岩体结构与岩体水力耦合计算模型

水力耦合作用是岩体中力学过程与渗流过程相互作用的物理过程。水力耦合机理的理解是水力耦合分析的关键问题,其耦合机理是由岩体结构特性决定的。在分析岩体水力耦合过程基础上,根据岩体的基本结构及代表性单元体（REV）是否存在提出了建立水力耦合模型的方法。当裂隙岩体中不存在代表性单元体（REV）时,提出了裂隙岩体多重介质流固耦合分析的全耦合数学模型,给出流固耦合模型数值方法求解的数学模型及有限元计算表达式。程序编制和验证工作正在进行中。     

Quantitative analysis of piping erosion micro-mechanisms with coupled CFD and DEM method

Piping, as one of the critical patterns of internal erosion, has been reported as a major cause for failures of embankment dams and levees. The fundamental mechanism of piping was traditionally investigated through experimental trials and simplified theoretical methods in macroscale. Nevertheless, the initiation and progressive evolution of piping is a microscale phenomenon in its essence. The current understanding of the micro-mechanism of piping erosion is limited due to a lack of quantitative analysis and visualized evidence. And in fact, seepage flows can affect the soil fabrics and the development of contact forces between particles. But how these fabrics and contact forces evolve under a critical hydraulic gradient is still not fully understood. In this paper, the detailed process of piping erosion is investigated by using a coupled computational fluid dynamics and discrete element method (CFD–DEM) approach. The treatment of soil–flow interactions in CFD–DEM is explained by exchanging the momentum between the two phases. During the simulation, the piping erosion process is initiated by incrementally ascending differential water head across the soil samples. The three main stages of piping erosion (initial movement, continuation of erosion and total heave) can be identified from monitoring the particle velocity and positions. In addition, the evolution of contact force, hydraulic force, coordination number and void fraction is inspected to provide insight into the micro-mechanism of piping erosion. Two cases are simulated, one with a uniform particle size and a relatively uniform porosity distribution and the other with specific particle size and porosity distributions. An interesting finding from this study is that piping does not always initiate from the free surface and the evolution of piping depends heavily on the particle size and porosity distribution.     

A stochastic model of soil erosion

The erosion model computes the rill and inter-rill flow over a surface with random roughness, and the erosion caused by this flow. The measured roughness of a surface is analysed and used to generate random surfaces for the simulation process. Computations are carried out over a number of time intervals; the steady state condition is assumed for each interval. Changes in the surface geometry due to erosion during an interval are used to revise the surface for the subsequent interval. The model includes simplified mechanisms to simulate ponding, deposition and failure of side slopes of rills.     

考虑重力侵蚀影响的分布式土壤侵蚀模型

黄土高原地区的土壤侵蚀具有水力侵蚀和重力侵蚀相伴发生的特点,在大多数针对黄土高原的侵蚀模型研究中未考虑重力侵蚀的影响,使得模型的模拟精度较差。针对黄土高原的侵蚀特点,采用量化影响重力侵蚀发生的主要因素,确定重力侵蚀发生的具体沟道栅格单元的方法,从而考虑了重力侵蚀对产输沙过程的影响。所建立的分布式土壤侵蚀模型以逐网格汇流的水文模型为基础,采用逐网格侵蚀输沙的模拟方式,能够模拟上方来水来沙对侵蚀输沙的影响。经小理河流域实测资料验证,模型具有一定的计算精度。     

A study on the uplift mechanism of Tongjiezi dam using a coupled hydro-mechanical model

During filling in 1992 of the Tongjiezi reservoir, in Southwest China, it was noticed that the dam body and the rock masses on the right side of the valley were uplifted up to 22.2 mm and 24.3 mm respectively. After reservoir filling in 1993, the uplift continued, but the rate decreased, and the measured maximum uplift increased from 22.2 mm to 27.5 mm in the dam body and from 24.3 mm to 28.9 mm in the rock masses from 1993 to 2004. Based on the geological features of the dam site and observed geomechanics data, a numerical method is used to evaluate the representative elementary volume (REV) and to identify the parameters related to the mechanical and hydraulic properties of the rock mass. Furthermore, a coupled hydro-mechanical model is presented to describe the uplift process. The simulated results agree well with those measured. In addition, the time-dependent deformation is studied in the laboratory and numerically. It is concluded that, with the specific hydrogeological conditions at the Tongjiezi dam site, hydro-mechanical coupling during and after the reservoir impoundment is the main factor contributing to the uplift, and the time-dependent deformation is due to the rheological behavior of rock masses under seepage pressure.     

