非饱和地震滑坡堆积体降雨破坏水-力耦合行为试验

降雨滑坡的水-力耦合过程和机制研究是开展准确预测预报的基础和前提。选取典型滑坡模型,采用足尺人工降雨模型试验模拟了典型砾石土滑坡堆积体破坏过程,并采用Brooks-Corey(BC)和Van Genuchten(VG)模型分别建立了降雨入渗吸湿过程的土-水特征曲线,再通过一维非饱和无限边坡稳定性分析模型对滑坡开展应力状态和稳定性分析,揭示了斜坡破坏过程是优先流和基质流的双渗流场共同作用的结果,而VG模型适用于吸湿条件下宽级配砾石土优先流的土-水特征曲线的重构;稳定性计算和试验过程的对比分析表明,试验中斜坡的变形破坏过程中观测到的含水率、基质吸力和地表倾斜角度的变化过程与计算的吸力应力、稳定系数的演进过程有显著的对应关系。通过试验还揭示了非饱和滑坡堆积体渗流潜蚀现象,定量分析了细颗粒迁移程度。基于物理模型试验,揭示了降雨激发条件下滑坡堆积体形成优先流作用的土-水特征曲线和变形破坏的水力耦合过程,可为地震滑坡堆积体非饱和失稳的预测预警提供参考和基础。     

An advanced hydro-mechanical constitutive model for unsaturated soils with different initial densities

This paper presents an advanced constitutive model for unsaturated soils, using Bishop’s effective stress (σ′) and the effective degree of saturation (S_e) as two fundamental constitutive variables in the proposed constitutive model. A sub-loading surface and a unified hardening parameter (H) are introduced into the σ′–S_e modelling framework to interpret the effects of initial density on coupled hydro-mechanical behaviour of compacted soils. Compared with existing models in the literature, the main advantage of the proposed model that it is capable of modelling hydro-mechanical behaviour of unsaturated soils compacted to different initial densities, such as the dependence of loading–collapse volume on initial void ratio and density effect on the shearing-induced saturation change. The proposed model requires 13 material parameters, all of which can be calibrated through conventional laboratory tests. Numerical studies are conducted to assess the performance of the model for a hypothetical soil under two typical hydro-mechanical loading scenarios. The proposed advanced unsaturated soil model is then validated against a number of experimental results for both isotropic and triaxial conditions reported in the literature.     

Modelling the effect of temperature on unsaturated soil behaviour

A simple thermohydromechanical (THM) constitutive model for unsaturated soils is described. The effective stress concept is extended to unsaturated soils with the introduction of a capillary stress. This capillary stress is based on a microstructural model and calculated from attraction forces due to water menisci. The effect of desaturation and the thermal softening phenomenon are modelled with a minimal number of material parameters and based on existing models. THM process is qualitatively and quantitatively modelled by using experimental data and previous work to show the application of the model, including a drying path under mechanical stress with transition between saturated and unsaturated states, a heating path under constant suction and a deviatoric path with imposed suction and temperature. The results show that the present model can simulate the THM behaviour in unsaturated soils in a satisfactory way.     

Model suggested for an important part of the hydro-mechanical behaviour of a water unsaturated bentonite

The hydro-mechanical behaviour of a clay-based buffer material for nuclear waste disposal has been investigated in a laboratory program. In this program, the main focus was on the influence of confinement on water uptake and swelling pressure during suction decrease. The laboratory program and some of the results are presented by Dueck [Dueck, A., 2006. Laboratory results from hydro-mechanical tests on a water unsaturated bentonite. submitted for publication.].

The results from the laboratory tests were used to find a relationship between water content, void ratio, swelling pressure and suction. Two equations for swelling pressure represent the outline of the model.

In the first equation, the swelling pressure developed during water uptake is normalised by a pressure corresponding to the swelling pressure at saturation. This is done in order to be independent of void ratio. A relationship between the normalised swelling pressure and the degree of saturation is suggested.

The second equation describes a relationship between the swelling pressure, the water content and the actual suction (or relative humidity). The equation is based on a thermodynamic relationship and includes the retention curve (i.e. water content vs. suction under free swelling conditions).

The model can be used for a state where two of the four variables; water content, void ratio, swelling pressure and suction are known and can thus be useful to evaluate field measurements and model late stages of the wetting process. An example of an application is given. The equations are mainly based on results from tests with increasing degrees of saturation under constant void ratio but are also suggested for use with increasing void ratio.     

