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1.
Post-Cyclic Undrained Behavior of Compacted Composite Clay Subjected to Various Cyclic Loading Paths
H. R. Tavakoli A. Shafiee M. K. Jafari 《Geotechnical and Geological Engineering》2011,29(6):1085-1097
Using a sequential procedure of undrained cyclic and post cyclic tests, the strength and stiffness degradation characteristics
of compacted composite clays are studied immediately after various cyclic loading paths by triaxial and hollow cylinder tests.
The effects of cyclic loading paths, sand contents, cyclic loading amplitude and confining pressure on the post-cyclic mechanical
behavior of the composite clays are evaluated. The results point out different peculiarities which can be of interest in assessing
the mechanical behavior of the composite clays under post seismic shaking. The results show that effect of cyclic loading
on post cyclic pore water pressure build-up is significant when pore water pressure build-up is considerably lower than the
associated value in monotonic loading. The effect of sand content and cyclic loading path on degradation of stiffness is more
remarkable than shear strength. Test results also reveal that the effect of sand content on the post cyclic pore water pressure
build-up is minor. However, when the aggregate content increases the shear strength increases. 相似文献
2.
A number of cyclic triaxial tests were carried out on mine tailings and natural sediment samples under undrained conditions
to investigate their resistance to cyclic loading. The tests were performed on more than 100 samples with a cyclic shear stress
ratio ranging from 0.10 to 0.40 under varying void ratio and the same confining pressure. It was observed that the axial strain
and excess pore water pressure increased with the number of loading cycles while the effective stress decreased with increasing
number of loading cycles. The liquefaction resistance of the tailings was also observed to be higher than that of natural
soils with similar particle size distribution, void ratio and plasticity index. It was observed that the influence of specific
gravity on the cyclic strength of mine tailings is significant. The results showed that the cyclic resistance of the tailings
was not strongly influenced by plasticity index for low plasticity tailings. A boundary relationship between void ratio and
normalized cyclic resistance ratio was established based on the results. 相似文献
3.
In dynamic geotechnical problems, soils are often subjected to a combination of sustained static and fast cyclic loading. Under such loading conditions, saturated and normally consolidated clays generally experience a build-up of excess pore water pressure along with a degradation of stiffness and strength. If the strength of the soil falls below the static stress demand, a self-driven failure is triggered. In this paper, a constitutive model is presented for the analysis of such problems, based on a general multisurface plasticity framework. The hardening behavior, the initial arrangement of the surfaces, and the nonassociated volumetric flow rule are defined to capture important aspects of cyclic clay behavior. This includes nonlinear hysteretic stress-strain behavior, the effect of anisotropic consolidation, and the generation of excess pore water pressure during undrained cyclic loading along with a degradation of stiffness and strength. The model requires nine independent parameters, which can be derived from standard laboratory tests. A customized experimental program has been performed to validate the model performance. The model predictions show a good agreement with test results from monotonic and cyclic undrained triaxial tests, in particular with respect to the strain-softening response and the number of loading cycles to failure. A procedure for a general stress-space implicit numerical implementation for undrained, total stress-based finite element analyses is presented, including the derivation of the consistent tangent operator. Finally, a simulation of the seismic response of a submarine slope is shown to illustrate a possible application of the presented model. 相似文献
4.
Soil liquefaction as a transformation of granular material from solid to liquid state is a type of ground failure commonly associated with moderate to large earthquakes and refers to the loss of strength in saturated, cohesionless soils due to the build-up of pore water pressures and reduction of the effective stress during dynamic loading. In this paper, assessment and prediction of liquefaction potential of soils subjected to earthquake using two different artificial neural network models based on mechanical and geotechnical related parameters (model A) and earthquake related parameters (model B) have been proposed. In model A the depth, unit weight, SPT-N value, shear wave velocity, soil type and fine contents and in model B the depth, stress reduction factor, cyclic stress ratio, cyclic resistance ratio, pore pressure, total and effective vertical stress were considered as network inputs. Among the numerous tested models, the 6-4-4-2-1 structure correspond to model A and 7-5-4-6-1 for model B due to minimum network root mean square errors were selected as optimized network architecture models in this study. The performance of the network models were controlled approved and evaluated using several statistical criteria, regression analysis as well as detailed comparison with known accepted procedures. The results represented that the model A satisfied almost all the employed criteria and showed better performance than model B. The sensitivity analysis in this study showed that depth, shear wave velocity and SPT-N value for model A and cyclic resistance ratio, cyclic stress ratio and effective vertical stress for model B are the three most effective parameters on liquefaction potential analysis. Moreover, the calculated absolute error for model A represented better performance than model B. The reasonable agreement of network output in comparison with the results from previously accepted methods indicate satisfactory network performance for prediction of liquefaction potential analysis. 相似文献
5.
