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1.
Settlements under consecutive series of cyclic loading 总被引：1，自引：0，他引：1
Hüseyin Y&#x;ld&#x;r&#x;m Hayreddin Eran 《Soil Dynamics and Earthquake Engineering》2007,27(6):577-585
In this study, consolidation settlements of soft clay caused by cyclic loading and the affecting factors such as number of cycles and stress level were experimentally investigated. A group of samples prepared in slurry consolidometer in the laboratory were tested using cyclic simple shear testing device. Normally consolidated samples were subjected to five consecutive series of cyclic loading and drainage for 60 min were applied between each cyclic loading stages. Cyclic tests were performed with stress controlled two-way sinusoidal wave loading with different stress levels and number of cycles. Frequency of cyclic loading was constant as 0.1 Hz. As a result of this study, it can be concluded that soft clays subjected to undrained cyclic loading and drainage cycles exhibit more resistance against subsequent cyclic shear stresses. The consolidation settlements, pore pressures and shear strains decrease after each stage of cyclic loading. 相似文献
2.
Cement stabilization is often used to improve the bearing capacity and compressibility of soft clays. The present paper aims to investigate the shear modulus degradation of cement treated clay during cyclic loading. A series of cyclic triaxial test was conducted on artificially cement treated marine clay to study the factors affecting the shear modulus degradation. The parameters considered for the study are cement content (2.5–7.5%), curing days (7–28), cyclic shear strain amplitude (0.3–1%), number of loading cycles (1–100) and loading frequency (0.1–0.5 Hz). As in the case of natural clays, cement treated clays exhibit stiffness degradation which depends on mix ratio, curing days and loading conditions. The results show that the shear modulus degradation decreases with increase in the shear strain amplitude, cement content and curing days. It is also noted that irrespective of the mix ratio and curing conditions, the degradation decreases with increase in loading frequency. An empirical relationship is proposed to predict the shear modulus degradation based on Idriss׳s degradation model. The performance of the proposed empirical model is validated with the present experimental results. 相似文献
3.
One of the key issues in cyclic behaviour of soft clays is gradual degradation of shear modulus. In most of the cyclic soil models such degrdation of shear modulus of soil with the progression of loading cycle was incorporated, addition to the standard non-linear backbone curves. Such cyclic degradation was usually represented by a parameter, degradation index, which is a function of loading cycles and cyclic shear strain amplitude. However it is well understood from the past experimental studies that the degradation index depends on various other factors as well. The present paper aims to develop a simple empirical model involving degradation index as a function of number of loading cycles, plasticity index, cyclic shear strain, overconsolidation ratio, loading frequency based on the experimental results. It is then fitted with the hyperbolic hysteretic model to estimate the modulus degradation for different cycles. Finally the damping ratio is calculated based on Masing rule with correction factors and validated through experimental results. 相似文献
4.
This paper investigates the cyclic and post-cyclic shear behavior of low-plasticity silt and the impact of additional clay content. Bentonite clay was added to the low-plasticity Mississippi River Valley (MRV) silt (PI=6) to increase the clay content of the soil. A series of triaxial tests were conducted in the laboratory to examine the shear and pore pressure behavior during and after cyclic loading. As the bentonite content in the reconstituted specimens increased, the excess pore pressure developed at a slower rate and the total excess pore pressure decreased at the end of cyclic loading. In contrast to the MRV silt, the specimens modified with bentonite experienced cyclic softening rather than initial flow liquefaction. The cyclic shear strength increased with an increase in bentonite content. The post-cyclic reconsolidation behavior was a similar to a virgin compression process, and not recompression. Adding bentonite to the MRV silt results in changes in permeability, compressibility, undrained shear strength, and initial stiffness. Additionally, the cyclic loading had a marked effect on the shear behavior of low-plasticity soil with a PI<6, but not noticeable with a PI>6. This study suggests that the behavior of the Mississippi River Valley silt changes from contractive sand-like material to clay-like behavior at a PI≈6 due to the addition of clay. 相似文献
5.
