KOCHI: The land subsidence that occurred in some parts of Idukki district recently is a consequence of soil piping or tunnel erosion — the erosion of sub-soil.
Experts who studied the process said soil piping had been taking place in the Udayagiri and Udumbanchola areas in the district since 2000.
The erosion, which takes place beneath the surface of the earth, often goes unnoticed. However, the indications of soil piping, besides the caving in or subsidence of land and the formation of craters, were noticed during August. A scientific assessment was also carried out in the areas after the reported incidents.
At present, a few subsidence craters can be seen in the areas. Two wells have also collapsed owing to the phenomenon, the experts said.
During soil piping, large quantities of sand and clay in the sub-surface are carried away by water, resulting in the formation of tunnel-like cavities beneath the earth's surface. The sand and clay are deposited in fields or stream channels in the valley portion. Large quantities of sub-surface soil are lost in the process, resulting in the formation of long funnel-shaped tunnels beneath the surface.
This process often goes unnoticed and when the eroded tunnel roof is not able to hold the burden, it collapses forming large cavities, G. Sankar, scientist of the Geo Science Division of the Centre for Earth Science Studies, Thiruvananthapuram, said. A similar incident had occurred at Velliyanimala earlier. It stabilised later.
Dr. Sankar and Eldhose, a technical officer at the centre, had carried out a preliminary assessment in the areas on September 16 and 17.
The inhabitants of a house in Thankamani village had stated that they could hear the sound of water flowing beneath the earth. It was later found that soil piping was taking place beneath the house. The district administration was alerted and asked to relocate the family immediately, he said.
Dr. Sankar, who studied the incidents in the district following a request from the district administration, said one of the visible signs of soil piping was the stable water level in wells despite copious rain. This indicated that the water was getting lost somewhere in the earth, he said.
Another indication was the mismatch between the quantity of rain received and the availability of water in the earth. If the difference was huge, one could reach the conclusion that water was getting lost in the earth. In such cases, a large volume of water is lost through the conduits in the earth, he said.
One of the major threats from soil piping is that the cavity formed can grow in size and result in more land subsidence. Arresting soil piping is an expensive process and in many cases, experts have expressed doubts over the effectiveness of the remedial measures.
One method employed in some foreign countries is the installation of filters that allow the flow of water while holding back sand.
A scientific report on the process will be submitted to the Idukki district administration next week, Dr. Sankar said.
Source:The HIndu,20-9-2010
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