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1.
Effects of Frequency on the Dynamic Properties of Intact Rock Samples Subjected to Cyclic Loading under Confining Pressure Conditions 总被引：2，自引：1，他引：1
A series of laboratory tests was performed to assess the effects of frequency on the dynamic properties of sandstone samples
subjected to cyclic loading in the confining stress state. Three levels of confining pressure (2.0, 10.0, and 40.0 MPa) and
three sets of frequencies (0.1, 1.0, and 3.0 Hz) were applied for the axial cyclic loading tests by the MTS-815 Rock and Concrete
Test System. The results from the cyclic loading tests indicate that frequency has a strong influence on the dynamic deformation,
the dynamic stiffness, and the failure mode at the same confining pressure. With an increase in the frequency, the axial strain
and the number of cycles at failure increased at the same confining pressure, the residual volumetric strain increased when
dilatancy occurred at the same confining pressure, and the number of cycles at failure increased. A new damage variable D was defined that describes the degradation process of sandstone samples upon dynamic cyclic loading. The larger the frequency,
the wider the localized band. Sandstone samples subjected to dynamic cyclic loading responded with a significantly higher
initial stiffness. The higher the initial stiffness, the greater the frequency. 相似文献
2.
A. D. Sharifabadi T. G. Joseph D. R. Schmitt 《Geotechnical and Geological Engineering》2010,28(6):727-743
The strain softening of oil sand in the underfoot of ultra class mobile mining equipment, due to the loading action of large
mobile mining equipment such as trucks and shovels, yields a highly unstable condition for the operation of this ultra-class
equipment. Soft ground conditions in oil sand, due to the low stiffness of the material a condition especially present in
the summer, can cause high rack, pitch, and roll in trucks, leading to fatigue failure in structural components. For shovels,
poor ground stability can cause twists in car bodies and undercarriages, resulting in major damages. Track and shovel frame
failures due to this instability result in high maintenance costs. The authors carried out a geophysical study of the oil
sand in order to evaluate the ground conditions under large mobile mining equipment. A geophysical investigation performed
in summer 2001 encountered 6–8 m of thick soft material, commensurate with very low velocities, caused by the loosening of
the surface material by heavy mining machinery and excavation; and a transition zone of up to 25–26 m depth approaching the
in situ oil sand below. The depth of the oil sand zones can be calculated by using the refraction analysis technique. Spectral
Analysis of Surface Waves was used to estimate the ground stiffness. A new technique is proposed to evaluate the changing
ground stiffness during the use of ultra-class mobile mining equipment. 相似文献
3.
Creep and rate of loading effects on geogrid- reinforced sand 总被引：1，自引：0，他引：1
This paper presents the results of studies of creep and rate of loading effects on geogrid-reinforced dune sand. Model plate load tests were conducted on reinforced sand beds under displacement and load controls. Displacement controlled set of plate load tests were carried out in the range 1–0.001 mm/min. Load controlled set of plate load tests were carried out in which the staged pressure was held constant for 6000 min. Results indicate that rate of displacement (strain rate) is very significant for potential improvement of geogrid-reinforced sand. Both stiffness and bearing capacity of geogrid-reinforced sand increases logarithmically with rate of displacement. Geogrid reduces the creep of sand especially in the working pressure range. A simple stiffness degradation and creep relationships were obtained for geogrid-reinforced dune sand. The results of the tests are reported, and the significance of each control variable is assessed. 相似文献
4.
This paper presents a series of cyclic 2D direct shear tests on sand–rough material interfaces under constant normal load (CNL) and constant normal stiffness (CNS) conditions. The aim of these tests is to describe the behavior of the soil–pile contact subjected to a large number of cycles due to environmental or anthropic loadings. These cycles (typically 104 or less due to an early rupture) are small (10, 20 and 40 kPa in terms of shear stress). A new interpretation of the direct shear tests is proposed. The sample of soil is schematically composed of a sheared interface and of a buffer under oedometric load. The problem of sand leakage between the shear box and the rough plate, classical phenomenon in this type of test, is focused. The effect of initial density, position of “center of cycles” in stress plane (mean cyclic variables) and cyclic amplitude is investigated. The cycles are defined by the initial mean cyclic normal stress, the level of initial mean cyclic stress ratio and the normalized cyclic amplitude. Under CNL condition, either dilation or contraction is exhibited, in agreement with the characteristic state developed by Luong (International symposium on soils under cyclic and transient loading, Swansea, 7–11 January, pp 315–324, 1980). The influence of a prescribed normal stiffness is especially considered. It can be highlighted that CNS cyclic paths are always contractive. This contraction results in a drop of mean cyclic normal stress often called degradation of friction. 相似文献
5.
