循环荷载下南京片状细砂的动应力——应变关系

以片状颗粒成分为主的片状结构砂与常用的圆形颗粒标准石英砂相比,在物理力学特性上有显著的差异。循环荷载作用下,饱和砂土振动孔压上升会导致土体刚度发生软化,当振动孔压累积达到一定水平时,会产生液化现象,从而引起土体结构发生破坏。采用英国WFI动三轴仪,研究了南京片状细砂在循环荷载作用下,静偏应力水平、循环应力比水平和循环次数对其动应力一应变关系的影响,考虑每一次循环过程中动应力—应变关系滞回曲线的卸载及再加载割线动剪切模量Gsec和最大割线模量Gmax的变化特性,建立了动剪模量软化的经验公式;静偏应力水平对动剪模量软化有显著影响,随着循环次数的增加,动应力—应变滞回圈逐渐向应变累积方向滑移和向应变轴方向倾斜,且彼此分离;考虑循环软化特性,采用修正的Masing准则,描述了循环荷载下南京片状细砂的动应力—应变关系。     

自由场典型液化特征数值模拟试验

应用FLAOD实现自由场液化数值模拟试验.试验结果验证了砂土液化典型特征:超静孔隙水压升高,有效应力降低,体积压缩积累增大.证实了液化的隔振作用:砂土在液化状态变为流体,不能传递剪力,液化时砂土位移、速度、加速度振幅显著降低,剪应力降低,动水向上渗流,土体向下沉降,水平残留不可恢复位移.球压应力、有效压应力、动水压力满...     

荆江大堤下伏饱和粉细砂动力特性试验研究

针对荆江大堤江陵段下伏地层广泛分布的饱和粉细砂,参照原位试验成果重塑粉细砂试样,按估算的固结应力比(Kc约为1.6)对试样动剪模量、阻尼比及总应力动强度进行测试,结果表明:(1)试样应力-应变骨干曲线与Hardin-Drnevich双曲线模型假设高度吻合,Hardin公式可很好地拟合动模量/阻尼比与动应变的关系。在研究试样密实度范围内,最大动模量随围压和密实度的增加而增加,但围压对动模量的敏感性更高,且相同围压下动剪模量比与动应变关系曲线近乎重合。围压增大或密实度升高均会引起阻尼比的降低,1%应变对应的阻尼比分布在0.15~0.21之间;(2)偏压状态下以累积轴向应变5%作为液化判别标准进行抗液化强度试验,随特征振次及测试围压的增大,液化动剪应力比相应减小,试样振动孔压比最高仅能达到0.8~0.9;(3)由总应力法求取的动内摩擦角与黏聚力均随设定特征振次的增加而下降,且内聚力并非约等于0,表明动力作用下该试样具有一定的黏滞性。     

基于累积耗损能量的饱和粉土液化特性试验研究

饱和粉土场地在强地震作用下易发生液化现象。开展饱和粉土的循环三轴试验,以循环加载的累积耗损能量为指标,探讨黏粒含量、密实度、有效围压和循环应力比等因素对粉土液化特性的影响,试验结果表明:粉土液化所需的耗损能量随黏粒含量的增加呈先减小后增大的趋势,当黏粒含量约为8%时粉土的液化耗损能量最低;液化耗损能量随粉土密实程度的增大而逐渐增加,并随初始有效围压的增长而增加,但粉土的液化耗损能量与循环应力比之间的关系不明显。     

基于OpenSees的砂土本构模型对比研究

饱和砂土液化是由地震引起的一种最常见的工程地质现象,也是造成重大地震灾害的主要原因之一。由于成因的复杂性和所造成灾害的严重性,饱和砂土液化一直是土动力学和岩土地震工程研究领域的重要课题。针对饱和砂土液化问题,基于开源地震工程数值计算平台OpenSees,对材料库中的4种砂土本构模型进行数值计算。采用二维u-p单元模拟土颗粒位移和孔隙水压力,分析和对比4种模型在循环动力荷载作用下的加速度、超孔隙水压力、位移、剪应力-剪应变和平均有效应力路径方面的响应结果。研究结果表明:(1)砂土对输入加速度表现出一定的放大效应,对于不同的模型,该放大效应存在一些差异;(2) Stress Density模型在循环动力荷载作用下易产生永久变形;(3)在循环动力荷载作用下,PDMY模型和CycLiqCPSP模型的强度逐渐降低,直到完全消失;(4) Stress Density模型和Manzari Dafalias模型在循环动力荷载下表现出明显的剪胀效应。研究成果对砂土液化的数值模拟问题具有重要的理论价值,可为饱和砂土的液化模拟和砂土本构模型的选取提供参考。     

