Land subsidence and lateral spread that lead to the occurrence of deep cracks extending several metres underground have emerged a serious threat to human habitation in the highlands of Kerala, a study conducted by the National Centre for Earth Science Studies (NCESS) here reveals.
Geoscientists who carried out the investigation in the wake of the heavy rains that caused widespread devastation in August found huge cracks across farmlands and dwellings.
Many houses had developed cracks on the walls and basement, rendering them unsuitable for habitation. In some places, the tiled floor of buildings had split up due to ground heaving.
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“The situation is alarming”, says V. Nandakumar, Head, Crustal Processes Group, NCESS who led the study. “It could aggravate during the northeast monsoon, unless remedial measures for slope stabilisation are adopted on a war footing,” he said.
The team conducted investigations in the Puthur, Madakkathara, Nadathara and Panancherry panchayats in Thrissur district and Iritty taluk and Kottiyoor panchayat in Kannur, all landslip-affected areas.
Lateral spread occurs when the soil on gentle slopes starts moving downhill, causing subsidence. It is caused by a process known as liquefaction.
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When soil loses strength due to saturation, it behaves like fluid, triggering a flow. Lateral spread is progressive and spreads rapidly, often culminating in a complex landslip.
“The unprecedented rainfall of 350 to 400 mm over a period of two to three days undoubtedly triggered the slope failure,” says Dr. Nandakumar. “Most of the slopes were used for raising crops and farmers had blocked the natural drainage systems.”
Rain pits
The construction of rain pits contributed to the percolation of water into the rock soil interface. Residents in the high ranges also reported water and mud gushing out of the ground for days during the monsoon season.
“Lateral spreading, subsidence and formation of cracks are quite unusual phenomena and need immediate attention,” says Dr. Nandakumar. “At many locations, holes had appeared prior to sliding and subsidence and water started percolating underneath, triggering huge landslips in a matter of days.”
The scientists recommend the formation of a trained task force for the highlands to monitor ground signatures like hollows, cracks and water spouts that often precede land subsidence, lateral spread and landslips.
They stress the need to provide adequate drainage in plantations on hill slopes and regulations on cutting the toe regions of slopes for construction of buildings.
The team advocates terracing and geotextile foundation for slope stabilisation in the landslip-affected areas.
It also calls for a moratorium on construction of rain pits in sloping terrain.
NCESS is planning to intensify the scientific investigations to understand the triggering factors of landslips in the Western Ghats. It is also working on a project to install a few landslip monitoring stations in the highlands.