Numerical simulation of flow processes in liquefied soils using a soil–water-coupled smoothed particle hydrodynamics method

Liquefaction can result in the damage or collapse of structures during an earthquake and can therefore be a great threat to life and property. Many site investigations of liquefaction disasters are needed to study the large-scale deformation and flow mechanisms of liquefied soils that can be used for performance assessments and infrastructure improvement. To overcome the disadvantages of traditional flow analysis methods for liquefied soils, a soil–water-coupled smoothed particle hydrodynamics (SPH) modeling method was developed to analyze flow in liquefied soils. In the proposed SPH method, water and soil were simulated as different layers, while permeability, porosity, and interaction forces could be combined to model water-saturated porous media. A simple shear test was simulated using the SPH method with an elastic model to verify its application to solid phase materials. Subsequently, the applicability of the proposed SPH modeling method to the simulation of interaction forces between water and soil was verified by a falling-head permeability test. The coupled SPH method produced good simulations for both the simple shear and falling-head permeability tests. Using a fit-for-purpose experimental apparatus, a physical flow model test of liquefied sand has been designed and conducted. To complement the physical test, a numerical simulation has been undertaken based on the soil–water-coupled SPH method. The numerical results correspond well with the physical model test results in observed configurations and velocity vectors. An embankment failure in northern Sweden was selected so that the application of the soil–water-coupled SPH method could be extended to an actual example of liquefaction. The coupled SPH method simulated the embankment failure with the site investigation well. They have also estimated horizontal displacements and velocities, which can be used to greatly improve the seismic safety of structures.     

基于三维并行SPH模型的土体流滑特性研究

土体的大变形流滑导致了许多地质灾害的发生,对人们的生命财产安全构成了极大的威胁,因此越来越多的研究开始关注土体的大变形流滑特性。其中,光滑粒子流体动力学（SPH）方法是常用的模拟方法之一,但SPH方法的粒子特性导致其计算时间过长,影响了在工程地质领域的进一步应用。对此,本研究基于SPH方法的基本原理、非牛顿流体理论和等效黏度概念,提出了适用于土体大变形流滑分析的三维SPH仿真模型。结合OpenMP并行计算原理,实现了SPH算法的并行优化。在此基础上,对土体流滑模型试验进行了二维和三维分析,得到了滑动距离、滑动冲击力和冲击力峰值等动力学参数,分析了计算维数和边界条件对流滑特性的影响机制。通过不同线程数下计算时间的对比,获得了计算效率随线程数的变化规律。结果证明了本文的OpenMP并行优化具有较高的计算效率,显著降低了三维SPH模拟的计算耗时,对工程地质数值方法的效率提升具有重要的借鉴意义。     

Numerical analysis on seepage failures of dike due to water level-up and rainfall using a water–soil-coupled smoothed particle hydrodynamics model

For seepage failures of dike due to water level-up and rainfall, surface infiltration and strength change induced by suction reduction are important factors; thus, numerical analysis should consider the coupling of water and soil, as well as the effect of saturation to obtain more precise failure mechanism. Based on the advanced smoothed particle hydrodynamics (SPH) method, this work proposed a two-phase-coupled SPH model in coordination with a novel constitutive model for unsaturated soils. Then, a triaxial compression test is simulated to check the applicability of the SPH method on the soil phase. After that, the failure test of a dike due to water level-up is discretized and simulated, from which the seepage process, the distribution of maximum shear strain, the slip surface, and pore water pressure are obtained. The two-phase-coupled SPH model is also applied to a slope failure test of heavy rainfall, and the results are compared to the model test. Finally, a dike failure test due to rainfall is analyzed using the proposed SPH model to reproduce the surface infiltration and suction reduction. The proposed SPH model provides several insights of seepage failures and can be a helpful tool for the analysis of dike failures induced by water level-up and rainfall.     

