Offshore wind turbines (OWT) subjected to long-term complex marine loads are prone to foundation damages, affecting their regular operation. However, the failure mode of the large-diameter monopile foundation under complex marine loads is not well understood. Hence, failure modes of monopile foundation for OWT in soft-soil seabed under unidirectional and multidirectional loads are investigated. Firstly, the mechanical characteristics of soft soil are tested indoors. Subsequently, the monopile foundation failure modes under unidirectional loads are examined, which indicates capacity failures of monopile foundation are progressive, and two distinct soil failure mechanisms are identified. Then, the failure modes and failure envelopes under multidirectional loads (horizontal load, vertical load, and bending moment）are explored. Results show that the failure envelopes in H–V and M–V load space are oval-shaped, and failure modes in H–V and M–V load space are characterized by wedge failure. More critically, the service performance of the monopile foundation in soft soil in a full lifecycle can be estimated by the location of the actual loads in the failure envelope. Finally, the effect of pile diameter on the bearing capacity of the monopile foundation for OWT under unidirectional and multidirectional loads is discussed.

Internal erosion at soil–structure interfaces is a dangerous failure pattern in earth-fill water-retaining structures. However, existing studies concentrate on the investigations of internal erosion by assuming homogeneous materials, while ignoring the vulnerable soil–structure-interface internal erosion in realistic cases. Therefore, orthogonal and single-factor tests are carried out with a newly designed apparatus to investigate the critical hydraulic gradient of internal erosion on soil–structure interfaces. The main conclusions can be draw as follows: (1) the impact order of the three factors is: degree of compaction > roughness > clay content; (2) the critical hydraulic gradient increases as the degree of compaction and clay content increases. This effect is found to be more obvious in the higher range of the degree of soil compaction and clay content. However, there exists an optimum interface roughness making the antiseepage strength at the interface reach a maximum; (3) the evolution of the interface internal erosion develops from inside to outside along the interface, and the soil particles at the interface flow as a whole; and (4) the critical hydraulic gradient of interface internal erosion is related to the shear strength at the interface and the severity and porosity of the soil.

This paper introduces a new memristor-based hyperchaotic complex Lü system (MHCLS) and investigates its adaptive complex generalized synchronization (ACGS). Firstly, the complex system is constructed based on a memristor-based hyperchaotic real Lü system, and its properties are analyzed theoretically. Secondly, its dynamical behaviors, including hyperchaos, chaos, transient phenomena, as well as periodic behaviors, are explored numerically by means of bifurcation diagrams, Lyapunov exponents, phase portraits, and time history diagrams. Thirdly, an adaptive controller and a parameter estimator are proposed to realize complex generalized synchronization and parameter identification of two identical MHCLSs with unknown parameters based on Lyapunov stability theory. Finally, the numerical simulation results of ACGS and its applications to secure communication are presented to verify the feasibility and effectiveness of the proposed method.

Owing to the repeatedly observed strong vertical ground motions and compressional damage of engineering structures in recent earthquakes, the multi-directional site response analysis is increasingly critical for the seismic design of important structures, such as nuclear power plants and high earth dams. However, the site response to the vertical component of the ground motion has not been the subject of detailed investigation in the literature. Therefore, in this paper, the vertical site response due to vertical ground motion is investigated by employing both analytical and numerical methods. First, a one-dimensional frequency domain analytical solution, which can be employed for vertical site response analysis in practice, is studied and compared against time domain finite-element (FE) analyses for the two extreme soil state conditions (i.e. undrained and drained conditions). The vertical site response is further investigated with hydro-mechanically (HM) coupled FE analysis, considering solid–fluid interaction. The parametric studies undertaken show that the predicted vertical site response is strongly affected by the parameters characterising the hydraulic phase, namely, soil permeability and soil state conditions, both in terms of frequency content and amplification. The subsequent corresponding quantitative investigation, of the frequency content and amplification function of the vertical site response, shows that depending on the soil permeability the response is dominated by the two types of compressional waves (fast and slow waves). Notably, the parametric studies identify a range of permeability that significantly affects dynamic soil properties in terms of P-wave velocities, damping ratios and vertical site response, and this range is relevant for geotechnical earthquake engineering applications. It is therefore recommended that coupled consolidation analysis is necessary to simulate this effect accurately at such permeability-dependent intermediate transient states between fully undrained and drained conditions. Finally, this work suggests a simple modification of standard total-stress site response analysis to account for vertical ground motion and solid–pore fluid interaction. In order to simulate the attenuation of the response due to solid–pore fluid interaction effects, it is suggested to employ additional HM viscous damping in total-stress analysis, further to the one used to account for hysteretic material damping. This additional viscous damping can be quantified based on the empirical curves proposed in the paper.