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1.
Expansive clays are widely prevalent all over the world as one of the most problematic soils. These soils undergo significant volume change with a change in the moisture regime, thereby posing problems to the stability of the structures founded on such strata. Efforts have been made to model the erratic behaviour of these soils at the macro, micro, and, to a lesser extent, nano levels. Micro and nano level fabrics, believed to have a central role in the overall behaviour of expansive clays, are only partially considered in the modelling concepts; natural clay fabrics with multiple clay minerals, silt and sand inclusions, micro fissures, cementation, overconsolidation, induration and other such features have never been considered. This paper covers a review of deficiencies in the existing constitutive models for the expansive characteristics of the natural clayey soils at macro, micro and nano levels. These shortcomings are discussed in the light of the understanding of the fundamentals including fabric and structure controlling the swelling mechanism of the expansive clayey soils at the molecular level. Finally, a framework based on authors’ work to incorporate molecular level behaviour in the constitutive modelling of expansive clays is presented. 相似文献
3.
This paper proposes a semi-analytical solution of undrained cylindrical cavity expansion in anisotropic soils with both isotropic and frictional destructuration. The rigorous derivation based on the general form of the SANICLAY model with destructuration is provided following a standardized solving procedure, and the features of anisotropy and structuration are then invoked in the cavity expansion solution by adopting the non-associated hierarchical model. Cavity expansion tests in both structured and unstructured clays with various overconsolidation ratio are conducted to investigate the evolutions of effective stresses, excess pore pressure, anisotropic parameters and structuration factors during cylindrical expansion. The results show that the effective stresses at the cavity wall are lower after expansion and the cavity excess pore pressure is oppositely higher in structured clays with slightly smaller plastic regions. The evolutions of anisotropy for structured clays appear to follow similar patterns to unstructured cases, whereas the degree of anisotropy is further developed with gradual loss of inter-particle bonds. Finally, the proposed solution is applied to predict the limit pressure of pressuremeter tests in Bothkennar clay, showing its ability for interpretation of in situ testing data in natural structured clays. 相似文献
4.
The results of a series of laboratory tests on unimproved and cement-improved specimens of two clays are presented, and the ability of a bounding surface elastoplastic constitutive model to predict the observed behavior is investigated. The results of the oedometer, triaxial compression, extension, and cyclic shear tests demonstrated that the unimproved soil behavior is similar to that of soft clays. Cement-improved specimens exhibited peak/residual behavior and dilation, as well as higher strength and stiffness over unimproved samples in triaxial compression. Two methods of accounting for the artificial overconsolidation effect created by cement improvement are detailed. The apparent preconsolidation pressure method is considerably easier to use, but the fitted OCR method gave better results over varied levels of confining stresses. While the bounding surface model predicted the monotonic behavior of unimproved soil very well, the predictions made for cyclic behavior and for improved soils were only of limited success. 相似文献
5.
On the basis of a double hardening model for clays and available experimental results, a new thermo-elasto-plastic constitutive
model for saturated clays is proposed to describe the effects of temperature and overconsolidation ratio on the mechanical
properties of saturated clays. Two hardening parameters are introduced: s c￠ {\sigma}_{{\rm c}}^{\prime} and α. The proposed model is then applied to simulate the relevant important features of saturated clays with different overconsolidation
ratios under different temperature and loading conditions. The model predictions are compared with available experimental
results to demonstrate its accuracy and usefulness. 相似文献
6.
天然沉积土在沉积过程中产生了结构性和各向异性,使其受力变形特性与重塑土存在明显的差异。实际工程中的天然土体往往在受荷过程中会经历不同的应力路径,因此,需要开展考虑结构性和各向异性影响的应力路径试验。通过研究不同应力路径下土体的力学特性,为建立复杂应力路径下的合理的本构模型提供试验依据。采用大直径PVC管取样器获取张家港地区地下2.5 m深的粉质黏土不扰动土样,通过GDS三轴仪对土样进行了K0固结不同排水应力路径试验。结果表明,应力路径对不扰动土样的体积变形和剪切变形均有显著影响,且球应力和偏应力对土的体应变和剪应变存在交叉影响。无论以体积变形为主还是剪切变形为主的应力路径下,应力-应变曲线都有明显的屈服性状。通过描绘试验所得各应力路径下的屈服点,获得张家港不扰动土样的屈服轨迹大致呈倾斜的椭圆形状,采用Wheeler模型的屈服面与试验屈服点的吻合程度要优于Nakano模型。 相似文献
7.
