The mechanical behaviors of granular soils at different initial densities and confining pressures in the drained and undrained triaxial tests are investigated micromechanically by three-dimensional ...discrete element method (DEM). The evolutions of the microstructure in the numerical specimen, including coordination number, contact force and anisotropies of contact normal and contact force, are monitored during the shearing. The typical shear behaviors of granular soils (e.g. strain softening, phase transformation, static liquefaction and critical state behavior) are successfully captured in the DEM simulation. It is found that the anisotropies of contact normal, normal and tangential contact forces comprise the shear resistance and show different evolution features during shearing. After large strain shearing, the microstructure of the soil will finally reach a critical state, although the evolution path depends on the soil density and loading mode. Similar to the macroscopic void ratio
e
and deviatoric stress
q
, the coordination number and anisotropies of contact normal and contact force at the critical state also depend on the mean normal effective stress
P
′
at the critical state.
Many engineering practices are carried out within the unsaturated soil. Soil-water retention curves (SWRCs) of many soils exhibit bimodal characteristics that significantly affect strength behavior, ...this issue still remains unresolved. This work employs suction stress concept to examine the uniqueness of strength parameters for coarse- and fine-grained unsaturated soils with bimodal SWRCs. Capillary degree of saturation is used to upscale pore-scale suction to macroscopic suction stress. In the suction stress-shear strength plane, both coarse- and fine-grained soils display the bi-linear strength envelope, differing from unsaturated soils with unimodal SWRC.
•The shear strength of unsaturated soils with bimodal water-retention curves are analyzed using suction stress concept.•The influences of soil type on the shear strength of soils with bimodal SWRC are discussed.•The unified strength criterion for soils with bimodal SWRC is manifested by a bi-linear relationship.
Considering anti-rotation of sand particles, two-dimensional Discrete Element Method (DEM) has been employed to reproduce direct shear behaviors of sand with different particle distribution sizes, so ...as to explore effects of anti-rotation of particle on responses of stress-displacement and dilatancy, the evolution law of shear stress, coordination number and vertical displacement of sand samples, and analyze the contact force chain, contact fabric and porosity of the samples after shearing.The results show that the anti-rotation ability of sand is enhanced, the torque of overcoming the relative rotation between particles is increased, and the peak shear stress, dilatancy and porosity in the middle of the sample are increased; with the increase of the anti-rotation coefficient, the coordination number decreases more obviously. The proportion of the contact number in the direction of 100°-160° to the total contact number decreases with the increase of the anti-rotation coefficient. The elliptical shape of the contact configuration becomes more flat, and the anisotropy of the contact force chain is more obvious; compared with fine sand, the coarse sand has greater shear capacity, more obvious dilatancy and larger porosity in the middle of the sample.The maximum minimum particle size ratio of the sample becomes larger, so that the shear strength of the sample is reduced, and the dilatancy is also weak.
This study aims to investigate the dynamic responses of layered poroelastic ground underlying rough pavement subjected to traffic loads. Particular attention has been given to the roles of the ...pavement roughness idealized as sinusoidal waves. The vehicle is simplified as a multi-rigid-body vibration system and the pavement roughness results in dynamic traffic loads in addition to the static component due to the vehicle weight. The pavement layer is modelled as a thin Kirchhoff plate, while the underlying base and the subgrade are treated as saturated two-layered poroelastic medium obeying Biot's dynamics theory. Frequency-wavenumber domain responses of the vehicle-pavement-underlying ground are solved by Fourier integral transform, transfer matrix method and considering stress and displacement compatibility conditions at the pavement-ground interface. The corresponding time-domain solution is numerically obtained from the inverse Fourier transform. Based on this semi-analytical solution, the influences of surface roughness, vehicle velocities and mechanical properties of pavement and base materials are discussed to provide practical guideline for the design of rough pavement system.
