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Application of fractal theory to unsaturated soil mechanics
XU Yongfu, TONG Lixin
Front. Struct. Civ. Eng.. 2007, 1 (4): 411-421.
https://doi.org/10.1007/s11709-007-0056-5
The mechanical properties of unsaturated soils are a function of the saturation degree or matric suction, and can be obtained based on currently available procedures. However, each procedure has its limitations and consequently, care should be taken in the selection of a proper procedure. The fractal approach seems to be a potentially useful tool to describe hierarchical systems and is suitable to model the structure and hydraulic properties of unsaturated soils. In this paper, the soil-water characteristics, unsaturated hydraulic conductivity function, unsaturated shear strength, swelling deformation and compression were derived from the fractal model for the pore-size distribution, and were expressed by only two independent physical parameters, the fractal dimension and the air entry value. The predictions of the proposed soil-water characteristics, unsaturated hydraulic conductivity, unsaturated shear strength, swelling deformation and compression were in good agreement with published experimental data. Comparisons between the experimental results of unsaturated hydraulic conductivity and the predictions of the both fractal model and the van Genuchten-Mualem model were also performed, and it was found that the predictions of the fractal model were better than that of the van Genuchten-Mualem model.
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Seismic ground motion analysis of Shanghai Pudong Airport site considering the effects of spatial correlation and irregular topography
JIANG Tong, CHEN Laiyun, XING Hailing, L? Xilin
Front. Struct. Civ. Eng.. 2007, 1 (4): 430-435.
https://doi.org/10.1007/s11709-007-0058-3
The terminal No. II of Shanghai Pudong International Airport is located at Pudong District of Shanghai City near shore of East China Sea, and the area of the long-span terminal is 400 m × 200 m. The construction site of the terminal locates on the irregular topography, and its alluvium achieves about 300 m in thickness. The spatial correlation of seismic ground motion, as well as the amplification of soft alluvium and the effect of irregular topography, should be considered. This paper uses a simplified method to obtain the response spectrum of the engineering bedrock under the irregular topography. The spectrum is used to generate the sets of spatially correlative horizontal and vertical seismic motions. The surface ground motion was calculated under incidence of the spatially correlative seismic motion by 2D finite element method (FEM) model considering nonlinear properties of the soil by means of the equivalent linear method. In order to compare the effect of 2D irregular topography, the seismic response analysis of 1D model is carried out by using the equivalent linear method. For indicating the effect of the spatial correlation of input motions, the horizontal uniform inputs, as well as the horizontal and vertical uniform input are carried out for the seismic response analysis of the site. Finally, some characteristics of seismic ground motion calculated for previously mentioned cases are compared.
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Explicit finite element method for calculation and analysis to the elasto-plastic dynamic response of fluid-saturated porous media
LI Liang, DU Xiuli, LI Liyun, ZHAO Chenggang
Front. Struct. Civ. Eng.. 2007, 1 (4): 436-442.
https://doi.org/10.1007/s11709-007-0059-2
In order to describe the elasto-plastic dynamic response of fluid-saturated porous media, the incremental elasto-plastic wave propagation equations of fluid-saturated porous media are developed by the fundamental theory of continuum mechanics and appointing to the characteristic of fluid-saturated porous media. Then, the space discretization of these equations is performed to get their Galerkin formula. At last, the time discretization of this formula is carried out with the integral method which consists of central difference method and Newmark constant average acceleration method to get the explicit time integral formula for solving the wave propagation equations of porous media. On the basis of the integral formula mentioned above, the time-domain explicit finite element method is developed for calculation and analysis of the elasto-plastic dynamic response of fluid-saturated porous media. In this method, the decoupling technique is adopted and it does not need to solve simultaneous linear equations in each time step, so the computational effort and memory requirement can be reduced considerably by using this method.
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New model of load transfer function for pile analysis based on disturbed state model
LIU Qijian, YANG Linde, WU Jun
Front. Struct. Civ. Eng.. 2007, 1 (4): 443-447.
https://doi.org/10.1007/s11709-007-0060-9
Based on the disturbed state concept (DSC), a new model of load transfer function for pile analysis is established by the idea that the deformed material between pile and soil can be treated as a mixture of two constituent parts, which are in intact or critical state and assumed to obey random distribution. Starting from the homogenization theory of heterogeneous materials and statistics method, a parameter D to describe the disturbance degree is proposed, and a formula to determine the parameter has been derived by using the plastic displacement of a pile as distribution variable. In the model, shear intensity of elements in an intact state are simulated by Duncan-Zhang model and that in a critical state by Mohr-Coulomb criterion. The model of this paper has few parameters, which can reflect the aspects of load transfer function, such as hardening, softening and the effects of confining pressure. The well agreement between the in-situ data and the predicted shows that the validity of the model herein. So the proposed model in this paper is easy to be used in engineering practice.
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