Incorporating hydro-mechanical coupling in an analysis of the effects of rainfall patterns on unsaturated soil slope stability

Rainfall-induced landslides can cause loss of life and damage to property, infrastructure, and the environment. Rainfall patterns affect the pore-water pressure of unsaturated soil slopes, and are related to the slopes’ stability. Four rainfall patterns were chosen to represent natural rainfall patterns for an examination of rainfall infiltration into soil slopes using numerical models incorporating coupled water infiltration and deformation in unsaturated soils. Our analysis showed that rainfall patterns play a significant role in the distribution of the pore-water pressure in soil slopes, and influence the slope stability. The pore-water pressure profile of soil slopes and the factor of safety are affected by the ratio of rainfall intensity and the coefficient of permeability. The depth and shape of the shallow sliding plane of the landslide is closely related to the rainfall pattern; moreover, the results showed a correlation between the factor of safety of the slope and the rainfall intensity. This relationship can be described by a dimensionless rainfall intensity. The nonlinear relationship can be used to estimate the slope stability resulting from rainfall infiltration when the hydro-mechanical coupling in unsaturated soil slopes is considered.     

Stress-strain behaviour of reconstituted illitic clay at different temperatures

Tests on specimens of reconstituted illitic clay have examined the influence of temperature on the mechanical behaviour of clay soils. The program involved consolidation to effective confining pressures up to 1.5 MPa, heating to 100°C, and tests on normally consolidated and overconsolidated specimens with OCR = 2. The tests included isotropic consolidation, undrained triaxial compression with pore water pressure measurement, drained tests along controlled stress paths to investigate yielding behaviour, and undrained tests which involved heating and measurement of the resulting induced pore water pressures. The large strain strength envelope is independent of temperature. However, peak undrained strengths increase with temperature because smaller pore water pressures are generated during shearing. An important contribution from the study is a series of results for the yielding of illitic clay at three different temperatures. For the first time, there is clear evidence of yield loci decreasing in size with increasing temperature. An associated flow rule can be assumed without serious error. The results contribute to the confirmation of a thermal elastic-plastic soil model developed by the authors from cam clay following the addition of a small number of extra assumptions. Depending on the initial stress state, heating under undrained conditions may produce shear failure.     

Review of fundamental principles in modelling unsaturated soil behaviour

An unsaturated soil is a state of the soil. All soils can be partially saturated with water. Therefore, constitutive models for soils should ideally represent the soil behaviour over entire ranges of possible pore pressure and stress values and allow arbitrary stress and hydraulic paths within these ranges. The last two decades or so have seen significant advances in modelling unsaturated soil behaviour. This paper presents a review of constitutive models for unsaturated soils. In particular, it focuses on the fundamental principles that govern the volume change, shear strength, yield stress, water retention and hydro-mechanical coupling. Alternative forms of these principles are critically examined in terms of their predictive capacity for experimental data, the consistency between these principles and the continuity between saturated and unsaturated states.     

非饱和土一维大变形固结模型

基于分段线性差分法,建立了一种非饱和土一维大变形固结模型。该模型可考虑土性参数非线性变化,可计算与分析大变形问题,并编制了Fortran计算程序。在现有解答和试验数据的基础上,对该模型进行了验证,瞬时加载情况下模型数值解与现有解答基本吻合,考虑加载过程下的数值解与试验数据吻合。进行了大变形算例分析,对比了加荷压密与消散固结阶段土层变形,探讨了孔隙气、水渗透系数比对土层沉降量、饱和度和不同应变情况下固结度的影响规律,分析了非饱和土大、小变形固结理论计算孔隙水（气）压和沉降量的差异。     

非饱和土拟二维平面应变固结问题的解析计算方法

基于Fredlund非饱和土一维固结理论,建立了二维平面应变条件下的固结方程组,并得到了单层非饱和土平面应变条件下的解析解。基于相关理论,假设体变系数和渗透系数都为常量,同时考虑到瞬时加载条件下,沿着土体深度方向上产生均匀或者线性分布的初始超孔隙压力,建立了二阶二元偏微分方程组。求解时,引入函数方法来降低方程的阶数,然后通过分离变量法获得方程的通解。在此基础上,结合一个针对单面排水条件下二维平面应变问题案例,通过与数值解对比,验证了所提方法的正确性。并采用所提方法计算获得了二维平面下超孔隙水压力、气压力沿垂直和水平方向消散的等时线,通过计算对比,分析了不同线性分布情况下,初始超孔隙压力对固结消散过程的影响。研究结果表明:初始超孔隙压力的不同分布对超孔隙气压力消散的影响几乎可以忽略,而对超孔隙水压力消散的影响更大。     