Probabilistic calibration of a coupled hydro-mechanical slope stability model with integration of multiple observations

Multiple types of responses, such as displacements, ground water level, pore water pressures, water contents, etc., are usually measured in comprehensive monitoring programmes for rainfall-induced landslide prevention. In this study, a probabilistic calibration method for coupled hydro-mechanical modelling of slope stability is presented with integration of multiple types of measurements. A numerical example of a soil slope under rainfall infiltration is illustrated to compare the effects of single and multiple types of responses on parameter estimation and model calibration. The results show that the soil parameters can be estimated with less uncertainty and total uncertainty bounds are narrower with multiple types of responses than with a single type of response. Model calibration based on multiple types of responses can compromise different responses and hence the means and standard deviations of model error are the smallest. A feasible correlation coefficient between soil modulus and permeability can be obtained from model calibration with multiple types of responses and single type of response as long as the responses include displacement data.     

均质土坝管涌发展过程的渗流场空间性状研究

堤坝发生管涌破坏时,渗流场具有强烈的空间特性,管涌发展过程的数值模拟对于堤坝工程管涌预防与治理都具有重要的参考价值。提出以一维通道嵌入三维块体的方法,克服了在三维问题有限元模拟方法上存在的困难。在探讨均质土坝管涌发展过程模拟方法的基础上,使用数值方法分析了土坝管涌发展过程的渗流场时空分布特性;揭示了管涌发展过程中渗流力集中状况及其变化规律。研究表明,改善渗流场的形态是防治堤坝工程管涌的关键措施。     

On the use of effective stress in three-dimensional hydro-mechanical coupled model

In the last decades, a number of hydro-mechanical elastoplastic constitutive models for unsaturated soils have been proposed. Those models couple the hydraulic and mechanical behaviour of unsaturated soils, and take into account the effects of the degree of saturation on the stress–strain behaviour and the effects of deformation on the soil–water characteristic response with a simple reversible part for the hysteresis. In addition, the influence of the suction on the stress–strain behaviour is considered. However, until now, few models predict the stress–strain and soil–water characteristic responses of unsaturated soils in a fully three-dimensional Finite Element code. This paper presents the predictions of an unsaturated soil model in a Three-dimensional Framework, and develops a study on the effect of partial saturation on the stability of shallow foundation resting on unsaturated silty soil. Qualitative predictions of the constitutive model show that incorporating a special formulation for the effective stress into an elastoplastic coupled hydro-mechanical model opens a full range of possibilities in modelling unsaturated soil behaviour.     

A coupled surface/subsurface flow model accounting for air entrapment and air pressure counterflow

This work introduces the soil air system into integrated hydrology by simulating the flow processes and interactions of surface runoff, soil moisture and air in the shallow subsurface. The numerical model is formulated as a coupled system of partial differential equations for hydrostatic (diffusive wave) shallow flow and two-phase flow in a porous medium. The simultaneous mass transfer between the soil, overland, and atmosphere compartments is achieved by upgrading a fully established leakance concept for overland-soil liquid exchange to an air exchange flux between soil and atmosphere. In a new algorithm, leakances operate as a valve for gas pressure in a liquid-covered porous medium facilitating the simulation of air out-break events through the land surface. General criteria are stated to guarantee stability in a sequential iterative coupling algorithm and, in addition, for leakances to control the mass exchange between compartments. A benchmark test, which is based on a classic experimental data set on infiltration excess (Horton) overland flow, identified a feedback mechanism between surface runoff and soil air pressures. Our study suggests that air compression in soils amplifies surface runoff during high precipitation at specific sites, particularly in near-stream areas.     

土质陡坡降雨侵蚀的数学模型及求解方法

建立了土质陡坡降雨侵蚀的数学模型,模型包括陡坡细沟间坡面流控制方程、变沟宽陡坡细沟流控制方程、陡坡细沟间坡面侵蚀泥沙连续方程及陡坡变沟宽细沟侵蚀泥沙连续方程。通过紊流冲击分布概率确定土壤剥蚀方程,确定了泥沙源/汇项的表达形式及细沟高程与宽度的表达式。通过有限元和有限差分相结合的方法求解该数学模型,利用有限单元法对数学模型的水动力方程、运动方程及泥沙连续方程进行空间上的离散,利用有限差分法解决时间域的问题,得出了顺序求解的数值计算公式及模型数值求解的具体方法和步骤。     

