A damage hypoplasticity constitutive model for cemented sand and gravel materials

Acta Geotechnica - A hypoplastic constitutive model is proposed to model the cementation-induced enhanced stiffness, strength and dilative behavior of cemented sand and gravel materials. The model...     

人工胶结砂土力学特性的离散元模拟

采用离散单元法（DEM）对胶结砂土力学特性进行模拟。将基于室内试验测得的理想胶结颗粒接触力学响应引入到开发的二维离散元程序（NS2D）中,模拟胶结砂土颗粒间的胶结作用。对不同胶结强度和围压的胶结砂土进行平面应变双轴压缩试验模拟,并将模拟结果与Wang和Leung[1]提供的人工胶结砂土的试验结果进行比较。最后对数值模拟中胶结试样的微观力学响应（接触力链、胶结点破坏率和位移场）进行分析。结果表明,离散元数值模拟能够有效地反映胶结砂土的主要力学特性,相比同一初始孔隙比的无胶结松散砂土,胶结砂土将具有更高的强度,应力-应变关系呈应变软化,体变为先剪缩后剪胀,且两者的差异随胶结强度的增大和围压的减小而越趋显著。此外,胶结砂土宏观力学响应（应力-应变关系和剪胀性）与其微观力学响应密切相关。     

Lightly cemented sand in tension and compression

Summary Laboratory test results for the stress-strain relationships for lightly cemented sand specimens are presented. The tests were conducted with the specimens under compression and tension without any confining pressure. The cement content was varied from 4% to 8%. Based on the test results, the variations of the stress and strain ratios at failure with cement content are presented for specimens under compression and tension.     

A hypoplastic model for clay and sand

The theory of hypoplasticity was developed initially for non-cohesive soils. However, sand and clay have many common properties; therefore arose the idea to extend the hypoplastic model to clay. The proposed model is able to describe the behaviour of cohesive soils with the incorporation of an appropriate structure tensor into the constitutive equation. This tensor is a stress-like internal parameter, also called back stress. This enables us to describe the behaviour of cohesive soils with the same material parameters for several states of consolidation and also to model barotropy and pycnotropy of sand. Numerical simulations of element tests are performed in order to check the performance of this hypoplastic model. Experimental data obtained with normally and overconsolidated clay and sand specimens with various densities are taken for comparison, and it is shown that the model is capable of describing the material behaviour of clay and sand. The determination of the material constants, the calibration method, is also presented in this paper.     

A basic hypoplastic constitutive model for sand

Hypoplastic constitutive models are based on nonlinear tensor functions and are characterized by simple formulation and few parameters. In its early stage, mainly basic hypoplastic constitutive equations were concerned, where the stress tensor is assumed as the only state variable. There followed some enhanced models based on the basic constitutive equation by including void ratio as an additional state variable. In this paper, we first show that the widely used hypoplastic model by Wolffersdorff is seriously flawed because the underlying basic equation does not perform properly. We proceed to develop a basic hypoplastic constitutive equation by introducing a new tensorial term, which preserves the critical state at large strain. The model performance is demonstrated by parameter study for some element tests. This simple and robust basic equation is well suited to build more sophisticated models.     

A simple hypoplastic constitutive model for sand

The paper presents a hypoplastic constitutive model for the three-dimensional non-linear stress–strain and dilatant volume change behaviour of sand. The model is developed without recourse to the concept in elastoplasticity theory such as yield surface, plastic potential and decomposition into elastic and plastic parts. Benefited from the non-linear tensorial functions available from the representation theorem the model possesses simple mathematical formulation and contains only four material parameters, which can be easily identified with triaxial compression tests. Comparison of the predictions with the experimental results shows that the model is capable of capturing the salient behaviour of sand under monotonic loading and is applicable to both drained and undrained conditions.     

