Currently,gravity anchor foundations are widely applied in offshore floating photovoltaic projects. Although conventional solid gravity anchors deliver satisfactory bearing performance, they consume massive materials and bring high construction costs. This work proposes a new type of perforated gravity anchor foundation and systematically explores its ultimate horizontal bearing capacity in clay ground. Combined with centrifuge model tests, finite element analysis and upper bound limit analysis, a two-dimensional numerical model of the perforated anchor is constructed. Good consistency between numerical predictions and centrifuge test results verifies the credibility of the established model. The bearing capacity coefficient grows remarkably with rising embedment ratio, whereas its growth rate declines obviously once the embedment ratio exceeds a critical threshold. Reasonable perforation layout can greatly lower the foundation’s self-weight and material consumption without severely weakening its bearing capacity. As the perforation ratio rises and hole spacing shrinks, the foundation’s failure mode transforms from integral sliding to segmental sliding, and the critical perforation ratio triggering such transition increases with embedment depth. We further derive an upper-bound analytical formula to rapidly calculate the foundation’s ultimate horizontal bearing capacity for engineering design. All findings supply theoretical references for the optimal design and on-site application of perforated gravity anchor foundations.

Seasonally frozen soil areas experience freeze–thaw cycles (FTCs), which affect the soil's mechanical properties and should be considered during geotechnical engineering applications. Therefore, understanding the micro-mechanism of soil's macro-mechanical characteristics exposed to the FTCs is necessary. This study explored the impact of FTCs on the macro–micro properties of silty clay by performing F-T tests, triaxial compression tests, and the corresponding Distinct Element Method (DEM) simulations. The effect of FTCs on the macroscopic shear properties of silty clay in Sichuan Province, including stress–strain behavior, peak deviatoric stress, cohesion, and internal friction angle, was investigated utilizing laboratory triaxial compression tests. In addition, a novel approach based on DEM, which considers the expansion–contraction influence of ice particles on the pores and the impact of FTCs on the structure of granular soils, was developed to simulate the fabric evolution of granular soils after FTCs. According to the results, macroscopically, failure strength, internal friction angle, and cohesion were in a decrement trend as FTCs increased. On the microscopic scale, with the increase of the FTCs, the degree of soil anisotropy decreased, and the contact and force fabric anisotropy coefficients decreased, which contributed to the decrease in macroscopic shear strength. In addition, a linear and unique relationship was detected between the macroscopic stress ratio q/p of the granular system and the microstructural stress ratio of the strong contact network Φ_d^s/Φ_m^s , independent of the number of FTCs. Increased FTCs caused a rearrangement of weakly contacted networks, resulting in macroscopically different mechanical behavior.

The mechanical characteristics of frozen soil under various temperatures have theoretical and engineering significance for improving the efficiency of permafrost excavation and ensuring the stability of permafrost area engineering. A novel approach based on Discrete Element Method (DEM) is proposed to simulate the temperature effect of frozen soil. An existing 3D contact model for cemented granular material is extended to include the effects of temperature on bond strength and modulus of ice cementation, which are verified by existing analytical solutions and experimental data, and a simplified method is employed to simulate the influence of ice crystal expansion on the void ratio of frozen soil, whereafter, the modified contact model is calibrated by the results of the temperature-controlled triaxial test. This approach was proven to be effective by comparing the simulation results with the experimental results. The micro mechanical behavior of frozen soil samples was studied. The results showed that different types of bond contact had different effects in the shearing stage. The structural 'weakening' and 'strengthening' of the frozen soil occur simultaneously during the loading process, and the structural damage is mainly caused by the plastic damage induced by the relative slip between soil and ice particles.