动荷载下砂土与EPS颗粒混合的轻质土变形特性的试验研究 总被引：3，自引：1，他引：2
砂土与EPS颗粒混合的轻质土（LSES）是一种通过减轻地基压力、提高地基承载力以处理不良地基的新型轻质土工材料。由于LSES主要用作路基填土,因此,其在动荷载下的变形特性值得关注。基于室内动三轴试验结果,讨论了围压、水泥掺量、EPS含量以及循环加载次数对LSES动变形特性的影响。当围压较低时,LSES会呈现出与EPS块体相似的线性动应力-应变关系;而在较高围压下,LSES的动应力-应变拟合曲线则为双曲线型。在给定的应变水平下,LSES的割线动弹性模量Esec随着围压和水泥掺量的增加而增加,随循环加载次数的增多而逐渐衰减,EPS含量增多时,Esec-εd曲线更趋于平缓。 相似文献
6.
为探究土工格室加筋路堤在循环荷载及静载下的各种性能,利用美国GCTS公司的USTX-2000加载装置进行加载,通过改变加筋层数、格室高度,格室焊距对土工格室加筋路堤进行一系列模型试验。对各种工况下加筋路堤极限承载力、长期循环荷载及固定振次循环荷载后极限承载力的变化进行研究。试验表明,土工格室加筋能显著提高地基极限承载力并能显著减小坡顶和坡中临界破坏时的法向累积变形,在加筋间距一定的情况下,加筋层数增加和格室高度增大均可不同程度提高极限承载力并减小临界破坏时坡顶法向累积变形,格室焊距的减小也可在一定程度提高极限承载力,格室焊距对边坡法向变形影响不大;长期循环荷载下固定间距加筋层数对路堤竖向累积沉降量影响不大,而对边坡坡顶法向累积变形有一定影响,格室高度增大和格室焊距减小均可不同程度减小路堤竖向累积沉降量和坡面法向累积变形;越靠近加载点处,路堤土压力值受加筋影响越显著,加筋提高了土体刚度和密实度,使加筋路堤土压力值较无筋路堤明显增大;对于无筋路堤,改变动载幅值和振次均导致振后极限承载力有不同程度的降低,而对于加筋路堤,当动载幅值≥30 kPa或动载振次≥1 000时,振后极限承载力均有不同程度的提高。 相似文献
7.
低围压循环荷载作用下饱和粗粒土的动力特性与骨干曲线模型研究 总被引：1，自引：0，他引：1
基床层是铁路路基的核心组成部分,一般为粗颗粒土,厚约2.5～3.0 m,长期直接承受行车荷载的反复作用,其在动载反复作用下的变形特性是评价路基工作性能的关键要素之一。为研究粗粒土在列车循环荷载作用下的应力-应变特性,开展了一系列应力控制的单向循环加载大型动三轴试验,模拟列车动载和路基粗粒料填筑实际情况,包括不同动应力幅值（模拟不同列车轴重）、不同围压（模拟不同埋深）的动三轴持续振动试验。结果表明,在循环荷载作用下,土体刚度变化与振动次数、围压关系密切。根据动应力幅值大小的不同,循环荷载作用下饱和粗粒土的动应变随振次的发展形态可分为3种类型：稳定型、破坏型和临界型。根据试验所得出的动应力-应变关系曲线特点,建立了含围压和循环振次的骨干曲线模型。与传统的骨干曲线模型相比,该模型能反映土体刚度随循环振次的变化,更能反映列车往复作用的实际情况;同时该模型能用于估算路基土体动强度,对铁路路基核心层的动力变形稳定性评价和基于动力变形控制的路基设计具有参考价值。 相似文献
8.