考虑循环载荷下饱和黏土软化的损伤边界面模型研究 总被引：1，自引：0，他引：1
研究表明,循环载荷作用下饱和黏土将发生软化,其机制主要有两个：一是孔压的积累;二是土体原有结构的不断损伤和新结构的不断重塑。针对上述机制,基于广义各向同性硬化准则建立了考虑饱和黏土循环软化的损伤单面模型。该模型在有效应力空间中引入损伤变量,表征土体结构的损伤和重塑程度,在连续的循环加载下,损伤不断累积,边界面则随着损伤的累积不断收缩,以模拟饱和黏土刚度和强度的软化;以应力反向点作为边界面的广义各向同性硬化中心和映射法则的映射中心,灵活地选择塑性模量的插值公式以模拟塑性变形和孔压的累积以及应力-应变的滞回特性。应用该模型对不排水循环三轴试验进行模拟,并且考查了循环周次、循环应力水平和固结历史对饱和黏土循环软化特性的影响,并与相关试验比较,验证了模型的有效性。 相似文献
6.
A. Janalizadeh Choobbasti Mohammad Javad Vahdatirad M. Torabi S. Firouzian A. Barari 《Arabian Journal of Geosciences》2012,5(4):545-554
Paying special attention to geotechnical hazards such as liquefaction in huge civil projects like urban railways especially in susceptible regions to liquefaction is of great importance. A number of approaches to evaluate the potential for initiation of liquefaction, such as Seed and Idriss simplified method have been developed over the years. Although simplified methods are available in calculating the liquefaction potential of a soil deposit and shear stresses induced at any point in the ground due to earthquake loading, these methods cannot be applied to all earthquakes with the same accuracy, also they lack the potential to predict the pore pressure developed in the soil. Therefore, it is necessary to carry out a ground response analysis to obtain pore pressures and shear stresses in the soil due to earthquake loading. Using soil historical, geological and compositional criteria, a zone of the corridor of Tabriz urban railway line 2 susceptible to liquefaction was recognized. Then, using numerical analysis and cyclic stress method using QUAKE/W finite element code, soil liquefaction potential in susceptible zone was evaluated based on design earthquake. 相似文献
7.
通过GDS循环三轴试验系统,对循环荷载作用下饱和软黏土的孔压变化规律进行了研究,分析了循环应力比,初始剪应力与振动频率对孔隙水压力的影响。研究结果表明：随着循环应力比的增加,孔压发展速度增快。循环荷载作用下饱和软黏土存在临界循环应力比,通过孔压也可以确定其值大小。在循环初期,孔压率较大,随着循环时间的增加,孔压率逐渐减小。随着循环应力比的增加,孔压率增加。振动频率对孔压比-循环次数关系影响明显,随着频率的增加,孔压比减小;然而,当振动频率大于1 Hz时,振动频率对孔压比-时间与孔压率-时间关系影响不明显。随着初始剪应力的增大,孔压增加。初始剪应力对应变率具有显著影响;随着初始剪应力的增加,应变率增加。在对数坐标下,孔压率与时间呈线性关系。在上述试验基础上建立了孔压率与时间关系表达式,通过积分得到了循环荷载作用下饱和软黏土的动孔压模型 相似文献
8.