砂土液化流动变形的简化方法

已有的液化砂土流动特性试验结果表明,砂土在液化流动状态下是剪切稀化非牛顿流体,可以用幂函数表示其剪应力-剪应变率的关系,从而建立了砂土液化流动的本构方程。基于FLAC3D程序的二次开发平台,将液化流动本构方程开发到FLAC3D中,建立了液化流动变形的简化分析方法。通过倾斜场地的液化流动变形分析,发现倾斜场地的液化变形曲线可以用正弦函数曲线描述,这与Towhata的理论分析成果一致,验证了本方法的合理性。分析了液化层坡度、稠度系数、流动指数以及弹性参数等变量对液化变形的影响。计算结果表明,液化变形随液化层坡度的增大而逐渐增大,液化砂土的稠度系数和流动指数对液化流动变形有重要的影响,而弹性参数对变形基本无影响,因此,在实际工程分析中,需要对流动模型参数进行深入研究。     

SPH approach for simulating hydro-mechanical processes with large deformations and variable permeabilities

A simulation framework based on Smoothed Particle Hydrodynamics (SPH) is introduced to model problems involving the interaction between flowing water and soil deformation. Changes in soil porosity and associated permeability are automatically adjusted within this framework. The framework’s capabilities are presented and discussed for three geotechnical problems caused by flowing water. The comparison between simulation results and experiments shows that SPH with the proposed concept is capable of quantitatively simulating the hydro-mechanical processes beyond limit state with satisfactory agreement. To improve the computational stability, a correction procedure and a new algorithm for the selection of the optimal time step are introduced.     

Identification of failure slip surfaces for landslide risk assessment using smoothed particle hydrodynamics

ABSTRACT

Location of failure slip surfaces plays a critical role in landslide risk assessment and mitigation, particularly for unstable slopes, because it is a key input to design of stabilisation measures for unstable slopes and it determines the volume of the sliding soil mass (i.e. landslide consequence). The failure slip surfaces in the numerical analysis (e.g. finite element/different method, FEM/FDM) are often identified using shear strength reduction (SSR) method. A careful examination of FEM results showed that, although the SSR method performs well for stable slopes, it might provide misleading results for unstable slopes. To properly locate failure slip surfaces for unstable slopes, this paper presents a particle-based numerical method called smoothed particle hydrodynamics (SPH), which is mesh-free, immune to the mesh distortion problem in FEM/FDM, and able to directly simulate large deformation of soils that occurs during landslides. A series of slope stability analyses is performed using an in-house SPH programme. Failure slip surfaces are properly identified by SPH for both stable and unstable slopes. Furthermore, because SPH provides a spatial distribution of the post-landslide large displacement of soils, the failure slip surfaces can be identified conveniently using soil displacement. A displacement-based criterion is proposed to locate the failure slip surfaces.     

Parallel simulations of pore-scale flow through porous media

Smoothed particle hydrodynamics (SPH) is a versatile technique which can be applied to single and multiphase flow through porous media. The versatility of SPH is offset by its computational expense which limits the practicability of SPH for large problems involving low Reynolds number flow. A parallel pore-scale numerical model based on SPH is described for modeling flow phenomena in porous media. Aspects of SPH which complicate parallelization are emphasized. The speed of the method is demonstrated to be proportional to the number of processors for test cases where load balance was achieved. The parallel algorithm permits the application of SPH to more complicated porous media problems than previously considered. For such problems, best performance is achieved when several soil grains are simulated by each processor. Finally, future applications of the method and possible extensions are discussed.     

An improved SPH method for saturated soils and its application to investigate the mechanisms of embankment failure: Case of hydrostatic pore‐water pressure

The method of smoothed particle hydrodynamics (SPH) has recently been applied to computational geomechanics and has been shown to be a powerful alternative to the standard numerical method, that is, the finite element method, for handling large deformation and post‐failure of geomaterials. However, very few studies apply the SPH method to model saturated or submerged soil problems. Our recent studies of this matter revealed that significant errors may be made if the gradient of the pore‐water pressure is handled using the standard SPH formulation. To overcome this problem and to enhance the SPH applications to computational geomechanics, this article proposes a general SPH formulation, which can be applied straightforwardly to dry and saturated soils. For simplicity, the current work assumes hydrostatic pore‐water pressure. It is shown that the proposed formulation can remove the numerical error mentioned earlier. Moreover, this formulation automatically satisfies the dynamic boundary conditions at a submerged ground surface, thereby saving computational cost. Discussions on the applications of the standard and new SPH formulations are also given through some numerical tests. Furthermore, techniques to obtain the correct SPH solution are also proposed and discussed throughout. As an application of the proposed method, the effect of the dilatancy angle on the failure mechanism of a two‐sided embankment subjected to a high groundwater table is presented and compared with that of other solutions. Finally, the proposed formulation can be considered a basic formulation for further developments of SPH for saturated soils. Copyright © 2011 John Wiley & Sons, Ltd.     

