Liquefaction proneness of stratified sand-silt layers based on cyclic triaxial tests

Most studies on liquefaction have addressed homogeneous soil strata using sand or sand with fine content without considering soil stratification.In this study,cyclic triaxial tests were conducted on the stratified sand specimens embedded with the silt layers to investigate the liquefaction failures and void-redistribution at confining stress of 100 kPa under stress-controlled mode.The loosening of underlying sand mass and hindrance to pore-water flow caused localized bulging at the sand-silt interface.It is observed that at a silt thickness of 0.2H(H is the height of the specimen),nearly 187 load cycles were required to attain liquefaction,which was the highest among all the silt thicknesses with a single silt layer.Therefore,0.2H is assumed as the optimum silt thickness(t_(opt)).The silt was placed at the top,middle and bottom of the specimen to understand the effect of silt layer location.Due to the increase in depth of the silt layer from the top position(capped soil state)to the bottom,the cycles to reach liquefaction(N_(cyc,L))increased 2.18 times.Also,when the number of silt layers increased from single to triple,there was an increase of about 880%in N_(cyc,L).The micro-characterization analysis of the soil specimens indicated silty materials transported in upper sections of the specimen due to the dissipated pore pressure.The main parameters,including thickness(t),location(z),cyclic stress ratio(CSR),number of silt layers(n)and modified relative density(D_(r,m)),performed significantly in governing the lique-faction resistance.For this,a multilinear regression model is developed based on critical parameters for prediction of N_(cyc,L).Furthermore,the developed constitutive model has been validated using the data from the present study and earlier findings.

Assessment of cyclic deformation and critical stress amplitude of jointed rocks via cyclic triaxial testing

Jointed rock specimens with a natural replicated joint surface oriented at a mean dip angle of 60were prepared,and a series of cyclic triaxial tests was performed at different confining pressures and cyclic deviatoric stress amplitudes.The samples were subjected to 10,000 loading-unloading cycles with a frequency of 8 Hz.At each level of confining pressure,the applied cyclic deviatoric stress amplitude was increased incrementally until excessive deformation of the jointed rock specimen was observed.Analysis of the test results indicated that there existed a critical cyclic deviatoric stress amplitude(i.e.critical dynamic deviatoric stress)beyond which the jointed rock specimens yielded.The measured critical dynamic deviatoric stress was less than the corresponding static deviatoric stress.At cyclic deviatoric stress amplitudes less than the critical dynamic deviatoric stress,minor cumulative residual axial strains were observed,resulting in hysteretic damping.However,for cyclic deviatoric stresses beyond the critical dynamic deviatoric stress,the plastic strains increased promptly,and the resilient moduli degraded rapidly during the initial loading cycles.Cyclic triaxial test results showed that at higher confining pressures,the ultimate residual axial strain attained by the jointed rock specimen decreased,the steadystate dissipated energy density and steady-state damping ratio per load cycle decreased,while steadystate resilient moduli increased.

Dynamic failure process of expanded polystyrene particle lightweight soil under cyclic loading using discrete element method

Expanded polystyrene(EPS)particle-based lightweight soil,which is a type of lightweight filler,is mainly used in road engineering.The stability of subgrades under dynamic loading is attracting increased research attention.The traditional method for studying the dynamic strength characteristics of soils is dynamic triaxial testing,and the discrete element simulation of lightweight soils under cyclic load has rarely been considered.To study the meso-mechanisms of the dynamic failure processes of EPS particle lightweight soils,a discrete element numerical model is established using the particle flow code(PFC)software.The contact force,displacement field,and velocity field of lightweight soil under different cumulative compressive strains are studied.The results show that the hysteresis curves of lightweight soil present characteristics of strain accumulation,which reflect the cyclic effects of the dynamic load.When the confining pressure increases,the contact force of the particles also increases.The confining pressure can restrain the motion of the particle system and increase the dynamic strength of the sample.When the confining pressure is held constant,an increase in compressive strain causes minimal change in the contact force between soil particles.However,the contact force between the EPS particles decreases,and their displacement direction points vertically toward the center of the sample.Under an increase in compressive strain,the velocity direction of the particle system changes from a random distribution and points vertically toward the center of the sample.When the compressive strain is 5%,the number of particles deflected in the particle velocity direction increases significantly,and the cumulative rate of deformation in the lightweight soil accelerates.Therefore,it is feasible to use 5%compressive strain as the dynamic strength standard for lightweight soil.Discrete element methods provide a new approach toward the dynamic performance evaluation of lightweight soil subgrades.

