Unified bounding surface model for monotonic and cyclic behaviour of clay and sand

Acta Geotechnica - This paper presents a constitutive model enabled to simulate monotonic and cyclic behaviour of clay and sand in a unified framework. The bounding surface concept has been...     

The impact of stress disturbance on undrained cyclic behaviour of a kaolin clay and settlement of tunnels under cyclic loading

Acta Geotechnica - The excavation of a pit or tunnel induces stress disturbances in soils under different stress paths. The cyclic deformation behaviour of soft clay would be affected by the...     

超固结黏土单调和耦合循环的剪切特性研究

针对超固结黏土空心试样,利用土工静力-动力液压三轴-扭转多功能剪切仪,在均等固结条件下进行了单调扭剪和三轴-扭转耦合循环剪切试验。试验结果表明：不同超固结比黏土的单调扭剪强度也可由正常固结黏土的单调扭剪强度得到,得到了不同超固结比下饱和黏土的强度及模量的退化规律;随着超固结比的增大,相同破坏循环次数的动应力比和临界循环应力比均线性增大;超固结比对耦合循环剪切的孔隙水压力的发展模式影响显著。参考Yasuhara的测量方法,采用荷载停止后继续采集孔压的方法可以更好地反映黏土在耦合循环荷载下产生的真实孔压和孔压的增长情况。提出的综合应变式同时考虑了剪切变化和正向偏差变形的共同效应,适合作为主应力连续旋转的耦合循环剪切试验的破坏标准。     

Numerical modelling of gassy sand behaviour under monotonic loading

Acta Geotechnica - The mechanical behaviour of gassy sand is rather complex owing to the inherent complex nature of sand and the occluded/dissolved gas. For better understanding of the behaviour of...     

DEM analysis of the onset of flow deformation of sands: linking monotonic and cyclic undrained behaviours

This study explores the link between the monotonic and cyclic undrained behaviour of sands using the discrete element method (DEM). It is shown that DEM can effectively capture the flow deformation of sands sheared under both monotonic and cyclic undrained loading conditions. When subjected to cyclic shearing, flow-type failure is observed for a loose sample, while cyclic mobility is observed for a dense sample. A strong correlation between the monotonic and cyclic loading behaviour that has been revealed experimentally is also confirmed in DEM simulations: (a) flow deformation occurs in the compressive loading direction when the cyclic stress path intersects the monotonic compression stress path prior to the monotonic extension stress path, and vice versa; (b) the onset of flow deformation in q–\(p^{\prime }\) space is located in the zone bounded by the critical state line and the instability line determined from monotonic simulations. Hill’s condition of instability is shown to be effective to describe the onset of flow failure. Micro-mechanical analyses reveal that flow deformation is initiated when the index of redundancy excluding floating particles drops to below 1.0 under both monotonic and cyclic loading conditions. Flow deformation induced by either monotonic or cyclic loading is characterized by an abrupt change of structural fabric which is highly anisotropic. The reason why the dense sample dilated during monotonic loading but showed cyclic mobility (temporary liquefaction) during cyclic loading is attributed to the repeating reversal of loading direction, which leads to the periodic change of microstructure.     

Improvement of a hypoplastic model to predict clay behaviour under undrained conditions

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.     

循环荷载作用下软土的孔压模式和强度特征

通过循环三轴剪切试验,研究原状软土的变形和孔压发展规律。建议了一个循环荷载作用下孔隙水压力的发展模式。研究了不排水循环荷载作用下软土的动强度以及作用后饱和软土静强度的衰减特征,讨论了周围固结压力和荷载作用频率对动强度的影响。研究表明,在某一轴向应变条件下,循环次数随循环剪应力比的增加而迅速减小;随循环剪切应力比的增大,饱和软土的静强度有一定衰减。     

原状和重塑海洋粘土经历动载后的静强度衰减

根据海洋粉质粘土原状和重塑土样的动三轴试验结果, 比较和分析了两种土样应力、应变、孔隙水压力和不排水抗剪强度行为, 得到了土样的静不排水抗剪强度衰减与波浪荷载作用下土样产生的动应变以及平均累积孔压之间的关系。 并将波浪荷载作用使土样内孔压升高, 有效应力降低, 形成拟超固结现象的理论, 应用到土样不排水抗剪强度衰减与平均累积孔压之间关系的分析;同时与重塑土样的超固结静态剪切试验结果进行比较, 得到了土样在波浪荷载作用后的归一化不排水抗剪强度与拟超固结比之间的关系式。 建议以少量原状土样, 配合大量重塑土样的动三轴试验结果, 实现对实际海洋粘土地基在波浪荷载作用后的静不排水抗剪强度衰化规律的评估。     

循环荷载作用下软黏土刚度软化特征试验研究

循环荷载作用下,软黏土将发生软化现象,而以往的研究中大多未考虑初始偏应力的影响。通过对萧山正常固结饱和软黏土进行应力控制的循环三轴试验,研究了循环荷载作用下土体的刚度软化情况。通过分析每一次循环过程中割线剪切模量的变化规律,探讨了循环次数、循环应力水平、初始偏应力对刚度软化的影响。结果表明,随着循环次数的增加,土体刚度逐渐减小;循环应力水平的提高,初始偏应力的施加将加快刚度软化。在试验的基础上推导了反映土体刚度软化规律的经验公式,得到了萧山软黏土的破坏刚度比。     

Investigation on the behavior of frozen silty clay subjected to monotonic and cyclic triaxial loading

