砂土液化后再固结体变规律表征与离心模型试验验证

针对一定相对密度的饱和砂土,首先开展单元体K₀固结试验和振动液化试验研究,发现饱和砂土液化后体变规律受再沉积和再固结两种机制制约：其中再沉积部分与所受振动历史密切相关,尤其是液化触发后的应变历史,土骨架累积剪应变比越大、再沉积体变越大;而再固结部分受先期固结历史和循环振动历史影响显著,再固结曲线会沿原有正常固结曲线趋势发展,其稳定段再压缩指数比相同条件下的正常固结曲线的压缩指数稍大。据此提出了考虑先期固结和振动历史的砂土液化后体变模型和简化算法,将再沉积和再固结两者统一表达成再固结体变,并建议了再固结压缩指数和假设起始应力的确定方法。进一步开展了水平场地地震液化离心机模型试验,监测模型固结和振动液化过程的沉降,从模型尺度进一步揭示砂土液化后体变规律,并初步验证了本文模型与简化算法的有效性。     

砂土液化大变形的弹塑性循环本构模型

循环剪切过程中饱和砂土的3个体积应变分量(有效球应力变化引起的体变、剪切引起的可逆性体变和不可逆性体变)的变化规律决定了液化后剪应变的发展。基于上述机理、对剪切引起的可逆性和不可逆性体变的数学描述、体积相容性条件以及边界面本构理论框架,建立了一个可描述饱和砂土液化后大变形的弹塑性循环本构模型。通过对饱和砂土排水和不排水循环扭剪试验结果的模拟表明,该模型不仅可以合理地模拟饱和砂土循环加载条件下从液化前到液化后、从小剪应变到大剪应变的变形发展过程,而且可以合理地模拟饱和砂土液化后再固结大体变的累积特性。本文的研究为定量描述砂土液化后大变形提供了一条合理而有效的途径。     

重塑粉质黏土的剪切蠕变特性及本构模型研究

通过室内三轴试验,对重塑粉质黏土的剪切蠕变特性进行研究,分析了粉质黏土在不同偏应力水平下和不同偏应力分级方式下的蠕变变形特性。试验表明,粉质黏土的偏应变蠕变变形与对数时间关系具有明显的非线性;在同样的剪切应力水平下,不同的加载分级方式对试样的最终偏应变量有明显的影响,加载分级越多,最终的偏应变量越小。基于试验数据,建立了适合描述重塑粉质黏土蠕变特性的双曲线模型,并对模型参数的物理意义和取值方法进行了讨论。分析表明,模型参数的取值与应力水平及试样的初始孔隙比有关;结合试验结果,给出了粉质黏土剪切蠕变双曲线模型参数取值建议。     

砂土液化后大变形的机理

基于试验观察和机理分析,揭示了不排水往返加载条件下饱和砂土初始液化后的剪切大应变和三个体积应变分量(有效球应力变化引起的体变、可逆性剪切体变和不可逆性剪切体变)之间的内在联系,阐明了三个体积应变分量的组合变化规律控制了饱和砂土液化后大变形的产生和发展,界定了液化后循环剪切大变形过程中交替出现的三种物态(摩擦接触状态、临界接触状态和悬浮状态)及其产生条件,解释了液化后流滑和再固结体变形成的机理,给出了一个物理概念明确并具有普遍性的建立液化后大变形弹塑性本构模型的合理途径。     

冻融循环下青藏粉砂土双屈服面本构模型研究

 基于青藏粉砂土在冻融循环下的常规三轴固结剪切试验,通过引入模量残余比和冻融循环次数,建立考虑冻融循环影响的双屈服面本构模型。试验结果表明,粉砂土的应力–应变关系呈应变硬化型,而在整个剪切过程中,体变呈现剪缩的特性。粉砂土的剪切模量随着围压的增大而增大,随着冻融循环的发展而出现先减小后增大的趋势。与未冻融粉砂相比,冻融以后的弹性剪切模量可降低约36%左右。其应力平面p-q上的剪切屈服面和体积屈服面可分别用过原点的线性函数和椭圆型曲线进行描述。对于剪切和体积硬化特性,建立与塑性应变及冻融循环次数相关的硬化参数,且均采用非相关联的流动法则。所提出的双屈服面本构模型能够很好地反映土体的应力–应变特性,模型计算值与试验值较为吻合,该模型能较为准确地预测不同围压及不同冻融循环次数下的应力–应变关系曲线,较好地反映冻融循环对粉砂力学性质的影响。     

