Soil–water–air fully coupling finite element analysis of slope failure in unsaturated ground

In this paper, a program of the finite element method (FEM), named as SOFT, using a finite element–finite difference scheme (FE–FD) for soil–water–air three-phase coupling problems, has been developed based on a rational and simple constitutive model for unsaturated soil proposed by Zhang and Ikariya (2011). In the program, similar to the works by Uzuoka et al. (2009) and Oka et al. (2010b), the FE–FD formulation in saturated condition of soil–water coupling problem, proposed by Oka et al. (1994), has been extended to unsaturated condition in soil–water–air fully coupling scheme, taking the saturation as a state variable. In order to verify the availability of the proposed numerical method, triaxial tests on unsaturated silty clay under fully undrained and unvented conditions, conducted by Oka et al. (2010a), are firstly simulated by the proposed method. The development of pore air pressure and pore water pressure measured in the specimen can be reproduced well by the proposed method. Furthermore, model tests on slope failure in unsaturated Shirasu, carried out by Kitamura et al. (2007), are also simulated by the same numerical method. From the simulation it is known that the slope failure behavior of the model ground observed in the tests can be described, on the whole, with satisfactory accuracy     

Failure mechanism of unsaturated landslide dam under seepage loading – Model tests and corresponding numerical simulations

Due to the rapid cracking, sliding and packing of its geomaterials, a landslide dam (LD) is usually weaker in structure than the undisturbed ground and is more vulnerable to seepage loading. In addition, since the naturally packed geomaterials of an LD are mostly in an unsaturated state, it is necessary to use a suitable constitutive model that can describe the mechanical behaviour of the soils under an unsaturated/saturated state in a unified way and whose material parameters can be determined in a rational way in order to accurately simulate the failure mechanism of the LD. In this paper, water retention tests and flume tests were conducted on model LDs prepared with three different ground materials. An unsaturated soil constitutive model was selected for the corresponding numerical simulations. Based on the results of water retention tests and triaxial tests, the parameters of the LD materials were properly determined. Using these parameters, soil-water-air coupling finite element analyses were conducted to simulate the flume tests on the model LDs. By comparing the calculated results with the test results, it was found that the numerical method used in this paper has satisfactory accuracy for describing the different failure mechanisms of the model LDs under seepage loading. The results indicate that the material properties of the LDs, especially the strength and the difference in void ratio between the unsaturated and the saturated states, play important roles. In addition, the influence of the rate of the rise in water head was also investigated by numerical tests. The purpose of this research is to provide a scientific basis for decision-making in the disaster mitigation process of LDs with a comprehensive method for understanding the failure mechanism of these LDs.     

Numerical Prediction of Seepage and Seismic Behavior of Unsaturated Fill Slope

This paper presents a case study on a numerical prediction of the seepage and seismic behaviors of an unsaturated fill slope. In this study, the numerical prediction is performed with a dynamic three-phase (soil, water and air) coupled analysis based on porous media theory and constitutive models. The weak forms of three governing equations, momentum balance equations of the overall three-phase material and mass and the momentum balance equations of the pore fluids (water and air), are implemented in a finite element model. The discretized equations are solved by the fully implicit method and the skeleton stress is also implicitly integrated. The in-situ observation at the target fill slope of the case study has been ongoing since 2006. First, distributions of water saturation in the fill slope are simulated by performing seepage analyses to reproduce the in-situ ground water level in the fill slope. Second, seismic responses of the fill slope during two past earthquakes are simulated, and the numerical method is validated by comparing observed acceleration records and numerical one. Finally, seismic responses of the fill slope during a future scenario earthquake are predicted under different moisture conditions of unsaturated fill. As a result, the following findings were obtained. 1) The shape of the ground water level was partially reproduced from the quite dry fill slope element by using virtual constant precipitation. However, the observation results of moisture distribution above the ground water level were not reproduced. 2) In the validation analyses after two earthquakes, the numerical method reproduced the positions of peak frequency of acceleration Fourier spectra. 3) The numerical results clearly showed that frequency characteristics of the seismic response and the residual displacement of the fill slope were affected by the initial distribution of the degree of saturation in the fill ground.     