This paper presents a new constitutive model for clays. The model is developed on the basis of generalized hypoplasticity principles, which are combined with traditional critical state soil mechanics. The positions of the isotropic normal compression line and the critical state line correspond to the Modified Cam clay model, the Matsuoka–Nakai failure surface is taken as the limit stress criterion and the non‐linear behaviour of soils with different overconsolidation ratios is governed by the generalized hypoplastic formulation. The model requires five constitutive parameters, which correspond to the parameters of the Modified Cam clay model and are simple to calibrate on the basis of standard laboratory experiments. This makes the model particularly suitable for practical applications. The basic model may be simply enhanced by the intergranular strain concept, which allows reproducing the behaviour at very small strains. The model is evaluated on the basis of high quality laboratory experiments on reconstituted London clay. Contrary to a reference hypoplastic relation, the proposed model may be applied to highly overconsolidated clays. Improvement of predictions in the small strain range at different stress levels is also demonstrated. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
8.
为了研究土体结构破坏对软黏土一维变形的时效特性的影响,在Bjerrum的等时间线体系基础上,提出了等黏塑性应变率线等黏塑性应变率线概念,建立了非结构性软黏土的一维弹黏塑性模型;为了描述土体结构渐进破坏特征,定义了结构性参数--结构应变,在非结构性模型的基础上推导了结构性软黏土一维弹黏塑性模型;讨论了通过试验法直接确定模型参数的方法,并利用新建模型对温州天然软黏土的一维常规压缩试验、天然Ariake 黏土的分级快速固结试验、结构性Berthierville黏土的一维等应变率压缩试验及长期蠕变试验进行模拟。模拟与试验结果的对比表明,该模型能较好地描述结构性软黏土一维压缩变形的时效特征。 相似文献
9.
Natural clays usually show anisotropic stiffness due to their deposition process and anisotropic in situ stress state. The stiffness anisotropy depends on both of the stress anisotropy and fabric anisotropy, while the latter can be quantified by the stiffness anisotropy at isotropic stress states. This paper measures the K0 value (i.e., stress anisotropy) and elastic shear stiffness anisotropy of natural Shanghai clay in a triaxial apparatus with horizontal and vertical bender elements. The results show that the K0 value of Shanghai clay lies in the range of 0.40–0.66, and an empirical equation is proposed to estimate the K0 value based on the plasticity index and initial void ratio. The fabric anisotropy of natural Shanghai clay lies in the range of 1.2–1.4 with a stronger fabric in the horizontal plane. Moreover, the experimental data of the stiffness anisotropy and fabric anisotropy of different clays in the literature are reviewed and analyzed. It reveals that the stiffness anisotropy generally increases, while the fabric anisotropy remains nearly the same during K0 consolidation. For normally consolidated clay, the fabric anisotropy generally lies in the range of 1.1–1.7. For overconsolidated clays, the fabric anisotropy generally increases as the overconsolidation ratio increases. Empirical equations are proposed to approximately estimate the fabric anisotropy of clays based on its stress normalized elastic shear stiffness. 相似文献
10.
改良土中土颗粒和水化物形成具有一定结构的聚合体而表现出较强的结构性。与重塑正常固结土相比,改良土的结构性更强且具有一定的超固结比。在变形发展过程中,由于聚合体结构逐渐破坏,黏聚强度逐渐损失,土体表现出应变软化的力学特性。基于适用于超固结重塑黏土的统一硬化模型,引入改良土黏聚强度及其随塑性变形的演化规律,对统一硬化参数进行了修正,并采用更适用于改良土的非关联的流动法则,建立了一个可以较好地描述改良土力学特性的弹塑性本构模型。通过与水泥改良土和石灰改良土的三轴剪切排水试验的结果进行对比,该模型能够较为合理地描述改良土加载过程中黏聚强度损失对其力学特性的影响。黏聚强度的存在导致土体表现出超固结土的特性,当黏聚强度损失时会加剧土体的软化速度。 相似文献
11.