Cyclic soil degradation and hardening affects soil stiffness and strength, and is linked to an increase or decrease in the
mean effective confining stress due to void ratio or pore pressure changes. This change of state can be explicitly modeled
by using effective stress methods, or implicitly modeled using total stress methods. In the latter, this is achieved by using
empirical functions based on the number of loading cycles that are derived from constant-amplitude stress or strain laboratory
tests. To suite generalized loading conditions, these functions must be extrapolated to variable-amplitude loading. This falls
under the general class of a fatigue-based problem. The main focus of this paper is to present a generalized consistent soil
fatigue formulation for soils under cyclic loading. The paper then goes on to discuss the implementation of various cyclic
soil degradation and hardening models reported in the literature, and highlights their important underlying assumptions, capabilities
and limitations. 相似文献
9.
采用大型动三轴试验仪进行重塑高岭土试样的循环三轴试验,试样直径为300 mm,高度为600 mm。圆柱体试样中心放置了一块竖向排水板,在循环加载试验进行时和结束后都可进行径向排水。试验结果验证了径向排水可以有效地消散循环荷载引起的孔隙水压力。通过结合径向固结理论与不排水循环加载土体模型,提出了一个循环荷载作用下径向固结模型,用来描述在允许径向排水的情况下软黏土在循环荷载作用下的孔压累积特性。模型中考虑了应力历史和孔隙水压力消散对孔隙水压力生成的影响,并用大型循环三轴试验结果进行验证。研究发现,当施加较大循环荷载时,径向排水减缓了孔隙水压力累积到临界值的速率,因而土体在破坏前可以经历更多次的循环荷载;当施加中等循环荷载时,径向排水可有效阻止孔隙水压力增长至临界值。除此之外,还研究了加载间歇期对孔压累积特性的影响,结果显示3组循环加载结束后,累积孔隙水压力开始减小,表明之后更多的循环加载并不会引起孔隙水压力的累积增长。 相似文献
10.
Cheng Wei Chen Ren-peng Hong Peng-yun Cui Yu-jun Pereira Jean-Michel 《Acta Geotechnica》2020,15(10):2741-2755
In thermal-related engineering such as thermal energy structures and nuclear waste disposal, it is essential to well understand volume change and excess pore water pressure buildup of soils under thermal cycles. However, most existing thermo-mechanical models can merely simulate one heating–cooling cycle and fail in capturing accumulation phenomenon due to multiple thermal cycles. In this study, a two-surface elasto-plastic model considering thermal cyclic behavior is proposed. This model is based on the bounding surface plasticity and progressive plasticity by introducing two yield surfaces and two loading yield limits. A dependency law is proposed by linking two loading yield limits with a thermal accumulation parameter nc, allowing the thermal cyclic behavior to be taken into account. Parameter nc controls the evolution rate of the inner loading yield limit approaching the loading yield limit following a thermal loading path. By extending the thermo-hydro-mechanical equations into the elastic–plastic state, the excess pore water pressure buildup of soil due to thermal cycles is also accounted. Then, thermal cycle tests on four fine-grained soils (natural Boom clay, Geneva clay, Bonny silt, and reconstituted Pontida clay) under different OCRs and stresses are simulated and compared. The results show that the proposed model can well describe both strain accumulation phenomenon and excess pore water pressure buildup of fine-grained soils under the effect of thermal cycles.
相似文献11.
通过等压固结静、动三轴试验,研究了渤海湾粉质黏土在循环荷载作用下的动力性质和循环荷载后不排水静力性质。试验结果表明,循环应力幅值比越大,平均轴向应变和轴向应变幅值越大;循环应力幅值比达到0.4时,平均轴向应变和轴向应变幅值随着循环周数增加迅速增大;循环应力幅值比相同,固结应力越大,轴向应变幅值越大,而平均轴向应变越小。在较大的循环应力幅值比下,平均孔压比值和孔压幅值比值随着循环周数的增大会达到稳定;循环应力幅值比越大平均孔压比值和孔压幅值比值均越大;相同循环应力幅值,固结应力越大平均孔压比值越小,而固结应力对孔压幅值比值影响较小。循环荷载的作用会导致循环荷载后不排水剪在q-p’平面上有效应力路径和孔压发展表现出超固结土的性质。 相似文献
12.
