The Hindu : Front Page : `Piping' creates underground tunnel

Online edition of India's National Newspaper
Monday, Nov 14, 2005

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`Piping' creates underground tunnel

T. Nandakumar

CESS scientists who visited the spot fear threat to human habitation in the area

BAFFLING TUNNEL: The gaping sub-surface tunnel stretching across Thirumeni village in Taliparamba taluk in Kannur district.

THIRUVANANTHAPURAM: Scientists are baffled by the discovery of a gaping underground tunnel stretching right across the Thirumeni village nestling on the slopes of the Kottathalachi hill in Kannur near the Kerala-Karnataka border.

A team of geoscientists from the Centre for Earth Science Studies (CESS) which visited the village recently has concluded that the gully was formed by `piping,' a phenomenon caused by subsurface erosion of soil. At some places, the tunnel is more than 10 feet wide and extends deep under the earth. Scientists fear that piping could pose a threat to human habitation, if the conduit is part of a larger network spreading all over the slopes.

On September 9, a local resident, Lisi Francis, and her family witnessed a huge chunk of soil sinking into the earth in a neighbouring plot of land. Villagers converging at the spot saw cavernous openings on both sides of the collapsed mass. They also reported hearing the sound of gushing water from within the tunnel. Alerted to the incident, Revenue officials persuaded Ms. Francis' family to move to a safer place and requested CESS to carry out a geological survey of the region.

Later, scientific examination revealed that the roof of the gully had caved in at a weak spot. The investigators also found water welling up through another outlet for the channel, about 250 metres down the slope.

The team leader G. Sankar said the tunnel could have been formed over a period of time, due to accelerated erosion of the clay-rich soil by an underground stream, possibly percolating down from the surface through a crack or the decayed root of a tree.

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. The narrow channel is carved out into a larger pipe. As the pipe enlarges, the flow becomes more concentrated and turbulent. "It is something like a giant underground vaccum cleaner," Mr. Sankar explains.

The scientists also found that the slopes were not holding water. "After a certain level, the entire groundwater is drained off from the soil surface. As a result, most of the wells in the region are heavily depleted. Ironically, this is why the Thirumeni hillside is less vulnerable to landslips than the neighbouring slopes," Mr. Sankar says.

The investigators are not ruling out the possibility that the erosion has penetrated the earth through a wide network spanning the entire slope. Subsurface pipes are known to extend some distance as a continuous channel or as a system of interconnected tunnels that form an extensive, branched network. "It is the worst case scenario but one that is likely, given the magnitude of the phenomenon," Mr. Sankar says.

In its report to the Kannur District Collector, the team has recommended a thorough geophysical investigation of the region using ground-penetrating radar to determine the extent of the threat. It has also called for hydrological studies to analyse the subsurface flow of water.

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