Study on Influence of Parameter Variability of Passive-Zone Reinforcement on Deformation Behavior of Soft Soil Deep Foundation Pits

To quantify the effects of spatial variability in the strength of passive-zone soil–cement reinforcement on the deformation response of deep excavations in soft ground, field investigations were conducted to obtain the strength characteristics of the cement-treated soil, and the spatial distributions of the unconfined compressive strength (UCS) in the hollow- and solid-pile sections were statistically characterized. The mean UCS values of the hollow- and solid-pile sections were 0.380 and 0.504 MPa, respectively, with corresponding coefficients of variation (COVs) of 0.909 and 0.6896. Based on the measured statistical characteristics, spatial random-field models were established for the two reinforcement zones and incorporated into a Monte Carlo stochastic finite element framework. A reliability-based assessment was then performed to quantify the effects of strength variability on the deformation response of the excavation retaining system. The results indicate that strength variability in the solid-pile section primarily governs the maximum horizontal displacement along the retaining-pile shaft, whereas variability in the hollow-pile section predominantly affects the horizontal displacement at the pile head. For the cases considering spatial variability in the hollow-pile section only, the solid-pile section only, and both sections simultaneously, the probabilities that the deterministic homogeneous analysis underestimates the maximum horizontal displacement, Umax, are 95.2%, 88.3%, and 93.6%, respectively. As the COV increases from 0.2 to 0.8, the fluctuation range of Umax increases from 0.9 to 5.3 mm for the solid-pile section and from 0.3 to 1.4 mm for the hollow-pile section. Despite the pronounced spatial variability of the soil–cement strength, its influence on the mean deformation response of the investigated excavation remains relatively limited because of the high overall stiffness of the pile–strut support system. These findings clarify the differentiated roles of strength variability in the solid- and hollow-pile reinforcement zones and provide a quantitative basis for reliability-based deformation assessment and refined design of passive-zone reinforcement in deep excavations in soft ground.