Evaluation of the cyclic shear modulus of soils is a crucial but challenging task for many geotechnical earthquake engineering and soil dynamic issues. Improper determination of this property unnecessarily drives up design and maintenance costs or even leads to the construction of unsafe structures. Due to the complexities involved in the direct measurement, empirical curves for estimating the cyclic shear modulus have been commonly adopted in practice for simplicity and economical considerations. However, a systematic and robust approach for formulating a reliable model and empirical curve for cyclic shear modulus prediction for clayey soils is still lacking. In this study, the Bayesian model class selection approach is utilized to identify the most significant soil parameters affecting the normalized cyclic shear modulus and a reliable predictive model for normally to moderately over-consolidated clays is proposed. Results show that the predictability and reliability of the proposed model out performs the well-known empirical models. Finally, a new design chart is established for practical usage. 相似文献
6.
Small strain shear modulus Gmax is an essential parameter in soil dynamics, and it is usually estimated based on the Hardin and Richart equation. However, many previous researches on sands have indicated that the Hardin and Richart equation does not consider the influences of cyclic loading history on Gmax. In this paper, effects of cyclic loading history on Gmax of saturated clays under undrained conditions are studied using a combination device of piezoelectric-ceramic bender element system and cyclic triaxial apparatus. The dynamic pre-loading includes both relatively high amplitudes of cyclic stresses and cyclic strains. Gmax without cyclic loading history is also investigated for the comparison purpose. Test results show that, at the same effective stress, both cyclic strain history and cyclic stress history will induce reduction of Gmax compared to the corresponding Gmax values with non-cyclic loading effects. In strain-controlled tests, the reduction of Gmax is slight and relatively stable; while in stress-controlled tests, the reduction of Gmax increases suddenly and remarkably when the effective stresses degrade to a certain degree. The comparison between double amplitude axial strain and residual excess pore water pressure behaviors show that the remarkable reduction of Gmax can demonstrate the cyclic failure of saturated clays. 相似文献
7.
Cyclic softening and strength loss of saturated clays during earthquakes is often an important consideration in engineering problems such as slope stability, dam/levee safety, and foundation bearing capacity. This study proposes a simplified procedure for evaluating cyclic softening (amount of strength loss) that may be expected in saturated clays during earthquakes and illustrates how to implement it in engineering analysis. The procedure has two main steps: (1) estimation of an equivalent cyclic shear strain amplitude and associated number of cycles induced in the soil mass by an earthquake; and (2) estimation of softening and strength loss in the soil mass. A key aspect of the proposed procedure is adoption of a strain-based approach to estimate cyclic softening as opposed to the widely used stress-based approach of liquefaction assessments. A threshold strain concept originating from the strain-based approach is first discussed and the development of a procedure is presented subsequently. The proposed procedure provides reasonable, first-order estimates of cyclic softening consistent with the other developed procedures. In addition, the capability of the procedure is demonstrated with two case histories identified as involving cyclic softening of clays. 相似文献
8.
Cyclic behavior of natural fine grained soils under a broad range of strains were investigated considering the effects of plasticity index and changes in confining pressures based on cyclic triaxial tests. A total of 98 stress controlled cyclic triaxial tests were conducted on normally consolidated and slightly overconsolidated samples. The investigation was divided into two parts. The first part consists of stress controlled cyclic triaxial tests under different stress amplitudes that were conducted to estimate the modulus reduction and the thresholds between nonlinear elastic, elasto-plastic and viscoplastic behavior. The second part involves the investigation of the undrained stress–strain behavior of fine grained soils under irregular cyclic loadings. The results showed that the elastic threshold is approximately equal to 90% of Gmax. Another transition point was defined as the flow threshold where the value of tangent of shear modulus ratio changes for the second time. Simple empirical relationships to estimate the dynamic shear modulus and damping ratio was formulated and compared with the similar empirical relationships proposed in the literature. The results provide useful guidelines for preliminary estimation of dynamic shear modulus and damping ratio values for fine grained soils based on laboratory tests. 相似文献
9.