不排水循环荷载作用过程中累积孔压对细砂弹性剪切模量Gmax的影响 总被引：1，自引：1，他引：0
在土工结构物地震有效应力分析中,往往采用Hardin经验公式计算动荷作用过程中由于孔压变化对砂土初始弹性剪切模量Gmax的影响,但由于以往试验手段的限制,这种方法的有效性至今没有得到验证。为此,采用多功能三轴仪和压电陶瓷弯曲元剪切波速测试系统,研究了不排水循环荷载过程中孔压对细砂Gmax的影响。试验结果表明,细砂Gmax可由 Hardin型经验公式计算,但材料参数应通过试验确定;不排水循环荷载作用过程中,细砂Gmax随孔压上升而降低,其值可由其实际平均有效应力和固结后的孔隙比通过由试验得到的Hardin型经验公式计算。 相似文献
13.
利用从现场取得的原状土样,通过室内动三轴试验对循环荷载作用下的变形、孔压和强度特性进行研究。试验研究主要考虑了周围固结压力、循环剪切应力比、荷载频率和循环次数等因素的影响。研究表明,在循环荷载作用下,孔隙水压力和轴向应变均是一个波动上升的过程。当循环应力比增大时,动孔隙水压力变化幅值显著增加,而残余孔压也较大。同时,孔隙水压力值也随着周围压力的增大而明显增大。当作用荷载频率比较大时,需要更多的循环次数才能达到小频率作用荷载能达到的孔隙水压力值。但是,随着循环荷载作用次数的增加,频率对孔隙水压力的影响有减小的趋势。 相似文献
14.
2008年汶川地震触发的大光包滑坡滑带形成背景是渗水层间错动带,该带强度变化是决定滑坡启动的主要因素,动孔压发展是土体材料强度劣化的根本原因,故基于系列室内动三轴试验研究该带材料孔压特性。结果表明,动载下层间错动带材料孔压快速增长,循环剪应力比越大,孔压增长越快,不同荷载条件下应变达到5%时动孔压比等于1,材料液化,根据动孔压比与振次比曲线的拟合关系提出幂函数应力模型。从能量角度描述层间错动带材料孔压增长特性,揭示出循环剪应力比对累积能量耗损与动孔压比曲线影响较小而围压对其影响较大,并进一步提出孔压增长的能量模型。 相似文献
15.
The influence of static shear stress on undrained cyclic behavior of nonplastic and low-plasticity silts has been studied
by means of undrained cyclic torque-controlled ring-shear tests. The cyclic and post-cyclic behavior of silty soils assumed
on sliding surface were investigated to assess the liquefaction potential and cyclically induced deformation of silty slopes.
Six different initial static shear stresses corresponding to slope angles from 0° to 25° were examined. To better understand
undrained cyclic behavior of silt governed by a change in clay content, three different mixtures were achieved by mixing of
nonplastic silt with 0%, 10%, and 20% of commercially available clay. These tests were conducted to simulate field conditions
prior to earthquake with initial static shear stresses corresponding to slopes and those with no initial static shear stresses
of level grounds. The gradual loss of mobilized undrained cyclic shear resistance after failure and pore water buildup in
relation to a number of cycles was observed. The undrained response of the soil to cyclic shear stress loading with the constant
amplitude revealed the significant effect of the initial static shear stress on the excess pore water pressure generation
and post-failure shear resistance. Test results showed that an increase in the initial static shear stress at the given initial
effective normal stress is associated with an increase of mobilized shear resistance at its peak state; however, the actual
resistance to liquefaction diminished for both nonplastic and low-plasticity silts. During both cyclic and post-cyclic stages
of loading, distinctly different types of shear deformation were identified. In order to evaluate mobility of landslides,
a modified conventional brittleness index for seismic loading, , was proposed and used to characterize unlimited deformation of silts. 相似文献
16.