Dynamic behaviour of weathered red mudstone in Sichuan(China)under triaxial cyclic loading

The construction of a high-speed railway(HSR) in Southwest China is being hindered by a severe shortage of high-quality subgrade materials. However, red mudstone is widely distributed in the Sichuan Basin of China. The ability to use weathered red mudstone(WRM) to fill subgrade beds by controlling its critical stress and cumulative strain would enable substantial savings in project investments and mitigate damage to the ecological environment. To better understand the dynamic behaviour of WRM, both monotonic and cyclic triaxial tests were performed. The evolution of the cumulative strain vs. increased loading cycles was measured. The influences of confining pressure and loading cycles on the dynamic modulus, damping ratio, critical cyclic stress ratio(CSR), and dynamic stress level(DSL) were investigated. The relationship between the CSR and loading cycles under different failure strain criteria(0.1%-1.0%) was analysed. The prediction model of cumulative strain was also evaluated. The results indicated that the shear strength of WRM sufficiently meets the static strength requirements of subgrade. The critical dynamic stress of WRM can thus satisfy the dynamic stress-bearing requirement of the HSR subgrade. The critical CSR decreases and displays a power function with increasing confining pressure. As the confining pressure increases, the DSL remains relatively stable, ranging between 0.153 and 0.163. Furthermore, the relationship between the dynamic strength and loading cycles required to cause failure was established. Finally, a newly developed model for determining cumulative strain was established. A prediction exercise showed that the model is in good agreement with the experimental data.

间歇性循环荷载下冻融风积土变形特性及分数阶预测模型

列车运营对路基的长期作用由振动加载、荷载间歇交替组成,已往研究多只考虑振动加载周期对路基土体强度和变形特性的影响,而忽略了间歇期的影响。为研究间歇性循环荷载作用下风积土路基变形特性,利用GDS DYNTTS冻土动三轴仪,开展不同有效固结围压σ_(3c)、冻融循环次数FT、动应力幅值σ_(d)^(ampl)和振动频率f的间歇性循环荷载下风积土动三轴试验,研究间歇性循环荷载下冻融风积土变形特性及其影响因素。试验结果表明:间歇性循环荷载作用下风积土累积塑性应变曲线呈稳定型、发展型和破坏型三种形态。间歇阶段能够在较大程度上削弱土体的应变累积,从而使其较连续荷载作用下变形减小。基于极差方法,确定动应力幅值是影响风积土累积塑性应变的最重要因素,其余依次为有效固结围压、冻融循环次数、振动频率。采用双Abel黏壶建立考虑间歇性循环荷载作用的冻融风积土分数阶累积塑性应变预测模型,并与试验结果进行了对比分析,二者吻合度较高,说明该文建立的分数阶累积塑性应变数学模型可合理预测间歇性循环荷载作用下风积土路基长期变形特性。研究成果可为季节性冻土地区路基工程设计和灾害防治提供科学依据。

