Part 4: A team from institute conducted tests at 12 locations around Joshimath to find cause for land subsidence
Photo: Sunny Gautam / CSE A report by the Indian Institute of Technology (IIT), Roorkee into the incidents of land subsidence in Joshimath, Uttarakhand, earlier this year found that more than half of Joshimath is at extremely high risk.
IIT-Roorkee is among the eight institutes that conducted an in-depth study into the landslides in the region. The reports recently came into public domain after the Uttarakhand High Court objected to the government’s decision not to make expert reports on Joshimath land subsidence public.
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This series by Down To Earth (DTE) is examining these reports one by one. The first report looked into the report by Wadia Institute of Himalayan Geology, followed by the Post Disaster Need Assessment paper by the National Disaster Management Authority (NDMA) and the Geological Survey of India (GSI) analysis. This is the fourth in the series.
According to the IIT Roorkee paper, the team of researchers conducted tests at 12 locations in Joshimath: Gandhinagar PG College (Ward 1); Marwari; near the gate of JP Colony (Ward 2); Lower Bazaar near Narsingh temple (Ward 3) and Singhdhar near Panchvati Sarai and near the parking plot (Ward 4).
Other areas that were tested were: near Manoharbagh Ropeway tower no. 1 (Ward 5); near Manoharbagh PWD guest house (Ward 5); near Manoharbagh CPWD office (Ward 5); near Upper Bazaar Municipality (Ward 6); near Sunil Shivalik Cottage (Ward 7), near Parsari AT Nala (Ward 8) and near Helipad in front of Ravigram NTPC Gate (Ward 9).
The team conducted six types of tests. The first was multichannel analysis of surface waves and the second involved taking soil samples by boring to a depth of 10 metres under the ground. These samples were then tested in the laboratory.
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The third test was dynamic cone penetration test, which also involves boring as deep as 10 metres. The fourth was a plate load test, where a 1.5 metre deep pit of 1.5 into 1.5 square metres was placed on a 300 into 300 square millimetres plate. The loading capacity of 60 tonnes per square metres was then tested.
The last two involved direct shear tests, where five tests were conducted on a 300 into 300 sq mm box and two tests on a 700 into 700 sq mm box to a depth of two metres.
After these tests, IIT-Roorkee has stated three main findings in its report. Joshimath region consists of a mixture of boulders and gravel and it was very difficult to examine the area at this stage, it said. “The final results indicate that more than 50% of Joshimath is in the very high-risk zone,” the paper stated.
The concluding remarks at the end of the report state:
Overall, the soil at Joshimath was found to be a complex mixture of boulders, gravel and clay. The matrix of the boulder is composed of gravel and clay. If there is any internal erosion in such soil then the entire structure becomes unstable and collision of boulders causes subsidence.
The second comment is very important. It is said that the main reason for subsidence is the lack of infrastructure for drainage. This water can be from rain, melting snow or dirty water coming out from houses and hotels.
Although subsidence is a frequent phenomenon, it can be reduced by controlling water seepage, which helps reduce internal erosion, the analysis said. The report also classifies various areas in Joshimath as high-risk, medium-risk and low-risk zones on the basis of its tests.
To read the first part of the series click here, click here to read the second part and click here for the part three.
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