Observed Behaviors of a Long and Deep Excavation Constructed by Cut-and-Cover Technique in Shanghai Soft Clay

TLDR

A 16–18 m deep cut‑and‑cover excavation for a metro station in Shanghai’s soft clay, surrounded by high‑rise buildings and utilities, behaved differently from other projects due to its unusually long shape. The study aimed to investigate the excavation’s performance and its impact on nearby buildings and utilities, serving as a case study to inform design and construction of long, deep cut‑and‑cover excavations in soft soils. A long‑term instrumentation program recorded diaphragm‑wall deflections, wall‑top vertical movements, ground settlements, and settlement of adjacent buildings and utilities. Analyses showed that overexcavation and prolonged construction produced large diaphragm‑wall deflections, but base and middle floor slabs mitigated post‑excavation deflections and ground settlements, indicating that both time and geometry influence deep‑excavation behavior in Shanghai soft clay.

Abstract

A 16–18 m deep excavation for a metro station with in-plane dimensions of approximately 20 m wide by 290 m long was constructed by using the cut-and-cover technique in soft clay in the Shanghai metropolitan area, in which many high-rise buildings and utilities exist. To investigate the performance of this deep excavation and the associated effect on the adjacent buildings and utility pipelines, a long-term comprehensive instrumentation program was conducted during construction. Field observations included deflections of diaphragm walls, vertical movements at wall tops, ground settlements, and settlements of surrounding buildings and utilities. Analyses of field data indicated that overexcavation (i.e., removing more soil than designed) and a long construction duration caused the diaphragm walls to develop substantial deflections. The base and middle floor slabs played dominant roles in suppressing postexcavation wall deflections and ground settlements. This excavation in general exhibited different behaviors than other excavation projects because of its relatively long shape. Behaviors of deep excavations in Shanghai soft clay are affected not only by construction duration (i.e., time effects), but also by their geometries (i.e., space effects). This project serves as a special case study and provides insights into the design and construction of a long and deep excavation that uses the cut-and-cover technique in soft soils and in a metropolitan environment.