循环荷载下重载铁路道砟磨耗破碎及力学特性试验研究

为深入研究重载列车循环动载作用下散粒体道砟的颗粒磨耗破碎和力学行为劣化等特征,开展3种不同动应力幅值下的室内大型动三轴试验。通过对染色道砟颗粒进行激光扫描和三维重构,量化分析不同循环加载应力状态下不同粒径道砟颗粒的磨耗破碎特征,深入分析循环加载下试样的宏观力学特性和力学行为劣化等。研究结果表明,循环加载后道砟颗粒普遍从原始粒径破碎至相邻粒径,偶尔出现破碎至相隔粒径的现象,另有质量占比1%左右的少量道砟颗粒破碎至最小粒径4.75~22.4 mm;在相同的循环加载动应力幅值下,随粒径的增大,道砟颗粒破碎至相邻粒径的质量占比总体呈增大趋势;循环加载过程中颗粒磨耗存在尖角磨损、棱边磨耗与表面纹理改变等3种典型类型,动应力幅值230 kPa下颗粒最大磨耗深度为0.36~5.2 mm,平均磨耗深度为0.03~0.62 mm,颗粒磨耗程度随粒径的增大而逐渐加剧,尖角磨损出现的数量更多且概率更大,其次为棱边磨耗;循环加载三轴试验前后各粒组颗粒含量变化幅度与级配曲线偏移距离随动应力幅值的增大而增大,Bg、Br与BBI 3种颗粒破碎指标均随动应力幅值的增大而增大,Br与BBI两者对动应力幅值的变化反应程度一致,并且更为灵敏;试样轴向累积塑性应变、阻尼比与回弹模量均随动应力幅值的增大而增大。研究结果可为重载铁路散粒体有砟道床长期服役性能劣化评估及其养护维修提供理论依据和技术参考。

循环荷载下软黏土阻尼比特性及其简化计算方法

土体的阻尼比是土层动力分析和评价必不可少的重要动力性能参数,但滞回曲线具有多样性和复杂性,利用规范法计算阻尼比时数据量过多导致数据的筛选和整理困难,适当地对滞回曲线进行简化有助于快速处理和分析试验数据。为研究南京地区长江漫滩淤泥质粉质黏土在地铁列车循环荷载作用下阻尼比的变化和特性,对土体进行动三轴试验,分析土体的阻尼比在不同固结围压和动荷载幅值下的变化过程,并根据土体动应力-应变滞回曲线的几何特征和循环荷载的物理意义,采用多个循环次数作为一个代表性循环单元的思路对阻尼比进行简化计算。结果表明:土体的阻尼比随着动应变的增加呈快速增长—缓慢增长—保持平稳的3阶段发展趋势;随着固结围压的增加,土体的阻尼比逐渐减小;随着动荷载幅值的增大,土体的阻尼比增加。简化方法中,代表性循环单元内循环次数越多,计算得到的阻尼比规范法所得数值越小,在精度合适的情况下,可以作为阻尼比计算的实用方法,以减少计算时的数据处理及计算量。

循环荷载作用下土工格栅加筋黄土动力特性研究

【目的】由于黄土所具有的结构疏松、抗剪性低,以及对水、力及震动都十分敏感等工程特性,故探究采用双向土工格栅对西宁地区黄土加筋前后土体动力性能的改善效果。【方法】采用GDS双向动三轴测试系统开展不同加筋层间距、不同围压、不同动应力幅值工况条件下加筋黄土的动三轴试验,分析循环荷载作用下加筋层间距对黄土动强度、动剪切模量、阻尼比等动力响应特性的影响,探讨分析加筋黄土动力响应特性的演化规律。【结果】结果显示:黄土动应变随着动应力的增大而增大,骨干曲线呈现应变强化型,并且骨干曲线随加筋层间距的减小逐渐由曲线型过渡为直线型;随着加筋层间距的减小,黄土强度随之增大,随着围压和加筋层间距的增加,黄土幅值剪应力明显增大,动剪应变减小,阻尼比明显减小。【结论】结果表明:围压和加筋层间距对加筋黄土的动力特性具有显著的增强作用,且存在一个最优加筋层间距。研究成果可为西宁地区加筋黄土道路设计提供参考依据。

