This study presents a novel constitutive model for granular soils within the framework of hyperplasticity theory. Based on fundamentally distinct mechanisms, two types of volume variations exist. The first type includes variations arising from changes in stress, accompanied by energy dissipation. The other type of variations is purely kinematic and can be measured using the induced dilatancy angle, which is transformed into a statedependent form. The state-dependent induced dilatancy angle decreases with decreasing material density, which enables the simulation of the softening phenomena and weakens dilatancy. After attaining the critical state, the incremental behavior reverts to an isotropic state. The compression and unloading curves considering grain crushing are introduced, and the corresponding free energy functions are derived under varying densities. In the proposed model, the different mechanical properties of loose sand and dense sand are derived from the state dependency of the kinematic constraint. Toyoura sand simulations reveal that the proposed model is able to accurately describe the stress-dilatancy relation of varying initial densities and confining pressures under both drained and undrained conditions. Triaxial compression tests on Leighton Buzzard sand and Rumei rockfills with different densities and confining pressures also demonstrate the validity of the model.

Cracks are one of the major characteristics of soil structure failure. Successfully performed numerical simulation of the soil crack evolution process is conducive to the prediction of potential dangers. In recent years, crack evolution simulation has been greatly facilitated in the field of fracture mechanics by the development of the extended finite-element method (XFEM). However, it is rarely used to simulate crack evolution in earth structures, because in most cases soil cannot be regarded as a quasi-brittle material. Based on the characteristics of stress concentration and redistribution at the tip of a soil crack revealed by numerical tests, this paper proposes a discriminating method for crack propagation that can reflect the influence of the stress field at the soil crack tip on the crack propagation direction, which is then coded to an XFEM program. Compared with existing methods, this new method is found to be more accurate and effective.

In high earth-rockfill dams, large shear displacement exists in the contact area between the earth core and the concrete cushion at the bottom and on both sides of the banks of a river valley. Exposed to a high hydraulic gradient, seepage erosion is likely to occur in this area. A practical measure is to compact a thin layer of clayey soil with high plasticity such that it lies between the earth core and the concrete cushion. It is crucial to evaluate the capabilities of a clayey soil for this thin layer. A new apparatus modified from a conventional triaxial apparatus is developed to test the seepage characteristics of the clayey soil-structure interface under large relative shear conditions. Compared to the previous device, the new device can achieve a larger relative shear displacement that is closer to the actual condition while providing stress and a seepage state with a simple testing procedure. This apparatus consists of a soil-structure model, a vertical loading system, two back-pressure systems, and a monitoring system. The vertical loading system allows the soil-structure displacement to develop at a constant speed. Two back-pressure systems can simulate the high hydraulic gradient and the consolidation state of the specimen. The monitoring system automatically records the force load, the volume of water outflow, and the volume change in the pressure chamber. Basic tests and shear-seepage tests were conducted using this device to validate its efficiency and reliability. The results indicate that the permeability of the soil-structure interface decreases with the shear displacement over a large range. A finite element (FE) analysis was carried out to reveal the strain-stress and seepage state inside the specimen to explain the seepage mechanism.

Purpose The stress–strain behaviors of rockfill materials in dams are significantly affected by the anisotropy and grain crushing. However, these factors are rarely considered in numerical simulations of high rockfill dams. This study intends to develop a reasonable and practical constitutive model for rockfill materials to overcome the above problems. Design/methodology/approach The effects of anisotropy and grain crushing are comprehensively considered by the spatial position of the reference state line. After the improved generalized plasticity model for rockfill materials (referred to as the PZR model) is developed and verified by laboratory tests, it is used with the finite element method to simulate the stress–strain behaviors of the Nuozhadu high core rockfill dam. Findings The simulated results agree well with the laboratory tests data and the situ monitoring data, verifying the reliability and practicability of the developed PZR model. Originality/value A new anisotropic state parameter is proposed to reflect the nonmonotonic variation in the strength as the major principal stress direction angle varies. This advantage is verified by the simulation of a set of conventional triaxial tests with different inclination angles of the compaction plane. 2) This is the first time that the elastoplastic model is verified by the situ monitoring data of high core rockfill dams. The numerical simulation results show that the PZR model can well reflect the stress–strain characteristics of rockfill materials in high core rockfill dams and is better than the traditional EB model.

