Construction of High Road Embankment Over Thick Soft Silty Clay- A Case Study-Paper

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The Calicut Bypass Phase-II project required construction of a highway embankment up to 7 m high over waterlogged areas containing 5–18 m of highly compressible soft clay. To reduce post-construction settlement and improve stability, prefabricated vertical drains (PVDs) were installed before embankment construction.<br /><br />PVDs, 100 mm wide and 3.5 mm thick, were installed through the full clay depth at 1 m spacing in a triangular pattern. An aggregate drainage layer at least 300 mm thick conveyed water from the drains. Woven geotextile served both as a separator and as basal reinforcement. Because the clay had low shear strength, the embankment was constructed in stages, with a minimum four-week waiting period between stages. Settlement gauges and piezometers monitored ground response and pore-pressure dissipation.<br /><br />At a representative location with 18 m of clay and a 7 m embankment, measured settlement reached 1,044 mm. By the end of the third loading stage, the recorded primary settlement had reached 100% of the total; the final 1 m fill stage produced no additional observed settlement. Monitoring generally showed settlement and pore pressures responding to changes in embankment loading.<br /><br />Careful scheduling was essential: most filling took place before the four-month monsoon period, allowing consolidation to continue while earthworks were suspended. The overall project, including two bridges, was completed in 18 months—six months ahead of schedule—without additional surcharge loading.<br /><br />The case demonstrates that PVDs, staged construction, reinforcement, instrumentation, and coordinated planning can enable safe, timely construction on thick soft clay. Geotechnical monitoring is important for assessing consolidation and guiding construction.

Keywords

Calicut Bypass

Phase-II project

prefabricated vertical drains

soft clay

staged embankment construction

soil consolidation

pore pressure dissipation

geotextile reinforcement

settlement monitoring

geotechnical instrumentation