Soils and Foundations最新文献

Particle shape affects the mechanical behaviour of soil and is thus a parameter of interest in geotechnical engineering. Shape is commonly described by form, angularity and roughness. Form describes the overall aspect ratio, angularity the sharpness of the edges and corners, and roughness the small surface irregularities. This work explores the characterisation of form and angularity of sand particles. Our results show that focus variation microscopy principles can be implemented in a conventional compound microscope to measure particle heights as small as 60 μm without having to observe a lateral view of the particle. The robustness of the procedure is demonstrated by implementing it on sand-sized particles from six different sources. Importantly, the compound microscope employed by the procedure is likely to be accessible to many soil laboratories. Heights measured using focus variation were used to assess particle form. Contrary to assumptions in previous works, form varied significantly within a given soiltype and a narrow particle size range. Regarding angularity, there is a systematic correlation between particle form and the angularity metric known as 'ellipseness'. Furthermore, while ellipseness is adequate to distinguish between angular and rounded particles, it cannot distinguish between sub-rounded and well-rounded particles.

The filtered tailings disposal (dry stacking) up to 300 m high is an alternative to overcome the drawbacks related to the slurry tailings storage in large impoundments as it is safer and demands smaller portions of the existing landform. Even so, the understanding of the denser and dewatered material response over a broad range of confining pressures is essential to safely design tall dry stacking tailings facilities. Accordingly, the present research assesses the mechanical behavior of compacted iron ore tailings through triaxial tests. A series of compression and extension drained and undrained triaxial tests were conducted over a wide spectrum of confinements (σ́₃ ranging from 75 to 8,000 kPa) to check possible particle breakage occurrence and effects. The influence of the initial density due to compaction was, as well, evaluated since the tests were performed using specimens molded at distinct dry unit weight values. The results were analyzed in the light of the critical state soil mechanics and have indicated the existence of a curvilinear critical state line in the ν: log ṕ plane. Small particle breakage has occurred and can be associated with reduction in surface roughness, breakage of asperities, and reduction in particle angularity. Moreover, a tendency for static liquefaction was observed amongst the loosest specimens sheared under the lowest confining levels.

The most widely used analysis method for the laterally loaded pile problem is the Winkler spring approach. Although researchers have proposed nonlinear formulations for the p-y curves, the contribution of the soil nonlinearity has not been thoroughly studied. The main drawback of the current approach is the use of a single stiffness in the p-y formulation. This study investigates the laterally loaded pile problem by employing the pressure-dependent hardening soil model with small-strain stiffness (HS-Small Model), where the degree of soil nonlinearity is better integrated. The parametric analyses are performed on the verified model for various pile and soil properties. A new p-y model is proposed for pile behaviour under monotonic loading based on the numerical analysis results. The model includes the initial stiffness, ultimate soil resistance, and degree of nonlinearity parameters. The validity of the proposed model is demonstrated by simulating a centrifuge and two field tests from the literature. The proposed model accurately accounts for soil nonlinearity and significantly improves the estimation of lateral displacements.

When designing a piled raft foundation (PRF) on clay, it is essential to understand the time-dependent behaviors of the foundation. However, little attention has been paid to this issue. On the basis of physical model tests, this study presents the long-term behaviors of piled raft foundations with different pile arrangement schemes. In the experiments, three foundation models with the same square raft but different numbers of piles were tested to observe long-term foundation behavior under different vertical load levels. During the observation time, the applied load, the PRF settlement, the axial forces along the piles, and the pore water pressure (PWP) beneath the raft base were carefully measured. The results show that the piles were effective at supporting the applied load and suppressing the settlement of the foundation when the applied loads were smaller than the bearing capacities of the corresponding pile groups. At the larger loads, the raft shared significant proportions of the increment parts of the applied load. The level of the applied load affected the load sharing not only between the raft and the piles, but also between the piles. The corner piles carried larger load at small applied loads but smaller load at larger applied loads, in comparison with the center piles. In addition, due to the variations in load sharing between the raft and the piles, the pile arrangement and the level of applied load affected the distributions of ground strength in both the magnitude and the depth of the affected zone in long-term load tests.

