A density‐dependent elastoplastic hydro‐mechanical model for unsaturated compacted soils

This paper presents a three‐dimensional elastoplastic constitutive model for predicting the hydraulic and mechanical behaviour of unsaturated soils. It is based on experimental results obtained from a series of controlled‐suction triaxial tests on unsaturated compacted clay with different initial densities. Hydraulic hysteresis in the water‐retention behaviour is modelled as an elastoplastic process, with the elastic part modelled by a series of scanning curves and the elastoplastic part modelled by the main drying and wetting curves. The effect of void ratio on the water‐retention behaviour is studied using data obtained from controlled‐suction wetting–drying cyclic tests on unsaturated compacted clay with different initial densities. The effect of the degree of saturation on the stress–strain‐strength behaviour and the effect of void ratio on the water‐retention behaviour are considered in the model, as is the effect of suction on the hydraulic and mechanical behaviour. The initial density dependency of the compacted soil behaviour is modelled by experimental relationships between the initial density and the corresponding yield stress and, thereby, between the initial density and the normal compression line. The model is generalized to three‐dimensional stress states by assuming that the shapes of the failure and yield surfaces in the deviatoric stress plane are given by the Matsuoka–Nakai criterion. Model predictions of the stress–strain and water‐retention behaviour are compared with those obtained from triaxial tests with different initial densities under isotropic compression, triaxial compression and triaxial extension, with or without variation in suction. The comparisons indicate that the model accurately predicts the hydraulic and mechanical behaviour of unsaturated compacted soils with different initial densities using the same material constant. Copyright © 2006 John Wiley & Sons, Ltd.     

非饱和上海软土水力和力学特性耦合弹塑性模拟

目前大多数非饱和土的弹塑性本构模型用非饱和击实土的试验结果进行验证,但现场其他类型的土,如沉积土经常有在非饱和状态下外部环境变化的情况。现有的非饱和土弹塑性模型是否适用于沉积土一类的现场土是需要研究的课题。进行非饱和上海第③层土的吸力控制排水排气三轴剪切试验,使用文中提出的能统一考虑非饱和土水力性状和力学性状的弹塑性本构模型,预测上述三轴试验结果,并与试验数据进行比较。比较结果显示,建立的本构模型能够很好地预测非饱和上海软土的水力和力学性质,说明该模型不仅可以适用击实土的预测,还能够很好地适用于其他类型非饱和土的水力和力学性质的模拟。     

不饱和土动弹塑性本构模型研究

以饱和度与有效应力为状态变量,通过引入描述不饱和与饱和土孔隙比差的状态变量,将Zhang等提出的饱和土体应力诱导各向异性动弹塑性本构模型推广到不饱和土体中,使其可描述不饱和土在动力循环荷载作用下的力学特性行为。通过对已有不饱和土体在完全不排水条件下的动三轴试验进行理论模拟,验证了所提出不饱和土本构模型的正确性。最后基于所提出本构模型,讨论了在不排水条件下初始饱和度对不饱和土动力特性研究。结果表明,不饱和土在动力荷载作用下,土体的孔隙比将减少,导致饱和度增加;当初始饱和度较高时,不饱和土会转化为饱和土,从而发生液化现象。该研究成果对研究不饱和土在地震等动力荷载作用下的力学特性行为具有重要意义。     

非饱和土的新非线性模型及其应用

为描述非饱和土的应力-应变特性,基于非饱和土三轴剪切试验,提出泊松比变化率（即切线泊松比随轴向应变的变化速率）的概念,发现非饱和土的切线模量和泊松比变化率均随轴向应变的增加呈指数衰减规律,基于此提出了一种描述非饱和土应力-应变关系的新非线性模型。该模型不仅能描述应变硬化,而且能描述应变软化;能对非饱和土三轴不固结不排水剪、固结排水剪、固结不排水剪试验的应力-应变关系进行描述;模型共包含6个参数,物理意义明确,确定方法简便。利用提出的模型对国内外文献中的三轴试验进行了模拟,结果表明,模拟结果与试验数据有很好的吻合度,从而验证了模型对试验数据的合理性和适用性。     

Three‐dimensional elasto‐plastic model for unsaturated compacted soils with different initial densities

