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1.
The process of dynamically induced liquefaction in two centrifuge soil models is analyzed. These models consist of saturated medium-dense sand overlain by a low permeability silt deposit, and represent prototypes of a level site and an embankment. The recorded lateral accelerations are employed to evaluate shear stress and strain histories at different elevations within the tested soil systems. These histories shed light on the involved liquefaction process, and the associated mechanisms of: (1) lateral deformation; (2) stiffness and strength degradation; and (3) possible densification and regain of stiffness, thereafter. The identified response patterns are found comparable to those documented by laboratory cyclic-loading tests.  相似文献   

2.
Downhole records of seismically-induced soil liquefaction are a valuable source of information on the associated mechanisms of stiffness degradation and lateral spreading. In this paper, free-field downhole array seismic records are employed to identify and model the recorded response at Wildlife Refuge (California, USA) and Port Island (Kobe, Japan) sites. The Wildlife Refuge site was instrumented in 1982 with a two-accelerometer array and six piezometers that recorded a case of seismically induced site liquefaction. At Port Island, a four-accelerometer down-hole array recorded strong motion during the recent 1995 Hyogoken-Nanbu earthquake. This earthquake resulted in widespread liquefaction and major ground deformations at Port Island. Using the recorded downhole accelerations at these two sites, the actual seismic shear stress-strain histories are directly evaluated. These histories provide valuable insight into the mechanisms of site liquefaction and associated loss of stiffness and strength. Based on the identified dynamic soil behaviors, computational simulations of the observed seismic response are performed. Optimization techniques are employed to estimate the necessary computational modeling parameters. This document constitutes the second part of a set of two companion papers about site amplification and liquefaction.  相似文献   

3.
Two centrifuge tests were designed to improve the understanding the response of liquefied sandy slopes beyond initial liquefaction. A distinctive dilative behavior of the soil was observed near the slope where static shear stresses are present. The corresponding drops in the piezometric records and simultaneous negative upslope spikes in the acceleration records were measured in the transducer raw data. This dilative response became stronger as the input acceleration increased and tends to limit the downslope accumulation and thus reducing the permanent lateral displacements. Therefore, the maximum permanent displacement was smaller in the model with the larger input motion, because it developed a stronger dilative response. The dilative response was not observed away from the slope, where no static shear stresses are present.  相似文献   

4.
Seismic downhole-array data provide a unique source of information on actual soil (and overall site) behavior over a wide range of loading conditions that are not readily covered by in-situ or laboratory experimentation procedures. In this paper, free-field downhole-array seismic records are employed to identify and model the recorded response at the Lotung (Taiwan) and Treasure Island (California) sites. At Lotung, a five-accelerometer array recorded the site response during 18 earthquakes (1985–1986). The Treasure Island site was instrumented in 1992 with an array of six accelerometers that recorded a low amplitude earthquake in 1993. Using this downhole data, correlation and spectral analyses are performed to evaluate shear wave propagation characteristics, variation of shear wave velocity with depth and site resonant frequencies and model configurations. In addition, the actual seismic shear stress-strain histories are directly evaluated from the recorded downhole accelerations. These histories provide valuable insight into the mechanisms of site amplification, damping and pore-pressure build-up. Computational simulations of these case histories are performed based on the identified mechanisms of site response. In a companion paper, two additional case histories of site liquefaction are analyzed using records of downhole seismic response.  相似文献   

5.
Assessment of liquefaction potential based on peak ground motion parameters   总被引：1，自引：0，他引：1  
Conventionally, evaluation of liquefaction potential of loose saturated cohesionless deposits as specified in Japanese design codes employs peak ground acceleration (PGA). However, recent large-scale earthquakes in Japan revealed that liquefaction at some sites did not occur even though large PGAs were recorded at or near these sites. As an alternative approach, an evaluation procedure based on peak ground motion parameters, i.e. incorporating both PGA and the peak ground velocity (PGV), is proposed. By performing parametric studies using one-dimensional seismic response analysis and formulating regression models, seismic-induced shear stresses within the deposit are expressed in terms of peak ground motion parameters at the surface, and these are used to calculate the factor of safety against liquefaction. Application to case histories in Japan indicates that the proposed two-parameter equation can adequately account for the occurrence and non-occurrence of liquefaction at various sites as compared to the conventional PGA-based approach. Moreover, analyses of several strong motion records at various sites show that liquefaction may occur when PGA≥150 gal and PGV≥20 kine, indicating that these values can serve as thresholds in assessing the possible occurrence of liquefaction.  相似文献   