A constitutive model for sand in triaxial compression

Although there is a large number of constitutive models for sand available in the literature it is believed that a fresh approach, striking a balance between complexity and theoretical rigour, is desirable. The approach here has certain conceptual links with the Cam Clay series of elastic–plastic models, but includes the more general starting assumption that the yield function, plastic potential and failure locus should be given quite distinct mathematical expressions. Possible physical bases for the proposed forms are discussed. Ways in which the parameters required to define the model may be determined are suggested and the use of the model is then demonstrated. Firstly, it is shown that, where a limited set of experimental data is available, the model is flexible enough to be able to match the test results. Secondly, it is shown that, where a wide range of test results has been produced, it is possible to determine the model constitutive parameters from a small number of tests and proceed to make satisfactory predictionsfor other, quite different, types of test. The model is developed for sand at a single initial density, but the way in which the constitutive parameters might be expected to vary with density is discussed. The model is described for conditions of triaxial compression, and extension to more general stress states will be needed before it can be put to the test of incorporation in, for example, a finite element program.     

不同级配珊瑚砂水泥胶结体的破坏行为分析

为探究不同级配的珊瑚砂水泥胶结体在静、动荷载下的破坏行为,利用落锤冲击试验机对不同级配的珊瑚砂水泥胶结体试样进行了一系列冲击试验,并结合静力压缩试验,综合分析了珊瑚砂水泥胶结体的力学行为及破坏形态。试验结果表明：珊瑚砂的级配区间越广,其水泥试块的抗压强度越大、抗冲击性能越强、受冲击破坏程度越小且破碎角逐渐递减;珊瑚砂水泥试块在不同种类的荷载作用下展现出不一样的破坏形态,区别于混凝土试块。在此基础上,建立了三维离散元模型,以模拟的应力-应变曲线与静力试验结果相吻合作为PFC细观参数的选取原则,并利用该细观参数对试块冲击试验进行数值重现。通过细观分析可以看出：从裂隙分布角度分析,随着级配区间越广、平均粒径的增大,均匀且分散的冲击微裂隙会越来越集中往某个方向发展,使得试块的裂缝数量减小、破损程度减轻。从系统黏结力角度分析,级配区间范围窄的试块内部黏结力较小且分布均匀,级配区间范围广的试块内部黏结力较大且分布不均匀。     

Investigating the micro mechanics of cemented sand using DEM

The discrete element method has been used to investigate the micro mechanics of cemented sand. High‐pressure drained triaxial tests are modelled in 3D using a flexible membrane that allows the correct deformation to develop. Simulations with up to 12 MPa confining pressure are presented, which are compared with laboratory experiments on a sand with a range of cement contents. Cementation is modelled using ‘parallel bonds’, and various parameters and strength distributions are investigated. Varying levels of cementation are successfully modelled, with the correct qualitative behaviour observed and the separate effects of cementation and confining pressures demonstrated. The triaxial behaviour is found to be highly influenced by the distribution of bond strengths. Copyright © 2014 John Wiley & Sons, Ltd.     

水泥偏高岭土复合稳定粉砂土渗透特性试验研究

水泥稳定粉砂土抗渗性能受粉砂土自身渗透性能、水泥用量、水灰比等因素影响显著,如何在提升其抗渗性能的同时降低水泥用量是提升工程经济效益的关键。通过开展不同水泥偏高岭土掺比、初始用水量、水泥偏高岭土总掺量以及养护龄期条件下的室内渗透试验,研究了上述因素对水泥偏高岭土复合稳定粉砂土抗渗性能的影响规律,探讨了上述因素及无侧限抗压强度与渗透系数之间的经验关系。结果表明：水泥与偏高岭土掺比为5:1时,水泥偏高岭土复合稳定粉砂土抗渗性能最佳,且该掺比不随水泥偏高岭土总掺量的改变而变化;水泥偏高岭土复合稳定粉砂土渗透系数随初始用水量增加呈非线性递增,随水泥偏高岭土总掺量增加和养护龄期发展呈先快后慢降低;基于试验结果归纳提出了4个关于初始用水量、水泥偏高岭土总掺量、养护龄期和无侧限抗压强度的水泥偏高岭土复合稳定粉砂土渗透系数经验模型。研究成果可为水泥稳定粉砂土抗渗性能提升提供理论参考与借鉴。     

