Pilot project for mitigation has been initiated at Ranni

The Department of Disaster Management has initiated a pilot project for mitigation of the threat posed by soil piping, the formation of underground tunnels due to sub surface erosion, in parts of the State.

The project, taken up by the Hazard Vulnerability and Risk Assessment (HVRA) Cell in collaboration with the Centre for Earth Science Studies (CESS), has been initiated at Ranni in Pathanamthitta district. It is part of a research programme funded by the National Disaster Management Authority (NDMA) to assess the extent of the underground network of tunnels that poses a serious threat to human habitation.

“Soil piping is a strange phenomenon. It is worrisome because if you do not discover it in time, there could be problems due to subsidence in a vulnerable place,” says T. Nandakumar, member, NDMA. Apart from studying the phenomenon, the research team will also test out a few mitigation methods.

Sekhar Kuriakose, head, HVRA cell, said the mitigating solution being tried out at Ranni, involved certain methods to fill up the sink holes and tunnels caused by subsidence.

“We found that filling up the holes with sand is useless because the subsidence recurs due to underground leaching. So we have come up with specific recommendations to prevent the recurrence,” Mr. Kuriakose said. The method involves the use of lime, rock fragments, old scooter tyres, and coir geotextile to fill the sink hole and planting deep-rooting grass or trees to stabilise the land. Mr. Nandakumar said the most feasible option, environmentally and otherwise, would be recommended to the Public Works Department (PWD) or another agency.

Meanwhile, the research team is carrying out resistivity imaging studies to assess the threat posed by soil piping. The study is expected to help scientists trace the tunnel network and water flow by applying small electric currents across an array of electrodes planted on the ground and measuring the variations in resistivity. Experts will gen erate mathematical models to identify the vulnerable areas.

Apart from Pathanamthitta, gaping sub surface tunnels have been discovered at many locations in Kannur, Idukki, Thrissur, Wayanad, and Kozhikode districts.

The cavernous gullies, some of them more than 10-ft wide, have caved in at some places, threatening to degrade the land. Scientists say the tunnels are formed due to the erosion of clay-rich soil by an underground water source, possibly a stream percolating down from the surface through a crack. Soil piping starts with the water cutting out a channel as it enters the earth. The flow triggers a suction force, drawing in soil from the sides. Over time, the narrow channel is carved out into a larger pipe. As the pipe enlarges, the flow becomes more concentrated and turbulent. Sub surface pipes are known to extend some distance as a continuous channel or as a system of interconnected tunnels that form an extensive, branched network.