Study on the mesostructure and mesomechanical characteristics of the soil–rock mixture using digital image processing based finite element method

The soil–rock mixture (SRM) is a kind of inhomogeneous geomaterial, which poses difficulties of in situ sample acquisition and in laboratory geomaterial tests; hence, the study of the SRM's mechanical properties is still at an early stage. In this paper, the technique of digital image processing based on the finite element method (DIP-FEM) is introduced to study SRMs in the Leaping Tiger Gorge Reservoir Area, China. Based on the DIP, the mesostructural characteristics of the SRM are analyzed statistically. The mesostructural concept model of SRM that can actually represent the inhomogeneity of SRM is built. By using geometry vectorizaiton transformation, the mesostructural model of SRM in the binary image format has been translated into a vector format (such as DWG or DXF format) which can be imported into the finite element software. By using the finite element method, two large-scale direct shear tests of inhomogeneous SRM and homogeneous soil are simulated. The numerical results indicate that the existence of “rock” blocks in SRM will greatly influence the distribution and the failure models of the internal stress field. As a result, three kinds of failure models of the SRM are put forward.     

三峡库区寨坝变形体的渗流变形有限元耦合分析

三峡库区寨坝民德小学变形体是山前坡积物、崩积物以及滑坡堆积物等的混杂体,中部存在孤立的块碎石含水层,地下水位较高。公路开挖引起了远场集中通道排水疏干和开挖面附近卸载,导致了不均匀变形破坏产生。本文给出了这种开挖卸载与排水疏干共同作用下,边坡饱和-非饱和渗流与变形耦合的有限元分析方法。研究表明,集中通道排水造成了坡体大范围的孔隙水压力下降,从而导致不均匀变形和地面开裂;其中通道排水诱发的不均匀变形将随着排水通道的封堵和地下水位的回升而停止,而纯粹的开挖卸载主要影响开挖面附近的土体变形。这一典型实例所揭示的机理模式,可以解释工程施工区通常出现的因开挖和开挖引起的排水所诱发的远近变形破坏现象的实质。     

Flume tests and corresponding numerical simulation of hydraulic/mechanical behavior of Tangjiashan landslide dam subjected to seepage loading

Landslide dams (LDs) are usually composed of natural debris materials and normally exhibit heterogeneous strata along both the depth and run-out directions. In addition, LDs usually have weaker structures than undisturbed ground and are more vulnerable to seepage loading. Considering that the surface layer of naturally packed LD materials is mostly in an unsaturated state, it is undoubtedly important to investigate the hydraulic/mechanical behavior of LDs with heterogeneous strata subjected to seepage loading. In this paper, a systematic flume test program was firstly conducted, in which the Tangjiashan LD was carefully referenced for its model design. Three different water level rising rates and two different stratal arrangements were considered in the flume tests. Then, soil-water-air coupled finite element analyses were conducted to simulate the flume tests, and all the material parameters of the LD materials were carefully determined based on the results of element tests. A comparison of the test and calculated results showed the possibility of using the proposed numerical method to predict the occurrence of dam breaching and to estimate the risk of LD failure. Moreover, the hydraulic/mechanical behavior of the LD materials and the heterogeneous strata of the LD were proven to be very important to the stability of the LD. Finally, from an engineering viewpoint, the possibility of utilizing a naturally formed LD, and not destroying it after its formation, is also discussed.     

