Numerical Modeling of the Dynamic Response of an Elastoplastic Seabed Under Wave-Current Interactions

Wave-induced liquefaction of the seabed is a geohazard frequently encountered in shallow waters.Although widely discussed,most studies paid attention to the seabed response under a single sequence of wave loading.However,the seabed suffers from repeated‘wave loading–dissipation’phases in a real ocean environment.In this study,a homogeneous sandy seabed model is established to investigate the mechanism of wave-induced liquefaction by considering the existence of currents.Finite element analyses are conducted by incorporating a kinematic hardening elastoplastic model into the commercial package Abaqus.The constitutive model is validated against centrifugal wave tests.Parametric studies are conducted to demonstrate the effects of relative densities,current,and wave-loading history on the seabed response.The predicted excess pore pressure,effective stress paths,and associated variation of relative density are discussed in detail.The results show that the densification of soils significantly enhances the resistance against liquefaction,which provides new insight into the mechanism of residual liquefaction during wave sequences.

Strength and dilatancy behaviors of deep sands in Shanghai with a focus on grain size and shape effect

With rapid development of infrastructures like tunnels and open excavations in Shanghai,investigations on deeper soils have become critically important.Most of the existing laboratory works were focused on the clayey strata up to Layer 6 in Shanghai,i.e.at depth of up to 40 m.In this paper,Layers 7,9,and 11,which were mostly formed of sandy soils at depth of up to 150 m,were experimentally investigated with respect to physico-mechanical behaviors.The stressestrain behaviors were analyzed by the consolidated drained/undrained(CD/CU)triaxial tests under monotonic loading.One-dimensional(1D)oedometer tests were performed to investigate the consolidation properties of the sandy soils.Specimens were prepared at three different relative densities for each layer.Also,the micro-images and particle size analyzers were used to analyze the shape and size of the sand grains.The influences of grain size,density,and angularity on the stressestrain behaviors and compressibility were also studied.Compared to the other layers,Layer 11 had the smallest mean grain size(D50),highest compressibility,and lowest shear strength.In contrast,Layer 9 had the largest mean grain size,lowest compressibility,and highest shear strength.Layer 7 was of intermediate mean grain size,exhibiting more compressibility and less shear strength than that of Layer 9.Also,the critical state parameters and maximum dilatancy rate of different layers were discussed.

Settlement mechanism of piled-raft foundation due to cyclic train loads and its countermeasure

In this paper, numerical simulation with soil-water coupling finite element-finite difference（FE-FD） analysis is conducted to investigate the settlement and the excess pore water pressure（EPWP） of a piled-raft foundation due to cyclic high-speed（speed： 300km/h） train loading. To demonstrate the performance of this numerical simulation, the settlement and EPWP in the ground under the train loading within one month was calculated and confirmed by monitoring data, which shows that the change of the settlement and EPWP can be simulated well on the whole. In order to ensure the safety of train operation, countermeasure by the fracturing grouting is proposed. Two cases are analyzed, namely, grouting in No-4 softest layer and No-9 pile bearing layer respectively. It is found that fracturing grouting in the pile bearing layer（No-9 layer） has better effect on reducing the settlement.

Investigation of the overconsolidation and structural behavior of Shanghai clays by element testing and constitutive modeling

The mechanical properties and constitutive modeling of Shanghai clays are very important for numerical analysis on geotechnical engineering in Shanghai,where continuous layers of soft clays run 30-40 m deep.The clays are divided into 5 major layers.A series of laboratory tests are carried out to investigate their mechanical properties.The top and bottom layers are overconsolidated hard clays,and the middle layers are normally consolidated or lightly overconsolidated sensitive marine clays.A constitutive model,which can describe the overconsolidation and structure of soils using only 8 parameters,is modified to simulate the test results.A rational procedure to determine the values of the material parameters and initial conditions is also proposed.The model is able to effectively reproduce both one-dimensional(1D)consolidation and drained/undrained triaxial test results of Shanghai clays,with one set of parameters for each layer.From element testing and constitutive modeling,two findings are obtained.First,the decay rates of overconsolidation are smaller in overconsolidated layers than in normally consolidated layers.Second,the natural microstructure of layer 4 is relatively stable,that is,a large degree of structure is still maintained in the specimen even after 1D consolidation and drained triaxial tests.The modified model and obtained parameter values can be used for numerical analysis of geotechnical projects in Shanghai.