Large strain static and dynamic semisaturated soil behaviour

The theoretical and computational aspects of large strain static and dynamic partially saturated soil analysis are presented. The soil is assumed to be a three-phase material with a solid phase which may yield according to a generalized plasticity theory, whose liquid phase flows according to Darcy's law and where gaseous phase remains equal to the external ambient pressure. Permeability varies not only as a function of the degree of water saturation, but also as a function of the void ratio. These aspects are shown on several examples.     

基于平面应变试验的非饱和膨胀土强度分析

介绍了非饱和土强度理论的研究现状,总结了FREDLUND提出的吸力内摩擦角 的性质与规律研究,根据FREDLUND非饱和土强度理论和吸力内摩擦角 的计算方法,利用原状膨胀土的平面应变等应力比卸荷试验,分析得到土样破坏时应力状态下得到的吸力内摩擦角 > ,与FREDLUND提出的吸力内摩擦角 的变化规律不一致,分析认为,是由于卸荷试验过程导致微裂隙的张开,引起基质吸力的降低,有待于继续深入研究。     

干密度对非饱和膨胀土水分迁移参数的影响

研究了非饱和膨胀土的两个水分迁移参数--水体积变化系数 和渗透系数kw,得出非饱和膨胀土的水分迁移参数 和kw并非恒值,是基质吸力的函数,而基质吸力与含水率密切相关。因此,其参数大小随含水率变化而变化。此外,还明显受到干密度的影响,随干密度的不同而变化。取得了考虑干密度和含水率的水体积变化系数 、渗透系数kw和基质吸力之间的非线性关系,为进一步研究非饱和膨胀土地基水分场变化提供基础资料。     

土层结构对非饱和毛细水盐运移的影响

层状土层之间孔隙结构和水力学性质的不连续性对土体水盐运移有显著影响。基于现场调查,设计了两种不同粒径土层二元结构组合（黄土-砂质粉土和黄土-粉质黏土）的室内土柱试验,通过模型试验讨论了不同地下水补给条件下土层结构对毛细水分布和盐分累积的影响。试验结果表明：在毛细水补给条件下,黄土-砂质粉土构成的上细下粗型土层结构有利于毛细水盐运移,经过60 d蒸发后,其表层${\rm{SO}}_{\rm{4}}^{{\rm{2 - }}}$含量是上粗下细型土柱的两倍。在无毛细水补给条件下,砂质粉土层中水盐向上迁移总量和迁移速率大于粉质黏土层,最终上细下粗型土柱中上覆土层各层位离子含量均大于上粗下细型土柱。研究结果为层状土区盐渍土病害的防治提供了试验参考。     

A unified mechanical and retention model for saturated and unsaturated soil behaviour

The coupled mechanical and water retention elasto-plastic constitutive model of Wheeler, Sharma and Buisson (the Glasgow coupled model, GCM) predicts unique unsaturated isotropic normal compression and unsaturated critical state planar surfaces for specific volume and degree of saturation when soil states are at the intersection of mechanical (M) and wetting retention (WR) yield surfaces. Experimental results from tests performed by Sivakumar on unsaturated samples of compacted speswhite kaolin confirm the existence and form of these unique surfaces. The GCM provides consistent representation of transitions between saturated and unsaturated conditions, including the influence of retention hysteresis and the effect of plastic volumetric strains on retention behaviour, and it gives unique expressions to predict saturation and de-saturation conditions (air-exclusion and air-entry points, respectively). Mechanical behaviour is modelled consistently across these transitions, including appropriate variation of mechanical yield stress under both saturated and unsaturated conditions. The expressions defining the unsaturated isotropic normal compression planar surfaces for specific volume and degree of saturation are central to the development of a relatively straightforward methodology for determining values of all GCM parameters (soil constants and initial state) from a limited number of laboratory tests. This methodology is demonstrated by application to the experimental data of Sivakumar. Comparison of model simulations with experimental results for the full set of Sivakumar’s isotropic loading stages demonstrates that the model is able to predict accurately the variation of both specific volume and degree of saturation during isotropic stress paths under saturated and unsaturated conditions.     