Water retention behavior of clayey soils usually exhibits a hysteretic phenomenon, which can be attributed to the ink-bottle effect, different contact angle during wetting and drying process, entrapped air etc. For expansive soils, along the wetting and drying path, significant microstructure change is usually observed. The effect of microstructure change on the water retention hysteretic phenomenon was studied in this paper for an intact expansive clay from China, Mile clay. The soil water retention curve of Mile clay was obtained at the full suction range. The evolution of microstructure along wetting and drying path for Mile clay was characterized by pore sized distribution obtained from mercury intrusion porosimetry tests. Test results show that a strong hysteretic phenomenon was observed for suction ranging between 40 kPa and 15 MPa. This hysteretic phenomenon was mainly contributed to the different microstructure of specimens along wetting and drying paths with similar water ratio. For higher suction, as adsorption mechanism mainly contributed to the water retention properties, for specimens with similar water ratio, even with different maximum filled entrance pore sizes, the corresponding suction were similar with each other. For the lower suction, due to the completely drying historical state of specimens on the main wetting path, slightly different pore size distributions were observed for specimens on the main wetting and drying path with similar water ratio.

Dynamic soil-water coupling analyses, based on the u-p formulation, are inapplicable to highly permeable soils, causing numerical instability. In this study, it is demonstrated that theoretical solutions to the u-p formulation itself certainly exhibit unconditional convergence regardless of the permeability coefficient. This suggests that the instability is only numerical and can be observed in a temporally discretized system. Firstly, the linearized governing equation for the u-p formulation was proven to be reduced to a damped wave equation under a one-dimensional condition, similar to the Full formulation. Secondly, theoretical solutions for the u-p formulation were derived and their unconditional convergence was confirmed. Then, the essential characteristics of the u-p theoretical solutions, that is, the underestimation of permeability, overestimation of compression wave celerity, and occurrence of negative pore water pressure against positive load application, were described and compared with theoretical solutions for the Full formulation.

Improving soft grounds with cement or lime is commonly used to increase their strength and deformation characteristics. However, the properties of cement/lime-treated soil deteriorate in seawater because magnesium salts accelerate calcium leaching. In this study, changes in the unconfined compressive strength of cement-treated soil samples with various water contents, amounts of added cement, and curing times were investigated after immersion in a highly concentrated Mg solution. Subsequently, a thermogravimetric-differential thermal analysis and scanning electron microscopy were used to determine the strength reduction mechanism based on the changes in the hydrate composition as the cement-treated soil deteriorated. The results indicate that the cement-treated soil lost more than 80% of its strength after immersion in the Mg solution. The initial conditions strongly influenced the strength of the deteriorated soil, and higher strength was observed in the samples with larger amounts of added cement and longer curing times. Furthermore, calcium silicate hydrate (C-S-H) and ettringite were not present in the deteriorated soil, implying the presence of magnesium silicate hydrate (M-S-H). Therefore, it was postulated that the loss in strength of the cement-treated soil in a seawater environment was caused by the transformation of C-S-H to M-S-H.

The flooding of embankments used for rail and other infrastructure has the potential to cause lasting weakening of slopes via the movement of fine particles induced by seepage. In laboratory experiments, internal erosion was induced in granular soil samples, with properties consistent with those used to construct transportation embankments, to assess how particle migration through, and out of, samples caused shear wave velocity, strength, stiffness and permeability changes. Shear wave velocity changes, measured using horizontal bender elements, of up to 19 % were observed following fine particle removal of up to 1 % of initial sample mass. Shear wave velocity change was found to be an indicator for identifying the development of permeability change during seepage-induced particle migration. Median measured permeability changes were +5 % and −34 % for samples containing 15 % and 30 % fines, respectively. The largest directly observed permeability and shear wave velocity changes occurred during the initial stages of seepage. Negative correlation was observed between mass of material removed from samples and peak friction angle. Following seepage, soils displayed a dual stiffness behaviour. Stiffness and strength changes were attributed to redistribution of fine particles and opening of pore spaces. Our results have implications for the monitoring of earthworks affected by flooding and seepage as the associated redistribution of fine particles may lead to large changes in slope properties.

Based on the electroosmotic reinforcement mechanism of pulsating direct current (PDC) with a high energy efficiency ratio, the calculation method of PDC electroosmosis drainage rate was verified under different potential gradients using two forms of voltage loading, i.e., constant direct current (CDC) and PDC. The drainage weight and electric current were achieved by laboratory tests, and then the energy efficiency ratio, soil resistivity and contact resistance was calculated. The energy consumption of each test group was analyzed by considering the initial potential gradient. The obtained results show that under the same potential gradient, the difference in soil resistivity and electroosmotic drainage between PDC and CDC is not significant, but there is a significant difference in contact resistance, which leads to low current intensity and high energy efficiency ratio in the PDC test group. The expression of the electroosmotic drainage rate of the PDC is described with the coefficient μ, and then the energy efficiency ratio versus potential gradient curve is calculated, which is in good agreement with the experimental results. The reason for the lower energy consumption of PDC electroosmosis compared to CDC is described in terms of the drainage mechanism of electroosmosis.