•Analytical solutions of coupled vehicle-pavement-layered ground system is developed.•Different pavement structural components such as pavement, base and subgrade are considered.•The proposed model is suitable for both flexible and rigid pavement system.•Pavement roughness and vehicle speed (especially high speed) significantly affect dynamic responses of pavement system.
The mechanical properties of deep rock masses are significantly influenced by temperature and other factors. The effect of temperature on the strength of deep rock masses will pose a serious ...challenge to deep resource exploitation and engineering construction. In this paper, the thermal-mechanical coupling calculation model is established by particle flow code (PFC2D) to study the uniaxial compression response of rock masses with microcracks after temperature load. The strength of failure, microcracks, and strain was analyzed. The results show that: (i) When the soft rock thickness ratio Hs/H < 0.5, the displacement caused by the applied temperature is concentrated at the structural plane, and the contact force is concentrated at the end of the initial microcrack. When Hs/H ≥ 0.5, the displacement caused by the applied temperature is concentrated on both sides of the initial microcrack, and the contact force is concentrated in the hard rock area. (ii) The number of microcracks decreases with the increase of soft rock thickness under different working conditions. When the soft rock thickness ratio Hs/H < 0.5, the relationship curve between the number of microcracks and the vertical strain shows two stages of change. When Hs/H ≥ 0.5, the relationship curve between the number of cracks and the vertical strain changes shows three stages of change. (iii) When the soft rock thickness ratio Hs/H < 0.5, the failure strength decreases with the increase of soft rock thickness ratio at T = 100°C and 200°C. When T = 300°C and 400°C, the failure strength decreased first and then increased. When Hs/H ≥ 0.5, the failure strength increases with the increase of soft rock thickness at T = 200°C, 300°C, and 400°C. At T = 100°C, the failure strength decreases with the increase of soft rock thickness.
Dual-porosity structures of fine-grained soils can noticeably affect their ability to retain water. This work jointly employs axis translation technique, filter paper method, and vapor equilibrium ...technique to study the soil–water retention curve (SWRC) over a wide suction range of Nanyang expansive soil characterized by double porosity. Mercury intrusion porosimetry tests are carried out to investigate the correlations between the aforementioned water-retention response and underlying pore structure characteristics. The test data show that dual-porosity distribution leads to bimodal SWRC. The change in void ratio mainly affects the median size of the inter-aggregate pores and consequently the portion of SWRC at low suction range. Based on these experimental observations, this work presents an SWRC equation for fine-grained soils with dual-porosity structures. Attracting water through capillary and adsorptive processes is explicitly distinguished. The capillary water is described by a relation that includes the characteristics of both inter- and intra-aggregate pore size distributions as parameters for representing bimodal characteristics. The adsorbed water is modeled by a relation that considers capillary condensation within intra-aggregate pores and allows for the decoupling between adsorptive water-retention mechanism and void ratio change. The latter feature is the foundation for the model to include the void ratio effect on SWRC in a way consistent with how it affects the pore structures of soils. By simulating test data in this work and in the literature, the proposed model is shown to be capable of representing the water-retention behavior of fine-grained soils with dual-porosity structures under different void ratios. To include the aforementioned key factors that influence the SWRC of fine-grained soils, seven parameters are required in the proposed model. This feature can reduce the practical applicability of the model. Future directions to enhance this aspect are discussed.
Discrete element modeling was used to investigate the effect of particle size distribution on the small strain shear stiffness of granular soils and explore the fundamental mechanism controlling this ...small strain shear stiffness at the particle level. The results indicate that the mean particle size has a negligible effect on the small strain shear modulus. The observed increase of the shear modulus with increasing particle size is caused by a scale effect. It is suggested that the ratio of sample size to the mean particle size should be larger than 11.5 to avoid this possible scale effect. At the same confining pressure and void ratio, the small strain shear modulus decreases as the coefficient of uniformity of the soil increases. The Poisson＇s ratio decreases with decreasing void ratio and increasing confining pressure instead of being constant as is commonly assumed. Microscopic analyses indicate that the small strain shear stiffness and Poisson＇s ratio depend uniquely on the soil＇s coordination number.