孔隙比对砂类土抗剪强度影响的定量分析

砂类土的密实程度是影响其抗剪强度的重要指标,然而已有研究并没有直观给出砂类土抗剪强度随孔隙比变化的定量规律。控制砂类土的初始孔隙比得到土体所需的抗剪强度,这在土工相似模型试验中具有一定的应用前景。基于砂土的临界状态理论,本文建立了砂类土抗剪强度与孔隙比关系的理论表达式。针对尾矿库现场取回的铜、钨尾砂开展了直剪试验和固结试验,并利用所推导的理论表达式对尾砂在不同状态时的抗剪强度进行了分析。结果表明,随着孔隙比的增加,砂类土的抗剪强度非线性减小,孔隙比越大时抗剪强度减小的幅度越平缓。试验结果证实砂类土抗剪强度与孔隙比关系的理论表达式能够合理描述尾砂在不同孔隙比下的抗剪强度。     

Progression of backward erosion piping with sudden and gradual hydraulic loads

Acta Geotechnica - With the utilization of underground space, backward erosion piping (BEP) has been observed in many underground structures. Different from BEP in embankments or river dikes, the...     

粘性土土洞形成的水化学侵蚀实验

一般认为地下环境中土洞的形成，进一步导致坍塌，其主要成因是地下水动力条件的急剧变化，而目前的研究尚考虑水体化学侵蚀这一因素，作者通过粘土性崩解实验认为，不同水质的水体对土壤崩解的影响存在明显差别，水动力作用对其具有促进作用。在地面坍塌的灾害研究中，由于各种因素导致的地下水体化学成分的变化对土体崩解，土洞形成的影响是不能忽视的。     

A mathematical model for predicting coupled heat and water movement in unsaturated soil

An investigation was conducted to develop a comprehensive moisture model for predicting non-isothermal moisture conditions in soils. An extensive literature review indicated that a model based on the Philip and de Vries equations for non-isothermal moisture movement and heat conduction would give the best results. By using numerical methods, the implicit finite difference approximations to the moisture movement and heat-transfer equations were programmed for computer solution of water content and temperature in the soil with time. Validation studies indicate that the moisture model can be used to predict accurately moisture conditions in the soil. The model was validated by using hydraulic data from laboratory studies conducted on soil columns compacted with AASHO A-3 and AASHO A-4 soil. The application of the moisture model to the study of non-isothermal moisture movement in the field is demonstrated. The influence of parameters such as water table depth, precipitation, and soil hydraulic properties on soil moisture content are shown by use of the moisture model. The model is shown to be applicable to a wide range of boundary conditions and that it predicts the moisture-temperature regime with time in soils utilizing climatic input data.     

土壤侵蚀模型研究现状与展望

土壤侵蚀已成为极为严峻的环境问题之一,对它进行准确的预报具有重要的实践意义.本文对国内外土壤侵蚀模型的研究现状进行了详细的回顾,并对我国土壤侵蚀模型研究中相对薄弱的环节进行了深入的分析.旨在借鉴国外研究经验,促进我国土壤侵蚀模型研究的进展.     

A finite element method for localized erosion in porous media with applications to backward piping in levees

This paper presents a computational method able to effectively model both the simultaneous processes typically observed in backward erosion piping, ie, the pipe tip propagation and the conduit cross-section enlargement. The numerical method is based on the novel formulation of a problem of localized erosion along a line propagating in a multidimensional porous medium. In this line, a conduit with evolving transverse size is embedded, which conveys a multiphase flow. The two systems, porous medium and pipe, are bridged by exchange terms of multiphase fluid mass and by a shared fluid pressure field. On the contrary, different fields are considered to describe flows, which are assumed as Darcian in the porous medium and turbulent in the conduit. These two flows drive pipe propagation and enlargement, respectively, as modeled by means of proper erosion kinetic laws. The corresponding numerical formulation is based on the combination between one- and multidimensional finite elements, to model the erosion conduit and the porous medium, respectively. Several simulations are proposed to demonstrate the ability of the proposed approach in reproducing available experimental data of real-scale tests on levees. Our results point out the crucial role played by the combined influence of pipe propagation and enlargement, as well as of three-dimensional (3D) effects. We also assess the mesh independence of the proposed numerical solution, particularly as concerns the calculated pipe propagation history.     

