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    Abstract

    The Hybrid A-Frame Micropile/MSE (mechanically stabilized earth) Wall suitable for mountain roadways is put forward in this study: a pair of vertical and inclined micropiles goes through the backfill region of a highway MSE Wall from the road surface and are then anchored into the foundation. The pile cap and grade beam are placed on the pile tops, and then a road barrier is connected to the grade beam by connecting pieces. The MSE wall’s global stability, local stability and impact resistance of the road barrier can be enhanced simultaneously by this design. In order to validate the serviceability of the hybrid A-frame micropile/MSE wall and the reliability of the numerical method, scale model tests and a corresponding numerical simulation were conducted. Then, the seismic performance of the MSE walls before and after reinforcement with micropiles was studied comparatively through numerical methods. The results indicate that the hybrid A-frame micropile/MSE wall can effectively control earthquake-induced deformation, differential settlement at the road surface, bearing pressure on the bottom and acceleration by means of a rigid-soft combination of micropiles and MSE. The accumulated displacement under earthquakes with amplitude of 0.1‒0.5 g is reduced by 36.3%‒46.5%, and the acceleration amplification factor on the top of the wall is reduced by 13.4%, 15.7% and 19.3% based on 0.1, 0.3 and 0.5 g input earthquake loading, respectively. In addition, the earthquake-induced failure mode of the MSE wall in steep terrain is the sliding of the MSE region along the backslope, while the micropiles effectively control the sliding trend. The maximum earthquake-induced pile bending moment is in the interface between MSE and slope foundation, so it is necessary to strengthen the reinforcement of the pile body in the interface. Hence, it is proven that the hybrid A-frame micropile/MSE wall system has good seismic performance.


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    Abstract

    It is a challenge to suggest a constitutive model for describing the stress-strain behavior of sand-fines mixtures due to that these granular mixtures exhibited very complex behaviors at different densities, pressures and fines contents. In this study, an elastoplastic constitutive model within the framework of the bounding surface plasticity and critical state theories was proposed for sand-nonplastic-fines mixtures by using the concept of the equivalent-skeleton void ratio and equivalent-skeleton void-ratio state index. The proposed model with a set of material constants calibrated from a few tests could be used to model the fines-dependent and state-dependent behaviors of the sand-nonplastic-fines mixture including the strain-softening and volumetric-expansion behaviors in the drained triaxial compression tests, and also the effects of fines content on the critical state lines in both the deviatoric stress versus mean effective stress and the void ratio versus mean effective stress planes.


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    Abstract

    Binary granular soils, mixtures of carbonate sands and nonplastic fines, are widely used for constructions of foundation, airport and embankment in island and coast. Impact load (e.g., sea wave, aircraft landing, pile driving and dynamic compaction during foundation, et al.) is frequently exerted to the mixtures. It is therefore of extreme importance to investigate the evolutions of particle size distribution, particle breakage and volumetric deformation of the mixtures under impact load due to that the grains of carbonate sands are easily to be crushed, which may significant affect its mechanical behavior. Three mixtures (i.e., 100% carbonate plus 0% fines (by dry weight), 90% carbonate plus 10% fines and 80% carbonate plus 20% fines) were prepared to analyze the effect of fines content on particle breakage and volumetric deformation under impact load. It was observed that a unique fractal grading could be obtained for all the mixtures when the blow number was large enough (\(N>20,000\)). The void ratio of the mixtures converged to be a constant ultimate value as the mixture reached the fractal state. The volumetric strain and relative particle breakage with respect to the blow number could be described by hyperbolic functions, indicating that the volumetric strain and relative particle breakage progressively increased to ultimate values with increasing the blow number.


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    Abstract

    In order to visually observe the displacement characteristics of internal soil, model tests on the penetration of six flat-ended piles in synthetic transparent soil were carried out. The granular-soil particles were registered by laser speckles, thus the penetration process of the piles was visualized. The movement of pile-soil interaction was determined by comparing the images before and after penetrations. Based on the test results, the vertical and horizontal displacement of the granular caused by a sequence of jacked piles was discussed. The sheltering effect of the adjacent piles during the penetration of the follow-up pile was analyzed. The responses of the granular soils during the penetration of piles were discussed, and test data was compared with DEM analyses. The pile heave at different stages of the penetration and the ground uplift at different positions were evaluated and compared.