Numerical modeling for the coupled thermo-mechanical processes and spalling phenomena in spPillar Stability Experiment (APSE)

In this paper, the coupled thermo-mechanical (TM) processes in the sp Pillar Stability Experiment (APSE) carried out by the Swedish Nuclear Fuel and Waste Management Company (SKB) were simulated using both continuum and discontinuum based numerical methods. Two-dimensional (2D) and three-dimensional (3D) finite element method (FEM) and 2D distinct element method (DEM) with particles were used. The main objective for the large scale in situ experiment is to investigate the yielding strength of crystalline rock and the formation of the excavation disturbed/damaged zone (EDZ) during excavation of two boreholes, pressurizing of one of the boreholes and heating. For the DEM simulations, the heat flow algorithm was newly introduced into the original code. The calculated stress, displacement and temperature distributions were compared with the ones obtained from in situ measurements and FEM simulations. A parametric study for initial microcracks was also performed to reproduce the spalling phenomena observed in the APSE.     

加筋砂土挡墙承载力及渐进性变形破坏的有限元分析方法

 为开发一种可以合理模拟加筋砂土挡墙承载力及渐进性变性破坏特征的数值分析方法,利用可考虑应变局部化的非线性弹塑性有限元法对一系列的加筋砂土挡墙模型试验结果进行从小变形到破坏的全过程数值分析。有限元分析中,砂土的本构关系采用基于修正塑性功的硬化–软化弹塑性模型,该模型引入应变局部化参数S用以描述砂土单元峰值以后的局部剪切破坏效果。模型试验结果与有限元计算结果比较表明：建议的非线性弹塑性有限元分析不仅可以较好地模拟分析加筋砂土挡墙基础底面的平均压力与沉降之间的关系,而且也能合理地模拟加筋砂土挡墙基础的剪切破坏的发生与发展状况、筋材的拉力以及挡墙面板的水平土压力分布等,它可以定量化地分析加筋砂土挡墙的渐进性变形破坏特征以及条带加筋材料的加固机制。     

考虑渗流和锚固作用的强度折减有限元法研究

 针对目前岩土工程中锚杆数值模型存在的局限性,探索适合于强度折减有限元法计算和参数优化的新的锚杆计算模型,研制出一个考虑渗流场和由于泡水含水量变化引起岩土体强度参数降低及锚杆支护作用的三维强度折减有限元程序,该锚杆计算模型能够考虑锚杆和岩土体间黏结力不够的锚杆失效形式。通过实例计算并与传统极限平衡法实例计算分析结果对比研究,对抗剪强度折减有限元法分析边坡稳定问题的适用性进行评价。结果表明,采用三维强度折减有限元法确定考虑渗流场和由于泡水含水量变化引起岩土体强度参数降低及锚杆支护作用的边坡稳定性安全系数是可行的。     

A Seepage-Deformation Coupled Analysis of an Unsaturated River Embankment using a Multiphase Elasto-Viscoplastic Theory

A multiphase deformation analysis of a river embankment was carried out using an air-soil-water coupled finite element method capable of considering unsaturated seepage flow. A numerical model for unsaturated soil was constructed based on the mixture theory and an elasto-viscoplastic constitutive model. The theory used in the analysis is a generalization of Biot's two-phase mixture theory for saturated soil. An air-soil-water coupled finite element method was developed using the governing equations for three-phase soil based on the nonlinear finite deformation theory, i.e., the updated Lagrangian method. Two-dimensional numerical analyses of the river embankment under seepage conditions were conducted, and the deformation associated with the seepage flow was studied. We have found that the occurrence of large deformations corresponds to the large values of the hydraulic gradients at the toe of the embankment, and that the overflow of river water makes the embankment more unstable. It has been confirmed that seepage-deformation coupled three-phase behavior can be simulated well with the proposed method.     