An iron ore mine at Anhui Province of China is in the transition from open pit to underground mining. The closed open pit of this mine will be filled with mine waste for land reclamation. This is conducive to the sustainable development of the mine. However, filling the open pit may affect the exploitation of lower mineral resources. Therefore, the influence of the filling of the pit on the underground mining operation should be investigated. A numerical model is thus established to study the mechanical response of the pit and underground stope to the open pit filling. The strain–time evolution of backfill with binder content and stress–strain data of backfill with curing time are investigated and thus used to compare with the relevant outcomes predicted by the developed model. The comparison results can prove the validity and applicability of the developed model. The validated model is then applied to study the evolutions of stress and displacement of the pit and stope during the filling process. The simulation results indicate that stress concentration of the slope, pit bottom and underground stope evolves during the filling process. The displacement of the stope roof is affected by the horizontal distance between the stope and the pit bottom. Filling the open pit can reduce the maximum stress on the stope roof. The obtained results can be combined with the field monitoring data in the future, which will provide guidance for the better production arrangement of the open pit filling and underground stope mining.

Cracks are the visual manifestation of soil failure. Exploring the propagation process of soil cracks is of great significance to the safety of geotechnical engineering. However, this part of the work is limited by the simulation technology and complex properties of soils. Whether indoor or numerical experiments are in the exploratory stage, the existing results often rely on a specific engineering background. As a result, there is no universal cognitive conclusion. In this paper, the evolution process of cracks in soil was simulated under the condition of loading and unloading and different stress paths with specially developed experiment equipment, and the objective phenomena in the process of crack propagation are systematically summarized. Then, based on the Mohr-Coulomb theory, the stress distribution near the tip of the soil cracks was simulated and analyzed by the extended finite-element method. On the one hand, the reasonableness of laboratory experiments is verified. However, the propagation mechanism of soil cracks is revealed. The whole work has a positive guiding significance for the research and prediction of the crack evolution process in soil.

高心墙堆石坝在心墙和岸坡混凝土垫层间常设有一层接触黏土.这层接触黏土在缓解心墙与混凝土之间较大的不协调变形的同时还应具有较强的抗渗能力.接触面附近土体在高水头作用下产生大剪切变形,处于复杂的应力、变形和渗流状态.工程界普遍对此处是否更容易发生渗透破坏抱有疑问.接触面剪切-渗流试验的结果显示,在土体与结构之间发生较大剪切错动的情况下,渗透性呈现减小的趋势,但对其机理尚不清楚.本文在Biot固结理论的基础上,结合剪切渗透系数模型反映物理状态和变形对渗流场的影响,利用接触面单元反映试验中不同部件之间的接触关系,建立多体-多场耦合分析方法,对接触面剪切-渗流试验进行了数值模拟.通过分析计算结果,揭示了在接触面剪切-渗流的过程中,土体内部的应力-变形和孔隙比的演化过程以及渗透特性的内在机理.

该文综述了国内外岩土真三轴仪的发展、堆石料力学特性及机理相关的研究现状.重点介绍了该研究团队开发的大型岩土静动真三轴试验机及在堆石料力学特性试验研究方面取得的初步成果.按加载方式,土的真三轴仪可以分为3类:刚性加载真三轴仪、柔性加载真三轴仪和混合型加载真三轴仪.堆石料真三轴试验具有试样尺寸大、承压高、各方向相互干扰强、试样变形后荷载对中难、加压板与试样间摩擦效应强、试样安装和量测复杂等诸多困难.因而,目前适用于堆石料的真三轴仪和试验成果均较少.考虑堆石料真三轴及复杂应力路径条件下的颗粒破碎、各向异性等,研究其力学特性并开发相应的本构模型是该课题发展的趋势.清华大学大型岩土静动真三轴试验机中联合采用了椭圆形试样帽和异形乳胶膜的封样方式,可很好地解决真三轴试样的高压密封、拆装、量测和耐久性等一系列难题.对堆石料进行了一系列真三轴复杂应力路径试验,结果表明真三轴试验中堆石料表现出更明显的应力诱导各向异性和应力状态依赖性,特别是对小主应力-应变关系、球应力-体应变关系和广义剪应力-剪应变关系的影响更显著.球应力和广义剪应力对体积应变和广义剪应变之间存在着明显的交叉影响.