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    Abstract

    A soil improvement method based on a microbially induced carbonate precipitation (MICP) process has been developed in recent years. In this method, calcium carbonate is precipitated in-situ to act as a cementing agency. Calcium chloride is normally used as the calcium source for the MICP process. The use of calcium chloride causes two problems. The first is chloride is corrosive to concrete, and the second is the cost of calcium chloride is relatively high. An improvement to this method is to use other alternative calcium sources. A method to produce soluble calcium using calcium rich calcareous sand and use it as a calcium source for the MICP process to improve the properties of soil has been proposed in this paper. A comparative study between the effect of MICP treatment using soluble calcium produced from calcareous sand and that using calcium chloride with the same concentration of calcium was carried out. The results from both series of tests showed that with increasing amounts of cementation solutions, the strength and stiffness of the treated calcareous sand increased and the permeability decreased. The scanning electron microscopy (SEM) and X-ray diffraction analyses revealed that the aragonite crystals with an acicular mineral morphology were formed when the soluble calcium was used, whereas the calcite crystals with a rhombohedral mineral morphology were formed when calcium chloride was used. This study also shows that it is feasible to treat calcareous sand using a MICP method with soluble calcium produced from calcareous sand.


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    Abstract

    In order to fully develop the South China Sea, a large number of reclamation projects using calcareous sand have been carried out in this area recently. A deep understanding of the physical and mechanical properties of calcareous sand is of critical importance. Therefore, the calcareous sand near a certain reef of the South China Sea is used in this study to investigate the effect of three-dimensional (3-D) particle morphology and gradation on the compressibility characteristics of calcareous sand. This paper proposes a 3-D mesoscope observation method to obtain the average 3-D angularity parameter Sd and 3-D aspect ratio Td of calcareous sand with different particle sizes. It is found that the morphology of coarse particles (diameter: 5 ~ 1 mm) is significantly multi-angular, while the morphologies of middle particles (1 ~ 0.25 mm) are mostly dendritic and schistic. Compared to the 3-D Sd of quartz sand, the calcareous sand’s particle morphology is much more irregular and multi-angular, which makes it easy for the calcareous sand to form large pores and, thus, be more compressible. In order to systematically study the effect of gradation on the calcareous sand’s compressibility characteristics, a number of compression tests on calcareous sand with different gradations are taken. The influential mechanism is then discussed by analyzing the test results from a mesoscopic viewpoint. It is found that changing the coarse fraction content is the most efficient way to reduce the compressibility of the calcareous sand. That is because of the coarse fraction’s high angularity, which makes the skeleton-bearing capacity of the calcareous sand sensitive to the change of coarse fraction content. An empirical formula is proposed to evaluate the compressibility of the calcareous sand with different coarse fraction contents.


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    Abstract

    In this technical note, evolutions of the particle size distribution, particle breakage, volume deformation and input work of carbonate sands with varying relative densities were investigated through performing a series of one-dimensional compression tests. Loading stress levels ranged from 0.1 to 3.2 MPa. It was found that the initial relative density could greatly affect the magnitude of particle size distribution, particle breakage, volume deformation and input work. Particularly, it was observed that the specimen at a lower relative density underwent much more particle breakage than that at a higher relative density. This could be attributed to the change of the coordination number with the initial density. However, a unique linear relationship between the particle breakage and input work per volume could be obtained, which is independent of the initial relative density.


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    Abstract

    This work presents an analysis of the influence of stress anisotropy on cylindrical cavity expansions in an undrained elastic-perfectly plastic soil. This problem was formulated by assuming a large strain in both the elastic and plastic zones around the cavity and a plain strain condition during the cavity expansion process. The solutions for the limit pressure, stress, and excess pore pressure were obtained by introducing the anisotropic initial stress coefficient K0 into the conventional cylindrical cavity expansion method. The proposed solutions were then used to interpret the piezocone penetration test, and the suitability of the solutions was verified by comparing the prediction with the piezocone penetration test data. Subsequently, parametric studies were carried out to investigate the influence of stress anisotropy on the stress, excess pores pressure distributions around an expanding cylindrical cavity, and limit pressure. The results show that the proposed cylindrical cavity expansion method under stress anisotropy is suitable and can be used to investigate the piezocone cone test. The present work improves upon the conventional theoretical framework of cavity expansion and can be applied to the determination of the stresses around axially loaded piles and around in-situ testing devices such as penetrometers.