This paper aims to assess the characteristics of the deformation and strength behavior of frozen soils at different temperatures under monotonic and cyclic triaxial conditions. The deformation and failure patterns of the specimens change from ductility to brittleness with decreasing temperatures under both monotonic and cyclic loadings. The development of axial strain and stiffness with increasing number of cycles for the soils under cyclic loading is presented and analyzed in detail. A collapse behavior in strength and stiffness is observed in tests of frozen soils at ??5 °C, ??7 °C and ??9 °C. The difference in frictional sliding between the samples with high ductility and those with high brittleness is attributed to the different patterns of deformation and failure. The dynamic modulus is plotted versus axial strain, and the state where the stiffness begins to decrease is employed as the criterion of cyclic failure. The proposed criterion of cyclic failure is verified to be more suitable for frozen soils with high brittleness and seems to be consistent with the peak strength under monotonic loading. Finally, the cyclic stress ratios are plotted against the number of cycles up to this failure criterion, and the effect of temperatures on cyclic strength is evaluated.
相似文献9.
为了研究桩在水平动荷载作用下的受力和变形规律,自制了一套能施加动荷载的加载装置,并对两根承力盘位置不同的模型桩进行了水平动荷载试验,研究了桩顶所加荷载幅值、加载频率、承力盘的位置等因素对支盘桩桩身弯矩和桩侧土压力的影响规律。结果表明:随着桩顶荷载幅值的增加,桩身上部正弯矩和弯矩的最大值均呈现增大趋势,桩侧土中压力亦逐渐增大;随着加载频率的增大,桩身上部弯矩先变大,后又减小,土中压力一直呈增大趋势;承力盘位置靠上的桩在同样条件下桩身正弯矩和最大弯矩值较小,桩侧土中的压力也较小。 相似文献
10.
为了解黄土三轴剪切过程中的微观变化,采用PFC3D建立黄土三轴试验模型,模拟围压分别为0、50、150、300 kPa的三轴剪切试验,并与室内试验进行对比分析。分析结果表明：PFC3D能够较好地模拟出不同围压下从开始到破坏到残余变形整个过程中应力-应变的变化规律,且位移场及接触应力场的变化规律与室内三轴试验宏观现象较一致;发现弹性模量、泊松比及峰值强度与数值模型中微观参数有着密切的联系,如法向刚度kn控制试样宏观弹性模量,kn /ks值控制泊松比,摩擦系数控制峰值强度;通过体应变-轴向应变曲线发现,随着围压的增加应变能增大,试样呈现出由体积膨胀到体积减缩的变化规律。其研究结果为进一步探究黄土的应力-应变性状及抗剪强度特性提供参考。 相似文献
11.
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
12.
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
13.
New excavation or tunnelling affects the stress state of soils in ground. The change of stress state due to excavation may affect the cyclic behaviour of soils. Cyclic loading, such as traffic and earthquake loading, induced ground deformation may be greater than expected if such effect is not considered. A series of cyclic triaxial tests were performed on Sydney sand with different relative densities. The effect of unloading sequence on deformation of the sand under cyclic loading was simulated by reducing lateral stress in steps between loading cycles. The dependence of strain accumulation on the magnitude of confining pressure reduction and on unloading stress paths was studied. The results indicate that the sand has a memory of stress history and the stress history of such unloading enlarges the strain accumulation during the subsequent cycles, and the greater the reduction of lateral stress, the greater the accumulated strain. Under cyclic loading, the accumulated axial strain could increase nonlinearly or linearly with the ratio of unloading magnitude to initial mean effective stress, depending on the stress state before cyclic loading. The unloading stress paths have limited effects on the final accumulated strain if the initial and final stress states are the same. The variation of strain accumulation direction attributes to the change of average stress ratio resulting from lateral stress reduction, but hardly depends on relative density and unloading stress paths. The strain accumulation direction after unloading roughly agrees with the modified Cam Clay flow rule.
相似文献14.
15.
循环荷载下饱和软黏土的动骨干曲线模型研究 总被引：2，自引：1，他引：1
通过饱和软黏土室内不排水动三轴试验,研究了循环周次和动应力幅值对土体动力特性的影响。结果表明,在循环荷载作用下,随着循环周次的增加,土体均存在不同程度的刚度软化现象。根据动应力幅值大小的不同,循环荷载作用下饱和软黏土的动应变发展形态可分为3种类型:稳定型、破坏型和临界型。根据动应力-应变关系曲线特点,提出了含动应力幅值、固结围压和循环周次等影响因素的动骨干曲线模型。该模型所得的拟合值与试验值比较吻合。与传统骨干曲线模型相比,该模型能够体现土体在循环荷载作用下的刚度软化特性,更加切合实际。 相似文献
16.