非饱和土水-力本构模型及其隐式积分算法

在已有工作基础上建立了水力-力学耦合的非饱和土本构模型,在硬化方程中考虑饱和度的影响,同时在土水特征曲线中考虑了塑性体变的影响,从而使模型可以反映非饱和土中的毛细现象与土中弹塑性变形现象的耦合行为。采用隐式积分方法,建立了非饱和土耦合模型的数值模型,并推导了得到了水力-力学耦合的非饱和土的一致切线模量。利用该算法编制了本构模型计算的子程序,使其能向外输出切线刚度矩阵,用于有限元计算。为了验证该算法和程序的正确性,用所编制程序对不同路径下的土体行为进行了预测。通过预测结果与试验结果相对比,表明程序预测结果与试验数据相吻合,模型可以较好地模拟土体的水力-力学耦合行为特性。     

Fully coupled elastoplastic hydro-mechanical analysis of unsaturated porous media using a meshfree method

This paper presents the first application of an advanced meshfree method, ie, the edge-based smoothed point interpolation method (ESPIM), in simulation of the coupled hydro-mechanical behaviour of unsaturated porous media. In the proposed technique, the problem domain is spatially discretised using a triangular background mesh, and the polynomial point interpolation method combined with a simple node selection scheme is adopted for creating nodal shape functions. Smoothing domains are formed on top of the background mesh, and a constant smoothed strain, created by applying the smoothing operation over the smoothing domains, is assigned to each smoothing domain. The deformation and flow models are developed based on the equilibrium equation of the mixture, and linear momentum and mass balance equations of the fluid phases, respectively. The effective stress approach is followed to account for the coupling between the flow and deformation models. Further coupling among the phases is captured through a hysteretic soil water retention model that evolves with changes in void ratio. An advanced elastoplastic constitutive model within the context of the bounding surface plasticity theory is employed for predicting the nonlinear behaviour of soil skeleton. Time discretisation is performed by adopting a three-point discretisation method with growing time steps to avoid temporal instabilities. A modified Newton-Raphson framework is designed for dealing with nonlinearities of the discretised system of equations. The performance of the numerical model is examined through a number of numerical examples. The state-of-the-art computational scheme developed is useful for simulation of geotechnical engineering problems involving unsaturated soils.     

Investigation of the hydro-mechanical behaviour of GMZ bentonite pellet mixtures

Bentonite pellet mixtures are considered as one of the candidate sealing materials for deep geological disposals of radioactive waste. One of the particularities of this material is the initial heterogeneous distribution of pellets and porosity within the mixture, leading to complex hydro-mechanical behaviour. In this paper, the hydro-mechanical properties of GMZ bentonite pellet mixtures were investigated in the laboratory by carrying out water retention tests on pellet mixtures under constant-volume condition and single pellets under free swelling condition, as well as a infiltration test on a column specimen of pellet mixture. In the infiltration test, the relative humidity and radial swelling pressure were monitored at five heights, the axial swelling pressure was also recorded. The instantaneous profile method was applied to determine the unsaturated hydraulic conductivities. Results show that, in high suction range (>?10 MPa) the water retention curve of pellet mixture under constant-volume condition was comparable to that of a single pellet under free swelling condition, while in low suction range (<?10 MPa) the latter exhibits a much higher water retention capacity. Due to clogging of large pores, the unsaturated hydraulic conductivity decreases as suction decreases to around 25 MPa. However, with further suction decrease, the hydraulic conductivity increases continuously until the value at saturated state, as in the case of most unsaturated soils. The radial swelling pressure at different heights develops with local sudden increase and decrease, which was attributed to local rearrangement of pellets upon wetting. By contrast, as the axial swelling pressure was measured on the global surface of the specimen, it develops in a more regular fashion.

     

不同应力路径下非饱和黄土的结构变化特性

以陕西杨陵非饱和黄土为研究对象,对不同初始含水率的原状黄土及相应的重塑黄土进行不同应力比下的等应力比加载试验,求取非饱和黄土在不同应力状态下的综合结构势参数,研究了黄土在等应力比压缩试验状态下的结构变化特性。结果表明,当初始含水率不变时,应力比(?=?3/?1)对黄土的结构性有较大的影响,表现为应力比愈小,黄土的结构性愈弱;当应力状态不变时,含水率对结构性参数影响明显。含水率愈低,同一应力状态下对应的结构性参数愈大,尤其在低含水率区域,含水率变化强烈影响原状黄土的结构性。