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    Abstract

    A series of dynamic large-scale model tests and three-dimensional finite element analyses for XCC pile composite foundation are conducted to investigate the dynamic behavior and the settlement of XCC pile composite foundation of existing expressway under traffic load. The test and FE results are presented in the variation of dynamic stress, distributions of skin friction, deviator stress, and the settlement of XCC pile composite foundation. The test results reveal the transfer mechanism of dynamic stress, and a linear relationship between the transferred stress and traffic load is found. Also, XCC piles can improve the stability of composite foundation because of lower neutral point and less sensibility to the traffic load. The distribution characteristics of deviator stress in the horizontal and vertical direction have been found by the numerical simulation. A modified model for predicting the traffic-load-induced settlement of XCC pile composite foundation is proposed. The asymmetric settlement of XCC pile composite foundation is revealed.


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    Abstract

    Piled embankments, which offer many advantages, are increasingly popular in construction of high-speed railways in China. Although the performance of piled embankment under static loading is well-known, the behavior under the dynamic train load of a high-speed railway is not yet understood. In light of this, a heavily instrumented piled embankment model was set up, and a model test was carried out, in which a servo-hydraulic actuator outputting M-shaped waves was adopted to simulate the process of a running train. Earth pressure, settlement, strain in the geogrid and pile and excess pore water pressure were measured. The results show that the soil arching height under the dynamic train load of a high-speed railway is shorter than under static loading. The growth trend for accumulated settlement slowed down after long-term vibration although there was still a tendency for it to increase. Accumulated geogrid strain has an increasing tendency after long-term vibration. The closer the embankment edge, the greater the geogrid strain over the subsoil. Strains in the pile were smaller under dynamic train loads, and their distribution was different from that under static loading. At the same elevation, excess pore water pressure under the track slab was greater than that under the embankment shoulder.


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    Abstract

    MICP (Microbially Induced Calcite Precipitation) technique is recently known as a new research area in geotechnical engineering. This technique provides a more environmentally way to enhancing the soil strength and stiffness by the MICP process in the soil pores. However, the spatial uniformity of MICP in the treated sands, which determines the effectiveness of MICP technique, remains a challenging issue even in the laboratory tests, especially for low-strength biocemented sands. Noting that the MICP process could be greatly inhibited under low temperatures before the homogeneous conditions of MICP reactions is achieved in sands, a temperature-controlled MICP method is proposed in this paper to improve the MICP uniformity in low-strength biocemented sands. A series of temperature-controlled MICP tests are made and the results are compared with the MICP tests under a constant temperature.


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    Abstract

    Calcareous sand as one kind of fragile geotechnical materials, however it is often under dynamic loading in engineering. In this study, discrete elements method (DEM) simulation was used to investigate the fracture characteristics of single and contacting spherical calcareous sand particles under dynamic compression. Calcareous Sand particle was simulated by cluster made up of elemental ball. The DEM models included two kinds of particle arrangements with one and five particles bounded in a cylindrical wall, and the dynamic compression loading were applied by a drop hammer made of clumps. The numerical simulation results of single particle under dynamic compression included stress-strain relation and Weibull distribution were coincide with the experimental results. For five particles numerical model, grain size distribution was obtained for all five cluster to differentiating the degree of breakage of particles in different positions. The transfer of force between particles was also exposed through the observation of the force chain evolution during compression.


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    Abstract

    The thermal–mechanical behavior of the energy pile under three kinds of climatic conditions was investigated in this study. A small-scale floating energy pile and a small-scale end-bearing energy pile, which were embedded in normally consolidated clay, were employed. The energy piles were subjected to cyclic heating/cooling, heating/recovery and cooling/recovery to simulate the energy pile work in the regions of warm/cold balanced climate, warm-dominated climate and cold-dominated climate, respectively. The thermal response and the mechanical response of the energy pile under different climatic conditions, as well as the different response between the floating energy pile and the end-bearing energy pile, were analyzed and discussed comprehensively. The results show that the thermo-mechanical performance of energy pile depends on the types of climatic conditions, and the behavior of the floating energy pile is different from the end-bearing pile. Larger irreversible displacement could be induced by thermal cycles for the floating energy pile compared to the end-bearing energy pile, while irreversible tip resistance could be induced for end-bearing energy pile. Under warm/cold balanced climate, the largest irreversible tip resistance and pile displacement could be induced for end-bearing energy pile and floating energy pile, respectively, and the smallest thermally induced irreversible displacement was observed when the energy pile was under cold-dominated climate.


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