This paper presents an elasto‐plastic model for unsaturated compacted soils and experimental results obtained from a series of suction‐controlled triaxial tests on unsaturated compacted clay with different initial densities. The initial density dependency of the compacted soil behaviour is modelled by establishing experimental relationships between the initial density and the corresponding yield stress and thereby between the initial density and the location and slope of normal compression line. The model is generalized to three‐dimensional stress states by assuming that the shapes of the failure surface and the yield surface in the deviatoric plane are given by the extended SMP criterion. A considerable number of the isotropic compression, triaxial compression and extension tests on unsaturated compacted clay with different initial densities were performed using a suction‐controllable triaxial apparatus, to measure the stress–strain–volume change in different stress paths and wetting paths. The model has well‐predicting capabilities to reproduce the mechanical behaviour of specimens compacted under different conditions not only in isotropic compression but also in triaxial compression and triaxial extension. Copyright © 2003 John Wiley & Sons, Ltd.     

非饱和土弹塑性模型参数的试验确定及有限元法

简要介绍广泛应用的非饱和土Alonso模型的原理与发展,并通过3组共7个试样的非饱和土三轴试验,得到模型的11个参数。在饱和土Cam本构关系的基础上,推导出一个基于非饱和土Alonso模型的应力-应变增量方程的计算公式,该方程与饱和土的本构方程形式相同。编制了能够考虑净应力和吸力二者对土体硬化规律影响的有限元程序,从而为非饱和土弹塑性计算提供一条途径,便于进一步应用于实际工程。     

Limitations of suction‐controlled triaxial tests in the characterization of unsaturated soils

Behavior of unsaturated soils is influenced by many factors, and the influences of these factors are usually coupled together. Suction‐controlled triaxial (SCTX) tests are considered to allow researchers to investigate influences of individual variables on unsaturated soils under specified stress path with controls of stresses, pore water, and air pressures. In the past 50 years, SCTX testing method has been established as a standard approach to characterize constitutive behavior of unsaturated soils. Most important concepts for modern unsaturated soil mechanics were developed upon results from the SCTX tests. Among these, one of the most important contributions in the constitutive modeling of elasto‐plastic behavior for unsaturated soils is the Barcelona basic model (BBM) proposed by Alonso et al. in 1990. The BBM successfully explained many features of unsaturated soils and received extensive acceptance. However, the SCTX tests are designed based upon the divide‐and‐conquer approach in which an implicit assumption is used: soil behavior is stress‐path independent. However, it is well‐established that unsaturated soil behavior is elasto‐plastic and stress‐path dependent. It is found that the SCTX tests in fact cannot control the stress path of an unsaturated soil during loading. This incapability, in combination with complicated loading/collapse behavior of unsaturated soils, makes the SCTX tests for characterizing unsaturated soil questionable. This paper discusses the limitations of the SCTX tests in the characterization of unsaturated soils. A possible solution to the problem was proposed based on a newly developed modified state surface approach. The discussions are limited for isotropic conditions. Copyright © 2015 John Wiley & Sons, Ltd.     

非饱和土毛细滞回与变形耦合弹塑性本构模型

在修正剑桥模型的基础上,提出了一个非饱和土毛细滞回与骨架变形耦合的弹塑性本构模型。该模型考虑了基质吸力与饱和度对屈服应力的影响,可以同时描述非饱和土的弹塑性变形特性与毛细循环滞回效应。根据塑性体变的产生使非饱和土进气值增大的特点,建立了变形对土-水特征曲线影响的数学描述。该模型有效地考虑了饱和度对前期屈服应力的作用,准确地反映了土体在不同土-水状态条件下（脱湿和吸湿过程）强度特性的变化,而且还可以有效地描述水力循环历史对土体变形的影响。通过与试验数据对比,证明了该模型能够模拟非饱和土的主要力学特性。     

非饱和土三轴压缩应力路径试验的数值模拟研究

应力路径对土的强度和变形性质具有重要影响。相对于饱和土而言,控制吸力条件下的非饱和土三轴压缩状态的应力路径研究更加复杂。随着非饱和土本构理论的不断发展,理论和试验研究结果表明,非饱和土弹塑性本构模型可以用来近似地描述非饱和土的强度和变形性质。因而,运用非饱和土弹塑性本构模型对控制吸力条件下的3种非饱和土三轴压缩应力路径试验进行数值模拟是一种有效的理论研究手段。采用Barcelona模型能够对此类试验进行较好的数值模拟,其研究结果表明,在控制吸力条件的三轴压缩状态下应力路径对非饱和土的强度和变形性质具有重要影响。     