Experimental investigations and modeling of nonlinear elasticity of fiber-reinforced soil under cyclic loading at small strain are conducted in this paper. The investigations include three aspects. First, cyclic shear tests are conducted using conventional triaxial apparatus. Twenty-seven specimens with three different fiber contents are employed to conduct triaxial cyclic shear tests under different confining pressure and loading repetition. Effects of geofiber, confining pressure and loading repetition on elastic shear modulus of reinforced soil are studied and analyzed. Second, a hyperbolic function is introduced to describe the nonlinear stress–strain skeletal curve under cyclic loading. Nonlinear elastic modulus is expressed as a function of shear strain and two variables A and B that are related to the initial tangential modulus and ultimate cyclic loading stress, respectively. In the present paper, variables A and B both are further assumed to be functions of geofiber content, confining pressure and loading repetition. Finally, eight constitutive coefficients of the nonlinear elastic model are calibrated using stress–strain curves from cyclic triaxial shear tests. The calibration of parameters is conducted using the technique of the linear regression for multiple variables. Impacts and effects of geofiber, confining pressure and loading repetitions on soil nonlinear elastic behavior are discussed. 相似文献
10.
A. Caserta S. Martino F. Bozzano A. Govoni F. Marra 《Bulletin of Earthquake Engineering》2012,10(4):1133-1162
The presence of peats and high compressibility inorganic clays within alluvial fills on the left-side tributaries of the Tiber River, close to Rome’s historic center in Italy, is well documented in literature. Nevertheless, few literature data exist until now on the dynamic properties of these deposits by undisturbed bore-hole samples. The Galba test-site was set up to characterise dynamic properties of the alluvial deposits by using lab-tests as well as to derive velocity profiles by seismic noise measurements. These were performed in the Giustiniano Imperatore area located in the Grottaperfetta valley, about 2?km south of Rome’s historic centre. The alluvial deposits filled a paleovalley excavated in the bedrock during the Würm glacial (18–20 ky). The stratigraphic setting of the alluvial body was reconstructed along three geological cross-sections by means of the available logs; seven lithotecnical horizons can be distinguished within the alluvial body, some tens of meters thick, based on both log-stratigraphic data and in-site geotechnical tests. These horizons include peaty layers (T) and high compressibility inorganic clays (AGI), which characterise the alluvial deposits in the Grottaperfetta valley. They do not have direct correlation with the alluvial horizons which constitute the alluvial body of the main Tiber valley in Rome’s historical centre. These alluvial horizons which are distinguished and characterised at the Galba test-site can be regarded as typical of other lateral valleys of the Tiber River in Southern Rome. They are characterised by the presence of similar high compressibility clayey deposits as well as peaty layers up to some meters thick. Undisturbed samples were also obtained at the Galba test-site for dynamic testing via resonant column and cyclic torsional shear tests. In order to attribute dynamic properties to the alluvial body at the Grottaperfetta valley, an extrapolation process was performed based on a detailed engineering-geology model of the alluvial body which was reconstructed along three transversal geological sections of the valley using bore-hole data. Three subsoil profiles, considered representative of the geological setting along the three reconstructed transversal cross sections, were analysed by seismic noise measurements performed specifically to derive S-waves velocity profiles. The results obtained show a very low velocity (<180 m/s) for the layers T and AGI. 1D modelling of seismic shaking was performed by the code Shake91, in order to evaluate the influence of the low-velocity strata on maximum shear strains induced within the alluvial deposits under the maximum expected seismic action. The results of the numerical modelling indicate that the AGI and the T layers play a key role in: (i) concentrating the maximum shear strain along the subsoil profiles, even though the volumetric threshold is never exceeded; (ii) increasing the maximum shear strain along the subsoil profiles; (iii) causing the resonance frequency of the alluvial fill to assume an almost constant value (about 1Hz) which is quite similar to that measured in the main Tiber River valley, despite a significant change in thickness of the alluvial body along the Grottaperfetta valley. 相似文献
11.