人工胶结砂物理力学特性试验研究

采用二次掺水法制备氧化钙胶结砂样,为加速碳酸化将试样放置在充满干冰的养护箱中养护,养护完成后对不同氧化钙掺入比的试样进行固结排水三轴试验和碳酸钙定量化学试验,分析了氧化钙掺入量对胶结砂物理力学特性的影响,定义了反映胶结物生成量的化学指标 ,利用 修正了摩尔-库仑强度理论。结果表明：二次掺水法可有效控制人工胶结砂的初始含水率,CaCO3胶结物的形成对试样物理力学特性有极大影响。在不同围压下,试样均表现出应力软化。随氧化钙掺入比的增加,试样应力软化现象逐渐增强,并且黏聚力及内摩擦角增量逐渐增大。胶结砂具有压硬性,围压、氧化钙的掺入比对试样剪胀有抑制作用。随着 的增加,试样黏聚力、摩擦角增量均有明显提高,通过试验数据得到了 与试样黏聚力、内摩擦角增量之间的函数关系,进而对摩尔-库仑强度理论进行了修正,修正后的公式能够反映胶结物的形成对胶结砂强度的影响。     

A state-dependent critical state model for methane hydrate-bearing sand

Methane hydrate exists in the pores of methane hydrate-bearing sand (MHBS) and is considered to be a potentially significant source of methane and thus energy for mankind. However, before conducting a large-scale extraction of methane from MHBS, it is crucial to simulate the mechanical behaviour of MHBS and evaluate its stability during drilling and methane production. In this paper, a state-dependent critical state model for MHBS is presented. The critical state of MHBS is discussed, and critical state line formulations are introduced as functions of hydrate saturation. A simple nonlinear bonding and linear debonding law is incorporated considering the cementing mechanism of hydrate. A modified state-dependent dilatancy is proposed to account for the effects of stress level, internal state (density), bonding strength and hydrate saturation. Determination of the model parameters is described in detail. The proposed model is employed to predict results of drained triaxial compression tests on MHBS. Satisfactory performance is demonstrated, i.e., the model can adequately capture the stress–strain and volume change behaviours of MHBS over a wide range of hydrate saturations, confining pressures and densities using a unified set of parameters.     

A constitutive model for sand with anisotropic hardening rule

An anisotropic hardening model for sand or other granular materials is presented. The state of initial densification is represented by a configuration surface and the state of loading by a loading surface in the stress space. The domain of elastic response may not exist and the irreversible strain is assumed to occur for both active and reverse loading. The derived constitutive equations are applicable both for dense and loose sands. Some model predictions are compared with available experimental data for triaxial compression and extension under undrained condition.     

胶结砂及半成岩粘土的岩土力学特征分析--以湘阴湘江大桥桥位区为例

胶结砂及半成岩粘土目前处于一种由土向岩石转化的状态.它们和正常意义上的土及岩石均有明显的区别.在组成、结构和构造等方面,它们既具有部分土的特征,又具有部分岩石的特征.本文对湘阴湘江大桥桥位区产出的胶结砂及半成岩粘土的土力学和岩石力学特征进行了分析,对其成因和应用进行了探讨.     

Triaxial behaviour of a cemented gravely sand, Tehran alluvium

Cemented coarse-grained alluvium is present in a vast area of Tehran city, Iran including its suburbs. This deposit consists of gravely sand to sandy gravel with some cobbles and is dominantly cemented by carbonaceous materials. In order to understand the mechanical behaviour of this soil, a series of triaxial compression tests were performed on uncemented, artificially cemented and destructured samples. Hydrated lime was used as the cementation agent for sample preparation to model the Tehran deposit. The tests were performed on cemented samples after an appropriate time for curing. The tests on cemented samples show that a shear zone appears as the shear stress approaches the peak shear strength. During shearing these samples undergo dilation at confining stress lower than 1000kPa. However, the uncemented and destructured samples show contraction during shearing. Peak shear strength is followed by strain softening for all cemented samples. The shear strength increases with increasing cement content but the influence of the cementation decreases as the confining stress increases. With increasing cementation the stress-strain behaviour of samples tend towards the behaviour expected of high-density soils. Test results indicate that the failure envelope for cemented samples is curved and not linear.     

