Stress-strain transformations and liquefaction of sands

Presented in this paper are the results of the laboratory tests of sands performed for the purpose of defining the characteristics of the dynamic shear stress-shear strain relationships. For this purpose, the transformation of the initial stress-strain characteristics of undrained saturated sands was investigated separately. These transformations take place under conditions of an increase in pore pressure under the effect of sufficiently intensive dynamic excitations. The process of occurrence and development of liquefaction was investigated simultaneously. The obtained results show that the transformation of the stress-strain relationships leads to intensive reduction in the initial dynamic characteristics of sands. At the moment of occurrence of initial liquefaction, for the selected strain, shear moduli are considerably reduced in respect to their initial values. These parameters tend to be further reduced in the phase of post-initial liquefaction. It is concluded that the process of liquefaction of sands can be completely defined through the transformation of the stress-strain relationships.     

小应变硬化土模型参数的确定与敏感性分析

地下工程施工引起的土体扰动区可分为剧烈扰动区、扰动区、微扰动区和未扰动区。为全面反映土体在扰动下的应力路径和力学响应,必须考虑全应变范围的土体特性,尤其是小应变范围内的力学响应,因此对小应变硬化土本构模型关键参数(初始剪切模量和剪应变阀值)的确定方法进行介绍。开展上海典型软土的三轴固结排水剪切试验和固结试验研究,给出确定上海软土小应变硬化土模型(HSSmall)参数的方法,建议采用原位测试的方法确定土体的初始弹性模量。基于土单元数值模拟进行初始弹性模量和剪应变阀值的参数敏感性分析。随着初始弹性模量的增大,初始压缩曲线与卸载-再压缩曲线的斜率均增大。由于对应的回弹模量不变,初始弹性模量与回弹模量的差值增大,应变与偏应力试验曲线的回滞环宽度也随之增大。随着剪应变阀值的增大,初始压缩曲线和再压缩曲线的近似直线段增长,在同样剪应力情况下,土体的应变值减小,土体保持初始弹性模量刚度的区间增大。     

双向动荷载下重塑红黏土动变形特性研究

利用SDT-20型三轴仪探究双向动荷载下红黏土动变形特性。结果表明:相位差为0其他条件相同时红黏土动剪切模量随径向动荷载幅值增加而减小;双向动荷载下红黏土动剪应变与振动次数近似呈指数型关系增长,并存在一个临界循环次数;随含水率升高和固结应力增大,重塑红黏土破坏模式由受拉破坏变为受压破坏;径向动荷载幅值的增加使重塑红黏土更容易发生受拉破坏;双向动荷载下阻尼比随动剪应变增加无明显规律,动剪应变小于1%时阻尼比的变化无规律,动剪应变大于1%时随动剪应变增加阻尼比处于稳定平衡阶段。含水率对阻尼比变化规律有明显影响,含水率小于20%时阻尼比随含水率增大而增大,含水率大于20%时其对阻尼比影响可以忽略。     

Simplified dynamic analysis to evaluate liquefaction-induced lateral deformation of earth slopes: a computational fluid dynamics approach

Lateral deformation of liquefiable soil is a cause of much damage during earthquakes, reportedly more than other forms of liquefaction-induced ground failures. Researchers have presented studies in which the liquefied soil is considered as viscous fluid. In this manner, the liquefied soil behaves as non-Newtonian fluid, whose viscosity decreases as the shear strain rate increases. The current study incorporates computational fluid dynamics to propose a simplified dynamic analysis for the liquefaction-induced lateral deformation of earth slopes. The numerical procedure involves a quasi-linear elastic model for small to moderate strains and a Bingham fluid model for large strain states during liquefaction. An iterative procedure is considered to estimate the strain-compatible shear stiffness of soil. The post-liquefaction residual strength of soil is considered as the initial Bingham viscosity. Performance of the numerical procedure is examined by using the results of centrifuge model and shaking table tests together with some field observations of lateral ground deformation. The results demonstrate that the proposed procedure predicts the time history of lateral ground deformation with a reasonable degree of precision.     