Hanifi Missoum Mostefa Belkhatir Karim Bendani Mustapha Maliki 《Geotechnical and Geological Engineering》2013,31(1):279-296
In a number of recent case studies, the liquefaction of silty sands has been reported. To investigate the undrained shear and deformation behaviour of Chlef sand–silt mixtures, a series of monotonic and stress-controlled cyclic triaxial tests were conducted on sand encountered at the site. The aim of this laboratory investigation is to study the influence of silt contents, expressed by means of the equivalent void ratio on undrained residual shear strength of loose, medium dense and dense sand–silt mixtures under monotonic loading and liquefaction potential under cyclic loading. After an earthquake event, the prediction of the post-liquefaction strength is becoming a challenging task in order to ensure the stability of different types of earth structures. Thus, the choice of the appropriate undrained residual shear strength of silty sandy soils that are prone to liquefaction to be used in engineering practice design should be established. To achieve this, a series of undrained triaxial tests were conducted on reconstituted saturated silty sand samples with different fines contents ranging from 0 to 40 %. In all tests, the confining pressure was held constant at 100 kPa. From the experimental results obtained, it is clear that the global void ratio cannot be used as a state parameter and may not characterize the actual behaviour of the soil as well. The equivalent void ratio expressing the fine particles participation in soil strength is then introduced. A linear relationship between the undrained shear residual shear strength and the equivalent void ratio has been obtained for the studied range of the fines contents. Cyclic test results confirm that the increase in the equivalent void ratio and the fines content accelerates the liquefaction phenomenon for the studied stress ratio and the liquefaction resistance decreases with the increase in either the equivalent void ratio or the loading amplitude level. These cyclic tests results confirm the obtained monotonic tests results. 相似文献
17.
18.
采用自主设计的竖向循环加载装置,通过室内模型试验研究竖向循环荷载作用下砂土中单桩承载特性和桩周土体变形机制。根据试验结果,桩体累积位移可以划分为不发展区域、渐变发展区域、破坏区域3种区域;滞回曲线的滞回圈包络面积随着循环次数的增加,呈现逐渐减小的趋势,滞回曲线由不闭合发展为闭合曲线,桩周土体由弹塑性变形逐渐转变为弹性变形。采用粒子图像测速（particle image velocimetry,简称PIV）技术对循环荷载作用下桩周土体变形进行实时量测,得到桩周土体完整的位移场和剪切应变场。结果表明：循环周期、幅值和密实度为桩周土体变形的主要影响因素,随着循环周期的增加,剪切破坏带在接近土体表面处呈现内敛趋势,剪切破坏面最终近乎平行于桩-土界面。循环荷载幅值越大,表层土体在循环荷载作用下越趋于密实,侧向土压力增大,位移影响区域减小,对应剪切应变场呈现“耳”状分布,幅值比循环周期更容易导致桩周土体出现沉陷。不同砂土密实度中的桩体累积位移随着循环周期呈现出不同的特征,松砂状态下桩周土体位移场呈现倒截锥形,密砂则呈现圆柱形。 相似文献
19.
Literature regarding the pore pressure generation characteristics and in turn the cyclic resistance behaviour of silty sand
deposits is confusing. In an attempt to clarify the effect of nonplastic fines on undrained cyclic pore pressure response
of sand–silt mixtures, an experimental programme utilising around 289 stress-controlled cyclic triaxial tests on specimens
of size 50 mm diameter and 100 mm height was carried out at a frequency of 0.1 Hz. Specimens were prepared to various measures
of density through constant gross void ratio approach, constant relative density approach, constant sand skeleton void ratio
approach, and constant interfine void ratio approach to study the effect of nonplastic fines on pore pressure response of
sand–silt mixtures. The effect of relative density, confining pressure as well as the frequency and magnitude of cyclic loading
was also studied. It was observed that the pore pressure response is greatly influenced by the limiting silt content and the
relative density of a specimen corresponding to any approach. The influence of other parameters such as relative density,
confining pressure and magnitude of cyclic loading was as usual but an increase in frequency of cyclic loading was seen to
generate excess pore pressure at a higher rate indicating an impact load type of behaviour at higher frequency. Utilising
the entire test results over a wide range of parameters a new pore pressure band for sand–silt mixtures in line with Lee and
Albaisa (1974) has been proposed. Similarly another pore pressure band corresponding to 10th cycle of loading as suggested by Dobry (1985) and up to a shear strain of around 25% has been proposed. These two bands can readily be used by researchers and field engineers
to readily assess the pore pressure response of sand–silt mixtures. 相似文献
20.
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