6.
In the first part of this study, a series of stress-controlled hollow cylinder cyclic torsional triaxial shear tests were conducted on loose to medium dense saturated samples of clean Toyoura sand to investigate its liquefaction behavior. A uniform cyclic sinusoidal loading at a 0.1 Hz frequency was applied to air-pluviated samples where confining pressure and relative density was varied. Cyclic shear stress–strain changes, the number of cycles to reach liquefaction and pore pressure variations were recorded. Results indicate that the liquefaction resistances of uniform sands are significantly affected by the method of sample preparation and initial conditions.  相似文献   

7.
振动频率对饱和砂土液化强度的影响   总被引：4，自引：0，他引：4  
郭莹  贺林 《地震学刊》2009,(6):618-623
采用“土工静力-动力液压三轴-扭转多功能剪切仪”对饱和砂土进行了一系列动三轴实验,探讨了振动频率对液化强度数值的影响程度。在1．0、1．5固结比和0．05、0．10、1．00Hz振动频率条件下,针对相对密实度分别为70％、28％的密砂和松砂进行了100、200、300kPa围压和100kPa围压条件下的液化强度实验。实验结果表明,饱和密砂和松砂在各种固结条件下,液化强度随着振动频率的增大而增大,相同破坏振次时,各种实验条件下的液化强度与振动频率的关系在双对数坐标上均符合线性关系;振动频率由0．05Hz变化到1．00Hz时,液化强度相差达25％以上;动强度指标翰值随振动频率的增大而增大,最大相差12．2％;随着振动频率的增大,砂土达到液化破坏所需的时间明显缩短;振动频率对松砂液化强度的影响比对密砂的影响更为显著。  相似文献   

8.
This paper intended to evaluate the behavior of saturated sand and sloped ground subjected to flow failure with seepage of pore water in the ground after earthquake and the resultant liquefaction. Triaxial compression tests of sand with constant deviator stress but changing of pore pressure and volume of the specimens were conducted in this study. It was revealed that the relation between the volume change and the amount of shear strain during deformation depended on the initial density of the sand but it did not much depend on shear stress and initial confining stress levels. Based on this test results and numerical analysis of the seepage of pore water in liquefied ground, a methodology was proposed to predict the deformation of inclined ground due to liquefaction.  相似文献   

9.
Contrary to many laboratory investigations, common empirical correlations from in situ tests consider that the increase in the percentage of fines leads to an increase of the cyclic liquefaction resistance of sands. This paper draws upon the integrated Critical State Soil Mechanics framework in order to study this seemingly not univocal effect. Firstly the effect of fines on the Critical State Line (CSL) is studied through a statistical analysis of a large data set of published monotonic triaxial tests. The results show that increasing the content of non-plastic fines practically leads to a clockwise rotation of the CSL in (e–ln p) space. The implication of this effect on cyclic liquefaction resistance is subsequently evaluated with the aid of a properly calibrated critical state elasto-plastic constitutive model, as well as a large number of published experimental results and in situ empirical correlations. Both sets of data show clearly that a fines content, less than about 30% by weight, may prove beneficial at relatively small effective stresses (p0<50–70 kPa), such as the in situ stresses prevailing in most liquefaction case studies, and detrimental at larger confining stresses, i.e. the stresses usually considered in laboratory tests. To the extent of these findings, a correction factor is proposed for the practical evaluation of liquefaction resistance in terms of the fines content and the mean effective confining stress.  相似文献   

