Analysis of the Mechanical Behavior of Double-row Steel Sheet Pile Support Structures for Underwater Urban Riverside Conduit Excavations

In the construction of ultra-long underwater conduits along urban rivers, excavation pits on the riverside are typically subjected to markedly unbalanced loading conditions: the fill and water–soil pressure on the landside of the embankment are significantly greater than those on the waterside. Under such pronounced eccentric loading, conventional single-row sheet-pile systems are inadequate to effectively resist and equilibrate the unbalanced earth and water pressures, making it difficult to ensure pit safety. To address this issue, this study first qualitatively analyzes the mechanical behavior of traditional single-row sheet-pile support under eccentric loading, and then proposes a double-row sheet-pile support system arranged on the waterside of the pit, with a sandbag cofferdam infilled between the two rows.In the proposed system, the spacing between the two rows of sheet piles is 4.9 m. The inner row consists of 18 m long Larssen IV sheet piles and the outer row of 12 m long Larssen IV sheet piles. Sandbags are compactly filled between the rows to form a counter-pressure cofferdam, thereby balancing the landside eccentric loading. The mechanical response and deformation behavior of this support system are analyzed using the geotechnical finite element software MIDAS GTS NX. The results indicate that the double-row sheet-pile–sandbag cofferdam system effectively mitigates the eccentric loading from the embankment side. The maximum horizontal displacement of the support structure toward the river is approximately 48 mm, occurring at the top of the cofferdam, and the maximum calculated stress in the sheet piles is about 36.4 MPa, which is far below the design strength of the steel. The support structure thus remains in a safe state with deformations within a controllable range.This study demonstrates that, for ultra-long underwater conduits constructed along urban rivers, adopting a double-row sheet-pile support system with an infilled sandbag cofferdam on the waterside is an effective technical solution to cope with significantly unbalanced water–soil pressure, and can provide valuable reference for the design and construction of similar projects.