饱和南京细砂孔压增量模型的普适性研究

循环荷载作用下饱和砂土的孔压增长规律是土动力学的核心内容之一。基于笔者在给定相对密度、均等固结条件下饱和南京细砂的不排水等幅循环三轴试验结果建立的孔压增量模型,进一步进行了不同相对密度、不同固结比条件下饱和南京细砂的不排水等幅循环三轴试验,将上述均等固结的孔压增量模型拓展为适用于不同相对密度、均等和非均等固结条件的孔压增量模型。采用拓展后的孔压增量模型对试验结果进行分析的结果表明：通过该孔压增量模型预测验证试验的孔压与验证试验测试的孔压具有较好的一致性,说明该孔压增量模型具有普适性。     

Simulation of torsional shear test results with neuro-fuzzy control system

In the first part of this study, a series of stress-controlled hollow cylinder cyclic torsional triaxial shear tests were conducted on loose to medium dense saturated samples of clean Toyoura sand to investigate its liquefaction behavior. A uniform cyclic sinusoidal loading at a 0.1 Hz frequency was applied to air-pluviated samples where confining pressure and relative density was varied. Cyclic shear stress–strain changes, the number of cycles to reach liquefaction and pore pressure variations were recorded. Results indicate that the liquefaction resistances of uniform sands are significantly affected by the method of sample preparation and initial conditions.     

复杂应力条件下松砂振动孔隙水压力与体变特性的试验研究

利用新研制的土工静力-动力液压三轴-扭转多功能剪切仪,在5种初始主应力方向角与5种中主应力系数相组合的初始固结条件下,对饱和松砂进行了不排水循环扭剪试验。讨论了初始固结条件对不排水条件下饱和松砂孔隙水压力变化规律及对剪胀、剪缩、卸荷体缩等体积变化过程的影响。试验研究表明：（1）分别以稳定残余孔隙水压力和破坏时循环次数归一化后的残余孔隙水压力比和循环次数比之间的关系可以用双曲线模式表达。其参数主要依赖于初始主应力方向,中主应力系数对参数的影响并不显著。归一化后的孔隙水压力比与广义剪应变之间的关系也可以用双曲线模式表达,其中的2个待定参数依赖于初始主应力方向,与中主应力系数无关;（2）在三向非均等固结条件下的不排水循环扭剪试验中,饱和松砂表现出卸荷体缩特性,不同初始主应力方向时,饱和松砂剪缩、剪胀、卸荷体缩呈现出不同的交替变化模式。     

含微生物气泡高饱和砂土循环三轴剪切CFD-DEM模拟

为探究砂土液化的微观机理,根据室内试验中微生物反硝化反应气泡的生成速率,建立数值模拟的时效性关系,分别制取微生物处理0天、2天、3天和5天的高饱和砂土试样,采用CFD-DEM耦合方法模拟不同工况下砂土试样的循环三轴不排水剪切试验.依据砂土试样的力链分布、抗液化振次、孔压比、轴向应变和力学配位数在加载过程中的变化情况,从...     

复杂应力条件下饱和松砂单调与循环剪切特性的比较研究

本文利用大连理工大学新引进与开发的“土工静力-动力液压-三轴扭转多功能剪切仪”,针对福建标准砂,在不排水条件下同时进行了单调剪切试验与循环剪切试验,进而对其进行了对比分析。通过比较表明,应力-应变关系的应变软化和硬化特性与流滑变形和循环流动特性密切相关,当循环剪切应力水平高于单调剪切过程中应变软化阶段最小强度时将会发生流滑变形。无论在单调剪切中,还是在循环剪切中,稳定状态时的有效偏应力比随着大主应力方向与竖向之间夹角的增大而减小,在中主应力系数相同的条件下,循环剪切中呈现显著剪胀时的有效偏应力比和最终稳定状态时的有效偏应力比峰值分别与单调剪切中达到相变状态时的有效偏应力比和最终稳定有效偏应力比基本上一致。然而不排水条件下单调与循环剪切过程中孔隙水压力的增长特性却并不相同,循环剪切中的最大孔隙水压力随着初始主应力方向角的增大而减小,单调剪切中的最大孔隙水压力却随着主应力方向角的增大而增大。     