Cyclic shear response of channel-fill Fraser River Delta silt 总被引：2，自引：0，他引：2
The cyclic shear response of a channel-fill, low-plastic silt was investigated using constant-volume direct simple shear testing. Silt specimens, initially consolidated to stress levels at or above the preconsolidation stress, displayed cyclic-mobility-type strain development during cyclic loading without static shear stress bias. Liquefaction in the form of strain softening accompanied by loss of shear strength did not manifest regardless of the applied cyclic stress ratio, or the level of induced excess pore water pressure, suggesting that the silt is unlikely to experience flow failure under cyclic loading. The cyclic shear resistance of the silt increased with increasing overconsolidation ratio (OCR) for OCR>1.3. The silt specimens that experienced high equivalent excess cyclic pore water pressure ratios (ru>80%) resulted in considerable volumetric strains (2.5%–5%) during post-cyclic reconsolidation implying potentially significant changes to the particle fabric under cyclic loading. 相似文献
12.
Pre-shear history has been shown to be a critical factor in the liquefaction resistance of sand. By contrast to prior experimental studies in which triaxial shear tests were used to examine the effects of pre-shear on the liquefaction resistance of sand, hollow cylinder torsional shear tests were used in this study to avoid the influence of the inherent anisotropy that is inevitably produced during the sample preparation process because of gravitational deposition. A series of cyclic undrained shear tests were performed on sand samples that had experienced medium to large pre-shear loading. The test results showed that the liquefaction resistance of sand can be greatly reduced by its pre-shear history, and a pre-shear strain within the range from 0.1% to 5% can cause sand to be more prone to liquefaction. During the cyclic shear tests, the samples that had experienced pre-shear loading exhibited different behaviors when cyclic shear loading started in different directions, i.e., the clockwise direction and the counterclockwise direction. If the cyclic loading started in the identical direction as the pre-shear loading, then the mean effective stress of the sand was almost unchanged during the first half of the loading cycle; if the cyclic loading started in the direction opposite to that of the pre-shear loading, then the mean effective stress decreased significantly during the first half of the loading cycle. However, this anisotropic behavior was only remarkable during the first loading cycle. From the second cycle onward, the speeds of the decrease in the mean effective stresses in the two types of shear tests became similar. 相似文献
13.
Gonghui Wang Runqiu Huang Masahiro Chigira Xiyong Wu Sérgio D. N. Lourenço 《地球表面变化过程与地形》2013,38(3):265-274
Here, we propose that an earthquake can trigger the failure of a landslide mass while simultaneously triggering liquefaction of runout‐path materials before the arrival of the landslide mass, thus greatly increasing the size and mobility of an overriding landslide. During the 2008 Wenchuan earthquake, about 60 000 landslides were triggered, directly resulting in about 20 000 casualties. While these landslides mainly originated from steep slopes, some landslides with high mobility formed in colluvial valley deposits. Among these, the most catastrophic was the Xiejiadian landslide in Pengzhou city, which traveled hundreds of meters before coming to rest. Through field investigation and laboratory testing, we conclude that this landslide primarily formed from colluvial deposits in the valley and secondarily from failure of slopes in granitic rock located uphill. Much of the granitic slope failure was deposited in the upper part of the travel path (near the slide head); the remainder was dispersed throughout the main landslide deposit. Superposition of deposits at the landslide toe indicates that landslide debris derived from colluvial soil was deposited first. The deposits at the landslide toe displayed flow characteristics, such as fine materials comprising basal layers and large boulders covering the deposit surface. We hypothesize that the main part of the landslide resulted from seismogenic liquefaction of valley colluvium, rather than from liquefaction potentially caused by undrained loading from the granitic slope failures impacting the colluvium. To examine the likelihood that seismogenic liquefaction occurred, we took samples from different areas of the landslide deposit and performed undrained cyclic shear tests on them in the laboratory. The results showed that the sandy soils that comprise most of the deposit are highly liquefiable under seismic loading. Therefore, we conclude that liquefaction of the colluvium in the valley during the earthquake was the main reason for this rapid (~46 m/s) long‐runout (1·7 km) landslide. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
14.