Behavior of geogrid–reinforced sand and effect of reinforcement anchorage in large-scale plane strain compression

This paper presents experimental investigations on the behavior of geogrid–reinforced sand featuring reinforcement anchorage which simulates the reinforcement connected to the wall facings in numerous in-situ situations. A series of large plane strain compression tests (the specimen 56 cm high × 56 cm wide × 45 cm long) was conducted. Standard Ottawa sand and 4 types of PET geogrids exhibiting 5% stiffness in the range of 750–1700 kN/m were used in this study. The specimens were tested by varying the relative density of sand, confining pressures, geogrid types, and reinforcement-anchorage conditions. Experimental results indicate that relative to unreinforced specimens, both anchored and non-anchored geogrid reinforcements can enhance the peak shear strength and suppress the volumetric dilation of reinforced soil. The studies on anchorage revealed that anchoring the reinforcement can restrain the lateral expansion of reinforced specimens, resulting in a substantial increase in shear strength and a reduction in volumetric dilation. The strength ratios of non-anchored specimens appeared to be insensitive to the reinforcement stiffness, whereas the strength ratios of the anchored specimens increased markedly with increases in soil density, reinforcement stiffness, and system deformation (i.e., axial stain). Geogrid anchorage contributed a large percentage of the total shear-strength improvement, nearly 3-times more than the contribution of the soil–geogrid interaction in non-anchored specimens. Lastly, an analytical model was developed based on the concept that additional confinement is induced by reinforcement anchorage, and the predicted shear strength of the anchored soil was verified based on the experimental data.     

Stress–dilatancy relationships of sand in the simulation of volumetric behavior during cyclic torsional shear loadings

In order to describe the volumetric behavior of soil subjected to shearing, a relationship that deals with the ratio of plastic strain increments to stress ratio (i.e. a stress–dilatancy relationship) is required in addition to the stress–shear strain relationship. In view of the above, stress–dilatancy relationships during cyclic torsional shear loadings were experimentally investigated in the current study. Based on the experimental results, a bilinear non-unique stress–dilatancy model was proposed for stress controlled drained cyclic torsional shear loading. The stress–dilatancy relationships during virgin loading and subsequent cyclic loading were modeled separately by considering the effects of stress history (over-consolidation or normal consolidation). Then the volume change of Toyoura sand specimens subjected to cyclic torsional shear loading was simulated by combining the simulation of stress–shear strain relationship with the proposed stress–dilatancy relationships. It was observed from the comparison of the experiment results with the simulation of volumetric strain that, after combining with accurate modeling of stress–shear strain relationship, the proposed stress–dilatancy relationship can reasonably simulate the volumetric behavior of sand subjected to various drained cyclic torsional shear loadings.     

取决于材料状态的变形局部化现象

一种状态相关本构模型被置于耦合的有限元程序中，分析了平面应变条件下饱和砂土在排水剪切过程中的变形局部化问题。由于模型中固有的状态相关特性，初始有效平均正应力与土样密度对变形局部化的影响得到了详细研究。数值分析结果表明，砂土中的变形局部化取决于土样的状态，有效平均正应力越小，土样密度越大，变形局部化程度就越高，剪切带与大主应力面的夹角也就越大。     