The existence of various types of damage, small cracks, some large voids and the size of the sample in the rock will make the experimental results show great discreteness. In this paper, based on the ...results of laboratory experiments, a numerical model of large flawed rock samples is established by using particle flow software PFC2D, and the mechanical response of rocks with different length-diameter ratios and different flaw positions in uniaxial compression experiments is discussed. The results show that the specimen size has a significant effect on the crack characteristics, mechanical characteristics and energy characteristics of rock mass. From the perspective of energy and crack characteristics, the total number of cracks after the failure of the defective rock sample is slightly lower than that of the intact rock sample, resulting in a slightly lower peak strain energy during the rock failure process. From the mechanical properties of rock samples, the Poisson's ratio of intact rock samples is slightly smaller than that of defective rock samples. The strength of the defective sample is weakened relative to the complete rock sample, and the relationship formula between the weakening range and the aspect ratio is obtained through analysis. Moreover, different defect locations lead to different crack processes and crack modes, resulting in different uniaxial compressive strength.
Recently traffic induced geotechnical problems have been receiving increasing attention. Yet the silty clay from the third layer of Shanghai, which is within the main influence depth of traffic ...loading and overconsolidated, had not been sufficiently studied. A series of monotonic and cyclic undrained triaxial tests were thus carried out on reconstituted Shanghai silty clay with various overconsolidation ratios (OCRs). The silty clay specimens showed phase transformation behavior, which was characterized by stages of initial contraction, temporary phase transformation and later dilation. Such behavior could be termed as a transitional mode between the behavior seen for typical clays and that for sands. The dilation behavior in the later stage was shown more significant for overconsolidated specimens. Six different failure criteria were employed to study the friction angle. There was no observable effect of OCR on the friction angle for this silty clay as if it was a granular material. The undrained shear strength could be normalized with only a small deviation. The normalized shear strength increased with OCR and it behaved more as typical silts than clays at high OCRs. The development of permanent strain under cyclic loading was shown to depend significantly on the cyclic stress ratio and OCR. A higher OCR resulted in smaller permanent and resilient strains, and the decrease of the permanent strain with OCR was remarkable for lightly overconsolidated specimens. Such effect of OCR would be more significant in real soil condition where the soil is at least partially drained.
•The mechanical behavior of Shanghai silty clay is first extensively studied.•The silty clay shows phase transformation behavior, differently from typical clays.•No observable effect of OCR is found on the friction angle of this silty clay.•For undrained shear strength, it behaves more like a typical silt at high OCRs.•A higher OCR results in smaller permanent and resilient strains under cyclic loading.
By researching the distance between blasthole and interface of soft-hard rock strata, as well as the time of delay detonation, blasting effect of the rock mass will be more controllable. Firstly, ...validity of numerical method was authenticated from three angles: blasting coupled stress field, ratio of crushing zone radius to blasthole radius, and crack network state. Under the condition of soft-hard rock strata, numerical model of double-hole blasting was established by using PFC
. Then delay blasting experiments were carried out under different relative positions of blasthole and interface. Ultimately, results were analyzed from three perspectives: crack network, crack quantity and rock fragment. Results show that: (1) When detonated in hard rock, if between interface and blasthole distance is greater than twice crushing zone radius, the closer blasthole is to the interface, the more obvious the "hook" phenomenon between the two blastholes is. With increasing delayed initiation time, "hook" phenomenon will weaken or even disappear. (2) Based on the crack information initiated in hard rock, the law of crack number varying with thickness of hard rock and delay time is obtained. (3) For initiation in hard rock, crack extension range is large, but less fragments are formed. The law is opposite to that initiation in structural plane and soft rock. Fragmentation area increases exponentially with increasing soft rock thickness, and exponential function is obtained.
