This study evaluates the seismic performance of inclined tripod micropiles through physical modeling. A dynamic centrifuge model test examined the effect of inclination angle from 0° to 30° on the dynamic behavior of micropiles against seismic loads. The physical modeling results showed that the inclined micropiles exhibited better seismic performance than the vertical micropiles. The horizontal displacement and acceleration at pile heads decreased as the installation angle increased, but with minimal difference between 15° and 30°. The vertical displacement was the least at the 15° pile. The near-surface bending moments of the 15° and 30° piles were significantly less than that of the 0° pile. The presented results suggest that an installation angle of 15° is expected to be a proper choice for inclined micropile design for a limited space usage.

Seismic earth pressure acting on basement walls of buildings is greatly influenced by the dynamic interaction between the surrounding soil and the structure. In this study, a series of 1g shaking table tests were conducted to analyze the influence of the dynamic soil-structure interaction on seismic earth pressure acting on basement wall of building. The building model was prepared to satisfy the scaling laws for the natural frequency, mass, and length of a prototype building. Three models were made to have different conditions with no building, with a three-story building, and with a nine-story building. All models have same basement structure with 3-story. The model ground was prepared with a dense sand layer of about 80% relative density. The sine waves were used as input motion, varying in frequency within the range of 3 to 6 Hz and amplitude within the range of 0.1g to 0.3g. The magnitude and distribution of the seismic earth pressure acting on basement wall were measured using load cells. The test results showed that the magnitude of seismic earth pressure increased as the building height increased. The distribution shape changed from a triangular shape under no-building condition to inverted triangular shape as the building height increased.

Well foundations are one of the most commonly used deep foundations in bridges. Due to their ease of construction and high rigidity, these foundations are preferred in earthquake prone areas and in difficult soil conditions. As it is difficult to introduce plasticity in these massive structures, well foundations are designed to remain elastic during earthquake. These foundations have played a major role in the survival of bridges during the past earthquakes. But, there have been some failures in these foundations also, in terms of excessive displacements and rotations, due to shear fluidization of surrounding soils. This has motivated the researchers to study their response under combined vertical and lateral loads under seismic conditions. Under gravity and seismic loads, foundations are subjected to vertical load, V, shear force, H, and moment M. In the past, most researchers have studied the effect of vertical load, V, on shear, H, and moment, M, capacities of well foundations, individually. But in reality H and M are always combined, and the foundation capacity is significantly influenced by the combination of V-H-M loading. The aim of the present work is to study the characteristics features of the failure pattern due to different combinations of V, H and M loading on well foundation. ‘Probe’ analyses using finite element limit analysis (FELA) are used to develop the V-H-M capacity envelopes. The effect of seismic inertial forces, on the V-H-M capacity, is also investigated, using pseudo-static analyses, by applying constant seismic acceleration coefficient, a_h. The results of these analyses are compared with their static counterparts.

The main objective of this research is to investigate the response of the pre-piled offshore wind farm jacket structure when the structure undergoes operational cyclic (wind+wave) loads combined with earthquake loads. Hence, a two-layered soil condition was chosen, with the shallow-loose layer overlying a deep-denser layer. The scaled model of an exact jacket configuration was designed and tested in a centrifuge along with the wind tower and nacelle arrangement, which matches the dynamic properties of an equivalent 9MW turbine. Two cases were considered, one with lateral load (wind load) and another with no lateral load during the earthquake. The lateral load was applied at the nacelle level using an in-flight mass on a pneumatic piston arrangement to generate the equivalent pile head moment due to the wave and wind loads. The key findings are that the generation of excess pore pressures causes the soil to liquefy under strong earthquakes, eventually leading the structure to settle and rotate more than its allowable serviceability limit state (SLS). While it is evident that the lateral loads combined with seismic load cause excessive settlement and rotation, the structure also shows considerable settlement and rotation without any lateral load during the seismic event. Also, the effect of including operational wind and wave loads in combination with the seismic loads recommended by DNV-ST-0437 (2016) and DNV-RP-0585 (2021) are also discussed.

The offshore wind turbine structure is designed to have the first natural frequency between the rotor frequency (1P) and blade passing frequency (3P), a so-called soft-stiff design. Degradation of the pile-soil interaction stiffness can lead to a reduction of the first natural frequency, bringing it closer to the 1P and wave frequency. This leads to dynamic amplification of stresses in the structural steel. An analysis of the degradation of the pile-soil interaction stiffness and the associated reduction of the first natural frequency is therefore warranted. While 1D beam-column models are used for routine analysis of monopile-soil interaction, the modelling of the stress-strain response of the soil in non-linear 3D finite elements analysis with pore pressure generation/dissipation can offer improved insight into the cyclic degradation of soil stiffness and its effect on the monopile stiffness. In this work, a coupled dynamic analysis of a laterally loaded monopile located in the Belgian North Sea is presented. A 3D Finite Element (FE) soil-monopile model is developed in ABAQUS. The soil is modeled using the hypoplastic constitutive model, calibrated to advanced static and cyclic laboratory tests. The model makes use of the dynamic u-p formulation that allows for the generation and dissipation of pore pressures. The monopile is subjected to high-intensity loads (storm conditions) and the degradation of monopile lateral stiffness due to storm loading is investigated. The reduced stiffness is introduced in an integrated model of the wind turbine structure to quantify the reduction of the first natural frequency. It is found that cyclic loading leads to the degradation of the pile-soil stiffness, which in turn leads to the degradation of the wind turbine's frequencies. The effects are more pronounced on the frequency of the 2^nd mode of vibration.

The performance of existing pile groups and multiple pile group systems would be critical in ensuring the acceptable performance of the associated structures under the ever-increasing seismic hazard. It is argued that this could be assessed partly by understanding the development of plastic hinges in these pile group systems. This paper is to examine the development sequence of plastic hinges in pile groups and multiple pile group systems under lateral pushover, in different geotechnical conditions and pile types. An emphasis is put first on the spatial sequence of the plastic hinge development and second on the overall sequence of the development. A series of 3D numerical analyses is performed. The plastic hinges for single pile groups start to develop at pile head level, from perimeter piles and to center pile. For multiple pile group systems, the hinges start to develop in perimeter piles of the outer pile caps and to continue with firstly outer pile caps and subsequently the center pile cap. The plastic hinges at depths only appear to develop after plastic hinges at pile head level develop in all piles, and they develop in a similar pattern. Based on these findings, it is argued that pile retrofitting or repairing in pile groups would be possible, and therefore the performance of repaired damaged piles is briefly reported. In addition, the consequences of the plastic hinge development to the overall system load-displacement performance are discussed.

This paper focuses on the assessment of the critical acceleration for soil-foundation systems and the examination of the influences stemming from various relevant parameters. Using a formula recently proposed for the assessment of the seismic bearing capacity of shallow strip foundations, the critical acceleration of the soil-foundation system has been determined by imposing equilibrium between the limit load of the soil-foundation system and the applied load transmitted from the foundation to the ground surface. The proposed solution resulted in an iterative equation that allows for the numerical evaluation of critical acceleration. An extensive parametric analysis has been conducted to explore the effects of many relevant parameters, such as the angle of shear strength of the foundation soil, the value of the surcharge acting aside the footing, the static vertical load transmitted by the foundation, and the direction of the resultant inertial forces acting in the soil and superstructure. The results are presented in design charts for specific sets of relevant parameters, facilitating a quick evaluation of the critical acceleration of the soil-foundation system.