基于原位测试指标的砂土时间效应定量表征初步研究

时间效应对土体抗液化强度有显著影响,其定量表征是砂土地震液化判别的研究难点之一。基于土体小应变剪切模量Gmax和静力触探锥尖阻力qc的经验表达式,对土体密实度和有效应力作归一化处理,提出了砂土时间效应的表征指标AI。该表征指标在数学上表达为小应变剪切模量与大应变静力触探锥尖阻力的结构性参数的比值函数。通过分析已有室内试验数据发现重塑砂土具有稳定的AI值,而对现场数据的分析则揭示了砂土沉积年代越久、AI值越大的整体趋势。该表征指标能揭示土体力学特性随时间的演变规律却不含时间变量,因此是一个"表观时间"参数,为进一步研究时间效应对砂土抗液化强度的影响规律提供了基础。     

南京及邻近地区新近沉积土的动剪切模量和阻尼比的试验研究

通过对南京及其邻近地区河漫滩相成因的粘土、粉质粘土、淤泥质粉质粘土、粉质粘土与粉砂互层土、粉土、粉细砂等6类新近沉积土的自振柱试验，详细地探讨了围压大小、剪应变水平、土的颗粒组成和结构性对这6类新近沉积土的剪切模量G及阻尼比λ的影响。通过与Seed和Idriss建议的砂土G／Gmax-γ和λ-γ曲线变化范围的对比，结果表明，不能将粉质粘土与粉砂互层土简单地当作砂土或粉质粘土对待。通过试验和理论分析，给出了南京及其邻近地区6类新近沉积土动剪切模量和阻尼比随剪应变变化的平均曲线的拟合曲线、包络线及其参数的推荐值，对实际工程具有一定的借鉴作用。     

土层夯实过程模型和极限孔隙比：强夯加固机理探讨之四

提出一个强夯中土体被夯实的全过程模型，讨论了两个极限孔隙比问题。指出未饱水土强夯夯实的极限孔隙比受制于土的初始含水量W0，并与比重Gs和界限饱和度Sf有关。大体为e＝W0·Gs／Sf。饱水土夯实的极限孔隙比e受制于该土层排水固结最终可能的密实状态，对于亲水性不很强透水性不很小的土，此孔隙比e可以粗略预估而得。     

Insight into excess pore pressure generation leading to liquefaction of sand with stress history under saturated and unsaturated conditions

Assessments of the liquefaction resistance of clean sand still involve considerable uncertainties, which are a current research topic in the field of soil liquefaction. The factors considered and discussed in this study include the loading history, degree of saturation, and partial drainage. The effects of each of these factors on pore pressure generation and liquefaction resistance have been studied for decades in the laboratory, and empirical relationships have been derived. In this paper, an attempt is made to explain these effects using the unique index of volumetric strain. A pore pressure generation model is developed which is similar to that of Martin et al. (1975), but based on stress-controlled triaxial tests. The model is verified through comparisons of its results with those of laboratory tests. It is confirmed that the plastic volumetric strain that has accumulated in sand, either by drained or undrained cyclic loading, dominates the increase in the liquefaction resistance of the sand. However, the plastic volumetric strain caused by overconsolidation is less effective in reducing the volumetric strain potential for subsequent cyclic shearing, thus enhancing its resistance to liquefaction. The model provides a better understanding of the physical processes leading to the liquefaction of saturated and unsaturated sand with and without stress history.     

双向耦合循环剪切条件下饱和砂土体应变发展规律试验研究

砂土的剪胀与循环应力路径密切相关。针对饱和南京细砂,利用空心圆柱扭剪仪(HCA)进行了一系列均等固结条件下轴向–扭转耦合循环剪切排水试验,研究了复杂应力路径下饱和砂土的剪胀性及其体应变量化方法。研究表明:双向耦合循环剪切条件下饱和砂土的剪胀由一个完全可逆的循环体应变分量和一个不可逆的累积体应变分量构成,循环应力路径对累积体应变发展规律影响显著;以等效循环应力比ESR作为表征复杂应力路径下动应力大小的指标,饱和砂土累积体应变与ESR值具有事实上的唯一性关系,累积体应变随ESR的增加而线性累积;通过引入参数ESR,提出了双向耦合剪切条件下饱和砂土累积体应变规准化方法。验证性试验表明新的体应变增量模型的预测值与试验结果的吻合度较高,而基于循环直剪试验结果建立的Byrne模型对双向耦合剪切条件下饱和南京细砂的体应变预测偏小。     