Purpose The paper aims to find the correlation between the microparameters and the macroparameters of the soil. The study aims to calibrate the macroscale and microscale parameters of rolling resistance contact models to successfully apply the discrete element method to do some research of the geotechnical problem. Design/methodology/approach The paper opted for an exploratory study using the PFC3D to simulate the triaxial tests that include more than 50 cases and the coupling analysis method, which considering several effect of various factors. Findings The paper provides a quantitative relationship between the macroparameters and microparameters of the rolling resistance linear model and a method for fast calibration of macroscopic parameters is proposed and verified by a triaxial test example. Originality/value This paper provides the quantitative relationship of micro and macroparameters in the rolling resistance linear model by studying a single factor and considering the coupling effect of various factors and a fast method for the calibration of microparameters based on the rolling resistance linear model is proposed.

Understanding the mechanical characteristics of frozen soil is critical for engineering projects in cold regions. A series of case studies were conducted to explore the macroscopic and microscopic mechanical behavior of frozen soil during four different stress paths, i.e., isotropic, constant stress ratio, conventional triaxial, and true triaxial compression tests, using the distinct element method (DEM). The particle-scale mechanism and temperature effect of frozen soil are also taken into account in the DEM simulation. The results indicate that the peak stress/yield stress of frozen soil decreases with the increase of temperature or decrease of stress ratio or decrease of intermediate principal stress ratio b , which is related to the evolutions of bond breakage, bond stress, mechanical coordination number, and deviatoric fabric. When 0.25 ≤ b ≤ 0.75, the direction of strain increment deviates from the direction of stress. In addition, the relationship between the damage variable B and the volumetric strain ε v or shear stain ε s can be expressed as B=1-exp(-a·ε_v^n) or B=1-exp(-a·ε_s^n) under different stress paths. The simulation results and revealed microscopic mechanism will be valuable in developing breakage-mechanics-based constitutive models for frozen soil.

路基工程中常采用人工换填方式形成上硬下软的层状地基以增加地基承载力,但目前对于双层地基的极限承载力和破坏机制的宏微观研究尚不够深入.通过室内单元试验、室内模型试验对地基土体的单元力学特性及分层情况下地基的极限承载力进行研究,并采用离散-连续耦合数值模拟方法分别对4种不同上覆土层厚度的双层地基平板载荷试验进行模拟,研究上覆土层厚度对双层地基承载特性的影响及其对应的宏微观特性之间的关联.结果表明:上覆土层厚度对地基宏观力学特性及微观传力机制均有较大的影响;双层地基承载力及变形模量随着上覆土层厚度增加而增大;加载过程中,强力链主要分布在一个对称梯形区域内,且随着上覆土层厚度的增加,梯形区域的斜率增大,应力扩散角减小,受扰动区域变小;加载板正下方可观察到明显的应力集中,随着深度增加,应力集中的强度减小,随着上覆土层厚度的增加,应力集中区域减小.

The stability of methane hydrate depends on not only temperature and pressure but also the salinity of the environment where the methane hydrate exists. The variation in the stability of methane hydrate due to the change in salinity induces mechanical response in micro scale of the methane hydrate-bearing sediments (MHBS). However, it is really heard to quantitatively analyze this effect by controlling salinity of the MHBS in micro scale experiment. Therefore, our study applies Distinct Element Method to investigate the micro-scale mechanical response subject to the salinity variation. An existing contact model for cemented soils is extended to explicitly include the effects of salinity on bond strength and modulus, which are verified by experimental data. Using the proposed thermo-hydro-mechanical-chemical contact model, we conducted a series of case studies to investigate macroscopic and microscopic mechanical responses during three different stress paths, i.e. isotropic, triaxial and constant stress ratio tests. The results show that as the salinity increases, the shear strength and stiffness decrease while the deformation and bond breakage ratio accelerate. The results are valuable to develop constitutive models for MHBS and have safe exploitation of MH resources in the future.

