Deformation Characteristics of a Long Deep Metro Excavation in Ningbo Soft Clay Using the HSS Model

Abstract

Huishi Road Station on Ningbo Metro Line 6 was investigated to assess the deformation behaviour of a long, deep excavation in Ningbo soft clay. A three-dimensional PLAXIS model was developed to simulate staged excavation, diaphragm walls, reinforced-concrete struts, and bottom-soil improvement. Soil behaviour was represented using the Hardening Soil model with small-strain stiffness (HSS), and numerical predictions were compared with wall-inclinometer and ground-settlement monitoring. The measured wall-deflection profiles were generally bow-shaped, and deformation was greater in the middle of the long side than near the excavation ends, indicating a marked spatial effect. At the final excavation stage, maximum wall deflection at 48 inclinometer locations ranged from 0.23%H to 1.01%H, with a mean of 0.62%H. The model predicted a maximum wall displacement of 132.5 mm, approximately 0.8%H, while the mean absolute error for four representative monitoring sections was 12.3%. The maximum measured ground settlement was approximately 0.16%H, with an influence range of about 3H. Numerical settlement was larger and extended to approximately 3.5H. The results indicate that the adopted HSS parameters reproduce the principal wall-deformation pattern reasonably well, whereas settlement predictions are more conservative. Accordingly, wall deflection is suitable as a primary calibration quantity, while predicted settlement can provide a conservative reference for construction-stage deformation assessment in comparable Ningbo soft-clay excavations.