Soil Stiffness Parameters for Finite Element Simulation

Abstract

Numerical analyses are widely used in practical geotechnical engineering to assess the deformation behaviour of deep excavations, in particular when the influence on existing infrastructure, such as buildings or adjacent tunnels, has to be evaluated. The key issue of numerical analysis is selecting a applicable constitutive model and determining the input parameters. Some commonly used constitutive models are studied, and the adaptabilities and limitations of these models are also been analysed. It is found that the hardening soil model which can define the shear hardening, compression hardening and unloading-reloading characteristics gives a very reasonable prediction in deformation analysis. But the models which can only simulate the shear hardening behaviour, such as Mohr- Coulomb model, are not suitable for the deep excavation analysis. When the heave of the foundation after excavation of great interest, the hardening soil with small-strain gives a acceptable estimation, while the prediction from hardening soil has big errors. That may because the unloading-reloading modulus is not associated with stress and strain level in hardening soil model, and can not define the nonlinear unloading-reloading characteristics. Then, the determination for parameters of hardening soil model, especially for stiffness parameters is discussed in detail. A selection scheme is proposed on the basis of empirical correlations and back calculation for geotechnical engineering design with available soil data. Two case study are performed to help determining the soil parameters. It is found that the initial estimation of soil parameters need to be calibrated for undrained simulation, while it can be used directly for the drained case in deep excavation. Finally, the foundation excavation of Shanghai Bank building has been analysed. Finite element method software PLAXIS is employed to perform the numerical simulation. Three soil models are used in this study, and the soil parameters are determined by the selection scheme proposed before. The model results are compared with the on site measurements to prove the reasonableness of the parameters selecting scheme.