Seismic response analysis of the deep saturated soil deposits in Shanghai

The quaternary deposits in Shanghai are horizontal soil layers of thickness up to about 280 m in the urban area with an annual groundwater table between 0.5 and 0.7 m from the surface. The characteristics of deep saturated deposits may have important influences upon seismic response of the ground in Shanghai. Based on the Biot theory for porous media, the water-saturated soil deposits are modeled as a two-phase porous system consisting of solid and fluid phases, in this paper. A nonlinear constitutive model for predicting the seismic response of the ground is developed to describe the dynamic characters of the deep-saturated soil deposits in Shanghai. Subsequently, the seismic response of a typical site with 280 m deep soil layers, which is subjected to four base excitations (El Centro, Taft, Sunan, and Tangshan earthquakes), is analyzed in terms of an effective stress-based finite element method with the proposed constitutive model. Special emphasis is given to the computed results of accelerations, excess pore-water pressures, and settlements during the seismic excitations. It has been found that the analysis can capture fundamental aspects of the ground response and produce preliminary results for seismic assessment.     

Numerical study of localization in soil systems

A numerical study of the mechanical behavior of heterogeneous soil systems, consisting of a bulk of sand with embedded stiff gravel inclusions or soft clay inclusions, is performed. A solution scheme using parallel computing is employed when analyzing two different categories of problems. First, a homogenization problem is studied, where use of a single representative volume element subjected to plane strain compression offers the possibility to investigate the coupling between the response at a local scale and at a global scale. Second, a plane strain footing problem with different heterogeneous soil systems is analyzed using a traditional finite element formulation. The material model utilized for the soil is a large deformation formulation of non-associated elasto-plasticity with an isotropic hardening law, able to represent dilation. It was found that the shape of the gravel or clay inclusions in the systems had no significant effect on the global responses, whereas the strain localizations in the two different soil systems, sand–gravel and sand–clay, were found to have different character. The effect of the initial density on the response was clearly observed in the localization patterns.     

Investigating the effect of soil models on deformations caused by braced excavations through an inverse-analysis technique

In this study, a series of inverse-analysis numerical experiments was performed to investigate the effect of soil models on the deformations caused by excavation by using the finite element method. The nonlinear optimization technique that was incorporated into the finite element code was used for the inverse-analysis numerical experiments. Three soil models (the hyperbolic model, pseudo-plasticity model, and modified pseudo-plasticity model) were employed in the intended numerical experiments on a well-documented excavation case history. The results indicate that wall deflection due to excavation can be accurately back-figured by each of the three soil models, while the ground surface settlement can be reasonably optimized only by the pseudo-plasticity model and the modified pseudo-plasticity model. Importantly, the modified pseudo-plasticity model can yield more reasonable simulations when the wall deflection and the ground surface settlement are simultaneously back-figured. The results show that selection of an adequate soil model that is capable of adequately describing the stress–strain-strength characteristics of the soils is essentially crucial when predicting the excavation-induced ground response.     

Reliability-based semi-analytical solution for ground improvement by PVDs incorporating inherent (spatial) variability of soil

The design of soil consolidation via prefabricated vertical drains (PVDs) has been traditionally carried out deterministically and thus can be misleading due to the ignorance of the uncertainty associated with the inherent (spatial) variation of soil properties. To treat such uncertainty in the design process of soil consolidation by PVDs, stochastic approaches that combine the finite element method with the Monte Carlo technique (FEMC) have been usually used. However, such approaches are complex, computationally intensive and time consuming. In this paper, a simpler reliability-based semi-analytical (RBSA) method is proposed as an alternative tool to the complex FEMC approach for soil consolidation by PVDs, considering soil spatial variability. The RBSA method is found to give similar results to those obtained from the FEMC approach and can thus be used with confidence in practice.     

深基坑土钉支护的有限元数值模拟及稳定性分析

在平面应变条件下,对土钉支护的基坑进行了有限元数值模拟,在此基础上采用有限元稳定分析方法评价基坑的稳定性。研究了土钉拉力及基坑变形的变化规律,土钉长度及布置方式变化对基坑变形和稳定性的影响。结果表明,当土钉足够长时,基坑的潜在滑动面与最大拉力作用点连线位置通常是一致的     

Assessment of identifiable defect size in a drilled shaft using sonic echo method: Numerical simulation

The purpose of this paper is to study the capabilities of the sonic echo method in assessing the location of defects and the minimum possible detectable defect size in a drilled shaft. Three-dimensional axisymmetric finite element models were performed to investigate the effects of the defect size, defect depth-to-shaft diameter ratio, and shaft-to-soil stiffness ratio on the response of the sonic echo test. According to the results from the numerical simulations, a useful formula for defect size assessment in a shaft based on the sonic echo method is proposed, considering these factors simultaneously. Furthermore, a formula for predicting the detectable length-to-diameter ratio of a shaft was also recommended which correlates with the shaft-to-soil stiffness ratio and it varies from 10 to 32.     

Numerical analysis of a one‐dimensional infiltration problem in unsaturated soil by a seepage–deformation coupled method

A multiphase coupled elasto‐viscoplastic finite element analysis formulation, based on the theory of porous media, is used to describe the rainfall infiltration process into a one‐dimensional soil column. Using this framework, we have numerically analyzed the generation of pore water pressure and deformations when rainfall is applied to the soil. A parametric study, including rainfall intensity, soil–water characteristic curves, and permeability, is carried out to observe their influence on the changes in pore water pressure and volumetric strain. From the numerical results, it is shown that the generation of pore water pressure and volumetric strain is mainly controlled by material parameters α and n′ that describe the soil–water characteristic curve. A comparison with the laboratory results shows that the proposed method can describe very well the characteristics observed during the experiments of one‐dimensional water infiltration into a layered unsaturated soil column. Copyright © 2010 John Wiley & Sons, Ltd.