A bounding surface plasticity model for unsaturated soil at small strains

Most existing hydromechanical models for unsaturated soils are not able to fully capture the nonlinearity of stress–strain curves at small strains (less than 1%). They cannot therefore, for example, accurately predict ground movements and the performance of many earth structures under working conditions. To tackle this problem, a state‐dependent bounding surface plasticity model has been newly developed. Particularly, the degradation of shear modulus with strain at small strains ranging from 0.001% to 1% is focused. The proposed model is formulated in terms of mean average skeleton stress, deviator stress, suction, specific volume and degree of saturation. Void ratio‐dependent hydraulic hysteresis is coupled with the stress–strain behaviour. Different from other elastoplastic models for unsaturated soils, plastic strains are allowed inside bounding surfaces. In this paper, details of model formulations and calibration procedures of model parameters are presented. To evaluate the capability of the new model, it is applied to simulate a series of triaxial compression tests on compacted unsaturated silt at various suctions. Effects of suction, drying and wetting as well as net stress on unsaturated soil behaviour are well captured. The model shows good predictions of the degradation of shear modulus with strain over a wide range of strains from 0.001% to 1%. Copyright © 2015 John Wiley & Sons, Ltd.     

土的基本力学特性及其弹塑性描述

Cam-clay模型是在等向加载试验基础上建立起来的且能够描述正常固结土在常规三轴试验条件下应力-应变关系的最简单弹塑性模型,是建立各种土的弹塑性本构模型的基础。文中分别按加载方式的不同和土组构的不同,对描述与应力状态、应力历史、应力路径、持荷时间等因素有关的加载方式下土应力应变特性的弹塑性本构模型进行了归纳。对反映不同组构所形成的结构性、各向异性和颗粒破碎等特性的弹塑性本构模型进行了综述,并着重介绍了描述应力状态的三维化方法、反映应力历史影响的超固结土模型和模拟渐近状态路径的渐近状态模型等典型研究成果。     

A bounding surface model for unsaturated soils considering the microscopic pore structure and interparticle bonding effect due to water menisci

The interparticle bonding effect due to water menisci plays an important role in the hydromechanical coupling properties of unsaturated soils. This paper presents an unsaturated hydromechanical coupling model that considers the influence of matric suction, degree of saturation, and microscopic pore structure on the interparticle bonding effect. The enhanced effective stress and bonding variable are selected as constitutive variables. The bonding variable is correlated with the ratio between unsaturated void ratio and saturated void ratio. The deformation characteristics of unsaturated soils are described based on the bounding surface plasticity theory. A soil–water characteristic model that considers deformation and hydraulic hysteresis is integrated into the constitutive model to achieve hydromechanical coupling. The proposed model can effectively describe the hydromechanical coupling characteristics and the meniscus bonding force of unsaturated bimodal structure soils; the model parameters can be easily obtained through routine experiments. The experimental results of unsaturated isotropic compression, the wetting/drying cycle, and unsaturated triaxial shear tests are used to validate the capability of the proposed model.

     

Consistent Modelling of Expansive and Collapsive Response of Unsaturated Soils

In unsaturated soil modelling, collapse is viewed as an ordinary response, whereas swelling upon an increase of moisture is considered as a special class of problems. However, an investigation based on fundamentals of plasticity indicates that the two seemingly opposite responses are unified within the same ordinary elastoplasticity framework. In fact, similar to the well-known contractive and dilative responses in the critical state framework, the collapsive and expansive responses of unsaturated soils differ only in the sign of the dissipative hydrostatic stress. For a soil with a low applied net stress, the dissipative mean normal stress during wetting is likely to be negative, and the plastic volumetric strain is kept negative (swelling) as well, such that the dissipation is always positive, complying with the second law of thermodynamics. When the applied net stress is high, the dissipative mean normal stress may turn to be positive, resulting in a positive plastic volumetric strain (collapse). This paper explains the concept of this ‘ordinary’ modelling framework, and uses a bounding surface triaxial model to demonstrate the concept. The numerical results are compared with experimental observations reported in the literature.     