随机应力循环加卸载三轴试验土的变形稳定状态初探

为研究土在随机应力幅值循环加卸载条件下的应力-应变关系,基于土样全表面变形数字图像测量系统,以福建标准砂和硅微粉为研究对象进行一系列三轴试验。据此分析土样在不同随机应力循环加卸载过程的应力-应变行为,判断其是否达到变形稳定状态,同时与等应力幅值循环加卸载和阶梯应力幅值循环加卸载试验进行对比。结果表明:无论是恒定应力幅值、阶梯应力幅值还是随机应力幅值加卸载循环,当循环次数超过一定次数后,土样逐渐趋于变形稳定状态,且变形稳定状态对应的循环次数值不同;土在受到超过历史最大应力幅值时,应力-应变关系会回到初次单调加载的应力-应变曲线;在最大应力值小于破坏应力的条件下,应力加卸载时间过程幅值的随机特征,如幅值大小和分布对土的应力-应变行为,包括变形稳定状态没有影响。

Assessment of liquefaction potential based on shear wave velocity:Strain energy approach

Liquefaction assessment based on strain energy is significantly superior to conventional stress-based methods.The main purpose of the present study is to investigate the correlation between shear wave velocity and strain energy capacity of silty sands.The dissipated energy until liquefaction occurs was calculated by analyzing the results of three series of comprehensive cyclic direct simple shear and triaxial tests on Ottawa F65,Nevada,and Firoozkuh sands with varying silt content by weight and relative densities.Additionally,the shear wave velocity of each series was obtained using bender element or resonant column tests.Consequently,for the first time,a liquefaction triggering criterion,relating to effective overburden normalized liquefaction capacity energy(WL=s’c)to effective overburden stresscorrected shear wave velocity(eVs1)has been introduced.The accuracy of the proposed criteria was evaluated using in situ data.The results confirm the ability of shear wave velocity as a distinguishing parameter for separating liquefied and non-liquefied soils when it is calculated against liquefaction capacity energy(WL=s’c).However,the proposed WL=s’c-Vs1 curve,similar to previously proposed cyclic resistance ratio(CRR)-Vs1 relationships,should be used conservatively for fields vulnerable to liquefaction-induced lateral spreading.

Evolution of mechanical parameters of Shuangjiangkou granite under different loading cycles and stress paths

Surrounding rocks at different locations are generally subjected to different stress paths during the process of deep hard rock excavation.In this study,to reveal the mechanical parameters of deep surrounding rock under different stress paths,a new cyclic loading and unloading test method for controlled true triaxial loading and unloading and principal stress direction interchange was proposed,and the evolution of mechanical parameters of Shuangjiangkou granite under different stress paths was studied,including the deformation modulus,elastic deformation increment ratios,fracture degree,cohesion and internal friction angle.Additionally,stress path coefficient was defined to characterize different stress paths,and the functional relationships among the stress path coefficient,rock fracture degree difference coefficient,cohesion and internal friction angle were obtained.The results show that during the true triaxial cyclic loading and unloading process,the deformation modulus and cohesion gradually decrease,while the internal friction angle gradually increases with increasing equivalent crack strain.The stress path coefficient is exponentially related to the rock fracture degree difference coefficient.As the stress path coefficient increases,the degrees of cohesion weakening and internal friction angle strengthening decrease linearly.During cyclic loading and unloading under true triaxial principal stress direction interchange,the direction of crack development changes,and the deformation modulus increases,while the cohesion and internal friction angle decrease slightly,indicating that the principal stress direction interchange has a strengthening effect on the surrounding rocks.Finally,the influences of the principal stress interchange direction on the stabilities of deep engineering excavation projects are discussed.

循环荷载作用下含水合物粉细砂的残余变形

本文针对不同饱和度和不同围压水平的含水合物粉细砂进行了一系列不排水循环三轴剪切试验,研究了轴向应变和孔隙水压力的发展,分析了影响含水合物粉细砂残余应变的主要因素:循环应力比、有效围压和水合物饱和度。在Monismith公式的基础上,提出了一种新的经验公式,能够预测循环荷载作用下含水合物粉细砂的残余变形,预测结果与实验结果的对比表明该经验公式具有良好的适用性。