A critical state constitutive model for clean and silty sand

Acta Geotechnica - The mechanical behavior of silty sand is highly dependent on the percentage of fines in addition to the packing density and confining pressure. Properly modeling the diverse...     

A critical state sand plasticity model accounting for fabric evolution

Fabric and its evolution need to be fully considered for effective modeling of the anisotropic behavior of cohesionless granular sand. In this study, a three‐dimensional anisotropic model for granular material is proposed based on the anisotropic critical state theory recently proposed by Li & Dafalias [2012], in which the role of fabric evolution is highlighted. An explicit expression for the yield function is proposed in terms of the invariants and joint invariants of the normalized deviatoric stress ratio tensor and the deviatoric fabric tensor. A void‐based fabric tensor that characterizes the average void size and its orientation of a granular assembly is employed in the model. Upon plastic loading, the material fabric is assumed to evolve continuously with its principal direction tending steadily towards the loading direction. A fabric evolution law is proposed to describe this behavior. With these considerations, a non‐coaxial flow rule is naturally obtained. The model is shown to be capable of characterizing the complex anisotropic behavior of granular materials under monotonic loading conditions and meanwhile retains a relatively simple formulation for numerical implementation. The model predictions of typical behavior of both Toyoura sand and Fraser River sand compare well with experimental data. Copyright © 2013 John Wiley & Sons, Ltd.     

A unified plasticity model for large post-liquefaction shear deformation of sand

Based on previous experimental findings and theoretical developments, this paper presents the formulation and numerical algorithms of a novel constitutive model for sand with special considerations for cyclic behaviour and accumulation of large post-liquefaction shear deformation. Appropriate formulation for three volumetric strain components enables the model to accurately predict loading and load reversal behaviour of sand, fully capturing the features of cyclic mobility. Compliance with the volumetric compatibility condition, along with reversible and irreversible dilatancy, allows for physically based simulation of the generation and accumulation of shear strain at zero effective stress after initial liquefaction. A state parameter was incorporated for compatibility with critical state soil mechanics, enabling the unified simulation of sand at various densities and confining pressures with a same set of parameters. The determination methods for the 14 model parameters are outlined in the paper. The model was implemented into the open source finite-element framework OpenSees using a cutting-plane stress integration scheme with substepping. The potentials of the model and its numerical implementation were explored via simulations of classical drained and undrained triaxial experiments, undrained cyclic torsional experiments, and a dynamic centrifuge experiment on a single pile in liquefiable soil. The results showed the model’s great capabilities in simulating small to large deformation in the pre- to post-liquefaction regime of sand.     

适用于砂土循环加载分析的边界面塑性模型

基于临界状态土力学框架,建立了一个适用于砂土排水循环加载的边界面塑性模型。采用了考虑虚拟峰值应力比的偏应变硬化准则,初始加载阶段应力点位于边界面上,反向加载阶段以历史最大屈服面作为边界面,同时实现了对密砂软化现象的模拟和对历史所受最大应力的记忆。边界面采用修正的椭圆形,引入考虑密度与应力水平的状态相关剪胀函数,采用非相关联流动法则和以应力反向点作为映射中心的径向映射准则。模型仅有10个参数,通过常规三轴试验即可确定,并且使用一套参数可以模拟不同围压、密度的单调和循环加载情况。分别对饱和砂土的单调、循环排水三轴试验进行模拟,结果表明,该模型能够合理地反映饱和砂土排水条件下的应力-应变特性。