TY - BOOK

T1 - Modelling of excess pore pressure accumulation in sand around cyclically loaded foundations

AU - Saathoff, Jann-Eike Sören

N1 - Doctoral thesis

PY - 2023

Y1 - 2023

N2 - In particular during storm events an accumulation of excess pore pressures may occur in the soil around cyclically loaded offshore foundations. The excess pore pressure build-up reduces the effective stresses in the soil and, hence, may negatively affect the structural integrity by influencing the soil-structure interaction. Besides a loss in bearing capacity, large plastic deformations may occur to the structure. Especially for offshore wind turbines an accurate estimation of such deformations is of great importance. Even though the consideration of this degradation effect on the bearing capacity is commonly demanded by the involved certification or approval bodies, no general applicable and accepted method for the calculative verification currently exists. Over the past decades several researchers investigated the excess pore pressure build-up around offshore foundations due to environmental cyclic loads. They tried to capture the loss of bearing capacity, the accumulation of plastic rotation and the essential influence on the serviceability limit state and fatigue design. However, even if there are some sophisticated concepts, none of them is seen as the simple general applicable choice. Within this thesis a new numerical method – termed Excess Pore Pressure Estimation method (EPPE) – is presented in great detail. This method allows for the transfer of the soil behaviour obtained in cyclic simple shear tests to the bearing behaviour of the entire foundation. Herein, the numerical model accounts for the cyclic excess pore pressure accumulation by respecting the element-based mean stress and stress amplitude as well as an equivalent number of load cycles. The simulation of the excess pore pressure build-up due to certain cyclic loading is based on undrained conditions, i.e. the excess pore pressure build-up due to cyclic loading is derived by disregarding the simultaneous consolidation process. The respected transfer method, in the form of contour plots, enables the consideration of site-specific cyclic direct simple shear and triaxial test results from laboratory devices to elements within the finite element model. Each integration point is evaluated individually. Based on the derived excess pore pressure field, a consolidation analysis takes place in the second step. The actual accumulated excess pore pressure in each element at the end of the storm (or cyclic loading event) is then found by analytically superposing the excess pore pressure decay curves from the consolidation analysis. For a deeper understanding of cyclic soil behaviour, the cyclic response in different laboratory devices with different densities and under varying stress states was investigated by the author. A contour approach based on cyclic load- and displacement-controlled test results is derived to study the element response from the numerical point of view and use these for the calibration of an implicit model. Moreover, different explicit approaches are presented and compared in terms of their estimation behaviour of cyclic excess pore pressure generation, their predicted foundation capacity and their model assumptions. The intention is hence to examine existing approaches and their applicability by means of an elaborate comprehensive study. A simple modular explicit model is presented which can be easily assessed with engineering judgment. If needed, the different individual calculation steps can be exchanged with more sophisticated ones. For a reference sandy soil, results of cyclic laboratory tests are presented and used on a reference monopile foundation for a predefined storm event. The EPPE approach helps to quantify the risk of capacity degradation as well as to evaluate an appropriate safety margin. It is possible with the current methodology to evaluate the degradation potential for different sites quite easily and fast.

AB - In particular during storm events an accumulation of excess pore pressures may occur in the soil around cyclically loaded offshore foundations. The excess pore pressure build-up reduces the effective stresses in the soil and, hence, may negatively affect the structural integrity by influencing the soil-structure interaction. Besides a loss in bearing capacity, large plastic deformations may occur to the structure. Especially for offshore wind turbines an accurate estimation of such deformations is of great importance. Even though the consideration of this degradation effect on the bearing capacity is commonly demanded by the involved certification or approval bodies, no general applicable and accepted method for the calculative verification currently exists. Over the past decades several researchers investigated the excess pore pressure build-up around offshore foundations due to environmental cyclic loads. They tried to capture the loss of bearing capacity, the accumulation of plastic rotation and the essential influence on the serviceability limit state and fatigue design. However, even if there are some sophisticated concepts, none of them is seen as the simple general applicable choice. Within this thesis a new numerical method – termed Excess Pore Pressure Estimation method (EPPE) – is presented in great detail. This method allows for the transfer of the soil behaviour obtained in cyclic simple shear tests to the bearing behaviour of the entire foundation. Herein, the numerical model accounts for the cyclic excess pore pressure accumulation by respecting the element-based mean stress and stress amplitude as well as an equivalent number of load cycles. The simulation of the excess pore pressure build-up due to certain cyclic loading is based on undrained conditions, i.e. the excess pore pressure build-up due to cyclic loading is derived by disregarding the simultaneous consolidation process. The respected transfer method, in the form of contour plots, enables the consideration of site-specific cyclic direct simple shear and triaxial test results from laboratory devices to elements within the finite element model. Each integration point is evaluated individually. Based on the derived excess pore pressure field, a consolidation analysis takes place in the second step. The actual accumulated excess pore pressure in each element at the end of the storm (or cyclic loading event) is then found by analytically superposing the excess pore pressure decay curves from the consolidation analysis. For a deeper understanding of cyclic soil behaviour, the cyclic response in different laboratory devices with different densities and under varying stress states was investigated by the author. A contour approach based on cyclic load- and displacement-controlled test results is derived to study the element response from the numerical point of view and use these for the calibration of an implicit model. Moreover, different explicit approaches are presented and compared in terms of their estimation behaviour of cyclic excess pore pressure generation, their predicted foundation capacity and their model assumptions. The intention is hence to examine existing approaches and their applicability by means of an elaborate comprehensive study. A simple modular explicit model is presented which can be easily assessed with engineering judgment. If needed, the different individual calculation steps can be exchanged with more sophisticated ones. For a reference sandy soil, results of cyclic laboratory tests are presented and used on a reference monopile foundation for a predefined storm event. The EPPE approach helps to quantify the risk of capacity degradation as well as to evaluate an appropriate safety margin. It is possible with the current methodology to evaluate the degradation potential for different sites quite easily and fast.

U2 - 10.15488/13232

DO - 10.15488/13232

M3 - Doctoral thesis

CY - Hannover

ER -