Installation of flood defence works on the river bank however, three general assumptions common to every soil-structure numerical model are considered below. One of the first steps in any soil-structure interaction problem is to quantify the in-situ stress regime, i.e., to determine what the stresses are in the ground prior to con- struction-induced changes to the stress regime due to loading (e.g., foundations) or unloading (e.g., basement excavation). The in-situ stresses include the vertical and horizontal stresses, as well as the pore pres- sures. The vertical stresses can be derived from the stratigraphy of the site and the density of the various materials present; however, the lateral effective stress is a much more complicated parameter to quantify accurately. The lateral effective stress is influenced by the stress history and past geological events, such as glaciation. A K0 parameter, which is the ratio of the vertical to horizontal in-situ stress, is often used as an input in numerical modelling software, and this parameter can have a dramatic impact on the results of FEM analyses. Laboratory tests and correlations from field tests have been developed to quantify this parameter and these can be used to select an appro- priate K0 value for input in the soil-structure interaction analysis. Further consideration should be given to the variability of the value 70 • DEEP FOUNDATIONS • JAN/FEB 2017 of K0 within a stratum as it may be influenced by local properties, such as soil fabric, particularly in cohesionless material. One of the next steps in the modelling process is to determine whether the material is likely to behave as undrained or drained. This is a critical question as soil material may behave very differently depending on the assumption adopted. Undrained behaviour does not allow for volume changes or the dissipation of excess pore pressures, and, therefore, this typically represents a short- term response to loading/unloading. By contrast, drained behaviour represents the fully-equalised conditions where the pore pressures remain at their in-situ stress state, and this represents a long-term condition. Most real soil materials are neither perfectly drained nor perfectly undrained, but rather behave somewhere in between, in a partially drained state. However, depending on the problem being analysed, employing an assumption of drained or undrained behaviour may be more or less valid. For example, relatively impermeable clay is likely to behave undrained in short-term situations, where the stress conditions are changed, such as basement excavations, but the same clay may behave drained when considering the slope stability of a 150-year- old railway embankment. The third assumption that needs to be considered is the choice of constitutive model and the appropriate stiffness parameters for the soil within that model. For example, the Mohr-Coulomb model, which assumes a linear elastic relationship up to a linear plastic failure criterion, is one of the simplest material models available and is controlled by a single Young’s modulus which governs the soil stiffness. This model does not consider the nonlinearity of the soil material and the reduction in soil stiffness as the strain level increases. More advanced models, such as the small strain hardening soil model, are available, which capture three soil behaviour regimes. Initially, an elastic, very stiff soil response is observed in the small strain region followed by a nonlinear elasto-plastic response of the material in the larger strain zone, and, finally, a fully plastic response at soil failure. Most soils exhibit some degree of stiffness anisotropy between vertical and horizontal stiffness. Therefore, the possible influence of stiffness values in different directions within a soil mass should also be considered prior to deciding what geotechnical investigation testing will be carried out and what numerical analysis will be completed.