循环三轴试验加载频率和压实度对重塑砂土液化行为的影响

通过对各向同性固结不排水试样进行循环三轴试验,研究了饱和重塑砂土的液化行为。试验采用恒幅正弦载荷,通过测量剪切模量、阻尼比、轴向应变和超孔隙水压力比的变化,评估了加载频率(分别为0.1、0.5、1.0 Hz)和压实度(分别为0.95、0.90和0.80)对砂土液化行为的影响。试验结果表明,对于压实度为0.95的砂样,在5000次加载循环中,0.1 Hz的加载频率对液化过程影响甚微;而对于压实度为0.90和0.80的砂样,此频率的影响显著。对于压实度为0.80的砂样,当频率增加到0.5 Hz时几乎一加载就直接发生液化。这表明增加加载频率会加快液化速度,而增加压实度则会减慢液化速度。因此,通过一系列的试验可以模拟现场砂土在不同频率下的动荷载条件。试验结果表明,饱和砂土的液化行为由以下因素决定:(1)试样的压实度越低,液化越容易发生;(2)加载频率越高,液化越容易发生;(3)循环轴向应变越大,发生液化所需的循环次数越少;(4)超孔隙水压力随加载循环次数增加,直至试样完全液化。

Effect of Cyclic Loading Frequency on Undrained Behaviors of Undisturbed Marine Clay

Based on a series of cyclic triaxial tests, the effect of cyclic frequency on the undrained behaviors of undisturbed marine clay is investigated. For a given dynamic stress ratio, the accumulated pore water pressure and dynamic strain increase with the number of cycles. There exists a threshold value for both the accumulated pore water pressure and dynamic strain, below which the effect of cyclic frequency is very small, but above which the accumulated pore water pressure and dynamic strain increase intensely with the decrease of cyclic frequency for a given number of cycles. The dynamic strength increases with the increase of cyclic frequency, whereas the effect of cyclic frequency on it gradually diminishes to zero when the number of cycles is large enough, and the dynamic strengths at different frequencies tend to the same limiting minimum dynamic strength. The test results demonstrate that the reasons for the frequency effect on the undrained soil behaviors are both the creep effect induced by the loading rate and the decrease of sample effective confining pressure caused by the accumulated pore water pressure.

Experimental Study on Cyclic Mechanics Characteristic of Saturated Soft Clay Strata

Failure criterion of saturated soft clay is studied under cyclic loads through different experiments. A large number of cyclic torsional shear and cyclic triaxial tests on saturated soft clay under unconsolidated undrained condition are conducted. From the test result analysis, it is seen that the failure of saturated soft clay under static and cyclic loads satisfies Mises criterion. The result from different test stress states is not related to the test stress states or confining pressures. It can be applied in general stress states. Then according to the Mises criterion, the equivalent relationship on failure moment between the test stress state and the general stress state can be established. So the cyclic mechanics characteristics of saturated soft clay at failure moment are clarified in this paper. Furthermore, a theory basis is provided for using pseudo-static elasto-plastic cyclic strength model to evaluate cyclic bearing capacity.

Settlement and Stability Analysis on Soft Clay Under Cyclic Loading

The results of undrained cyclic triaxial tests on three types of clays are collected and a relationship among the accumulated strain, the initial stress state, cyclic stress in the soil, as well as the number of cycles is established based on the experimental data. With this relationship, a procedure is proposed for subsidence and stability analysis on soft clay under the action of cyclic loads.

Constitutive Model of Saturated Soft Clay with Cyclic Loads under Unconsolidated Undrained Condition

An equivalent visco-elastic model of saturated soft clay was studied under unconsolidated undrained (UU) condition, which can be used to evaluate the stability of ocean foundation. Cyclic triaxial compression and extension tests were conducted to study the parameters of the model. Results showed that the relationships of the damping ratio and the octahedral shear modulus with the octahedral cyclic shear strain were nearly unique, when the initial octahedral shear stress ratios of specimens were equal to 0.3, 0.5 and 0.7. The relationships of the damping ratio and the octahedral shear modulus with the octahedral cyclic shear strain determined from the cyclic triaxial compression tests were basically the same as those determined from the cyclic triaxial extension tests. Furthermore, the relationships were not related to the initial stress condition, the test stress state and the octahedral cyclic shear stress ratio. The relationships determined from the cyclic triaxial tests under no deviatoric stress were basically the same as those determined from the cyclic triaxial tests under deviatoric stress. The change of the octahedral cyclic accumulative strain with the number of cycles was unique under different tests stress states. An equivalent visco-elastic constitutive model of saturated soft clay under UU condition was initially established.