An efficient fiber beam-column element considering flexure–shear interaction and anchorage bond-slip effect for cyclic analysis of RC structures

In this paper, a fiber beam-column element considering flexure–shear interaction and bond-slip effect is developed for cyclic analysis of reinforced concrete (RC) structures. The element is based on conventional displacement-based Timoshenko beam theory, where the transverse shear deformation is included, and adopts the fiber model to describe the section force–deformation behavior. In the fiber model, shear deformation is assumed to be uniformly distributed along the section and is only resisted by concrete, thus the multi-dimensional concrete damage model is used for concrete fibers and therefore flexure–shear interaction is reflected naturally at the material level. Meanwhile, to account for the significant bond-slip effect at critical regions, the anchorage slip of bars at these regions is analytically derived. Then it is used to modify the uniaxial stress–strain model for steel fibers by assuming that the total strain can be treated as the sum of the bar deformation and anchorage slip, therefore the bond-slip effect is implicitly but simply represented. To validate the proposed element, a series of RC member and structure tests under cyclic loading are simulated. The results indicate that the proposed element can predict cyclic responses of RC structures, and can be used as a reliable tool for analysis of RC structures.     

不同应力水平下结构性黏土动力特性试验研究

对天津滨海地区典型结构性软黏土进行了压缩试验和不同固结应力下的循环三轴试验。结果表明:结构性软黏土的静力变形特性以先期固结应力和结构屈服应力为分界点可分为3个阶段;不同固结应力下的动应变-振次关系曲线上存在着一个临界动应力比,且该值随着围压的增大而降低;孔隙水压力的变化以结构屈服应力为转折点呈现出不同的变化规律;当振动频率为2Hz时固结应力的改变对轴向动应变的影响不明显。     

An experiment on temperature variations in sandstone during biaxial loading

The temperature response to stress–strain variations in rock is useful in developing an understanding of the thermodynamic property of crust. In this study, the temperature of sandstone during loading was investigated using laboratory biaxial testing. By changing the loading patterns, the deformation of a specimen was controlled to produce two distinct modes of strain: volume strain only and shear strain only. These strain modes were produced separately such that the temperature variation associated with the different deformation modes could be analysed. Experimental results indicate that temperature, as a scalar quantity, is notably sensitive to rock deformation. In the case of the volume strain, the temperature variation is positively correlated with the variation in the bulk stress. The temperature rises with the increase in hydrostatic pressure, and vice versa. In the case of the shear strain, experimental results repeatedly show two characteristics: firstly, there appears obvious increase in temperature in the area of pure shear strain, which is most likely related to local plastic deformation; secondly, the temperature drops in the area of tension during loading, whereas the temperature rises within the area of compression. This is to say, the state of crustal stress–strain should be obtained through the measurement of rock temperature.     

Flow deformation of liquefied sand under constant shear load and its application to analysis of flow slide of infinite slope

This paper intended to evaluate the behavior of saturated sand and sloped ground subjected to flow failure with seepage of pore water in the ground after earthquake and the resultant liquefaction. Triaxial compression tests of sand with constant deviator stress but changing of pore pressure and volume of the specimens were conducted in this study. It was revealed that the relation between the volume change and the amount of shear strain during deformation depended on the initial density of the sand but it did not much depend on shear stress and initial confining stress levels. Based on this test results and numerical analysis of the seepage of pore water in liquefied ground, a methodology was proposed to predict the deformation of inclined ground due to liquefaction.     

Nonlinear behavior of high-damping rubber bearings under horizontal bidirectional loading: full-scale tests and analytical modeling

Horizontal bidirectional loading tests are conducted for real-sized high-damping rubber (HDR) bearings with diameters of 700 mm (HDR700) and 1300 mm (HDR1300). The hysteresis loops of these bearings under bidirectional horizontal loadings are compared with those under unidirectional loadings. The results show that the bearing force measurement in the primary direction of loading increases when there is displacement in the orthogonal direction. Unusually, the maximum restoring force in the orthogonal direction to the primary loading direction occurs near zero displacement. On the basis of the observations of the restoring forces, a rate-independent model is proposed. This model simulates well the test results under both bidirectional loading and unidirectional loading. It can reproduce the irregular restoring forces characteristics around zero displacement as described above. Bidirectional loading induced twist deformation in the HDR bearings that increased local shear strains. This phenomenon results in an early failure as observed in HDR700. The additional shear strain is estimated based on the twist deformation measured by video image analysis. The comparison of the nominal total shear stress demonstrates that the increase of shear stress because of bidirectional loading occurs when the average shear strain is larger than about 200%. The larger the shear strain, the greater the bidirectional effect. It is shown that the nominal total shear stress of average strain of 350% under bidirectional circular loading pattern is approximately the same as the average shear strain of 400% under unidirectional loading. This means that the average shear strain of 350% under a bidirectional circular loading corresponds to a local shear strain of 400%. Copyright © 2012 John Wiley & Sons, Ltd.     