Hydraulic fracture will have a serious negative impact on the safety of core rockfill dams. Macroscopically, the hydraulic fracture of soils can be regarded as the failure process of further development of local cracks (weak surfaces) under the action of hydraulic wedge splitting. XFEM is a numerical simulation method which can effectively describe cracks. In this study, XFEM combined with the Biot’s consolidation theory is used to deal with the crack element, so that the hydraulic fracturing process of soils is described by both the dispersive crack state and the embedded crack shape. The method is verified by a model example and a practical engineering example of Hyttejuvet dam in Norway. The results of this work are helpful to understand the cause and process of hydraulic fracture of soils, and can be used for fluid-solid coupling failure analysis of soil structures.

A soil slope failure usually experiences the initiation, development, and run-through of the slip plane. The extended finite element method (XFEM) has been applied to simulate the process of slope failure in recent years. However, there are still some shortcomings to be overcome in practical applications of XFEM to landslides. For example, the initial position of the slip plane needs to be prescribed, and the extending direction of the slip front-end cannot be tracked accurately. In this paper, the mechanism of slip-plane initiation and development is introduced, and the process of automatically determining the initial position of the slip plane based on the stress history and the relative relationship between the tensile stress level and the shear stress level in the soil element is proposed and described in detail. Then, a new method for determining the front-end propagation direction of the slip plane by using the sector control domain and the circular control domain is introduced, which effectively improves the simulation precision of the XFEM for the landslide failure process. Finally, two typical slope cases with different failure types are analyzed to verify the reliability and rationality of the proposed method further.

Both experimental studies and engineering practices have indicated that shear effects significantly influence the permeability of clayey soil. In this study, to ameliorate the imperfections in a previous study, a series of tests were conducted on five different clayey soils by using the improved triaxial seepage apparatus and test method to measure the hydraulic conductivity of clayey soils under shear effect. Based on the test results, a modified mathematical model is proposed to describe the shear effect on the permeability of clayey soil. The new model uses generalized shear strain instead of stress level as the indicator of shear effect. Experimental results show that the new model fits well in predicting the hydraulic conductivity of clayey soil under shear, which indicates that the modified model has more general applicability.

土体的张拉破坏是岩土工程中常见的破坏形式之一,属于典型的非连续问题,扩展有限元法(XFEM)因在模拟这类问题方面具有更大的灵活性和兼容性近年来发展很快.然而,XFEM在模拟岩土工程中的三维张拉破坏方面还很少见.除了几何上的复杂性以外,开裂方向的判断是个难点.文章在已有的XFEM程序中引入开裂势函数方法以从宏观上更准确地追踪开裂方向,给出了三维裂缝的积分方案,建立了一套能够方便模拟三维条件下土体开裂过程的方法.首先介绍所使用的XFEM程序的理论基础,其次介绍开裂势函数方法在XFEM程序中的实现,再次介绍裂缝面的处理和积分,最后用加入开裂势函数法的XFEM程序对土石坝心墙横向开裂进行模拟,以验证方法的有效性.

To simulate the onset and growth of strong discontinuities (cracks or slip planes) and the failure mechanism in soil slopes with reasonable accuracy, a program was developed based on the existing extended finite-element method (XFEM). This program consists of a new analytical algorithm for strong-discontinuity initiation and propagation in soils considering the stress concentration and redistribution around the discontinuity tip and an integration scheme for the XFEM enriched discontinuities. Two different types of interfacial contact constitutive models are used to describe the interface behavior of the cracks or slip planes. Analyses of two slope failure examples are presented to demonstrate the reliability of this program and the reference values for engineering measurements. (C) 2018 American Society of Civil Engineers.