Dynamic properties and liquefaction behaviour of cohesive soil in northeast India under staged cyclic loading

Estimation of strain-dependent dynamic soil properties, e.g. the shear modulus and damping ratio, along with the liquefaction potential parameters, is extremely important for the assessment and analysis of almost all geotechnical problems involving dynamic loading. This paper presents the dynamic properties and liquefaction behaviour of cohesive soil subjected to staged cyclic loading, which may be caused by main shocks of earthquakes preceded or followed by minor foreshocks or aftershocks, respectively. Cyclic triaxial tests were conducted on the specimens prepared at different dry densities （1.5 g/cm3 and 1.75 g/cm3） and different water contents ranging from 8% to 25%. The results indicated that the shear modulus reduction （G/Gmax） and damping ratio of the specimen remain unaffected due to the changes in the initial dry density and water content. Damping ratio is significantly affected by confining pressure, whereas G/Gmax is affected marginally. It was seen that the liquefaction criterion of cohesive soils based on single-amplitude shear strain （3.75% or the strain at which excess pore water pressure ratio becomes equal to 1, whichever is lower） depends on the initial state of soils and applied stresses. The dynamic model of the regional soil, obtained as an outcome of the cyclic triaxial tests, can be successfully used for ground response analysis of the region.

Undrained dynamical behavior of Nanjing flake-shaped fine sand under cyclic loading

A series of dynamic behavior tests on Nanjing flake-shaped fine sand were performed by using the WFI cyclic triaxial apparatus made in England. The dynamic behaviors of Nanjing flake-shaped fine sand under different static deviator stress levels and cyclic stress ratios were studied. Through comparing the effective stress path under cyclic loading with static loading, the processes of liquefaction of saturated Nanjing flake-shaped fine sand with development of dynamic pore-water pressure, including the initial compact state, compression state and dilative state, were investigated. The variation of the shear stiffness with the number of cycles and cyclic strain was investigated by analyzing the secant shear modulus in each unload-reload loop of dynamic stress-strain relationship. And by means of the exponential function, the empirical equations of the relationship between secant shear modulus Gsec, shear modulus ratio Gsec/Gmax and cyclic strain ε were established based on series of test results. The results show that according to different combinations of static deviator stress and cyclic stress, two kinds of failure patterns with deviator stress reversal or no deviator stress reversal are observed in the samples tested in this series, including cyclic mobility and the failure of accumulation residual strain. In addition, the degradation of dynamic shear modulus is due to the development of vibration pore-water pressure and it is observed that the shear modulus reduces with the progressive number of cycles.

Cyclic deformation behaviour of natural K_0-consolidated soft clay under different stress paths

Characteristic of cyclic loading due to passing wheels is associated with one-way loading without stress reversal,which includes a simultaneous cyclic variation of vertical normal stress and horizontal normal stress lasting for a long period of time and generally takes place in partially-drained conditions.Therefore,it is of great practical relevance to study the deformation behaviour according to the characteristic of traffic loading.In this work,a series of one-way stress-controlled cyclic triaxial tests with a simultaneous variation of the vertical and horizontal stress components during cyclic loading were conducted to investigate the deformation behaviour of natural K_0-consolidated soft clay in partially-drained conditions.Test results demonstrate that not only the deviator part of the stress rules accumulation but also the volumetric part significantly contributes.While the deviator part of the stress amplitude is held constant,the increase amplitude of cyclic confining pressure will promote the development of both permanent volumetric strain and axial strain significantly.Furthermore,the effects of cyclic confining pressure on the deformation of natural K_0-consolidated soft clay was quantified.Finally,an empirical formula for permanent axial strain considering the effects of cyclic confining pressure was proposed which can be used for feasibility studies or for the preliminary design of foundations on K_0-consolidated soft clay subjected to traffic loading.