A shortcoming of the hypoplastic model for clays proposed by the first author is an incorrect prediction of the initial portion
of the undrained stress path, particularly for tests on normally consolidated soils at isotropic stress states. A conceptually
simple modification of this model, which overcomes this drawback, is proposed in the contribution. The modified model is applicable
to both normally consolidated and overconsolidated soils and predicts the same swept-out-memory states (i.e., normal compression
lines) as the original model. At anisotropic stress states and at higher overconsolidation ratios the modified model yields
predictions similar to the original model. 相似文献
12.
It is universally known that residual soils behave very differently from sedimentary soils. While the latter is widely known as cross-anisotropic, little is known regarding the strength anisotropy of residual soils. This study presents how the inherent anisotropy affects the strength of natural granite residual soils under generalized conditions, where intact specimens were carefully prepared and sheared under triaxial compression, extension, simple shear, and hollow cylinder torsional shear tests. The strength of natural residual soil, in terms of ultimate stress ratio M and undrained shear strength Su, is found to be significantly anisotropic in a different way from normally consolidated clays with the maximum strength obtained under triaxial compression and the minimum under simple shear or at intermediate principal stress direction. As a result, the existing method failed to measure the anisotropy degree of the studied soil. Two parameters were proposed accordingly to quantify the anisotropic strength under general conditions, taking the special strength anisotropy pattern and cohesive-frictional nature of GRS into account. The proposed parameters enable the direct comparison of strength anisotropy among soils. This study serves as a data set to better understand residual soils regarding their anisotropic behaviors under generalized conditions. Although specific to granite residual soils in China, this study is expected to be more widely applicable to other weathered geomaterials. 相似文献
13.
Expansive clay buffers in radioactive waste disposal designs experience cyclic drying and wetting paths during different stages of their design life. Clayey soils subjected to these processes develop swelling and shrinkage deformations, which give rise to the accumulation of compression or expansion strains during suction cycles. Experimental studies were undertaken using oedometer tests on an artificially prepared bentonite-sand mixture (80% bentonite by dry mass). In order to study these processes and to identify the most important features controlling soil behaviour, several wetting-drying cycles with suctions ranging between 130 and 4 MPa were applied using vapour equilibrium technique and covering a wide range of overconsolidation ratios (OCR). The tested samples showed cumulative shrinkage strains along the successive cycles, which became more significant at increasing vertical net stresses (low OCR values). However, no accumulation of expansion strains was detected at elevated OCR values. Test results were interpreted and predicted within the context of an elastoplastic model proposed by Alonso et al., 1999, [Alonso, E.E., Vaunat, J., Gens, A., (1999). Modelling the mechanical behaviour of expansive clays. Engineering Geology, 54, 173-183.] which takes into account the accumulation of strains. A good correspondence between measured soil response and model predictions was observed. The paper also presents the methodology to derive the constitutive parameters. 相似文献
14.
以大量试验结果为依据,运用物理意义明确、获取简便快捷、经济实用的土电阻率结构指标进行软黏土扰动变量的表征和测定。以三轴试验与电阻率测试结果为基础,进行了扰动函数的构建。在软黏土扰动状态描述中,引入旋转硬化因子,用以考虑应力诱发各向异性,进而构建了考虑各向异性的软黏土扰动状态概念本构模型。模型从一定程度上克服了传统扰动变量获取方法的弊端,克服了传统借用数学演绎手段建立扰动函数的不足,提高了模型应用的适用性。在模型构建中,融入初始应力各向异性的影响,从而使模型能更好地反映软土实际工程情况。三轴排水剪切试验验证结果表明,该模型可较好反映软黏土受荷下的结构变化情况和受力特性。 相似文献
15.
