Numerical Modelling of Liquefaction Under Embankments Subjected To Cyclic Loads By Wind Turbines

Abstract

Recent development have caused that the policy regarding the installation of onshore wind turbines on embankments has changed to 'Yes, provided that ...' in the Netherlands. (Simplified) approaches are available within professional practice to analyse the risk of liquefaction, but are considered to be conservative. In this thesis, a methodology is formulated to model the key aspects of liquefaction of a soil layer underneath a dike system subjected to cyclic loading by an onshore wind turbine. The key aspects are defined as: cyclic loading, soil-structure interaction and consolidation behaviour. A modification to the model of Seed and Rahman (1978) is implemented in this thesis.
The applicable modelling conditions (quasi-static or dynamic) are investigated for the current situation. The analysis is performed using analytic and numerical models (Plaxis 2D). Both models conclude that the loads induced by the wind turbine can be modelled as quasi-static as the loads are sufficiently slow to neglect inertia effects. The influence of the cyclic loads by the wind turbine on the liquefiable soil layer is determined by the Cyclic Stress Ratio (CSR). The influence due to the soil-structure interaction is determined using a Finite Element Analysis (Plaxis 2D). The Hardening Soil small strain model gives a more accurate prediction of the CSR in the soil, as the soil response is stiffer during un/re-loading. The presence of the embankment results in various modes of shearing in the soil, e.g. triaxial compression, triaxial extension and direct simple shear. The CSR does not include the effect of static shear stresses and various modes of shearing in the soil. This effect should rather be accounted by the Cyclic Resistance Ratio (CRR). The consolidation behaviour modelled using the model of Seed and Rahman (1978) which applies the consolidation equation with generation term. The model is able to model partial consolidation, which is defined as a state where excess pore pressures can be generated and dissipate simultaneously. The generation term is based on an empirical relationship of the development of pore pressures. The model is implemented using a Finite Difference Method in a cylindrical coordinate system to represent the dissipation behaviour of a granular soil. The method allows to model the consolidation characteristics in radial and vertical direction, a layered soil, the load intensity and the loading frequency. As a result, the maximum pore pressure ratio in the soil reduces. A limitation of the model is its uncoupled nature. Therefore, no strains are determined. The constitutive behaviour inherent to the liquefaction phenomena, such as plastic deformations, is lacking. Oostpolderdijk is used as a case study to compare the modified method of Seed and Rahman (1978) to the reference engineering method and the method of Boulanger and Idriss (2014). The comparison with Boulanger and Idriss (2014) suggests that the modified model gives reasonable results. The application of the modified method results in a more favourable result compared to the reference engineering method. However, a sensitivity analysis shows that liquefaction can’t be ruled out because the sensitivity to permeability is high.