循环振动对饱和粉土初始动剪模量的影响

初始动剪模量是土体多种结构性因素的综合反映,除固结压力和孔隙比(密度)这两个主要因素外,还受循环振动影响。以往对该因素的研究偏重于干砂,且预振时施加的剪应变幅多低于10-3;对于饱和土体,尤其是对经受过较大振幅振动后土体初始动剪模量的研究十分有限。本文利用安装弯曲元的动三轴仪,在等向固结条件下对不同相对密度的饱和粉土进行不排水循环三轴试验,同时监测试样的剪切波速,获得循环动力加载过程中土的初始动剪模量。为保证试验结果的可靠性,同时进行了共振柱试验,比较确定弯曲元接收信号的到达点;评估了不同孔压发展程度下试验暂停时间对测试结果的影响。结果表明:在一定次数大振幅循环荷载作用下观察到初始动剪模量有别于相同应力下固结后得到的模量值,出现附加衰减或增长,正是循环荷载作用下土微观结构变化的外部宏观表现。这种附加变化由土颗粒接触行为决定,受到试样初始条件和振动幅值等多种因素的影响,表现出不同的变化模式。     

Coupled analysis of bifurcation and shear band in saturated soils

The focus of this study is the analysis of the onset of a shear band in saturated soils under biaxial stress conditions taking into account bifurcation in both pore pressure and soil skeleton. By considering the saturated soil as a two-phase medium, the inhomogeneous displacement and pore pressure fields are assumed to be nonlinear and satisfy the continuity conditions of displacements, displacement gradients and excess pore pressure on the boundaries of the band. It is shown that bifurcation may take place in loose, contractive soils in the form of two-phase diffuse mode or localized mode of soil skeleton deformation. For low permeability saturated soils, the onset of a shear band occurs at peak friction, with the shear band parallel to Coulomb's failure plane. Numerical examples using a simple stress–strain model are given to demonstrate the occurrence of the onset of a shear band and its inclination, as a function of the void ratio and the initial consolidation pressure.     

In-Situ Evaluation of Strength and Dilatancy of Sands Based on CPT Results

In-situ tests have been increasingly used to estimate the shear strength of soils. In this paper, we propose methods to evaluate in-situ strength and dilatancy of sandy soils based on cone penetration test (CPT) results. It takes into account the silt content, relative density and stress state of the sand. A series of laboratory test results from fundamental property tests and triaxial tests are analyzed to develop methods for in-situ evaluation of strength and dilatancy for sands. Based on test results, modified and simplified dilatancy equations, in terms of the cone penetration resistance q_c and intrinsic soil variables, are proposed. Results from proposed and original dilatancy indexes show close agreements for various soil conditions. Values of intrinsic variables for the proposed dilatancy relationships were proposed as a function of silt content. Based on TX test results, a direct CPT-based correlation, applicable to both clean and silty sands, is proposed as well. In order to verify the proposed methods, calibration chamber CPT results obtained in this study and collected from the literature are adopted. It is observed that the results from the proposed methods show good agreement with the measured results.     

含黏粒砂土场地液化离心机振动台试验研究

提出了含黏粒砂土模型地基制备、饱和与均匀性监测技术,利用ZJU-400土工离心机振动台开展了相同相对密度含黏粒砂土（黏粒含量10%）和洁净砂的地震液化模型试验,再现了水平场地地震液化现象,揭示了含黏粒砂土场地液化灾变特点。弯曲元波速监测表明,模型制备均匀性良好,相同条件下含黏粒砂土剪切波速比洁净砂低。而根据超静孔压消散与固结沉降观测分析发现,含黏粒砂土渗透系数比洁净砂低一个数量级,从而影响其液化前后超静孔压响应和应力应变行为。渗透性差异导致模型内超静孔压产生模式和消散速率显著不同,振动时含黏粒砂土模型浅层超静孔压累积比洁净砂慢,而深层则相反;震后含黏粒砂土孔压消散时间是洁净砂的15倍。液化过程中含黏粒砂土剪应力应变响应比相同深度处的洁净砂更显著,液化后其滞回圈应变较大、割线模量较小且阻尼比较大。土体液化沉降主要发生在液化后超静孔压消散过程,含黏粒砂土模型超静孔压消散时间更长,沉降量更大。上述成果为进一步研究含黏粒砂土地震响应分析及其液化判别提供了科学依据。     