Experiments indicate that in one-dimensionally consolidated natural clays the elastic anisotropy is much stronger than the plastic strain anisotropy. Moreover, the elastic anisotropy appears to be dependent on the pre-consolidation strain. Coupled elasto-plastic constitutive law is shown to be able to simulate these anisotropy effects of natural clay deposits. In this law the elastic potential is not only a function of stress, but additionly of the plastic strain. The plastic strain comprises the geological process of pre-consolidation idealized as an one-dimensional plastic straining as well as a mechanically induced strain due to engineering activity. Calibration of the model and simulation of some stress paths are presented and related to the classical experimental results by Mitchell (1972). 相似文献
16.
This study presents a sophisticated elastoplastic constitutive model for unsaturated soil using Bishop-type skeleton stress and degree of saturation as state variables in the framework of critical state soil mechanism. The model is proposed in order to describe the coupled hydromechanical behavior of unsaturated soil irrespective of what kind of the loadings or the drainage conditions may be. At the same time, a water retention characteristic curve considering the influence of deformation on degree of saturation is also proposed. In the model, the superloading and subloading concepts are introduced to consider the influences of overconsolidation and structure on deformation and strength of soils. The proposed model only employs nine parameters, among which five parameters are the same as those used in Cam-Clay model. The other four parameters have the clear physical meanings and can be easily determined by conventional soil tests. The capability and accuracy of the proposed model have been validated carefully through a series of laboratory tests such as isotropic loading tests and triaxial monotonic and cyclic compression tests under different mechanical and hydraulic conditions. 相似文献
17.
天然岩土材料具有结构性和各向异性。在岩土破损力学的理论框架下,建立了初始应力各向异性结构性土的二元介质模型。岩土破损力学把结构性岩土材料抽象成由胶结强的胶结块（胶结元）和无胶结的软弱带（摩擦元）组成的二元结构体,变形过程中胶结块逐步破损并向软弱带转化。假定胶结块为横观各向同性的理想弹脆性体,胶结块破损后转化成的软弱带为可用邓肯-张模型描述的非线性弹性体。通过引入考虑各向异性影响的破损率和局部应变系数,建立了初始应力各向异性结构性土的二元介质本构模型,并给出了模型参数的确定方法。最后给出了模型的表现,且通过人工制备初始应力各向异性结构性土的三轴压缩试验结果验证了模型的适用性。计算结果表明,所提出的本构模型可以较好地模拟初始应力各向异性结构性土的应力-应变和体积变形特性。 相似文献
18.
天然土体的初始各向异性通常可对其后继循环特性产生显著影响。现有考虑循环载荷作用的土体弹塑性模型,往往采用类似修正剑桥模型的椭圆形屈服面,已有研究表明,该椭圆形屈服面因其拉伸弹性区域偏大,针对天然K0固结状态的土体,其计算精度较差。基于新近提出的广义各向同性硬化准则,在边界面方程中引入初始各向异性张量,并采用空间滑动面破坏准则（SMP）的变换应力法,建立了能考虑饱和黏土初始各向异性的循环边界面塑性模型。分别针对等压和偏压固结的饱和黏土静、动三轴试验进行模拟,结果表明,该模型能合理反映土体的初始各向异性及其后继循环动力特性。 相似文献
20.
在岩土破损力学基础上,基于微观破损机制,提出了考虑各向异性的结构性砂土本构理论。采用Lade-Duncan强度准则考虑中主应力对抗剪强度的影响;采用考虑颗粒排列组构的各向异性状态变量A反映各向异性对土体强度和变形的影响;通过相似扩大重塑土的屈服面反映结构性对土性的影响;通过引入非相关联流动法则考虑各向异性和结构性对土体塑性变形的影响。同时,将基于微观力学机制的损伤演化规律引入结构性土的硬化规律;该硬化规律同时考虑了塑性体积应变和剪切应变对各向异性结构性土强度的影响。然后将该模型用于模拟室内三轴压缩试验,初步验证了该模型的合理性和适用性。 相似文献
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