Performance-Based Design and Construction of a Semi-Top-Down Excavation in Soft Clay: A Case Study in Shaoxing, China

This paper presents a detailed case study of a semi-top-down excavation carried out for the Haowang Tower project in Shaoxing, China, where thick soft clay deposits dominate the subsurface profile. The excavation, covering approximately 10,000 m2 in plan area and reaching a depth of 12.35 m, posed significant challenges due to the presence of sensitive adjacent utilities and roads. In response, an integrated design–construction strategy was adopted, combining soldier pile retaining walls with a permanent first-floor slab serving as horizontal bracing. Several innovative structural features—including load-transfer beams, stress-reinforced strips, and soil molds—were introduced to address the specific demands of the semi-top-down method in soft ground. A multi-stage numerical analysis framework was implemented, employing the Hardening-Soil (HS) model within 2D and 3D finite element analyses (PLAXIS), alongside the subgrade reaction method (FRWS2006). Predicted wall deflections, ground settlements, and structural forces were systematically compared with field measurements. The 3D analysis showed good agreement for wall deflections (within 5% of the maximum measured value), validating the approach’s effectiveness. However, the analysis over-predicted ground settlements (e.g., sewage pipe settlement was over-predicted by 60%), indicating a need for more refined settlement prediction models or parameter calibration. Based on this finding, a correction factor of 0.6–0.7 is proposed for settlement prediction when using HS parameters derived from standard drained tests. The results also highlight the importance of spatial effects and the critical role of construction sequencing. This study offers both practical insights and validated numerical tools for similar deep excavations in urban soft clay environments.