Effects of small and large shear histories on multiple liquefaction properties of sand with initial static shear

It has been reported that soils belonging to slope grounds show different types of liquefaction behavior than those belonging to horizontal grounds. Some research has also revealed that liquefaction histories can significantly affect the shear behavior of sandy soils. However, the combined effects of the slope angle and the magnitude of past shear histories on the liquefaction properties of soils have not been studied comprehensively. Based on this background, several multiple liquefaction tests with initial static shear were conducted on Toyoura sand. In each of these tests, a single specimen was sheared several times up to small or large double amplitude shear strain under a constant volume condition using a specially designed stacked-ring shear apparatus. The behavior of the Toyoura sand observed in these tests was discussed considering various perspectives, such as the increase in relative density, the induced anisotropy, the change in liquefaction resistance, and the shear strain accumulation. The findings of this study established that shear histories of smaller magnitude had relatively less influence on densification and induced anisotropy than those of larger magnitude. Moreover, shear histories of smaller magnitude also resulted in the relatively higher liquefaction resistance of sand specimens against the next cyclic shear, while the opposite trend was observed in the case of specimens subjected to shear histories of larger magnitude. Finally, shear strain accumulated less easily in tests with small shear histories than in those with large shear histories.     

Deformation and cyclic resistance of sand in large-strain undrained torsional shear tests with initial static shear stress

This paper presents the findings from an experimental study focusing on the undrained cyclic behavior of sand in the presence of initial static shear stress. A series of undrained cyclic torsional shear tests was performed on saturated air-pluviated Toyoura sand specimens up to single amplitude shear strain ($\gamma$_SA) exceeding 50%. Two types of cyclic loading conditions, namely, stress reversal (SR) and stress non-reversal (SNR), were employed by changing the amplitude of the combined initial static shear and cyclic shear stresses. The tests covered a broad range of initial states in terms of relative density (Dr = 20–74%) and the initial static shear stress ratio (α = 0–0.30). The following five distinct modes of deformation were identified from the tests based on the density state, the transient undrained peak shear stress, and the combined cyclic and static shear stresses: 1) static liquefaction, 2) cyclic liquefaction, 3) cyclic mobility, 4) shear deformation failure, and 5) limited deformation. Of these, cyclic liquefaction and static liquefaction are the most critical. They occur in very loose sand (D_r ≤ 24%) under SR and SNR, respectively, and are characterized by abrupt flow-type shear deformation. Cyclic mobility occurs under SR in loose to dense sand with D_r ≥ 24%. Contrarily, shear deformation failure typically occurs under SNR in sand with 24 < D_r < 65%, and limited deformation may take place in dense sand with D_r ≥ 65%. In this paper, a stress-void ratio-based predictive method is proposed to identify the likely mode of deformation/failure in sand under undrained shear loading with static shear. Furthermore, the cyclic resistance is evaluated at three different levels of $\gamma$_SA (i.e., small, $\gamma$_SA = 3%; moderate, $\gamma$_SA = 7.5%; and large, $\gamma$_SA = 20%). The results show that, independent of the density state, the cyclic resistance continuously decreases with an increase in α at the small $\gamma$_SA level, while it first decreases and then increases for both loose and dense sand at the moderate and large $\gamma$_SA levels.     

Effects of specimen density and initial cyclic loading history on correlation between shear wave velocity and liquefaction resistance of Toyoura sand

Some previous studies have shown a good correlation between the shear wave velocity, V_s, and the cyclic resistance ratio, CRR. Recently, however, a V_s-based liquefaction assessment method has become an alternative and supplementary method to the conventional N_SPT-based method. It is known that the CRR is influenced not only by the specimen density, but also by the soil fabric. Unfortunately, there are concerns that different combinations of the effects of the specimen density and the soil fabric may generate different relations between V_s and the CRR even if the tested specimens are of the same soil material. In the current study, a series of V_s measurements and undrained cyclic triaxial tests is performed on Toyoura sand specimens with different soil fabrics for three different specimen densities. The fabric of the specimens is varied by applying initial cyclic loading. The results of the V_s measurements indicate that the V_s of the specimen is affected by the initial cyclic loading histories, and the results of the undrained cyclic triaxial tests show that there is a good correlation between V_s and the CRR. However, the correlation varies depending on the specimen density even when the tested material is Toyoura sand only. In other words, the soil-type specific correlation between V_s and the CRR depends on the specimen density. Therefore, the results indicate that both V_s and the specimen density are necessary parameters for an accurate assessment of the CRR.     

