Case Study: Challenges of Earthen Cofferdam in Deep Excavations for Waterfront Structures-Paper

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The paper examines an earthen cofferdam built for a 12 m deep intake-well excavation in the Aaundha River, Maharashtra. The site’s natural profile consisted of black cotton clay over basalt. The cofferdam was designed using clayey sand as fill, but nearby material was unavailable, so clay was used instead.<br /><br />The embankment was constructed in two stages: it reached 5 m in July 2017, was submerged during the monsoon, and was raised to 7 m the following February. Soil placed underwater could not be compacted; compaction was performed only above the waterline. After construction, six pumps lowered the water level inside the cofferdam by 6 m in seven days. On the eighth day, the inner embankment slope began to slide.<br /><br />PLAXIS-2D analyses assessed steady seepage and sudden drawdown conditions. The authors identify several likely contributors to the movement: the change from the designed fill material, softening during prolonged submergence, a weak interface between the old submerged and new dry fill, and rapid dewatering that generated excess pore-water pressure. The analyses did not account for all these construction-related conditions.<br /><br />Dewatering was stopped, the cofferdam crest was widened from 4 m to 7 m, the embankment was compacted, and unstable downstream soil was removed. Excavation resumed after conditions improved, and the project was completed that season.<br /><br />The study recommends checking local fill availability during design, building cofferdams in one season where possible, and carefully planning compaction and dewatering. It also advises considering an upstream HDPE lining to reduce seepage.

Keywords

cofferdam

earthen embankment

intake-well excavation

slope failure

sudden drawdown

seepage analysis

PLAXIS-2D

black cotton clay

dewatering

HDPE lining