For numerical studies of geotechnical structures under earthquake loading, aiming to examine a possible failure due to liquefaction, using a sophisticated constitutive model for the soil is indispensable. Such a model must adequately describe the material response to a cyclic loading under constant volume (undrained) conditions, amongst others the relaxation of effective stress (pore pressure accumulation) or the effective stress loops repeatedly passed through after a sufficiently large number of cycles (cyclic mobility, stress attractors). The soil behaviour under undrained cyclic loading is manifold, depending on the initial conditions (e.g. density, fabric, effective mean pressure, stress ratio) and the load characteristics (e.g. amplitude of the cycles, application of stress or strain cycles). In order to develop, calibrate and verify a constitutive model with focus to undrained cyclic loading, the data from high-quality laboratory tests comprising a variety of initial conditions and load characteristics are necessary. The purpose of these two companion papers was to provide such database collected for a fine sand. The database consists of numerous undrained cyclic triaxial tests with stress or strain cycles applied to samples consolidated isotropically or anisotropically. Monotonic triaxial tests with drained or undrained conditions have also been performed. Furthermore, drained triaxial, oedometric or isotropic compression tests with several un- and reloading cycles are presented. Part I concentrates on the triaxial tests with monotonic loading or stress cycles. All test data presented herein will be available from the homepage of the first author. As an example of the examination of an existing constitutive model, the experimental data are compared to element test simulations using hypoplasticity with intergranular strain. 相似文献
17.
利用GDS空心圆柱仪进行了一系列主应力方向角?d变化的轴向、扭转、内压和外压四向耦合不排水循环剪切试验。在均等固结条件下,着重研究了循环加载方向角?d0对饱和粉土动力特性的影响。试验结果表明：饱和粉土的双规准化孔压发展模式与?d0无关,但受循环应力比CSR的影响;广义剪应变的发展模式不受?d0的影响。在循环剪切过程中,循环加载方向的变化对粉土的不排水动强度有显著影响,饱和粉土的动强度CRR随着?d0的增大呈现出先减小后增大的变化趋势,且当?d0=45°时CRR最小。同时,建立了反映?d0与CSR影响的孔压、变形的模型,并给出了相应的动强度表达式。 相似文献
18.
For numerical studies of geotechnical structures under earthquake loading, aiming to examine a possible failure due to liquefaction, using a sophisticated constitutive model for the soil is indispensable. Such model must adequately describe the material response to a cyclic loading under constant volume (undrained) conditions, amongst others the relaxation of effective stress (pore pressure accumulation) or the effective stress loops repeatedly passed through after a sufficiently large number of cycles (cyclic mobility, stress attractors). The soil behaviour under undrained cyclic loading is manifold, depending on the initial conditions (e.g. density, fabric, effective mean pressure, stress ratio) and the load characteristics (e.g. amplitude of the cycles, application of stress or strain cycles). In order to develop, calibrate and verify a constitutive model with focus to undrained cyclic loading, the data from high-quality laboratory tests comprising a variety of initial conditions and load characteristics are necessary. It is the purpose of these two companion papers to provide such database collected for a fine sand. Part II concentrates on the undrained triaxial tests with strain cycles, where a large range of strain amplitudes has been studied. Furthermore, oedometric and isotropic compression tests as well as drained triaxial tests with un- and reloading cycles are discussed. A combined monotonic and cyclic loading has been also studied in undrained triaxial tests. All test data presented herein will be available from the homepage of the first author. As an example of the examination of an existing constitutive model, the experimental data are compared to element test simulations using hypoplasticity with intergranular strain. 相似文献
19.
20.
Observations from earthquakes over the past several decades have highlighted the importance of local site conditions on propagated ground motions. Downhole arrays are deployed to measure motions at the ground surface and within the soil profile, and also to record the pore pressure response within the soft soil profiles during excitation. The degradation of soil stiffness as excess pore pressures are generated during earthquake events has also been observed. An inverse analysis framework is developed and demonstrated to directly extract soil material behavior including pore water pressure (PWP) generation from downhole array measurements that can then be readily used in 1D nonlinear site response analysis. The self‐learning simulations (SelfSim) inverse analysis framework, previously developed for total stress site response analysis, is extended to extract PWP generation behavior of the soil in addition to cyclic response during ground shaking. A Neural Network based constitutive model is introduced to represent PWP generation during cyclic loading. A new analysis scheme is introduced that can use data from co‐located piezometer and accelerometer sensors. The successful performance of the proposed framework is demonstrated using four synthetic vertical array recordings. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献