初始剪应力对饱和粉土液化特性影响试验研究

为探寻饱和粉土的液化特性,利用GDS空心圆柱仪进行了一系列循环扭剪试验。在初始剪应力τs和循环剪应力τcy共同作用下,试样的最小剪应力τmin=τs-τcy存在3种类型:τmin0,τmin=0和τmin0。试验结果表明:当τmin≤0时,试样的孔压可以达到有效围压,其破坏模式为循环液化;当τmin0时,试样的孔压始终达不到有效围压,其破坏模式为过大的累计应变。饱和粉土的循环强度随着初始剪应力τs与初始有效平均主应力0p′之比值SSR(初始剪应力比)的增加呈现出先减小后增大变化趋势,且SSR=0.1~0.15时的循环强度最低。当SSR≤0.1时,孔压比的发展模式随着循环剪应力比的增加由"快—平稳—急剧"的增长模式向"快—平稳"的增长模式转变;当SSR0.1时,孔压比的发展呈现"快—平稳"的增长模式。     

Influence of strain histories on liquefaction resistance of sand

The liquefaction resistance of sand increases with cyclic pre-shearing and pre-shaking as a result of earthquakes if the strain level in the pre-shearing is small. When larger shear strains are imposed, liquefaction resistance decreases. These complicated effects of pre-shearing histories on the liquefaction resistance are investigated in this study through a series of cyclic triaxial tests. Various combinations of cyclic stress amplitude and number of cycles of pre-shearing are examined. The tested sand is Toyoura Sand at 45% relative density, under a confining pressure of 50 kPa. Test results indicate that for the range of shear strain amplitude in pre-shearing smaller than 0.35%, the liquefaction resistance increases with pre-shearing. The increase in the liquefaction resistance depends strongly on the volumetric strain in the pre-shearing, and several effects of the shear stress amplitude and number of cycles can be negligible. Small volumetric strain of the order of 1% doubled the liquefaction resistance. Meanwhile, in the range of shear strain amplitude larger than 0.6%, the liquefaction resistance decreases. The liquefaction resistance decreases as the shear strain amplitude increases. Shear strain amplitude is one of the factors dominating this degrading effect, and the volumetric strain exerts beneficial effects to a certain extent. In this study, another series of tests are conducted to investigate the combined effects of small and large strain amplitude pre-shearing. It is observed that small shear strain pre-shearing cycles subsequent to large shear strain cycles erased the degrading effect of the latter. However, a large shear strain pre-shearing after small strain cycles degrades the beneficial effect of the small shear strain pre-shearing cycles previously applied to the specimens; however, the effects of the former small strain pre-shearing remains.     

复杂应力路径下饱和砂土静态液化失稳预测

饱和砂土应力–应变关系具有各向异性和状态相关特性,因此其静态液化的触发与应力路径密切相关。现有文献已提出多种静态液化判别准则,但其准确性往往仅在三轴应力路径下被进行校验。对于涉及主应力方向旋转和不同中主应力比的复杂应力路径,现有判别准则是否可以准确预测静态液化的触发有待进一步验证。为此,基于状态相关各向异性砂土本构模型,结合前人的空心圆柱扭剪单元体试验,比较了二阶功、弹塑性刚度矩阵对称部分和失稳模量3种准则对复杂应力路径下砂土静态液化失稳预测的效果。发现包括:基于弹塑性刚度矩阵对称部分的失稳触发表达式不依赖于加载路径,具有更好的通用性,其预测的失稳点早于或与实际失稳点吻合;失稳模量理论可预测实际液化失稳的位置,但判定表达式因加载条件不同而变化。获得了复杂应力路径下的失稳线,分析了静态液化触发前砂土可发挥峰值摩擦角受中主应力、主应力方向等因素的影响。     

