Weak soil and trapped water likely behind faults on Lucknow-Kanpur highway: Initial investigation

NEW DELHI: Use of weak and highly plastic soil under the Lucknow-Kanpur Expressway appears to be one of the major reasons for premature failure (landslides) on one of its stretches, according to an expert assessment.Plastic soil is soil that can be easily molded when wet. The assessment by an IIT Kharagpur professor who examined the severely damaged parts of the expressway found that the main problem was in the soil beneath it. The report indicates that the section of the road itself does not present any major deficiency in the thickness of its different layers.He also said water trapped in deeply depressed sections, in part due to unpaved portions of shoulders intended for future expansion, could be restricting drainage.NHAI aims to complete the works by the end of September. Toll collection on the entire highway was suspended.The inspection found that there was generally water at the level of the bottom layer of concrete 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, noted a possible drainage problem near the paved shoulders. A layer of soil between the concrete drainage layer and the shoulders appeared to be restricting the movement of water, potentially slowing its flow into the drainage system.The expert, however, stated that the presence of water in the ground alone could not explain the type of failures observed in the section. It is the use of highly plastic soil that appears to be one of the main causes of the failures.The report said it was also necessary to examine whether the soil had been inadequately compacted and whether the highway’s design assumptions about the strength of its base layer were met in the soil.The panel has recommended rebuilding the affected stretches from the bottom up in places with large depressions and landslides. This reconstruction should be carried out particularly when tests show that the strength of the underlying soil is substantially below the level assumed in the original design.It also recommended a detailed study to identify sections with weak or highly plastic soil as early as possible, along with tests to evaluate the actual strength of the road and determine whether additional reinforcement is required.The assessment has emphasized that identifying weak soil and addressing the problem quickly would be crucial to prevent further premature failures and ensure the long-term durability of the highway.