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.

The deterioration depth (D) is a relatively simple index used to evaluate the deterioration degree of solidified soil. Based on the equivalent relationship between the accelerated deterioration tests and the conventional deterioration tests and the power function form of the D prediction equation, a rapid prediction method is proposed in this study for predicting the D of cement solidified marine soft soil. Deterioration test results of the cement solidified marine soft soil showed that increases in the concentration of seawater accelerated the rate of cement soil deterioration. Deterioration test results of the cement solidified marine soft soil showed that increases in the concentration of seawater accelerated the rate of cement soil deterioration. Additionally, the D obtained from indoor and in-site conventional deterioration tests is almost the same. A rapid prediction method for the D of cement stabilized marine soft soil was established with the equivalent relationship between the accelerated and conventional deterioration tests and a power function form of the deterioration depth prediction equation. The predicted D and the development trend were more consistent with the results of the indoor and in-site conventional deterioration tests.

The behavior of salt solutions infiltrated into compacted bentonite was investigated in this study, with particular attention paid to the ion concentration in the pore water, in order to improve the understanding of the bentonite behavior in geological disposal projects. A Japanese bentonite, Kunigel V1 (K_V1), was used to prepare specimens with a thickness of 2 mm and an initial dry density of 1.4 to 1.7 Mg/m³. For each density case, salt solutions (NaCl, KCl, and CaCl₂) of different amounts (0 to 2 mol/L) were supplied to the specimens. After infiltration, the basal spacing (d₀₀₁) and exchangeable cations of the montmorillonite in the bentonite and the leached cations from the bentonite were measured. Based on the test results, the ion concentration in the interlayer pore water of the montmorillonite or the interparticle pore water was discussed. The findings indicated that the infiltration capacities of the various salt solutions into the compacted K_V1 bentonite were in the order of KCl > CaCl₂ > NaCl. The K_V1 specimen with the highest initial dry density exhibited the strongest resistance to salt solution infiltration. After the infiltration of the NaCl solution into the compacted K_V1 bentonite, the increased sodium ions mainly remained in the interparticle pores, leading to an increase in the sodium ion concentration in the interparticle pore water. During the infiltration of the KCl and CaCl₂ solutions into the compacted K_V1 bentonite, the infiltrated potassium ions in the case of KC1 and the calcium ions in the case of CaCl₂ tended to penetrate the interlayer pore preferentially, thereby displacing the exchangeable sodium ions. After most of the exchangeable sodium ions that had initially existed in the montmorillonite had been replaced, the infiltrated potassium or calcium ions remained in the interparticle pores.

Natural volcanic soils containing pumice particles are commonly found in Hokkaido, Japan, and this type of soil is prone to landslides, internal erosion, and liquefaction. Therefore, the purpose of this paper is to summarise the hydro-mechanical response of volcanic ash soil subjected to internal erosion using the modified erosion triaxial apparatus, based on the literature and additional investigations. The results of the study show that the rate of erosion and shear strain during the erosion process are influenced by initial density, stress state, and hydraulic gradient. Notably, anisotropic consolidation is experienced by specimens under seepage flow. Additionally, the removal of fines leads to a slight decrease in the grading state index. Moreover, suffosion increases the maximum shear modulus and Poisson’s ratio of the soil, while increasing seepage time stabilises the peak shear strength of eroded specimens. Furthermore, the critical state line does not change much with internal erosion. To sum up, this study offers valuable insights into the behaviour of volcanic ash soil subjected to internal erosion and provides an integrated interpretation of hydro-mechanical response of volcanic ash on removal of fines.

The pile capacity is commonly calculated by the engineers as the lesser of its structural capacity and the ultimate resistance of ground supporting it using a generalized equation irrespective of the shaft type, socket diameter, socket length, rock type and grout strength. This equation may be over-simplified and risky if the pile/grout/rock interaction is not considered. Based on the loading tests of 6 instrumented socketed piles with 4–6 m rock socket by others and 35 non-instrumented socketed H-piles with 5–34 m rock socket by the author, the load-transfer mechanism in long rock socket is found dependent not only on the mobilization of shear resistance in soil and rock layers, but also largely on the steel/grout bond behavior. A side resistance distribution factor α_s is introduced as a simple and practical index to represent the load-transfer mechanism along the pile shaft and to the socket. It would increase with an increase in loading and pile length in soils, but decrease with an increase in socket length indicating that critical socket length does exist which is likely depending on the grout bond strength. Average bond stress reduces with increased socket length when the critical socket length is exceeded. Residual settlement is largely due to the slip and bond failure at the interface. Creep settlement is largely affected by the properties of grout mix and tends to increase with increased socket length.

Landslides are a multifaceted phenomenon triggered by rainfall infiltration as a consequence of the decrease in effective stress upon the development of porewater pressure. Although many studies concentrated only on rainfall infiltration as the source of the primary hydrological regime, the impact of groundwater dynamics has been relatively underexplored owing to its elusive nature. Field investigations after the landslide incidents provide insight into the influence of groundwater dynamics and speculate its effect as a secondary hydrological regime is immense. Therefore, this paper uses centrifuge modeling and numerical simulations to study groundwater dynamics in rain-induced landslides. Instrumented model slopes made of silty sand were tested to examine the hypothesis of pre-existing groundwater flow levels and surcharged groundwater flow conditions in rain-induced landslides. It was observed that swiftly rising porewater pressure along the soil–bedrock interface triggered landslides more rapidly under high groundwater flow and immediate surcharged groundwater flow conditions. Deformation analysis confirmed that a voluminous landslide could be expected if the role of groundwater dynamics is higher. A two–dimensional coupled hydromechanical finite element simulation was performed to back–analyze the experimental results and to discuss the failure mechanism. Upon validation, numerical simulation emphasized how the failure was accelerated under low-intensity rainfall if high groundwater flow exists. Furthermore, the study identified that surcharged flow profoundly affects landslide initiation if the slope has a low pre-existing groundwater flow. The outcomes highlighted that groundwater dynamics should be an integral part of the temporal predictability of landslides as they can also govern the magnitude of landslides.