冻融循环作用下黄土累积塑性应变演变规律

为探究黄土路基在冻融循环和交通荷载耦合作用下的累积塑性应变变化规律,选取西宁地区重塑黄土为研究对象,采用GDS双向动三轴测试系统对其进行一系列动三轴试验,研究不同冻融循环次数、围压、动应力幅值以及频率对累积塑性应变的影响规律,并通过引入拟合参数建立考虑多因素的累积塑性应变预测模型。结果表明：累积塑性应变随冻融循环次数的增大而增大,在6次冻融循环后增长速率减缓且趋于稳定;减小动应力幅值和增大围压能显著抑制累积塑性应变的发展;加载初期累积塑性应变随频率变化不明显,随着振次的增加,频率作用凸显,累积塑性应变随着频率的增大而减小;基于试样的累积塑性应变演变规律,分别采用幂指数模型和对数模型进行拟合,发现后者拟合效果好;综合考虑4种因素对累积塑性应变的影响,建立累积塑性应变预测模型,并对试验的实测值与预测值进行对比,验证模型的可行性。研究成果可为季冻区黄土路基永久变形的计算提供理论参考依据。     

Postcyclic behavior of low-plasticity silt with limited excess pore pressures

This paper investigates the postcyclic behavior of low-plasticity silt with excess pore pressure ratio (R_u) less than 1. The testing specimens were prepared from Mississippi River Valley (MRV) silt. Full and no reconsolidation were allowed after specimens were subjected to various excess pore pressure ratios due to cyclic loading in a cyclic triaxial cell, and then monotonic shear tests were conducted. The effect of the R_u on shear strength and stiffness at small and large deformation was investigated. It was found that a R_u greater than 0.70 is a prerequisite of large increase in volumetric strain and undrained shear strength for specimens with full reconsolidation. In contrast, a significant decrease in yield shear strength and initial stiffness was noted for specimens without reconsolidation. In comparison to published data for sands, the silt experienced significant volumetric strain due to reconsolidation at lower R_u, indicating that the specimen fabric was modified or strained at lower R_u.     

地基土粉砂层动力特性分析试验研究

本文通过对某核电厂取水明渠导流堤地基土粉砂层室内共振柱试验及动三轴液化试验,测定了动剪切模量、阻尼比与动剪应变幅的双曲线关系,分析了粉砂的动力变形特性,探讨了砂土的抗液化强度与液化振次之间的乘幂函数关系,确定了该地基土的抗液化强度指标。为评价导流堤的地震稳定和液化分析提供了相关参数,同时对堤坝工程场地的地震安全性评价和液化评判有良好的借鉴和参考价值。     

筑坝料动力变形特性试验与颗粒流模拟

对筑坝反滤料进行室内中型动三轴试验研究,探讨残余变形等动力特性,在此基础上,运用二维颗粒流方法,对相关试验进行数值模拟,为筑坝料的动力特性研究提供一种新的思路和途径。试验结果表明：（1）应力水平对关系曲线斜率无影响;（2）初期体积应变的读数会出现偏小的现象;（3）围压和动应力值过大,会导致模拟结果产生较大误差;（4）正确的细观参数应依据特定的试验结果反复推敲逼近而得。     

土体剪切带的模拟与机理分析

采用修正剑桥模型,研究在平面应变条件下正常固结土剪切带的产生机理和剪切带形状的多样性。研究结果表明,剪切带是由材料的不均匀变形引起,并由试样局部应变能释放的冲击作用形成的;试样端部摩擦约束与试样内部初始缺陷都能够引起试样的不均匀变形进而诱发剪切带。分析了不同边界约束条件、初始缺陷设置方式及二者共同作用对剪切带形式的影响,平面应变条件下土体的剪切带有单一型、交叉型和多段型等3种形式,多段剪切带的发展具有先后顺序。     

循环荷载下液化对土层水平往返变形的影响

采用多工况振动台实验研究液化对土层水平往返变形的影响.以干砂实验为参照,分析孔压增长与土层加速度和土层往返变形之间的关系.结果表明:液化将引起土表加速度显著降低,减小惯性力传递,但同时会引起土层往返剪应变明显增大.对往返变形而言,液化土层往返剪应变就可达到1%～5%的大变形状态,且液化土层往返剪应变沿深度呈下大上小分布.土层中孔压比0.4～0.8是往返变形出现放大的敏感段,在孔压比0.8左右而不是在1.0达到最大.作为其结果,土层液化将对刚性上部结构振动起减震作用,但同时增大的往返剪应变也易导致基础和地下结构破坏,特别是对液化层与下部非液化层交界处的构件更敏感.     