Quantitative analysis with plastic strain indicators to estimate damage induced by fault-slip

In the present study, methodologies to evaluate damage around an underground opening due to seismic waves arising from mining-induced fault-slip are examined. First, expressions for an associated flow rule with a failure criterion are developed for biaxial stress conditions, which are implemented into FLAC3D code. A three-dimensional (3D) mine model encompassing a fault running parallel to a steeply dipping orebody is constructed, whereby static and dynamic analyses are performed to extract stopes and simulate fault-slip in dynamic condition, respectively. In the analysis, the developed biaxial model is applied to the stope wall. The fault-slip simulation is performed, considering shearing of fault surface asperities and resultant stress drop driving the fault-slip. Two methodologies to evaluate damage caused by seismic waves arising from the simulated fault-slip are examined: (i) the ratio of dynamic plastic strain increment to elastic strain limit and (ii) plastic strain energy density. For the former one, two types of strain increments are tested, namely effective shear strain increment and volumetric strain increment. The results indicate that volumetric strain increment is a suitable index for detecting damage near the stope wall, while effective shear strain increment is appropriate for evaluating damage in backfill. The evaluation method with plastic strain energy density is found to be capable of assessing damage accumulated in an extensive area caused by rock mass oscillation due to seismic wave propagation. Possible damage to mine developments in the proximity of a stope is clearly described with the index. The comparison of the two methods clarifies that the former one assesses “instantaneous” damage, which is found to be different from “accumulated” damage calculated using plastic strain energy density, in terms of damage area and its location. It is thus concluded that the combination of the two methodologies leads to more accurate damage assessment as a proper measure against rockburst.     

非饱和砂土及黏土的水-力耦合双屈服面模型

使用精细的本构模型来同时分析非饱和砂土和黏土水力耦合特性依旧是一个挑战。提出了一种包含加载湿陷及剪切滑移双重塑性机制的水-力耦合模型。力学模型中选取Bishop应力作为应力变量,采用孔隙比和有效饱和度作为状态变量。模型中给出了孔隙比-平均土骨架应力的半对数坐标系内依赖于有效饱和度的临界状态线的表达式,其与剪切滑动屈服面的非关联流动准则的结合保证了模型可以更合理地预测非饱和土（包括砂土和黏土）的剪胀和剪缩性质。通过模拟粉砂和高岭土的三轴试验,对该模型预测非饱和土主要特征的能力进行了分析。     

液化循环流动土体动剪切特性试验研究

循环流动特性是剪胀性砂土液化变形的典型特征,为研究液化循环流动土体的动力剪切特性,在骨架相对密实度分别为35%、50%和80%的砂土中添加不同含量的细颗粒,以改变液化流动土体的重度,通过循环扭剪试验研究不同骨架密度、具有不同细粒含量的液化流动土体在大变形阶段的剪切模量及阻尼比的变化规律。试验结果表明：液化循环流动土体在流动大变形阶段仍具有一定的模量,模量随着应变的增大而逐渐减小;流动变形阶段的模量大小与液化土体的重度基本无关;强度恢复阶段模量与细粒含量及骨架相对密实度密切相关;液化大变形阶段卸载模量趋于稳定,其稳定值约为初始卸载模量的35%;阻尼比随剪应变的增大而先增大,当土体达到初始液化以后,阻尼比随剪应变的发展呈减小的变化趋势;对于相同骨架密度的土体,相变角随着细粒含量的增加而减小,临界状态线的斜率随着细粒含量的而增加而增大。