初始静孔隙水压力对砂土静动力剪切特性影响的试验研究

选取福建标准砂和滹沱河细砂,利用空心圆柱扭剪仪开展了一系列不同初始静孔隙水压力条件下的不排水循环扭剪试验和单调扭剪试验,着重探讨初始静孔隙水压力对超静孔隙水压力发展及其不排水抗剪强度的影响。试验结果表明:初始静孔隙水压力对超静孔隙水压力的发展产生显著的影响,从而影响砂土的静动力剪切特性。具体地,在不排水循环剪切过程中,初始静孔隙水压力越大,其超静孔隙水压力发展和变形发展越快;在不排水单调剪切过程中,初始静孔隙水压力越大,在砂土剪胀阶段产生负超静孔隙水压力越大,从而使砂土的强度显著提高。基于试验结果,初步探讨了初始静孔隙水压力对超静孔隙水压力及静动力剪切特性的影响机理。研究表明,研究地下水位以下土体(准饱和土)静动力剪切特性尤其是研究液化问题时,应充分考虑初始静孔隙水压力对砂土抗液化强度的影响,室内试验应根据砂土所处的地下水位深度来决定初始静孔隙水压力(反压)的大小。     

Microscopic investigation into liquefaction resistance of pre-sheared sand: Effects of particle shape and initial anisotropy

Liquefaction resistance of sand can be either increased or reduced due to an undrained cyclic pre-shearing depending on the degree of pre-shearing, which hinders a better prediction of liquefaction potential to be established. The mechanism of such changes in liquefaction resistance has been poorly understood. This contribution aims to gain micromechanical insights into pre-shearing effects on liquefaction resistance of sand using discrete element method (DEM) simulations. In particular, effects of particle shape and initial anisotropy on liquefaction resistance are investigated. The simulation results from samples consisting of non-spherical particles with an initial anisotropy can qualitatively capture the mechanical responses observed in equivalent laboratory experiments. The samples which yielded a qualitative agreement with the laboratory results are further analyzed micromechanically, and the relationships between liquefaction resistance and some microscopic parameters before cyclic loading are discussed. Microscopic analyses reveal that mean mechanical coordination number is well correlated with liquefaction resistance, whereas liquefaction resistance is less sensitive to anisotropy in particle orientation induced by pre-shearing.     

饱和砂土液化后的剪切吸水效应

饱和土在自然排水条件下随着剪切作用而发生的含水量增大的行为和过程被称为剪切吸水现象。本文发展了一种新的试验方法,成功地再现了从较松到密实状态的饱和砂土在振动液化后的剪切吸水现象,探讨了剪切吸水速率对饱和砂土液化后应力应变关系的影响。试验结果表明,具有相同密度的饱和砂土,其液化后的应力应变关系随剪切吸水速率的不同而呈现出硬化、理想塑性及软化的不同变化。文中给出了判定这些不同的液化后应力应变响应产生的条件     

Influence of inherent anisotropy on the seismic behavior of liquefiable sandy level ground

Inherent anisotropy is a crucial aspect to consider for an improved understanding of the strength and deformation characteristics of granular materials. It has been the focus of intense investigation since the mid-1960s. However, inherent anisotropy’s influence on ground seismic responses, such as liquefaction, has not been extensively studied. In this paper, inherent anisotropy’s influence on ground seismic responses is examined through a series of dynamic centrifuge model tests on liquefiable level sand deposits. During the model setup, five different deposition angles (0, 30, 45, 60, and 90 degrees) were achieved using a specially designed rigid container. The models were exposed to tapered sinusoidal input accelerations and the recorded results were fully investigated. It was found that deposition angle-caused inherent anisotropy significantly influenced the excess pore pressure responses during the shaking and dissipation phases. The amount of excess pore pressure build-up and the high excess pore pressure duration increased with the deposition angle, while the dissipation rate decreased as the deposition angle increased. The inherent anisotropy also influenced liquefaction-induced ground settlement, with volumetric strain increasing along with the deposition angle. With respect to response acceleration, inherent anisotropy’s effects depended on the amount of excess pore pressure build-up (i.e., degree of liquefaction). In view of these results, it was concluded that a sandy ground, deposited at a higher angle (i.e., closer to 90 degrees), is more susceptible to liquefaction and that inherent anisotropy’s influence should be considered when evaluating the liquefaction potential and performing effective stress analyses.     

Effects of initial static shear on liquefaction and large deformation properties of loose saturated Toyoura sand in undrained cyclic torsional shear tests

This study focused on the role which static shear plays on the large deformation behavior of loose saturated sand during undrained cyclic loading. A series of undrained cyclic torsional shear tests was performed on saturated Toyoura sand specimens up to single amplitude shear strain exceeding 50%. Three types of cyclic loading patterns, i.e., stress reversal, intermediate and non-reversal, were employed by varying the initial static shear level and the cyclic shear stress amplitude. The observed types of failure could be distinguished into liquefaction (cyclic and rapid flow) and residual deformation by comparing both monotonic and cyclic undrained behavior. It was found that the presence of initial static shear does not always lead to an increase in the resistance to liquefaction or strain accumulation; they could either increase or decrease with an increasing initial static shear level depending on the type of loading pattern and failure behavior. In addition, according to the failure behavior which the specimens exhibited, three modes of development of large residual deformation were observed.     

