the Nallamala Hills of the Amrabad Tiger Reserve, under which the Srisailam Left Bank Canal (SLBC) tunnel is being dug, is expected to take place next week.

The survey, a crucial step in finalising an alternate route for it to bypass the soft rock portion where a collapse occurred this February, is expected to provide rock strata data for up to a depth of 1,000 metres along the tunnel’s existing alignment.

For the tunneling work to restart, engineers and experts had concluded that a diversion would be required to avoid soft rock, or water-laden portions of the rock along the original alignment that could collapse as it did on February 11 this year in the inlet section the tunnel. There are some sections that could be very similar to the section that collapsed.

“Once the survey is completed, the data will give a clear picture of the underground rock structures which will help in devising plans for resuming work — first to design a diversion around the collapsed section and then to help prepare in calibrating tunneling for the rest of the 9.5 km of the tunnel that is still to be dug,” a senior irrigation department official said.

While work till this February involved the use of a tunnel boring machine that was destroyed in the accident, the rest of the tunneling will be done using the drill and blast method.

“In addition to the geological data from the survey, the modern drills used for punching holes in the rock for placing explosives, can also ‘read’ how the rock is ahead for the next five to 10 meters which will give even more real time information based on which just the right amount of explosive charges will be used to make the blasting process safe,” an official explained.