To investigate the mechanical characteristics of gravel under general stress conditions, a large-scale static and dynamic true triaxial apparatus was developed at Tsinghua University. The largest specimen was 200x200x400 mm, and the applied pressure can meet the research requirements of large-scale geotechnical structures. A set of verification experiments with cyclic loadings of different directions and frequencies was performed to verify the accuracy of the loading system. Then, a static true triaxial experiment with constant mean principal stress, p, and generalized shear stress, q, was carried out to study the deformation characteristics of gravel under changing Lode's angle, . The results show that changing Lode's angle, , causes plastic deformations in all principal stress directions. In addition, a series of experiments with both axial and lateral cyclic loadings was carried out to study the deformation characteristics of a gravel specimen under different lateral dynamic conditions. Results from the experiments indicate that lateral dynamic loads significantly change the residual and reversible strains in all three principal directions.

The variation of stress-strain state induces changes in the permeability of clayey soil and then affects the properties of seepage and consolidation. In previous studies, variations of the permeability of soil in shear process were mainly neglected, and there were no reasonable descriptions of the permeability in such a condition. To study the variation of the hydraulic conductivity of clayey soil in shear process, new triaxial seepage equipment was designed by changing the conventional triaxial apparatus. Based on a series of tests under different confining pressures carried out by this equipment, the influences of the stress-strain state of clayey soil on hydraulic conductivity were investigated. Furthermore, a mathematical model for the hydraulic conductivity of clayey soil under the condition of large shear deformation was proposed. The comparison between the seepage test results and the calculated values of the proposed model indicates that the mathematical model can calibrate the effects of the void ratio and the mesostructure on the hydraulic conductivity of soil in shear process very well.

The Pastor-Zienkiewicz-Chan model based on the generalized plasticity theory can simulate the stress-strain relationship of soils quite well. Based on the triaxial tests of the filling materials of an earth-rockfill dam, model parameters were obtained through fittings of model and test data. The fitting curves showed that the generalized plasticity model is capable of simulating the mechanical behaviors of rockfill materials. The model was then implemented into a finite element code to carry out 3-dimensional static and dynamic analyses of the earth-rockfill dam. Permanent displacement of the dam was gained and discussed. The results show that the generalized plasticity model can give a good description of dynamic response characteristics of earth-rockfill dams.

Nonlinear elastic model and elastoplastic model are two main kinds of constitutive models of soil, which are widely used in the numerical analyses of soil structure. In this study, Duncan and Chang's EB model and the generalized plasticity model proposed by Pastor, Zienkiewicz, and Chan was discussed and applied to describe the stress-strain relationship of rockfill materials. The two models were validated using the results of triaxial shear tests under different confining pressures. The comparisons between the fittings of models and test data showed that the modified generalized plasticity model is capable of simulating the mechanical behaviours of rockfill materials. The modified generalized plasticity model was implemented into a finite element code to carry out static analyses of a high earth-rockfill dam in China. Nonlinear elastic analyses were also performed with Duncan and Chang's EB model in the same program framework. The comparisons of FEM results and in situ monitoring data showed that the modified PZ-III model can give a better description of deformation of the earth-rockfill dam than Duncan and Chang’s EB model.

Previous studies ignored the possible contribution of the yellow clay interlayer to the failure mechanism of Carsington earth dam, and these studies were mainly based on conventional finite element method, which can neither describe discontinuous displacement field such as shear band visually, nor be able to investigate the formation of shear band and the evolution of the mechanics behavior of soils. In this paper, a self-developed simulation system for the evolution of shear band in soil based on the extended finite element method is applied to simulate the failure process of Carsington dam. The results show that the simulation system is able to trace the path and evolution of shear band in the central core of Carsington dam, and reveales the failure mechanism of Carsington dam reasonably, which proves the applicability of this simulation system in complicated structures and stress conditions.