流形元与有限元变形和渗流的非同步耦合分析方法及其应用

提出了一个用流形元与有限元的变形和渗流非同步耦合的分析方法，用于模拟不连续块体在水压作用下的变形过程。对一个岩石边坡上的裂隙岩块随地下水上升而产生滑动破坏的过程进行了计算模拟，将此数值计算结果与岩块的滑动极限平衡计算结果作了比较分析。计算结果显示了所提方法的可用性。     

Numerical simulation of sanding using a coupled hydro-mechanical sand erosion model

Mechanical failure of materials adjacent to the production cavity and material disaggregation caused by fluid drag are considered as the most important parameters that affect sand production.In light of such factors,the coupling of two mechanisms-mechanical instability and hydrodynamic erosion-is indispensable in order to model this phenomenon successfully.This paper examines the applicability of a coupled hydro-mechanical erosion criterion for simulating sand production using the finite element method.The porous medium was considered fully saturated.The onset of sanding and production of sand were predicted by coupling mechanical failure and subsequent erosion of the grain particles utilizing a sanding model.To consider the erosion process,the Papamichos and Stavropoulou(1998)'s sand erosion criterion was incorporated into the finite element code.Arbitrary Lagrangian-Eulerian(ALE) adaptive mesh approach was used to account for large amounts of erosive material loss.Besides,in order to address the problem of severe mesh distortion,the"mesh mapping technique" was employed.Sand production in a horizontal wellbore and in a field case was simulated to demonstrate capabilities of the proposed model.In addition,principal parameters affecting sand production,including in situ stresses,cohesion,perforation orientation,and drawdown were examined.The results indicated the efficiency of the model used in evaluation of sanding in the field.Parametric studies indicated that in situ stresses and formation cohesion could be considered as dominant factors affecting the amount of sand production.     

A comprehensive probabilistic model of chloride ingress in unsaturated concrete

Corrosion induced by chloride ions has become a critical issue for many reinforced concrete structures. The chloride ingress into concrete has been usually simplified as a diffusion problem where the chloride concentration throughout concrete is estimated analytically. However, this simplified approach has several limitations. For instance, it does not consider chloride ingress by convection which is essential to model chloride penetration in unsaturated conditions as spray and tidal areas. This paper presents a comprehensive model of chloride penetration where the governing equations are solved by coupling finite element and finite difference methods. The uncertainties related to the problem are also considered by using random variables to represent the model’s parameters and the materials’ properties, and stochastic processes to model environmental actions. Furthermore, this approach accounts for: (1) chloride binding capacity; (2) time-variant nature of temperature, humidity and surface chloride concentration; (3) concrete aging; and (4) chloride flow in unsaturated conditions. The proposed approach is illustrated by a numerical example where the factors controlling chloride ingress and the effect of weather conditions were studied. The results stress the importance of including the influence of the random nature of environmental actions, chloride binding, convection and two-dimensional chloride ingress for a comprehensive lifetime assessment.     

2012-07目录

固体材料受力后从弹性发展到塑性、再发展到破坏,表明屈服与破坏是不同的。本文简述了岩土材料的屈服准则,并提出岩土材料的屈服准则体系,提出材料应力场中点破坏和整体面破坏2种概念及其定义,并用传统极限分析方法定义材料的整体面破坏准则。回顾分析传统极限分析方法的求解特点,由此指出传统极限分析法与数值分析法的不足,两者结合形成近年发展的数值极限分析方法,使极限分析方法的适用范围大幅扩大。就传统极限分析法与数值极限法的内涵、特点做深入的分析,论证了方法的可靠性,同时指出数值极限分析法的优越性及其存在的问题,最后举例说明数值极限分析方法在岩土工程中的边(滑)坡、地基以及隧道工程中的广泛适用性。     

基于有限元强度折减法的边坡稳定分析

通过强度折减,使系统达到不稳定状态时,有限元计算将不收敛,此时的折减系数就是安全系数。本文对不同坡度的算例进行有限元分析把得到的结果与用简化Bishop法得到的结果进行比较,两种方法得到的安全系数基本一致。这说明将强度折减法用于分析匀质边坡稳定问题是可行的。     

静动力排水固结法地基处理数值模拟

结合广州南沙泰山石化成品油库区淤泥软基处理实践,运用有限元分析法对工程进行了数值模拟和预测,分析施工过程中地基位移的变化规律,实测期内模拟与实测资料较为接近,该数值分析对工程实践有一定指导意义。     

