Research On Dynamic Characteristics Of Saturated Loose Sand Under Random Loading

As the main dynamic loading on coastal engineering, ocean engineering and marine engineering, wave loading had a great effect on carrying capacity and dynamical stability of seabeds or offshore foundations. As a major factor inducing sandy seabed liquefaction, wave loading also guided the design of ports, oil platforms and subsea pipelines. As a matter of fact, wave loading was a kind of random loading, without the characteristics of random wave it’s impossible to describe wave loading precisely. So the research on dynamic test and numerical simulation under random wave loading was extremely important.Based on the generalized Biot consolidation theory, using linear wave superposition method, this paper established a porous medium two-dimensional finite element model of the elastic seabed, applyied regular wave loading and random wave loading through combination of spatial functions and time function. Calculations indicate that, under the pressure of random wave loading and regular wave loading, stress characters of seabeds had the same trend along seafloor depth direction, meanwhile the amplitude was slightly different. The state of stress changed rapdily near the sea surface, finally came to a steady state at a certain depth. The wave sequence can be used for the dynamic performance of the internal research in the dynamic test of the seabed.Random loading sequence was generated using average JONSWAP spectrum. The random wave loading and the corresponding representative wave loading were applied to saturated loose sand sample at the different confining pressures using CKC multi-functional stress path triaxial apparatus respectively. The differences of pore pressure and strain under the two types of loading conditions were investigated. Also, the influence of random wave loading shape on the dynamic characteristics of sand was examined through applying random loading in ascending and descending order. The experimental results reveal that the changing patterns of pore pressure and axial strain in soil sample under random wave loading are different to the patterns under the representative wave loading, and the time to reach liquefaction is significantly reduced. Under representative wave loading, the pore pressure cumulated linearly. While pore pressure increased irregularly and showed a segment of platform under random wave loading. When the stress amplidute was less than stress threshold value, wave vibration didn’t cause the accumulation of pore pressure and marine soils were in elastic state. Loading sequence of random waves has no obvious impact on dynamic behavior of sand.Made use of constitutive model based on generalized plasticity theory, the model response characteristics under cosine loading and random loading were calculated, and the calculated results with experimental data were compared. It turned out that fitting of simulation curve and test curve was very good, time to failure and changing pattern of the two were the same. The pore pressure rised oscillatory but evenly under cosine loading and increased unevenly under random loading. Large amplitude wave was the main cause of pore water pressure arise. Vertical strains were very small at the two kinds of loading pattern.