Dynamic instability of clays: an energy approach 总被引：1，自引：0，他引：1
The dynamic instability of any soil is based on the ability to accumulate from cycle to cycle a part of the energy to be dissipated. Using the general thermodynamic approach in combination with high resolution experiments it is possible to analyze the transformations of energy in soil under dynamic loading. Undisturbed specimens of two Norwegian clays, sufficiently different in their engineering properties, were used in a number of undrained cyclic triaxial tests to study the influence of confining stress, coefficient of earth pressure at rest (K0), dynamic stress amplitude and frequency of loading in the development of instability in the clays. This paper discusses the unit dissipated energy and the unit activation energy as possible energy criteria for evaluation of dynamic instability in soils. 相似文献
15.
复杂应力条件下饱和松砂单调与循环剪切特性的比较研究 总被引：4，自引：0，他引：4
本文利用大连理工大学新引进与开发的“土工静力-动力液压-三轴扭转多功能剪切仪”,针对福建标准砂,在不排水条件下同时进行了单调剪切试验与循环剪切试验,进而对其进行了对比分析。通过比较表明,应力-应变关系的应变软化和硬化特性与流滑变形和循环流动特性密切相关,当循环剪切应力水平高于单调剪切过程中应变软化阶段最小强度时将会发生流滑变形。无论在单调剪切中,还是在循环剪切中,稳定状态时的有效偏应力比随着大主应力方向与竖向之间夹角的增大而减小,在中主应力系数相同的条件下,循环剪切中呈现显著剪胀时的有效偏应力比和最终稳定状态时的有效偏应力比峰值分别与单调剪切中达到相变状态时的有效偏应力比和最终稳定有效偏应力比基本上一致。然而不排水条件下单调与循环剪切过程中孔隙水压力的增长特性却并不相同,循环剪切中的最大孔隙水压力随着初始主应力方向角的增大而减小,单调剪切中的最大孔隙水压力却随着主应力方向角的增大而增大。 相似文献
16.
Kiichiro Kawamura Takahiro Hosono Huda Mohamed Allawati Yujiro Ogawa Hidetsugu Taniguchi 《Island Arc》2005,14(4):564-570
Abstract Magnetic susceptibility and the anisotropy of magnetic susceptibility were measured on an 800-cm-thick succession of cumulate gabbro in the Sadm area of the Oman ophiolite. The section contained three distinct cumulate units. The susceptibility tends to decrease upward in each from a melanocratic layer (several tens of centimeters thick) to a leucocratic layer (a few meters thick). The susceptibility decreases in accordance with the decreasing number of magnetite grains, which are the alteration product mainly of olivine minerals. This suggests the cyclic downward accumulation of olivine in the cumulate gabbro. The apparent strain deduced from the patterns of magnetic and grain fabrics was the result mostly of simple shear, so that the layering of gabbro is understood to be formed primarily by a crystal cumulus process followed by simple shear deformation. 相似文献
17.