初始剪应力下剪切方向角对软黏土软化特性的影响

 边坡、挡土墙等结构中的土体一般存在初始剪应力的作用,同时在地震、波浪等动荷载作用下,土体的初始剪应力与动荷载之间往往会存在一定的夹角。为了研究初始剪应力与循环荷载之间的夹角(剪切方向角)对土体软化指数的影响,采用多向循环单剪仪对温州软黏土进行一系列的循环单剪试验研究。研究结果表明,在特定剪切方向角下,随着循环应力比rc的增加,软化指数?逐渐降低,且在相同软化指数条件下,相邻循环应力比对应的循环圈数相差的幅度逐渐减小;循环应力比一定时,当剪切方向角在0°～90°范围内增加时,软化指数依次递增,当90°＜θ≤180°时,软化指数的变化趋势与0°～90°时相反;循环应力比rc = 0.15时,在加载初期,θ = 120°,150°,180°时,软化指数均分别较θ = 60°,30°,0°时大。对试验数据进行回归分析,建立土体的软化指数模型和孔压–软化模型。     

饱和砂土地基液化特性振动台试验研究

饱和砂土地基试验是研究碎石桩复合地基抗液化性能振动台试验的先导和必要组成部分。本文采用自行研制的简易单向专用振动台和大型叠层剪切变形模型箱完成了两个饱和砂土地基模型的三次振动台试验,验证了模型箱的性能和模型地基内部的均匀性。通过量测振动过程中砂土的超静孔隙水压力,得到了饱和砂土地基液化规律以及振动加密对其抗液化能力的影响。同时,探索了饱和砂土地基液化大型振动台模型试验技术,如饱和砂土模型地基设计与制备、传感器布置、试验加载方案确定等,为今后开展此类试验提供一般的研究思路,并且为后续碎石桩复合地基振动台试验提供了必要的技术经验。     

Effects of fines on the cyclic liquefaction behaviour in unsaturated,well-graded materials

The liquefaction of cargoes of metallic ores during maritime transportation is believed to have caused a number of ships to capsize during the past 30 years. To minimise the risk of liquefaction, shipping standards specify a transportable moisture limit (TML), which is the maximum moisture content for ore cargoes to be loaded onto a ship. However, the mechanics leading to the liquefaction of these cargoes is not well understood. This study uses an unsaturated soil mechanics perspective to understand the cyclic liquefaction behaviour of partially saturated materials, similar in grading to iron ore fines, a metallic ore that is known to liquefy during shipping transportation. Iron ore fines are transported at relatively low densities and have variable gradings containing a wide range of particle sizes and fines contents. Therefore, the effects of the degree of saturation and the fines content on the cyclic liquefaction behaviour of well-graded materials have been investigated by performing unsaturated, compression-only cyclic triaxial tests on samples prepared with four different gradings containing particle sizes from 9.5 mm to 2 μm with fines (<75 μm) contents of 18%, 28%, 40% and 60%. The trends in the data are discussed and used to develop a simple method that can conservatively estimate the number of cycles that samples with different degrees of saturation and fines contents are able to resist. The use of this method to describe the liquefaction behaviour of cargoes containing iron ore fines, in practice, is discussed.     

Rate dependence on mechanical properties of unsaturated cohesive soil with stress-induced anisotropy

The strain rate during shearing has been shown in experimental studies to strongly affect the mechanical behaviour of soil. For saturated soil, sufficient knowledge has been obtained to achieve equilibrium conditions for the pore water pressure. Nevertheless, little is known about unsaturated soil. Therefore, this study used a hollow cylindrical torsional shear apparatus to investigate the rate dependence on the deformation and strength properties of unsaturated soil. First, unsaturated specimens were anisotropically consolidated with different directions of major principal stress to assess the rate dependence of the anisotropic behaviour. Then, the shear stress was removed to produce an isotropic stress state. Shearing was applied using the specimens to evaluate the strain rate effects on the mechanical properties of unsaturated cohesive soil. The results indicate that the secant shear modulus increased with the strain rate in both constant suction (CS) and constant water content (CW) conditions. The shear strength did not change with the strain rate under a CW condition, but it decreased with the strain rate under a CS condition.     

