Numerical Modelling of a Deep Excavation Shoring for Replacement of Control Structure at a Dam-Paper

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The paper presents a two-dimensional finite-element analysis of a braced sheet-pile cofferdam built to replace an inoperable control structure at the upstream toe of a dam. The enclosure had to resist pressure from approximately 14 m of lake water and excavation extending 5.5 m below the lakebed.<br /><br />The authors used SIGMA/W to model soil–structure interaction and the project’s staged construction sequence, including sheet-pile installation, underwater excavation, sealing, dewatering, and final excavation. Soils were represented with an elastic-plastic model, and wall–soil interfaces with elastic-plastic elements. Because the actual system behaves three-dimensionally, the analysis compared rigid, non-yielding braces with more flexible braces intended to approximate realistic support behavior.<br /><br />The results show that assuming rigid braces substantially underestimates wall movements and structural demands. In the flexible-support cases, predicted maximum horizontal displacement ranged from 17 to 46 mm, compared with 3.8 mm for rigid supports. Peak bending moments increased from 130 kN·m for the rigid case to as much as 427 kN·m, while peak shear forces rose from 113.5 to 182.5 kN. Brace and seal-slab forces also increased with assumed flexibility. The analyses provided construction-stage forces and moments for the structural engineer’s design.<br /><br />At the time of publication, excavation and brace installation were underway, but dewatering had not yet occurred. A concrete seal and well-point system were being installed to control uplift and prevent bottom instability. The authors emphasize that the numerical model had not yet been validated against field monitoring. Future work was planned to compare its predictions with traditional design methods and observed project performance.

Keywords

finite element analysis

braced sheet-pile cofferdam

soil-structure interaction

SIGMA/W

staged construction

sheet-pile wall

lake water pressure

excavation support

bending moment

dewatering