Assessment and Optimization of Soil Mixing and Umbrella Vault Applied to a Cross-Passage Excavation in Soft Soils

Abstract

Focusing on three-dimensional finite-element analysis (FEA) for the assessment and optimization of reinforcement treatment using deep soil-mixing and umbrella vault methods to stabilize a cross-passage excavation in soft alluvial soils, this paper presents parametric studies, interpretation analyses, and details of the modeling technique for the reinforcement treatment and the excavation process. Displacements at the crown of the cross passage are estimated and used as the basis for assessment and optimization for the reinforcement. Optimization could account for savings of about 50% in the cost of soil mixing. The analysis indicates that simulated displacements can be reduced significantly when both the soil-mixing and the umbrella vault techniques are used. Deep soil mixing is more effective and adaptable, whereas the umbrella vault method has to be used jointly with other techniques to stiffen the upper soil layers. The simulation results further demonstrate that the progress of the excavation influences the crown stability, but such influence also depends on the soil conditions after reinforcement. Based on the findings in this paper, it has been concluded that construction sequence and risk-control measures can be arranged based on the simulation outcomes to achieve savings in construction time and cost.