10.
In this study, cyclic hollow cylinder torsional tests were conducted on the reconstituted specimens of Toyoura sand in a practical range of initial density and stress states. The results were employed to evaluate the liquefaction resistance and residual pore water pressure of sand using the strain energy concept. A simple pore water pressure (PWP) model with two calibration parameters was developed for the prediction of residual pore pressure as a function of cumulative strain energy density and the capacity energy of sand. Capacity energy is defined as the cumulative strain energy that is required for liquefaction onset. Based on the results of the tests, an equation is then presented for the estimation of capacity energy in terms of relative density and initial effective confining pressure of sand. This equation is shown to work well as a state boundary curve, which can discriminate between the liquefied and non-liquefied field case histories. Several extra tests were also performed to investigate the effect of initial static shear stress on the proposed PWP model and capacity energy. The results show that initial shear stress has a minor effect on the trend of the proposed PWP model; however, it definitely affects the capacity energy. The final part of the paper aims to confirm reasonable performance of the proposed PWP model by the available observations of seismically induced pore water pressure in shaking table, centrifuge, and real site conditions.  相似文献   

11.
Residual strength after liquefaction: A rheological approach   总被引：1，自引：0，他引：1  
The motion of a sphere pulled through liquefied sand of low relative density has been observed. Long triaxial specimens were prepared around a 12.7 mm-diameter sphere loaded by a wire/deadweight system. Velocities and drag forces were measured over a large range of shear strains and strain rates. Typical behavior involved a rapid drop to a non-zero minimum drag resistance followed by an increase in resistance with velocity. These results suggest that residual strength of sand after liquefaction is rate-dependent, and that the Bingham Plastic model seems a reasonable fit to the observed behavior over a large range of sliding velocities. Shear stresses calculated from the hydrodynamics of the sphere are of the same order as those inferred from field case histories.  相似文献   

12.
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   

13.
—Forced torsional oscillation techniques have been used to explore the seismic-frequency shear mode viscoelasticity of specimens of two crustal rocks (Cape Sorell quartzite and Delegate aplite), cycled between room temperature and 700°C under conditions of moderate confining pressure. The anisotropy and intergranular inhomogeneity of thermal expansivity in these materials give rise to large deviatoric stresses, resulting in thermal cracking at temperatures above a pressure-dependent threshold temperature, associated with the onset of very pronounced temperature sensitivity of the shear modulus, in general accord with the predictions of fracture mechanics models. For Delegate aplite in particular, the shear modulus behaves reproducibly during multiple thermal cycles at different confining pressures, consistent with the notion that the thermal cracks are of low aspect ratio (minimum/maximum dimension), and are therefore readily closed by the prevailing confining pressure once the thermal stresses are removed. Marked frequency-dependent dissipation of shear strain energy is observed on heating each rock to temperatures ≥ 500°C, although the attenuation varies significantly with prior thermal history, probably as a result of progressive dehydration and relaxation of deviatoric stresses. Temperature and pressure dependent crack densities for Delegate aplite have been estimated by comparison of the observed shear moduli with those expected for a crack-free aggregate. In parallel with the forced oscillation tests, measurements have been made of the rate at which (argon) pore pressure equilibrium is re-established following a perturbation. Combination of these results, which provide a proxy for permeability, with the inferred crack densities indicates that the variation of permeability with crack density is well described by a percolation model with a threshold crack density of ～0.2.  相似文献   

14.
To investigate the seismic response of a pile group during liquefaction, shaking table tests on a 1/25 scale model of a 2 × 2 pile group were conducted, which were pilot tests of a test project of a scale-model offshore wind turbine with jacket foundation. A large laminar shear box was utilized as the soil container to prepare a liquefiable sandy ground specimen. The pile group model comprising four slender aluminum piles with their pile heads connected by a rigid frame was designed with similitude considerations focusing on soil–pile interaction. The input motions were 2-Hz sinusoids with various acceleration amplitudes. The excess pore water pressure generation indicated that the upper half of the ground specimen reached initial liquefaction under the 50-gal-amplitude excitation, whereas in the 75-gal-amplitude test, almost entire ground was liquefied. Accelerations in soil, on the movable frames composing the laminar boundary of the shear box, and along the pile showed limited difference at the same elevation before liquefaction. After liquefaction, the soil and the movable-frame accelerations that represented the ground response considerably reduced, whereas both the movable frames and the piles exhibited high-frequency jitters other than 2-Hz sinusoid, and meantime, remarkable phase difference between the responses of the pile group and the ground was observed, all probably due to the substantial degradation of liquefied soil. Axial strains along the pile implied its double-curvature bending behavior, and the accordingly calculated moment declined significantly after liquefaction. These observations demonstrated the interaction between soil and piles during liquefaction.  相似文献   

