NEW DELHI: Use of weak and highly plastic soil beneath the Lucknow-Kanpur Expressway appears to be one of the main reasons for premature failures (slippage) on one of its stretches, an expert assessment has found.Plastic soil is soil that can be easily moulded when wet. The assessment carried out by an IIT Kharagpur professor that examined the severely damaged portions of the expressway found that the main problem pertained to the soil beneath it. The report said the highway stretch itself did not show any major deficiency in the thickness of its different layers.It also said that water getting trapped in deeply depressed sections, partly because of the unpaved portions of the shoulders, meant for future expansion, could be restricting drainage.NHAI targets completing the works by Sept end. Toll collection on the entire expressway has been suspended.The inspection found that water was generally present at the level of the lower concrete layer and in the soil beneath it, but there was no widespread accumulation of water in the upper layers of the road. Waterlogging was found mainly in deeply depressed portions.The report, however, pointed to a possible drainage problem near the paved shoulders. A layer of soil between the concrete drainage layer and the hard shoulders appeared to be restricting the movement of water, potentially slowing its flow into the drainage system.The expert, however, said the presence of water in the soil alone could not explain the kind of failures witnessed on the stretch. It is the use of highly plastic soil that appeared to be one of the primary causes of the failures.The report said it also needed to be examined whether the soil had been inadequately compacted and whether the road’s design assumptions about the strength of its base layer were achieved on the ground.The panel has recommended rebuilding the affected stretches from bottom up at places with major depressions and slippage. Such reconstruction should be undertaken particularly where tests show that the strength of the underlying soil is substantially below the level assumed in the original design.It has also recommended a detailed survey to identify stretches with weak or highly plastic soil at the earliest, along with tests to assess the actual strength of the highway and determine whether additional strengthening is required.The assessment has stressed that identifying weak soil and addressing the issue quickly would be crucial for preventing further premature failures and ensuring the long-term durability of the expressway.