振动频率对饱和砂土液化强度的影响

采用“土工静力-动力液压三轴-扭转多功能剪切仪”对饱和砂土进行了一系列动三轴实验,探讨了振动频率对液化强度数值的影响程度。在1．0、1．5固结比和0．05、0．10、1．00Hz振动频率条件下,针对相对密实度分别为70％、28％的密砂和松砂进行了100、200、300kPa围压和100kPa围压条件下的液化强度实验。实验结果表明,饱和密砂和松砂在各种固结条件下,液化强度随着振动频率的增大而增大,相同破坏振次时,各种实验条件下的液化强度与振动频率的关系在双对数坐标上均符合线性关系;振动频率由0．05Hz变化到1．00Hz时,液化强度相差达25％以上;动强度指标翰值随振动频率的增大而增大,最大相差12．2％;随着振动频率的增大,砂土达到液化破坏所需的时间明显缩短;振动频率对松砂液化强度的影响比对密砂的影响更为显著。     

Cyclic liquefaction strength of sands

Through an energy approach, a model is proposed to predict the cyclic liquefaction strength of saturated sands in terms of their static shear strengths. Plots of cyclic liquefaction strength versus relative density and also versus modified standard penetration resistance are presented for various uniformity coefficients and different numbers of stress cycles. The predicted cyclic liquefaction strength values are converted to cyclic stress ratios and compare favourably with Seed's empirical correlations.     

Physical modeling of lateral spreading induced by inclined sandy foundation in the state of zero effective stress

The state of zero effective stress is a situation at which the effective stress of saturated sand decreases to zero in the process of liquefaction. In the state of zero effective stress, sand particles suspend in water and the foundation is vulnerable to much large lateral deformation. The state of zero effective stress can be achieved through dynamic loading tests, but the obtained state is difficult to sustain a steady situation. To simulate the suspended sand in water under fully liquefied condition, plastic sand, characterized by small specific gravity, is used instead of quartz sand to build an inclined foundation. Salt water with similar density is used to pass in slowly near bottom of the foundation. As observed in tests, the plastic sand is able to suspend in sodium chloride solution (salt water) of a specific density and thus this model can be used to simulate the lateral spreading of a foundation under zero effective stress state. Lateral deformation occurs within a certain depth beneath the ground and the magnitude increases from the bottom up, showing nonlinear behaviors. This paper presents a physical modeling approach for achieving the state of zero effective stress under static laboratory condition.     

南京片状细砂不排水循环三轴试验的数值仿真

基于软件ABAQUS的二次开发平台,引入Byrne修正的Martin和Finn孔压应变模型,并与土体动力黏塑性记忆型嵌套面模型相结合,通过编制Fortran子程序,嵌入到软件ABAQUS中,实现了软件ABAQUS的有效应力算法,对南京片状细砂土的不排水循环三轴试验进行三维数值仿真。通过与循环三轴试验结果的比较,发现数值模拟结果基本上能再现南京片状细砂的不排水循环特性,如不排水循环三轴试验的动应力?应变关系、应变累积、应变软化等;且数值仿真结果较好地反映了试验结果的振动孔压发展特征。研究的意义在于理想三维数值模拟试验能够突破常规的室内土工试验能力及其局限性,这为今后饱和砂土不排水循环特性的宏、细观机理的进一步研究奠定了基础。     

Cyclic shearing and liquefaction of soil under irregular loading: an incremental model for the dynamic earthquake-induced deformation

The paper presents a mathematical model for the deformation of soil under irregular cyclic loading in the simple-shear conditions. The model includes the possible change in the effective pressure in saturated soil due to the cyclic shearing, the reciprocal influence of the effective pressure on the response of the soil to the shear loading, and the pore pressure dissipation due to the seepage of the pore fluid. The hysteresis curves for the strain–stress relationship are constructed in such a way that they produce both the required backbone curve and the required damping ratio as functions of the strain amplitude. At the same time, the approach enables the constitutive functions involved in the model to be specified in various ways depending on the soil under study. The constitutive functions can be calibrated independently of each other from the conventional cyclic shear tests. The constitutive model is incorporated in the boundary value problem for the dynamic site response analysis of level ground. A numerical solution is presented for the dynamic deformation and liquefaction of soil at the Port Island site during the 1995 Hyogoken-Nambu earthquake.     

