Accurate prediction of excavation deformation and stress affects the safety of excavation engineering and the surrounding environment. However, the traditional calculation method ignores the influence of soil shear action and its nonlinear deformation characteristics. Therefore, this paper proposed a coupled analytical method for braced excavation considering the continuity of soil deformation and nonlinear pile-soil interaction. A nonlinear Pasternak two-parameter foundation model was developed based on the Pasternak foundation model and nonlinear p-y curves. The control differential equations for the excavation in the critical and embedded sections were derived. Also, the numerical solutions of excavation deformation and force under different boundary conditions were obtained by the finite difference method and Newton's iteration method. Further, the excavation calculation procedure considering the construction process and nonhomogeneity of soil was suggested. Through finite-element (FE) and engineering case analyses, the traditional calculation method overestimated the excavation deformation and internal force, while the proposed methods were consistent with the measured results. Finally, the effects of soil shear stiffness and initial foundation reaction modulus on the excavation were discussed, and we found that the two parameters had more significant impact on the wall bending moment than displacement. The results provide some reference for the design calculation of braced excavation.

The recent construction of an underground mass rapid transit (MRT) station in Singapore involved 21 m deep excavations within underconsolidated marine clay. The lateral earth support system comprised 1 m thick diaphragm walls socketed into the underlying Old Alluvium and 4 levels of preloaded cross-lot struts. Deep soil mixing (DSM) and jet grouting piles (JGP) were used to improve up to 15 m thickness of the marine clay formation. Field monitoring data showed that these ground improvement processes caused large outward deflections of the diaphragm wall panels at some locations prior to the excavation and may have caused yielding within the wall panels. In this paper, the impacts of these prior wall deformations on the subsequent performance of the excavation support system are investigated. The measured performance at two indicative cross sections is compared with results from simplified 2D finite element analyses. The analyses simulate the effects of ground improvement through prescribed boundary pressures and represent the yielding of the diaphragm wall panels through zones of reduced bending stiffness. We show that large outward wall deflections and curvature observed during jet grouting at one contribute to higher inward wall movements and strut loads measured during excavation, while smaller movements (and curvature) prior to excavation at a second similar cross cause negligible change in the performance of the temporary earth retaining system. The results highlight (1) the importance of controlling ground movements associated with ground modification processes such as jet grouting, (2) the uncertainties in estimating mechanical properties for the improved soil mass, and (3) the need to improve the representation of non-linear, flexural properties (M-kappa) of reinforced concrete diaphragm panels.

A great challenge in the construction of braced excavation in unsaturated residual soil is the emergency caused by rainfall-infiltration, which may affect the safety of the braced excavation and the serviceability of adjacent underground structures. Therefore, prediction of the deformation of braced excavation induced by rainfall-infiltration is becoming one of the major tasks in the design of underground engineering. Numerical simulation is gaining popularity, and many constitutive models are available nowadays to analyze excavation problems. However, it is not clear to select a suitable soil constitutive model to describe responses of the braced excavation under rainfall-infiltration in unsaturated soil. This paper investigated the optimal selection of different soil constitutive models. These models included the hardening soil model (the HS model), the Mohr-Coulomb model (the MC model), and the Tresca model. A series of laboratory tests back-analysis using the above three soil constitutive models were conducted to evaluate the ability to predict the mechanical response of soils. And a case of braced excavation in unsaturated soil was simulated by using the different constitutive models. The optional selection of constitutive models was discussed by comparing simulation results of wall deflection, earth pressure, and stress path of soils. These investigations confirmed that the HS model had the best predictions of soil stress state and excavation deformation. This result may be since the HS model could more reasonably predict the pore water pressure and critical failure state of soils under rainfall and excavation conditions.