桥梁隔震支座压缩剪切变形状态下竖向变形量研究

本文通过对不同参数的橡胶支座在压缩剪切变形状态下的竖向变形量进行了试验,分析了在压缩剪切变形状态下竖向变形量的特点。得出对RB支座,由剪切和弯曲产生的竖向变形随剪切应变增加而增大较快;对LRB支座,在达到一定剪切应变时,竖向变形量的增加缓慢。     

秦祁接合带造山缝合带磁组构特征及其构造意义

结合构造及磁化率各向异性研究详细解剖了秦祁接合带唐藏—关子镇—武山和新阳—元龙造山缝合带的应变及岩组特征.41个采点168个构造岩样品的平均磁化率全部较低,磁化率椭球形态分析表明其以平面和压扁应变为主,磁化率各向异性度普遍较高,属强变形岩石组构类型,结合野外观察认为其与变形强度明显正相关.此外,磁化率各向异性参数T、P′可能受岩石类型一定程度的影响.磁化率椭球主轴方位与变形密切相关,提供了丰富的岩组信息.两构造带具有类似的岩组特征,磁面理大致分为呈共轭形态的两组,暗示高应变剪切带在平面上可能以网格状形态出露;高倾伏角磁面理与占优势的低倾伏角、近水平磁线理表明了构造带明显的走滑特征,部分高角度磁线理可能与构造带的挤压和（或）转换挤压相关;磁组方法不能简单用于判别复杂强变形带的运动指向,糜棱面理的复杂变化及K_min与构造带夹角过高使其判别结果意义不明,而野外及显微构造观察都表明了构造带的右行走滑特征.上述结果表明,沿缝合带大规模的右行转换挤压形成了秦祁接合带反“S”型的平面构造形态,暗示在南北板块拼合过程中,西秦岭诸中、小块体一定程度的向西挤逸.     

Influence of a cyclic and dynamic loading history on dynamic properties of dry sand, part I: cyclic and dynamic torsional prestraining

Initial fabric of a soil induced by its cyclic strain history is an important parameter together with the void ratio, state of stress and amplitude in respect to further accumulation of deformations under drained cyclic loading. It is of importance for the further deformation prediction to determine the initial fabric of the grain skeleton or the cyclic loading history of the soil. An attempt is made within this paper to correlate small strain stiffness of non-cohesive soil with its cyclic loading history. The results of performed cyclic and dynamic torsional tests show that small strain shear modulus is only moderately affected by cyclic prestraining even if high amplitudes are applied. A signature of prestraining history is observed in the tests since the sand memorizes its prestraining amplitude and the number of applied cycles.     

Experimental study on dynamic properties of sand with emphasis on the degree of saturation

Cyclic drained and undrained tests were performed in a modified simple shear device in order to specifically investigate the effect of saturation ratio on shear modulus and damping parameters of sand. In addition to the degree of saturation, the effects of number of cycles, vertical consolidation stress, shear strain amplitude and relative density on dynamic properties of sand were investigated. Dynamic stiffness and damping characteristic were found to be substantially independent of saturation ratio in the range of 25−75%. However, by approaching the full saturation state, the values of modulus fall sharply and damping of loose samples increases dramatically from corresponding values of unsaturated levels. Results of drained tests revealed that most of increase in shear modulus and decrease in damping take place in the first 30 cycles. It was found that vertical consolidation stress is the most important parameter which affects shear modulus and damping ratio of sand. Similarly, relative density or void ratio significantly affects dynamic stiffness and damping under undrained conditions. Moreover, the influence of shear strain amplitude should be considered for both drained and undrained tests.     

The effect of plastic fines on the pore pressure generation characteristics of saturated sands

Pore water pressure generation during earthquake shaking initiates liquefaction and affects the shear strength, shear stiffness, deformation, and settlement characteristics of soil deposits. The effect of plastic fines (kaolinite) on pore pressure generation in saturated sands was studied through strain-controlled cyclic triaxial tests. In addition to pore pressure generation, this experimental study also focused on evaluating the threshold shear strain for pore pressure generation and the volumetric compressibility of specimens during pore pressure dissipation. The results reveal that specimens having up to 20% plastic fines content generated larger values of pore water pressure than clean sand specimens. At 30% fines content, the excess pore water pressure decreased below that of clean sand. The threshold shear strain, which indicates the strain level above which pore pressures begin to generate, was assessed for different kaolinite–sand mixtures. The threshold shear strain was similar for 0–20% fines (γ_t0.006–0.008%), but increased to about 0.025% for 30% fines. The volumetric compressibility, measured after pore pressure generation, was similar for all specimens. The transition of behavior at fines contents between 20% and 30% can be attributed to a change in the soil structure from one dominated by sand grains to one dominated by fines.