Statistical Analyses on a Database of Deep Excavations in Shanghai Soft Clays in China from 1995–2018

This paper presents the results of statistical analyses on a database comprising 592 deep excavations in Shanghai soft clays from 1995 to 2018. First, the general trends and patterns over time are outlined for seven different excavation methods and seven types of earth-supporting systems. Then, the major design factors are characterized in a statistical manner. Based on analyses of the collected data, the frequency distributions of δhm/He data (δhm = maximum lateral wall displacement; He = final excavation depth) corresponding to each excavation method and each earth-supporting system are presented; furthermore, the correlations of δhm/He with the major design factors are explored. In this new database, 90% of δhm/He data featured magnitudes no more than 1.0%, which frequently lie within 0.10%–0.55%, indicating better performance than other databases worldwide. Via extensive comparisons, both the optimum construction method corresponding to each type of earth-supporting system and the optimum earth-supporting system corresponding to each excavation method for controlling excavation performance were determined. Last, the major design and construction defects as well as natural hazards pertaining to abnormally large δhm/He=1.01−7.27% tracked at 23 individual excavations are identified and summarized. These include excessive excavation duration, long wall exposure lengths, retaining walls toed in soft muddy clays, overexcavation at deep levels, surcharges behind retaining walls, inadequate supporting system stiffness, and intense rainfall; among these factors, walls toed in soft muddy strata are shown to be detrimental to the performance of excavations. For a deep excavation that is to be carried out in sensitive clayey strata, safety redundancy can be enhanced substantially by accounting for the degradation of soil strengths due to construction disturbance and/or rainfall infiltration in design. The findings and lessons from this study will be useful to professionals worldwide in efficiently achieving cost-effective designs and/or making reliable evaluations regarding the performance of deep excavations in soft clays.