Deformation Behaviour of Secant Pile Walls in Layered Soil–Rock Profiles: Lessons Learned from Deep Excavations in Copenhagen

The performance of deep excavations in layered stiff ground conditions (overconsolidated soil overlying rock) remains largely unexplored, with few published case studies available, hindering the understanding of performance and key indicators for deep excavations in such ground conditions. This study addresses this gap by comprehensively investigating seven deep excavations for a new metro line in Copenhagen. These excavations utilized temporary retaining walls constructed via the bottom-up method in stiff, layered ground conditions embedded in limestone. Through a long-term instrumentation program, the major factors influencing deformation behaviour were examined, including secant pile wall deflections, anchor and strut axial forces, and groundwater levels inside and outside the pits. During the cantilever stage, the maximum normalized deflections (u_h,max/H_e) ranged from 0.05 to 0.25%, aligning closely with other reported case studies in the literature. Ground stiffness emerged as the critical factor affecting wall deflections, both in the cantilever and supported stages. The relationship between soil stiffness and deflections transitioned from linear to non-linear as construction progressed. Predictive equations for supported cases are derived based on monitoring data, and 3D effects were estimated in four excavations, consistently with existing studies in layered soft soil. This paper expands the literature on secant pile wall behaviour in stiff ground conditions, offering theoretical support for similar projects.