Large tracts of land in Kerala are sitting on a sprawling network of underground tunnels that could cave in any time, posing a threat to infrastructure and human habitation, an expert committee appointed by the government has warned.
The vulnerability to land subsidence has been attributed to soil piping, the formation of underground tunnels due to subsurface erosion. Considering the extent of the threat, the government has issued an order declaring soil piping as a State-specific calamity.
The study by experts from the National Centre for Earth Science Studies (NCESS) and the State Emergency Operations Centre (SEOC) has reported that the phenomenon is a serious problem across the highlands and foothills in Kerala, except in Thiruvananthapuram, Kollam and Alappuzha districts.
The committee has expressed concern over the safety of the Banasurasagar dam in Wayanad, the largest earthen dam in India, due to soil piping in the locality. In its report to the National Disaster Management Authority, the panel has also recommended detailed studies to assess the possibility of the tunnels extending to the border districts in Karnataka.
The team headed by G. Sankar, former scientist, NCESS, has unearthed evidence that the tunnels are spreading across the highlands. At many places, the network has grown beyond any possibility of mitigation.
The scientists came across gaping underground tunnels in the worst affected districts of Kasaragod, Kannur and Idukki. The cavernous gullies, some of them more than 10-ft wide, have caved in at some places, damaging roads and farmlands and affecting groundwater storage.
Geotechnical studies have indicated that laterite soils and deforested land are more prone to soil piping.
Sekhar L. Kuriakose, Head (Scientist), SEOC, who was part of the study team said more focussed research on soil piping would be needed to understand the complexity of the process and take up mitigation measures in affected localities.
The report has called for a village-level survey to map the affected areas. It has also recommended soil studies before taking up major development projects in the highlands.
The erosion typical of soil piping starts with water, possibly originating from a stream, percolating down from the surface through a crack and 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.