Lagrangian meshfree particles method (SPH) for large deformation and failure flows of geomaterial using elastic–plastic soil constitutive model

Simulation of large deformation and post‐failure of geomaterial in the framework of smoothed particle hydrodynamics (SPH) are presented in this study. The Drucker–Prager model with associated and non‐associated plastic flow rules is implemented into the SPH code to describe elastic–plastic soil behavior. In contrast to previous work on SPH for solids, where the hydrostatic pressure is often estimated from density by an equation of state, this study proposes to calculate the hydrostatic pressure of soil directly from constitutive models. Results obtained in this paper show that the original SPH method, which has been successfully applied to a vast range of problems, is unable to directly solve elastic–plastic flows of soil because of the so‐called SPH tensile instability. This numerical instability may result in unrealistic fracture and particles clustering in SPH simulation. For non‐cohesive soil, the instability is not serious and can be completely removed by using a tension cracking treatment from soil constitutive model and thereby give realistic soil behavior. However, the serious tensile instability that is found in SPH application for cohesive soil requires a special treatment to overcome this problem. In this paper, an artificial stress method is applied to remove the SPH numerical instability in cohesive soil. A number of numerical tests are carried out to check the capability of SPH in the current application. Numerical results are then compared with experimental and finite element method solutions. The good agreement obtained from these comparisons suggests that SPH can be extended to general geotechnical problems. Copyright © 2008 John Wiley & Sons, Ltd.     

地震动加速度幅值对土坡失稳后滑坡体大小及致灾范围的影响

地震滑坡的致灾范围是判断滑坡能否会对已有建构筑物造成损失、确定预警疏散范围的重要依据,因此对地震土坡破坏后的滑坡体大小和致灾范围进行研究具有重要的意义.本研究基于SPH动力分析方法,结合弹塑性本构模型和固体力学控制方程建立了地震土坡破坏的动力分析模型;通过设置振动边界粒子和自由场边界粒子,实现了地震动加速度的施加以及自...     

Simulation of debris flow on an instrumented test slope using an updated Lagrangian continuum particle method

We present an updated Lagrangian continuum particle method based on smoothed particle hydrodynamics (SPH) for simulating debris flow on an instrumented test slope. The site is a deforested area near the village of Ruedlingen, a community in the canton of Schaffhausen in Switzerland. Artificial rainfall experiments were conducted on the slope that led to failure of the sediment in the form of a debris flow. We develop a 3D mechanistic model for this test slope and conduct numerical simulations of the flow kinematics using an SPH formulation that captures large deformation, material nonlinearity, and the complex post-failure movement of the sediment. Two main simulations explore the impact of changes in the mechanical properties of the sediment on the ensuing kinematics of the flow. The first simulation models the sediment as a granular homogeneous material, while the second simulation models the sediment as a heterogeneous material with spatially varying cohesion. The variable cohesion is meant to represent the effects of root reinforcement from vegetation. By comparing the numerical solutions with the observed failure surfaces and final free-surface geometries of the debris deposit, as well as with the observed flow velocity, flow duration, and hot spots of strain concentration, we provide insights into the accuracy and robustness of the SPH framework for modeling debris flows.