A laboratory study on the undrained dynamic behavior of saturated clays in cyclic triaxial tests with a variable confining pressure (VCP tests) is presented. Tests were performed on remolded clayey samples using a dynamic triaxial device where the deviatoric stress and confining pressure can be varied simultaneously. Various cyclic stress paths have been applied on the specimens through varying the ratios or phase differences between the cyclic deviatoric stress and cyclic confining pressure. Specifically, the stress paths used in the present study were designed to simulate the coupling effects of simultaneously varying shear and normal stresses in clays due to earthquakes and other vibration sources. Test results obtained from this study show that the undrained response of saturated clays is strongly influenced by the variation of confining pressure, in terms of pore water pressure, development speed of cyclic strain and magnitude of cyclic strength. It is found that when strong P-waves are propagating in soil layers, VCP tests are more appropriate for the simulation of in situ stress fields than the conventional cyclic triaxial tests with a constant confining pressure (CCP tests). 相似文献
18.
针对人工制备的大连饱和黏土,在静应力与循环应力联合作用下进行了多组循环剪切试验,着重探讨了初始静力大小及加载速率对循环剪切变形特性的影响.试验表明静应力与循环应力作用下饱和黏土具有循环蠕变特性,总应变由初始静应变和循环蠕变两部分组成,其中循环蠕变主要受循环应力幅值、初始静应力大小及其加载速率的影响,循环蠕变的逐次增加量... 相似文献
19.
Francesca Bozzano Paolo Mazzanti Marco Anzidei Carlo Esposito Mario Floris Gianluca Bianchi Fasani Alessandra Esposito 《地球表面变化过程与地形》2009,34(11):1469-1486
New detailed data about the morphology of the submerged slopes of Lake Albano (Rome, Italy) have been collected by a sonar multibeam survey financed by the Italian Department of Civil Protection. These data allow for investigation of the subaqueous slope dynamics of the lake, which partially fills a volcanic depression, and the elucidation of the relationships between subaqueous and subaerial slope processes. Subaerial, submerged and combined subaerial/submerged landslide‐related morphologies were detected around the inner slopes of the lake. In the submerged slopes, several gravity‐induced landforms were recognized: landslide scar areas, landslide accumulations, erosional chutes and channels, block fields, isolated blocks, scarps and slope breaks. An attempt to evaluate the state of activity of the submerged slopes was carried out by taking into consideration the relative freshness of some selected landforms. Interpretation of bathymetric data, as well as direct surveys of the subaerial slopes, was used to assess the morphometric features and interpret the type of movement of the landslides. We propose a comprehensive classification based on the landslide's size and type of movement. We recognized rock fall/topples, debris flows, rock slides and slump, complex rock slides/channelled flows and debris slide and slump. The volume of the main landslides ranged between 101 and 103 m3, while a few rock and debris slides have volumes ranging between 103 and 105 m3. Two large palaeo‐landslides with volumes on the order of 106 m3 were identified in the southern and northern part of the lake, respectively. Velocities of the recognized landslides range from rapid to extremely rapid. Two main landslide hazard scenarios have been depicted from the results of the integrated analysis of both subaerial and submerged gravity‐induced landforms. The most hazardous scenario involves extremely rapid large volume events (>106 m3) that could, if they interacted with water, induce catastrophic tsunamis. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
20.
During earthquake events, low-plasticity silt undergoes a reduction in shear strength and stiffness due to development of excess pore pressure induced by cyclic loading. With reconsolidation, during which process excess pore pressure is dissipated, the shear strength and stiffness can be regained. However, due to the low permeability of silts (compared to sands), the dissipation of excess pore pressure and the reconsolidation of low-plasticity silt takes much more time. This paper investigates the postliquefaction shear behavior of Mississippi River Valley (MRV) silt at various degrees of reconsolidation using triaxial tests. Test results indicate that there was a steady increase, in shear strength and stiffness, at both large and small deformations, with increase in the degree of reconsolidation. The postliquefaction silt showed the effect of the apparent OCR, which had a close effect on postcyclic shear behavior as did the OCR on the static behavior. The critical state lines of MRV silt were different for pre- and post-liquefaction conditions. 相似文献