堆积软岩的黏弹塑性本构模型及其数值计算应用

 堆积软岩的主要力学特性有应变软化、流变、受中间主应力影响等,为了尽可能完整地描述这些力学特性,提出一个全新的黏弹塑性本构模型。该模型基于下负荷面和tij概念,以超固结状态与正常固结状态之间的孔隙比差r为状态变量,并在该状态参量的演化律中引入非齐次函数,使模型能综合描述软岩的应变软化、流变和受中间主应力影响的力学特性。新模型仅比剑桥模型多了4个参数,而且4个参数都具有明确物理意义,可通过常规三轴试验确定。以降雨引起的软岩边坡的渐进性破坏为工程背景,用水土耦合有限元来探讨2种不同地下水位变化对边坡破坏的影响,从坡体的剪应变分布、位移矢量、破坏的传播等方面分析边坡的破坏机制。     

Coupling of smoothed particle hydrodynamics and finite element method for impact dynamics simulation

This paper presents an alternative method for coupling smoothed particle hydrodynamics (SPH) and finite element method (FEM) in a Lagrangian framework. The attachment and contact between SPH particles and finite elements are calculated. FE nodes are added to the SPH neighbor list for the attachment, and the continuity of the interface is guaranteed. The contact force on SPH particles and FE nodes is calculated with the same approach used in SPH particle to particle contact algorithm, and the identification of the contact surface and the surface normal is not required. Background particles are assigned in the position of FE nodes to facilitate particle approximation. The perforation of a cylindrical Arne tool steel projectile impacting a plate Weldox 460 E steel target is simulated in 3D to demonstrate the performance of the SPH-FEM coupling algorithm. The coupled computational model of viscoplasticity and ductile damage and Gruneisen EOS are used for the target plate. A particle-kill algorithm is used to invalidate the damage particles. Good agreement between the numerical simulations and the experimental results is obtained, and the ballistic limit velocity obtained from the SPH-FEM coupling algorithm gives a deviation of 2% from the experimental data.     

降雨入渗对非饱和土边坡稳定性的影响

研究降雨入渗对边坡稳定性的影响规律。采用有限元法进行非饱和土边坡的二维非稳态渗流计算,考虑基质吸作用利用极限平衡法进行非饱和土边坡稳定安全系数计算,进而通过算例计算,分析了降雨过程中及降雨之后,边坡内孔隙水压分布、潜在滑裂面位置以及边坡稳定安全系数的变化情况。着重分析了降雨强度和降雨持续时间的影响,并特别注意分析降雨结束后的边坡稳定性。算例表明某些情况下边坡安全系数最小值出现在降雨之后的数小时或数天,而非降雨的过程中或降雨刚刚结束之时。     

Stability analysis of bank slope under conditions of reservoir impounding and rapid drawdown

Stability of an ancient landslide in a reservoir area is analyzed by using centrifugal model tests, soil laboratory tests and numerical analysis. Special attention is paid to variation in water level, simulation of large-scale heterogeneous prototype slope, and strength reduction of sliding zone soils after slope sliding. The results of centrifugal model test show that reservoir impounding can reduce sliding resistance at the slope toe, followed by toe collapsing and front cracking of slope. Rapid drawdown can produce hydrodynamic pressure towards reservoir at the front of slope. Deformation is observed in the middle and upper slope, which reduces the slope stability further and forms the pull-typed landslide trend. Reinforcement of slope toe is effective for preventing the progressive failure. The results of laboratory test show that slope toe sliding will lead to the redistribution of soil density and moisture content, which will reduce the shear strength of soil in sliding zone, and the cohesion of immersed soil is reduced gradually and finally vanishes with time. The numerical results show that the strength reduction method used in finite element method (FEM) is very effective in capturing the progressive failure induced by reservoir water level fluctuations, and the evolution of failure surface derived from numerical simulation is very similar to that observed in centrifugal model test.