饱和砂土液化的动三轴试验判断与评价

基于动三轴砂土液化试验,采用抗液化剪应力判剐方法和地震反应分析计算结果与地震剪应力时程相结合的综合判别方法,对某长江大桥工程场地的饱和砂土液化进行了判断,并对其液化危害程度进行了等级划分,给出了不同超越概率下的预测结果。     

Influence of fines content on liquefied strength of silty sands

Critical state soil mechanics is a useful framework to understand sand behavior. In this paper, a relationship is developed for estimating undrained critical shear strength of sands based on the critical state framework. The relationship is validated by comparison with laboratory test results and sand liquefied strength from field liquefaction failure case histories. Using this relationship, the influence of fines content on undrained critical shear strength is studied for different combinations of effective stress and density. The parametric study indicates that depending on soil void ratio, effective stress, and the shape and mineralogy of the fine particles, undrained critical strength may increase, remain the same, or decrease as the amount of fines content increases. Both the susceptibility to liquefaction and the severity of strain-softening are affected by adding fines. It is suggested that the critical state parameter is inadequate for describing the behavior of liquefiable sands and sand shearing-compressibility should be taken into account in liquefaction analysis.     

Experimental investigation on fluid characteristics of medium dense saturated fine sand in pre- and post-liquefaction

Liquefaction of saturated loose sand is a major cause of extensive damage to buildings and infrastructures during large earthquakes. A better understanding of the behaviour of liquefied soil is becoming increasingly necessary to mitigate earthquake damage, and the fluid method has become an increasingly popular means to study the behaviour of liquefied soils. The purpose of this study is to determine the fluid characteristics of liquefied fine sand. In this paper, the apparent viscosity was measured as an index of fluid characteristics using the shaking table tests of pre-liquefaction behaviour of saturated fine sand at approximately 45 % relative density; the relationship of apparent viscosity and shear strain rate on liquefying fine sand was indicated as a power-law shear-thinning non-Newtonian fluid; and liquefying fine sand has the alternating behaviour of shear dilatancy and compressibility during cyclic loading. Additionally, a series of a monotonic axial compression loading tests in an undrained manner were performed to measure the shear stress and excess pore pressure ratio relationship on the post-liquefaction saturated fine sand at approximately 50 % relative density. The fluid characteristics of post-liquefaction fine sand exhibits rate dependence and can be described by a combined fluid model of time-independent and time-dependent power-law functions; the time-independent viscous resistance is not relevant to the excess pore pressure ratio; but the time-dependent frictional resistance is closely related to the excess pore pressure ratio. Furthermore, the results of the verification tests demonstrate that the proposed fluid model has good applicability for the fluid behaviour of the post-liquefaction fine sand.     

Stiffness degradation of natural fine grained soils during cyclic loading

Cyclic behavior of natural fine grained soils under a broad range of strains were investigated considering the effects of plasticity index and changes in confining pressures based on cyclic triaxial tests. A total of 98 stress controlled cyclic triaxial tests were conducted on normally consolidated and slightly overconsolidated samples. The investigation was divided into two parts. The first part consists of stress controlled cyclic triaxial tests under different stress amplitudes that were conducted to estimate the modulus reduction and the thresholds between nonlinear elastic, elasto-plastic and viscoplastic behavior. The second part involves the investigation of the undrained stress–strain behavior of fine grained soils under irregular cyclic loadings. The results showed that the elastic threshold is approximately equal to 90% of G_max. Another transition point was defined as the flow threshold where the value of tangent of shear modulus ratio changes for the second time. Simple empirical relationships to estimate the dynamic shear modulus and damping ratio was formulated and compared with the similar empirical relationships proposed in the literature. The results provide useful guidelines for preliminary estimation of dynamic shear modulus and damping ratio values for fine grained soils based on laboratory tests.     

列车振动荷载作用下南京细砂动强度变化规律研究

以南京细砂为研究对象,采用空心圆柱扭剪仪模拟列车振动荷载作用对应的土体单元所受应力路径,考虑试样围压、加载幅值和排水条件,研究列车振动荷载作用下土体动强度的变化规律。试验结果分析表明:在模拟列车振动荷载作用对土体竖向应变的影响时,采用椭圆应力路径来代替心形应力路径是切实可行的,能克服试验仪器高频加载时无法有效模拟心形应力路径的缺点;其次,当围压和加载幅值都较小时南京细砂主要表现为在振动初期强度强化特征,当振动次数达到一定数量后强度也达到一个稳定阶段;当试验围压较大时,随振动次数增加,南京细砂的强度变化主要以强度弱化阶段为主;同时,排水条件对其强度变化的主要影响表现为对其振动前期强度强化阶段的影响,对其强度弱化阶段的影响并不明显。