This paper aims to assess the characteristics of the deformation and strength behavior of frozen soils at different temperatures under monotonic and cyclic triaxial conditions. The deformation and failure patterns of the specimens change from ductility to brittleness with decreasing temperatures under both monotonic and cyclic loadings. The development of axial strain and stiffness with increasing number of cycles for the soils under cyclic loading is presented and analyzed in detail. A collapse behavior in strength and stiffness is observed in tests of frozen soils at ??5 °C, ??7 °C and ??9 °C. The difference in frictional sliding between the samples with high ductility and those with high brittleness is attributed to the different patterns of deformation and failure. The dynamic modulus is plotted versus axial strain, and the state where the stiffness begins to decrease is employed as the criterion of cyclic failure. The proposed criterion of cyclic failure is verified to be more suitable for frozen soils with high brittleness and seems to be consistent with the peak strength under monotonic loading. Finally, the cyclic stress ratios are plotted against the number of cycles up to this failure criterion, and the effect of temperatures on cyclic strength is evaluated.

     

Discrete element modelling of cyclic behaviour of granular materials

Discrete Element Modeling (DEM) of cyclic behavior of granular material has been attempted to understand liquefaction behavior of sands. A series of cyclic biaxial tests in both undrained and drained conditions with constant stress and strain amplitudes were performed on assemblage of loose and dense systems. Tests are conducted on monodisperse (uniform) and polydisperse (well graded) samples. From this study, it has been shown that DEM can simulate the cyclic behavior of sands very satisfactorily. Characteristic features, i.e., occurrence of large plastic strains and changing over from contractile to dilative behavior beyond the phase transformation angle, anisotropy of reduced strength on the extension side etc are very well reflected in numerical simulations. Liquefaction of loose assemblage seems to be mainly due to continued and cumulative loss of co-ordination number under each cycle as there is a reversal of loading direction and hence a continuous reorientation of fabric. There is no cumulative loss of co-ordination number in dense states because the stress ratios are mostly higher than the phase transformation level where the fabric has reached a limiting orientation. Micro mechanical explanations to the macroscopic behavior of the disc assemblage under cyclic loading are presented in terms of the force and fabric anisotropy coefficients.     

珠江口海洋软土不排水抗剪强度及循环弱化特性试验研究

为了获得广东珠江口海洋软土的不排水抗剪强度,分别开展了不固结不排水剪切和固结不排水剪切三轴试验、无侧限抗压试验、现场取土离心机正常固结模型地基的T-bar静力贯入试验以及现场十字板剪切试验和静力触探试验（CPT）,并采用土力学经验公式进行计算,确定了珠江口海洋软土不排水抗剪强度沿深度的分布规律,并综合评价了各种方法的有效性。为了获得该区域软土的循环弱化规律,分别开展了循环动三轴试验、现场取土离心机正常固结模型地基的T-bar循环贯入试验和现场单桩水平循环加载试验,揭示了软土循环动模量比与循环次数之间的双对数线性关系,获得了3种试验条件下珠江口海洋软土的循环弱化因子。其研究结果可为该区域海洋建筑物设计提供直接依据,也可为其他类型工程提供借鉴。     

循环荷载下软黏土不排水强度弱化分析的动力有限元ABAQUS实现

在波浪循环荷载作用下饱和软黏土地基出现孔隙水压力升高,并导致不排水强度弱化,严重影响防波堤的承载性能。考虑静偏应力影响,基于最大孔隙水压力发展模型和正常固结软黏土不排水强度公式,推导出软黏土不排水强度随循环荷载作用次数和应力水平变化的动态折减规律。结合软黏土不排水强度动态折减规律和M-C屈服准则,在有限元软件ABAQUS上实现软黏土不排水强度循环弱化分析的数值开发和动力运算过程。运用该动力有限元方法对天津港防波堤地基软黏土的动、静三轴试验进行数值模拟运算。结果表明,最大孔隙水压力发展曲线以及循环荷载作用后不排水强度的数值预测结果与动三轴试验结果吻合良好。另外,动力有限元方法（DFEM）能够表示土体强度在循环荷载作用下的具体弱化过程。     

高温下混凝土水力、热及变形耦合数学模型

针对受高温载荷的混凝土提出了一个水力-热-变形耦合的数学模型,模型中采用了3种不同的连续介质：水蒸汽与干燥空气的混合气体相,液态水以及固态混凝土骨架。在连续介质的Navier-Stokes方程基础上,推导了各相介质的质量守恒方程、动量守恒方程以及混合物整体的能量守恒方程。为了简化问题,引入了若干假设,给出了基于文献资料数据拟合的某些变量的用基本未知量表达的经验公式,建立了基于物理定律的各类运动学变量与其热力学共轭力的本构关系,并给出了系统偏微分方程。     

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。     

Correction to: Investigation on the behavior of frozen silty clay subjected to monotonic and cyclic triaxial loading

Acta Geotechnica - In the original publication, the figures 7 to 12 are incorrect. The correct figures 7, 8, 9, 10, 11 and 12 are provided below:     

Elastoplastic model with loading memory surfaces (LMS) for monotonic and cyclic behaviour of geomaterials

A new elastoplastic model called loading memory surface based on the critical state concept and the multi‐surface framework is proposed for geomaterials. The model uses a hypoelastic formulation and two plastic mechanisms. The formulations of the model are made in three‐dimensional stress–strain space and work under both monotonic and cyclic loadings. A newly introduced formalism makes it possible to obtain the cyclic response directly from the monotonic loading one. This formalism gives a three‐dimensional generalization of the well‐known Masing rule. The model has been validated against test results of Hostun sand under several conditions: monotonic and cyclic, drained and undrained, tests in compression and in extension, and at different confining pressures and different densities. Copyright © 2014 John Wiley & Sons, Ltd.