Methane hydrate (MH) is regarded as a potential future clean energy source, which has attracted lots of attention in recent decades. The sediment containing MH is called as Methane Hydrates Bearing Sediment (MHBS). The evolutions of mechanical behaviors of MHBS from macro and micro scales are needed in detail for safe MH exploitation (e.g., chemical injection method). For this purpose, a novel 3D thermo-hydro-mechanical-chemical (THMC) bond contact model was proposed and implemented into a DEM commercial software to capture the mechanical behavior of MHBS under exploitation stress path. Firstly, MHBS samples were compressed to different deviator stress levels, which represents the different initial stress state. Then the chemical injection process was simulated by increasing the salinity from 3wt% to 14wt% to allow MH dissociate while the deviator stress remained constant. After which, the recovery stage was simulated by decreasing the salinity from 14wt% to 3wt%. The results show that the salinity can affect the mechanical behavior of MHBS by changing the properties of inter-particle methane hydrate. During chemical injection process, the axial strain increases with the increase of salinity. The chemical injection method will result in soil collapse when initial deviator stress is larger than the strength of pure sand at exploitation stage but slight deformation at recover stage.

The discrete element analysis of the true triaxial and wetting tests is conducted to study the macroscopic and microscopic mechanical properties of unsaturated structural loess under complex stress states. The change of axial strain after wetting and the variation of void ratio and bond breakage number with water content were studied in this work. The results show that the collapsing failure occurs when the deviatoric stress approaches or exceeds the peak shear strength of the corresponding saturated structural loess sample. There is little difference in the axial strain between the quick wetting (QW) method and gradual wetting(GW) method, and the volumetric strain of the samples wetted by QW are larger than that by GW specimen. The bond breakage number of the samples wetted by QW are slightly higher than that by GW. However, the number of bond breakage wetted by the two wetting methods is slightly higher than that of the corresponding saturated structural loess sample.

Liquefaction can be considered to occur in saturated sand widely under seismic load, which will cause serious disaster, including road damage, ground subsidence, cracking of houses. Thus, it is quite necessary to study the liquefaction characteristics of saturated sand. For this aim, the undrained simple shear test on saturated sand under cyclic loading were simulated by three-dimensional distinct element method (DEM), where the stress-strain relationship, excess pore pressure ratio, mechanical coordination number and contact normal direction were analyzed. The results show that the liquefaction of saturated sand is manifested by the accumulation of excess pore pressure ratio. In addition, the mechanical coordination number gradually reduces and the sample anisotropy slightly fluctuates before the saturated sand reaches initial liquefaction. When the specimen approaches initial liquefaction, the mechanical coordination number drops abruptly and the sample anisotropy obviously increases.

The evolution of bond degradation is essential for analyzing the macro-and micro-scopic behaviors and establishing constitutive models for structured soils with cementation bond which is a kind of bonded granular material. The discrete element method is employed to analyze the evolution of bond degradation on account of the disadvantage of laboratory tests in bond breakage quantitative analysis. First, the discrete numerical sample is generated by installing a relatively completed bond contact model incorporating the interparticle rolling and twisting resistances and the influences of bond size on the contact stiffness and strength. The DEM simulation reproduces the key mechanical behaviors of one-dimensional compression, isotropic and anisotropic compressions, conventional triaxial and true triaxial tests on the DEM sample. The results show that the evolution of the degradation variable B0 is stress-path-dependent, while a new degradation variable Bσ is roughly stress-path-independent. An exponential function is recommended for Bσto describe the degradation of soil structure.

This study investigates the shear behaviors of unsaturated structured loess in direct shear test by the discrete element method (DEM). A bond contact model characterized by the consideration of inter-particle attraction and van der Waals force was used. The direct shear tests were simulated under different suctions and vertical pressures. The simulation results were analyzed in terms of stress–strain relationships, volumetric responses, bond breakages and contact fabric. It is shown that: 1) The shear strength of structural loess samples can be enhanced by decreasing water content and increasing vertical pressure; the strain softening and dilatancy behaviors of structural loess enhance with the decrease of water content and vertical stress, respectively. 2) The bond breakage rate in shear band is related to the macroscopic mechanical response. 3) The anisotropy of the contact fabric changes more significantly in the shear band during shearing, it is of great significance to study the microcosmic properties of particles in shear band.