     

液化流动条件下隧道结构动力分布场研究

将液化流动的土体视为流体,运用流体力学原理,基于矢量符号运算法进行液化场地的动力场分析,求解出动力场解析解。采用ABAQUS/CFD进行液化场地流体动力学有限元分析,根据流动基本控制方程,计算出动力场的数值解。分析结果表明,液化土体横向流动时,隧道结构周围的应力场既包括由表面压力引起的压力阻力,也包括由剪应力引起的摩擦阻力;隧道结构周围的液化土体流动速度很小,但在隧道结构下方存在流动速度加强区;理论计算的动力场解析解大于有限元计算的数值解,但总体的分布变化趋势基本一致;隧道结构附近的应力场变化较为密集,所受到的应力主要分布在液化土体流动的迎面位置。     

Simulation of earthquake‐induced slope deformation using SPH method

This paper presents the development, calibration, and validation of a smoothed particle hydrodynamics (SPH) model for the simulation of seismically induced slope deformation under undrained condition. A constitutive model that combines the isotropic strain softening viscoplasticity and the modified Kondner and Zelasko rule is developed and implemented into SPH formulations. The developed SPH model accounts for the effects of wave propagation in the sliding mass, cyclic nonlinear behavior of soil, and progressive reduction in shear strength during sliding, which are not explicitly considered in various Newmark‐type analyses widely used in the current research and practice in geotechnical earthquake engineering. Soil parameters needed for the developed model can be calibrated using typical laboratory shear strength tests, and experimental or empirical shear modulus reduction curve and damping curve. The strain‐rate effects on soil strength are considered. The developed SPH model is validated against a readily available and well‐documented model slope test on a shaking table. The model simulated slope failure mode, acceleration response spectra, and slope deformations are in excellent agreement with the experimental data. It is thus suggested that the developed SPH model may be utilized to reliably simulate earthquake‐induced slope deformations. This paper also indicates that if implemented with appropriate constitutive models, SPH method can be used to model large‐deformation problems with high fidelity. Copyright © 2013 John Wiley & Sons, Ltd.     

Numerical simulation of air–soil two-phase flow based on turbulence modeling

Soil flow and induced air blasts are of great harm to humanity, and historically they have caused a lot of damage to infrastructure. However, these phenomena cannot be described by traditional analog modeling methods that limit their use in disaster prevention efforts. Computational fluid dynamics (CFD) is an applied technique commonly used in a range of fields including the chemical industry, and aircraft and automobile manufacturing, but little is reported on the use of this method to simulate flowing soil in geotechnical engineering applications. The CFD method can effectively make up for the deficiency of normal calculation methods in the analysis of soil flow and air blasts. This paper uses the FLUENT (version 6.3) CFD calculation software to simulate the processes of soil flow and induced air blast changes during soil flow with an Eulerian air–soil two-phase model included in a standard k-ε turbulence model. Velocity vectors of air blasts at different times during soil flow are obtained, and the characteristics of turbulent flow can be found based on the velocity vectors. The numerical simulation techniques adopted in this paper captured precise configurations of soil flow. The results show that the CFD method is especially suitable for simulating the process of soil flow; hazard assessments can be implemented, and the performance of structures involved with disaster prevention can be improved based on the numerical simulation of changing air blasts.     

基于HBP本构模型的泥石流动力过程SPH数值模拟

本构模型是描述泥石流流变特性的关键,也是决定其动力过程数值模拟准确性的核心问题之一。泥石流流体属多相混合物,现有的研究已证实其存在剪切增稠或剪切变稀的现象,传统基于Bingham及Cross线性本构关系的数值模型难以准确描述泥石流流变特性。文中探讨了Bingham模型在低剪应变率下的数值发散问题,在光滑粒子流体动力学（SPH）方法框架上建立了整合Herschel-Bulkley-Papanastasiou(HBP）本构关系的稀性泥石流动力过程三维数值模型。相比传统基于浅水波假设的二维数值模型,所述方法从三维尺度建立SPH形式下的泥石流浆体纳维?斯托克斯方程并进行数值求解,可获取泥石流速度场时空分布及堆积形态,同时采用HBP本构关系描述泥石流流变特性,能在确保数值收敛的前提下反映泥石流流体在塑性屈服过渡段及大变形状态下应力?应变的非线性变化。为验证提出方法的合理性,结合小型模型槽实验观测进行了对比,结果表明数值模拟与实测结果基本吻合。     

