Analysis and Behavior of Double Sheet Pile Cofferdam-Paper

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The Philadelphia Water Department required a temporary cofferdam to separate the east and west basins at Belmont Raw Water Basin while an existing earthen divider was replaced with a narrower concrete structure. The west basin had to remain near full capacity, and the cofferdam needed to retain approximately 35 feet of water and soil over a 500-foot length.<br /><br />The selected system used two rows of sheet piles with soil and stone infill between them, providing stability and water cutoff. Subsurface investigation found shallow bedrock near the planned excavation bottom, limiting sheet-pile embedment. To compensate, the design incorporated tie rods connecting the walls and three rows of tiebacks anchored into competent bedrock.<br /><br />Because standard limit-equilibrium methods could not adequately represent the system’s construction stages and component interactions, the team used two-dimensional finite element modeling. The analysis simulated staged excavation and tieback installation, and helped optimize anchor locations and pretensioning while limiting sheet-pile deflections and component stresses. Predicted tieback and tie-rod loads remained close to their design values, and calculated infill-soil stresses were below the estimated soil strength.<br /><br />Inclinometers and survey markers monitored the cofferdam during construction. Measured sheet-pile deflections generally agreed with the finite element predictions; local differences reflected variations in bedrock elevation. The project demonstrates that a double-wall cofferdam with tiebacks can provide a stable, effective water cutoff where shallow rock restricts conventional embedment. Finite element analysis also provided useful insight into deformation and interaction among the walls, infill, anchors, and retained ground throughout construction.

Keywords

Philadelphia Water Department

Belmont Raw Water Basin

temporary cofferdam

double-wall cofferdam

sheet piles

tiebacks

finite element modeling

bedrock

excavation support

water cutoff