The microbially induced calcite precipitation is a promising technology to improve ground, and the treated soil can be regarded as the structural one. In this study, firstly, based on the three-dimensional (3D) contact model for granulates incorporating rolling and twisting resistances and 3D bonds failure criteria, and considering both the slight plastic deformation of particles during collisions and the rate-dependency, a cyclic bonded contact model is established. A time-dependent relationship is then proposed to describe the denitrification reaction in reinforced sand. Next, the mechanical responses of microbially treated sands at different cementation and bubble contents are investigated by the coupled CFD-DEM in undrained-consolidated cyclic triaxial tests. The effects of biological bond and biological bubbles on the liquefaction resistance of sands are analyzed in link with the mechanism from macroscopic and microscopic scales. The results show that the coexistence of cementation and bubble does not increase the liquefaction resistance as expected in the form of "1+1=2". The presence of cementation enhances the liquefaction resistance of unsaturated sands evidenced by the decrease of excess pore water pressure ratio and axial strain, and the increase of coordination number. However, the presence of bubbles reduces the liquefaction resistance of cemented sands, where the number of cycles to the initial liquefaction decreases, the axial strain increases significantly in the tensile direction, and the coordination number decreases significantly.

In this study the Distinct Element Method (DEM) with a size-dependent bond contact model was employed to investigate the fracture mechanism induced by TBM cutting in rock mass with a group of cross joints, where the effect of joint inclination angle under two cutters was analyzed. The center joint outcrop is fixed at a distance of 20 mm with the rock mass center. The DEM results show that the process of rock fragmentation can be divided into three stages: loading stage, unloading stage and residual jumping stage. The peak normal thrust decreases gradually with the decrease of joint angle α. During the process, the number of bond failure increases gradually with the increase of invasion depth. There are two distributions of bond failure in different joint angles. One is the bond failure concentrates between the cutter and the shallow joint. The other one is the bond failure crosses the shallow joint and reaches the next joint

An adaptable three-dimensional (3D) contact model is necessary to perform discrete element simulation (DEM)on unsaturated structural loess, in which the effects of adhesive force and chemical cementation bond have to be taken into account. A 3DDEM contact model is introduced to simulate the macroscopic and microscopic mechanical behaviors of unsaturated and structural loess by considering the effects of adhesive force and chemical cementation. Interparticle adhesive force is composed of van der Waals attractions and capillary forces. Both bond stiffness and strength are associated with the bond size to represent unrecoverable chemical cementation. By using the relationships between water content and suction, a new 3D contact model for loess is established considering the coupling effects of water content-void ratio-suction. The DEM is used to simulate the triaxial and wetting tests at different deviator stress levels and the results show that the 3D contact model can well reproduce the macroscopic mechanical behaviors of unsaturated structural loess.

Abstract It is well known that the mechanical properties of Methane Hydrate-Bearing Sediments (MHBS) are complex and highly influenced by the surrounding temperature, pore pressure, effective stress and salinity. This paper proposes a three-dimensional(3D) bond contact model which incorporates the effects of temperature, pore pressure, effective stress and salinity. The model is then implemented in a distinct element method(DEM) code which can be employed for thermal–hydro–mechanical-chemical (THMC) analysis. The mechanical behaviour of MHBS was investigated by simulating a series of triaxial compression tests on MHBS with various MH saturations , effective confining pressures and salinity. The results show that the DEM with the proposed contact model is able to capture the salient properties of MHBS, such as the effects of hydrate saturation, effective confining pressure and salinity. The numerical results show that: the shear strength and secant modulus increase as the methane hydrate saturation or effective confining pressure increases, which are in good agreement with the experimental observation. The peak shear strength decreases significantly while the residual shear strength decreases slightly with the increase of salinity.