Soils and Foundations最新文献

Evaluation of seismic bearing capacity is to be vital for design of strip foundations in earthquake areas. Combining the upper bound theorem of limit analysis, the discrete technique is successfully extended in this study to investigate the seismic ultimate bearing capacity of shallow strip foundations on rock masses considering the Rayleigh waves, in which the nonlinear HB failure criterion is used to describe the constitutive relation of rock masses. The failure model of foundation soil is generated using the discretization method, a “point by point” technique. The variations of shear modulus G of rock masses and seismic acceleration varying with the depth are taken into consideration. The generalized tangential technique is employed to avoid the difficulty resulting from the nonlinear HB failure criterion. A linear corresponding to the Mohr–Coulomb failure criterion, tangent to the nonlinear Hoek–Brown failure criterion, is used to derive the objective function that is to be minimized. By comparing with the existing results, the present approach is verified. The widely parametric studies are made to investigate the effect of different parameters, e.g. shear modulus G, m_i, GSI, ${\sigma }_{\mathrm{ci}}$, γ, D, V_R, on the seismic bearing capacity of strip foundations. The present method provides a reference for strip foundations designed in earthquake areas.

Literature review revealed that effects of particle segregation and silt uniformity on the liquefaction resistance of sand–silt mixtures are not well understood. Therefore, cyclic direct simple shear tests were conducted to investigate effects of silt uniformity and stratified structures on the liquefaction resistance of sand–silt mixtures with 0%–40% fines content (FC). For all uniform sand–silt mixtures, as FC increased up to 20%, liquefaction resistance decreased, while it increased as FC increased from 20% to 40%. The liquefaction resistance of the samples with uniform silt only in the top and bottom layers was slightly higher than that of a uniform sample (USM), while the cyclic strength of the samples with silt concentrated in the middle layer was greater (up to 23%) than that of other nonuniform samples. USM exhibited the least liquefaction resistance. In addition, the number of silt layers (NoSLs) substantially affected the liquefaction resistance of stratified structures: as NoSLs increased from 1 to 3 layers, the cyclic resistance ratio was reduced by 20%, 10%, and 7% for FC values of 20%, 30%, and 40%, respectively. The liquefaction resistance of the stratified samples was greater than that of USM. To quantify the effect of silt uniformity and NoSLs, the nonuniformity index (NUI) was introduced herein; the calculated NUI values showed that the increase in liquefaction resistance was well correlated with the increase in the NUI.

Ground deformation on the Earth’s surface layer is strongly affected by the nonlinearity of geomaterials. However, the formation process of such deformation has yet to be described in a unified manner based on mechanics. The present study focuses on the normal faults in a submarine ground with highly developed soil skeleton structures and attempts to reproduce the process of normal fault formation associated with the tilting of a horizontally deposited submarine ground using an elastoplastic finite element simulation. The simulation was conducted using the soil–water coupled finite deformation analysis code GEOASIA, which incorporates an elastoplastic constitutive equation of the soil skeleton based on the modified Cam-clay model and the soil skeleton structure concept. The key findings are as follows:

1) Normal faults are formed from the ground surface to depth as shear bands, where shear strain is localized while exhibiting softening behavior with plastic volume compression.

2) Multiple normal faults are almost equally spaced and parallel to each other, with the inter-fault blocks rotating backward. The morphology of normal faults formed by the tilting of the ground shows domino-style characteristics.

3) The degree of the soil skeleton structure influences the formation of normal faults.

This study demonstrates that elastoplastic geomechanics can explain the formation process of ground deformation, which has usually been interpreted from the perspectives of geomorphology and geology.

With the development of computer vision technology, structure from motion and multiview-stereo (SfM-MVS) approach has been widely applied in the geotechnical field. However, as a method that utilizes a series of images to reconstruct a 3D model, errors often occur due to insufficient feature points in the images. In this study, soil blocks, rubber specimens, and a sand particle ranging in size from 10 cm to 0.3 mm were utilized for synthetizing 3D model by the SfM-MVS approach. Additionally, an artificial background containing various colored blocks was introduced during photographing process to improve this approach. Moreover, the application of this approach was extended to process optical microscope images and scanning electron microscope (SEM) images by using two artificial backgrounds. Experimental comparison suggested that using artificial backgrounds could optimize the depression areas between the specimen and sample holder of the three-dimensional (3D) model generated by the SfM-MVS approach, especially in the depression portions with acute angles. And the reconstructed model from the SfM-MVS approach was comparable to that generated by X-ray computed tomography (CT). It was also found that increasing the image resolution and decreasing voxel size can improve the accuracy of the 3D model. And these improvements have been quantitatively demonstrated by tests. When using optical microscopy and SEM, the application of artificial backgrounds significantly increased the success rate of constructing 3D models, compared to the near impossibility of achieving successful reconstruction without them in the practice. It was mainly attribute to sufficient feature points in artificial backgrounds can be captured from artificial backgrounds in the camera tracking and point-matching processes of the SfM-MVS approach. With the proposed method in this study, the applicability of the SfM-MVS approach was extended in laboratory geotechnical experiments.

Since the soil–water characteristic model relates the matric suction and the water content, it cannot describe changes in the water content when the suction is zero and constant, i.e., all the pore air is trapped air. To reasonably describe changes in the water content due to air entrapment and the compressibility of trapped air, this paper presents a deformation analysis method based on the mixture theory for a four-phase mixture consisting of the soil skeleton, capillary water, trapped air, and continuous air, in which the pore air phase is divided into trapped air and continuous air phases. Specifically, considering the mass conservation equation and the equation of motion for each phase of trapped air and continuous air, and considering the mass exchange between the trapped air and continuous air phases, governing equations were derived for the initial and boundary value problems of the four-phase mixture in a finite deformation field using a rate-type equation of motion.

Two examples are provided to validate the new method. Firstly, experiments and analyses of soil water retention tests were conducted under multiple drying-wetting cycles. A comparison shows that, even if hysteresis is not considered in the relationship between the effective degree of saturation and suction, the new method can successfully describe the gradual decrease in the degree of saturation at a suction of 0 kPa with multiple drying-wetting cycles, indicating that the pore air gradually becomes trapped in the pore water, by modelling the mass exchange between the trapped air and continuous air phases. Secondly, analyses of an unexhausted and undrained triaxial compression test under zero suction were conducted, comparing the new and previous soil–water-air coupling methods. The results show that the new method, unlike the previous method, can successfully simulate the experimental result. This is because the new method is able to describe the compressibility of trapped air as the change in the capillary water degree of saturation, which is a novel state variable defined as the ratio of the volume of capillary water to the total volume of capillary water and trapped air.

The new method contributes to the simplification of the soil–water characteristic model and enables evaluations of the soil deformation behavior due to the compressibility of trapped air, such as a countermeasure against liquefaction caused by unsaturation.