Catchment-scale soil erosion and sediment yield simulation using a spatially distributed erosion model

Increasing rainfall intensity and frequency due to extreme climate change and haphazard land development are aggravating soil erosion problems in Korea. A quantitative estimate of the amount of sediment from the catchment is essential for soil and water conservation planning and management. Essential to catchment-scale soil erosion modeling is the ability to represent the fluvial transport system associated with the processes of detachment, transport, and deposition of soil particles due to rainfall and surface flow. This study applied a spatially distributed hydrologic model of rainfall–runoff–sediment yield simulation for flood events due to typhoons and then assessed the impact of topographic and climatic factors on erosion and deposition at a catchment scale. Measured versus predicted values of runoff and sediment discharge were acceptable in terms of applied model performance measures despite underestimation of simulated sediment loads near peak concentrations. Erosion occurred widely throughout the catchment, whereas deposition appeared near the channel network grid cells with a short hillslope flow path distance and gentle slope; the critical values of both topographic factors, providing only deposition, were observed at 3.5 (km) (hillslope flow path distance) and 0.2 (m/m) (local slope), respectively. In addition, spatially heterogeneous rainfall intensity, dependent on Thiessen polygons, led to spatially distinct net-erosion patterns; erosion increased gradually as rainfall amount increased, whereas deposition responded irregularly to variations in rainfall.     

Assessment of soil erosion risk using SWAT model

Soil erosion is one of the most serious land degradation problems and the primary environmental issue in Mediterranean regions. Estimation of soil erosion loss in these regions is often difficult due to the complex interplay of many factors such as climate, land uses, topography, and human activities. The purpose of this study is to apply the Soil and Water Assessment Tool (SWAT) model to predict surface runoff generation patterns and soil erosion hazard and to prioritize most degraded sub-catchment in order to adopt the appropriate management intervention. The study area is the Sarrath river catchment (1,491 km²), north of Tunisia. Based on the estimated soil loss rates, the catchment was divided into four priority categories for conservation intervention. Results showed that a larger part of the watershed (90 %) fell under low and moderate soil erosion risk and only 10 % of the watershed was vulnerable to soil erosion with an estimated sediment loss exceeding 10 t?ha^?1?year^?1. Results indicated that spatial differences in erosion rates within the Sarrath catchment are mainly caused by differences in land cover type and gradient slope. Application of the SWAT model demonstrated that the model provides a useful tool to predict surface runoff and soil erosion hazard and can successfully be used for prioritization of vulnerable areas over semi-arid catchments.     

Fully coupled hydro-mechanical numerical manifold modeling of porous rock with dominant fractures

Coupled hydro-mechanical (HM) processes are significant in geological engineering such as oil and gas extraction, geothermal energy, nuclear waste disposal and for the safety assessment of dam foundations and rock slopes, where the geological media usually consist of fractured rock masses. In this study, we developed a model for the analysis of coupled hydro-mechanical processes in porous rock containing dominant fractures, by using the numerical manifold method (NMM). In the current model, the fractures are regarded as different material domains from surrounding rock, i.e., finite-thickness fracture zones as porous media. Compared with the rock matrix, these fractured porous media are characterized with nonlinear behavior of hydraulic and mechanical properties, involving not only direct (poroelastic) coupling but also indirect (property change) coupling. By combining the potential energy associated with mechanical responses, fluid flow and solid–fluid interactions, a new formulation for direct HM coupling in porous media is established. For indirect coupling associated with fracture opening/closure, we developed a new approach implicitly considering the nonlinear properties by directly assembling the corresponding strain energy. Compared with traditional methods with approximation of the nonlinear constitutive equations, this new formulation achieves a more accurate representation of the nonlinear behavior. We implemented the new model for coupled HM analysis in NMM, which has fixed mathematical grid and accurate integration, and developed a new computer code. We tested the code for direct coupling on two classical poroelastic problems with coarse mesh and compared the results with the analytical solutions, achieving excellent agreement, respectively. Finally, we tested for indirect coupling on models with a single dominant fracture and obtained reasonable results. The current poroelastic NNM model with a continuous finite-thickness fracture zone will be further developed considering thin fractures in a discontinuous approach for a comprehensive model for HM analysis in fractured porous rock masses.     

Effects of coupled hydro-mechanical model considering dual-phase fluid flow on potential for shallow landslides at a regional scale

Natural Hazards - A total of 36 rainfall-induced shallow landslides occurred on July 26 and 27, 2011, on Halmidang Mountain, Gyeonggi Province, South Korea. To precisely analyze these shallow...