主动侧向受荷桩桩周土的影像观测与数值模拟

在室内模型试验中对模型桩顶施加水平荷载,分别采用数字照相无标点变形量测系统及配套量测分析软件形象地再现了不同密实度砂土中桩周土体及地表土体水平位移分布规律。对双桩的地表水平位移分布情形进行观测分析,结合桩顶位移与水平荷载的关系研究了桩间距对桩与桩之间的相互作用的影响。应用颗粒流程序PFC2D模拟一定深度平面内桩周土体位移,揭示了主动桩桩周土的颗粒流动性状及桩间距对桩-土相互作用的影响。结果表明,主动受荷桩桩周砂土位移场呈两个纺锤体状;砂土密实度增加,桩前砂土变形范围增大;桩间距越小,相邻桩相互影响越明显。     

基于CFD的地震液化研究新进展

综述了近年来关于液化土体流体动力学特征的试验发展状况,以及基于计算流体动力学（简称CFD）的地震液化数值模拟现状,重点介绍了目前比较活跃的可以较高精度模拟液化土体流动状态的三次伪质点数值方法（简称CIP法）。通过对CFD和传统固体力学在地震液化研究中的应用比较,指出了应用CFD的三大优势,即土体大变形问题、液化土体参数分析以及液化土体中结构物的变形应用CFD分析,均可获得较好的结果。进一步提出,在地震液化应用中,未来CFD的发展应该考虑整合液化前的土体性状研究和地震液化中桩-土-结构物的综合分析。     

基于GIS空间数据的滑坡SPH粒子模型研究

针对光滑粒子流体动力学方法（SPH）在滑坡模拟中建立粒子模型的难题,提出了基于地理信息系统（GIS）栅格数据的粒子排列与插入方法。根据该方法,建立了滑坡SPH粒子模型及相关粒子生成程序,进一步以结合摩尔-库仑破坏准则的SPH宾汉流体模型为核心,实现了运用SPH方法模拟滑坡破坏后三维运动的过程。该SPH模型在对唐家山滑坡的模拟中得到了验证,并预测了金坪子滑坡破坏后的影响范围。结果表明：基于GIS空间数据的滑坡SPH粒子模型具有可行性与良好的适用性。以GIS数据库为基础,开展滑坡灾害的模拟研究,将大大提高对滑坡等地质灾害的仿真分析,为滑坡灾害的预测与防治提供参考。     

液化砂土流动效应的振动台试验研究

采用振动台激励使饱和砂土发生液化,并侧向拖拽埋入砂土中的铝管,模拟液化土体与管体相对运动以分析液化砂土流动的力学效应。引入流体力学理论与方法,推导出以拉力反算表观黏度的表达式以及液化土体作为流体对管壁作用的黏滞剪切力。分析和比较了振时拖动、振后拖动下土体的流体性质及其流动效应的率相关性和孔压相关性,探讨了砂土密实度对土体流动效应的影响。结果表明,土体初始密实度与液化后土体的表观黏度正相关;液化土体的表观黏度以及因流动产生的黏滞剪切力与孔压反相关;液化砂土流动产生的黏滞剪切力具有强烈的率相关性。针对可液化场地中的结构抗震分析,应考虑土-结构率相关相互作用。     

Visual simulation of landslide fluidized movement based on smoothed particle hydrodynamics

Earthquake-induced landslides can cause a large number of casualties and great economic loss. Presently, research methods for studying landslides are largely based on a framework of solid mechanics. In this work, visualization software, known as Visual SPH, is developed in Visual Basic 6.0, and can be used to analyze the fluidized movement of landslides based on smoothed particle hydrodynamics (SPH). First, the accuracy of the software is verified through simulating a benchmark problem of a dam-break. Then, fluidized movement analysis of the Tangjiashan landslide is carried out using this software. The simulation derives plots of displacement versus time, reproduces the entire flow process of the Tangjiashan landslide, and determines the run-out, which coincides exactly with the characteristics of flow-like landslides observed in the field. All of these are essential in the design of supporting structures and site selection for reconstruction in earthquake-prone regions.