橡胶掺量对格栅–橡胶砂界面宏细观剪切特性影响

为了研究土工格栅–橡胶砂界面的剪切特性,通过室内大型直剪仪,分析了不同橡胶掺量、相对密实度和竖向应力下界面的剪应力–剪切位移关系及体变特性;并基于室内试验,建立关于PFC^3D的纯砂和橡胶砂直剪模型,揭示土工格栅加筋橡胶砂在直剪过程中的细观力学机理。研究表明：在相同密实下橡胶砂剪切强度随着橡胶掺量增加先增大后减小,其最优橡胶掺量为10%,且越松散由橡胶掺量引起的剪切强度差(|τ₀-τ₅₀|)越大;在纯砂中掺入一定量橡胶颗粒能增大其似黏聚力和内摩擦角,但影响其剪切强度以内摩擦角为主;橡胶颗粒掺入致使剪切带相对厚度变小,橡胶颗粒对砂颗粒的转动起抑制作用,使得橡胶砂的孔隙率的变化量小于纯砂;橡胶砂组构分布能够反映接触力的分布,整体上颗粒间接触点数量比纯砂的略多,法向接触力和切向接触力比纯砂小;橡胶颗粒掺入增大了体系的阻尼耗能,提高了混合物的弹性。     

低含水率非饱和砂土抗剪强度研究

在引进附加内压力概念的基础上 ,将非饱和砂土中的基质吸力划分为毛细吸力和附加内压力两部分 ,进一步将低含水率非饱和砂土中的有效应力区分为由基质吸力引起的有效应力及由重力和其它外力产生的有效应力两部分 ,基此推导出了基质吸力与抗剪强度之间的关系 ,第一次成功地解释了关于非饱和砂土抗剪强度的Donald实验现象 :非饱和砂土的抗剪强度随含水率降低、基质吸力增大而升高 ,但升高的速度逐渐减缓 ,并当基质吸力超过某一阀值时 ,抗剪强度开始随基质吸力的增大而下降     

A new model for evaluating the dynamic shear strength of rocks based on laboratory test data for earthquake-resistant design

The dynamic shear strength of rocks is required for the earthquake-resistant design of nuclear power plants in Japan.This research aims to propose a mathematical model for estimating the dynamic strength and to validate the model.Two different types of specimens were prepared for the model validation,and the monotonic and cyclic loading tests were conducted to obtain the mathematical model parameters.Subsequently,multistep cyclic loading tests were performed,followed by simulations using the mathematical model.The test results demonstrated that the dynamic shear strength exceeded the static shear strength,which agreed with previous researches.Furthermore,the dynamic shear strength calculated using the mathematical model was generally consistent with that obtained from the experimental data.     

Effect of reinforcing technique on strain-dependent dynamic properties of reinforced earth walls

Due to lack of sufficient understanding of the strain-dependent dynamic properties of reinforced mass, existing soil and reinforcement models are not capable of accurately representing the seismic response of reinforced soil structures in numerical and analytical analyses. In the present study, to evaluate the effect of reinforcing technique on strain-dependent dynamic properties of reinforced earth walls, the values of equivalent shear modulus (G_e) and damping ratio (D) were estimated for soil-nailed wall (SNW) and steel-strip reinforced-soil wall (SSW) as a function of shear strain level using 1-g shaking table tests. The results obtained showed that the variation trend of G_e and D versus γ is strongly influenced by the type of reinforcing technique and confining pressure, so that the variation trend of these two parameters versus shear strain can be well expressed as an exponential equation with a high correlation coefficient for each type of reinforced earth wall.     

交通荷载引起的静偏应力对饱和软黏土不排水循环性状影响的试验研究

为研究交通荷载引起的静偏应力对饱和软黏土的不排水循环性状的影响,设计了室内不排水动三轴试验,将动应力简化为静偏应力与正弦荷载的叠加,研究了静偏应力对饱和软黏土变形、孔压发展规律以及动强度的影响。研究表明,静偏应力对土体动力特性影响很大,加快了塑性变形、残余孔压发展,降低了动强度曲线,减小了最小极限循环强度。且静偏应力的影响随着静偏应力的增大而显著。研究结果说明,交通荷载引起的动应力作用下,饱和软黏土更容易破坏。