荆门非饱和膨胀土的变形与强度特性试验研究

通过压力板试验,对比分析了荆门原状膨胀土与石灰改良土持水特征的差异性;应用非饱和土三轴仪,开展了原状膨胀土、石灰改良膨胀土与重塑膨胀土的变形及强度特性试验。结果表明：①石灰改良膨胀土与原状膨胀土相比,其进气值有显著降低,残余含水率显著升高,土-水特征曲线两个特征点的斜率较为平缓,说明其水稳定性较好,土体性状更为稳定;②原状膨胀土和石灰改良膨胀土在非饱和与饱和状态的应力-应变关系曲线均呈应变软化型,随净围压的增大,应变软化的程度趋缓;随含水率的减小,峰值应力增大,应力峰值点有所提前,应变软化现象更加显著。饱和重塑膨胀土的应力-应变关系呈应变强化型,非饱和重塑膨胀土则呈现为应变软化型,但其应变软化的程度较前两类土大为趋缓;③经石灰改性后,膨胀土强度参数值有大幅度提高,即使在湿化饱和后,石灰改良膨胀土仍保持了相对稳定的力学性质和较高的抗剪强度参数值。相比之下,无论是重塑膨胀土,还是原状膨胀土,对湿化作用均十分敏感,其强度参数值或波动较大,或整体水平较低。     

饱和度对非饱和土力学性质的影响

现在被广泛公认的由Fredlund提出的非饱和土力学的双参数理论,即净应力和吸力为非饱和土的应力状态量,不能直接考虑饱和度或含水率对非饱和土的应力-应变关系和强度的影响。在非饱和土三轴试验结果表明,即使在净应力和吸力路径相同的条件下,具有不同饱和度试样的应力-应变关系和强度也是不同的。其他条件相同时,试样饱和度越高,其应力比-应变关系曲线越高,强度越大。最新的水力-力学特性耦合的弹塑性本构模型可以定量地表示上述非饱和土的性质     

Validity Criteria of Oedometric and Triaxial Test Results

Oedometric and triaxial tests are very much used in soil mechanics. However, in spite of their frequency in the world, these tests present some limitations related to remolding due to sampling process and to implemented experimental procedures. These limitations contribute to decreasing the resistance and deformability mechanical properties of soils, which condition their limit and critical states behavior. This paper proposes four validity criteria of oedometric and triaxial test results (compressibility criterion based on the oedometric test results; consolidation, resistance and rigidity criteria based on the triaxial test results) to appreciate the disturbance of test specimens. Impact of the sample disturbance on the yield envelope of clayey soils according to these validity criteria is then analyzed.     

红砂岩风化土湿化特性的三轴试验研究

针对红砂岩风化土遇水易湿化、崩解,从而影响路基填筑体稳定性的问题,选取某高速公路红砂岩风化料,采用单线法,在两种密度条件下进行了三轴湿化变形试验,分析了密度、围压和应力水平对湿化轴向应变的影响规律,并对湿化后试样的后续剪切强度进行了研究。试验结果表明:随着围压的增大和应力水平的提高,红砂岩风化料的湿化轴向变形有明显提高;而随着密实度的增加,湿化轴向变形有所减小。同一围压下,湿化轴向应变随着应力水平的增长而增大,两者近似呈线性关系。湿化后试样的峰值强度随应力水平的增加略有降低,而且普遍低于饱和状态试样的峰值强度。     

A hypoplastic model for mechanical response of unsaturated soils

A new constitutive model is developed for the mechanical behaviour of unsaturated soils based on the theory of hypoplasticity and the effective stress principle. The governing constitutive relations are presented and their application is demonstrated using several experimental data from the literature. Attention is given to the stiffening effect of suction on the mechanical response of unsaturated soils and the phenomenon of wetting‐induced collapse. All model parameters have direct physical interpretation, procedures for their quantification from test data are highlighted. Quantitative predictions of the model are presented for wetting, drying and constant suction tests. Copyright © 2008 John Wiley & Sons, Ltd.     

基于状态曲面的非饱和土强度准则及其验证

考虑体变对非饱和土土-水状态的影响,将状态曲面函数引入传统的Vanapalli强度公式得到与孔隙比相关的抗剪强度准则,新准则使用饱和土的强度参数和两条不同孔隙比对应的土-水特征曲线。选择一种尾矿砂和高岭土的混合土料为研究对象,进行一系列的土-水特征曲线试验、吸力控制的等向压缩和三轴剪切试验。试验结果表明,新准则能更准确地预测非饱和土的强度,证明了传统强度预测的误差主要来源于忽略了体变导致的土-水状态变化,并提出在不同应力空间内精确地获得抗剪强度包线的方法,合理地解释了强度包线斜率在净应力-强度平面内随吸力增大、强度包线形状在吸力-强度平面内随净应力发生变化的特性。