15.
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   

16.
Estimating severity of liquefaction-induced damage near foundation   总被引：1，自引：2，他引：1  
An empirical procedure for estimating the severity of liquefaction-induced ground damage at or near foundations of existing buildings is established. The procedure is based on an examination of 30 case histories from recent earthquakes. The data for these case histories consist of observations of the damage that resulted from liquefaction, and the subsurface soil conditions as revealed by cone penetration tests. These field observations are used to classify these cases into one of three damaging effect categories, ‘no damage’, ‘minor to moderate damage’, and ‘major damage’. The potential for liquefaction-induced ground failure at each site is calculated and expressed as the probability of ground failure. The relationship between the probability of ground failure and the damage class is established, which allows for the evaluation of the severity of liquefaction-induced ground damage at or near foundations. The procedure presented herein represents a significant attempt to address the issue of liquefaction effect. Caution must be exercised, however, when using the proposed model and procedure for estimating liquefaction damage severity, because they are developed based on limited number of case histories.  相似文献   

17.
The linear and non-linear responses of surface soil layers have been predicted through the simultaneous simulation test against the observed ground motions at the six sites in Kobe City during the 1995 Hyogo-ken Nanbu earthquake. The total stress analysis method and the effective stress analysis method have been applied for the rough and detailed verification of the predicted non-linear dynamic behavior at the PIS and RKI sites including the liquefaction phenomenon. The shear strain distribution along depth, the ratio of excess pore water pressure to initial effective stress, the liquefaction strength parameters to initial effective stress, and the stress–strain curve during the earthquake at the PIS site have been investigated when the predicted ground motion could simulate successfully the observed acceleration time histories and response spectra in the non-linear range.  相似文献   

18.
In this study the stress–strain characteristics of sand-ground rubber mixtures are investigated in the sandlike zone,at different confining pressures,using hollow cylinder specimens subjected to torsional monotonic and cyclic loading.Under monotonic loading a mixture of sand-ground rubber with 10% and 25% rubber content show more contraction behaviour than that observed in a pure sand specimen.Phase transformation point in these mixtures are located on a larger shear strain.As expected,the shear strength of specimens decreases with increase of ground rubber content.However,with increasing of effective confining pressure,the loss in shear strength of the mixture is decreased.In addition,a mixture with 25% ground rubber shows a smaller loss in shear strength compared to a mixture with 10% ground rubber mixture.Under cyclic loading mixtures with 10% and 25% ground rubber have similar liquefaction resistance,especially at confining pressures of 110 k Pa and 260 k Pa.Therefore,by using of the mixture with 25% ground rubber,a larger volume of scrap tires could be recycled.The addition of ground rubber to sand would affect the shear strain variation and excess pore water pressure trends,and this effect was further intensified with increasing ground rubber percentage.  相似文献   

19.
The deformation characteristics, shear moduli and damping ratios of reconstituted lacustrine carbonate silt samples were investigated in different kinds of laboratory tests. Material from a landslide that occurred in a seismically very active area in Slovenia was used in the tests. The initial values of this material's deformation characteristics and the extent to which these values change due to increasing shear strain depend on a number of factors, such as the degree of saturation, the initial void ratio, over-consolidation, and the effective stresses. The use of a volumetric threshold strain has proved to be the best way of describing the shear modulus reduction curve irrespective of any other factors, and it fits well with the data obtained in the experiments. The critical state and consolidation lines for the investigated material are presented in the paper as deformational characteristics at critical strain levels. The material's liquefaction potential was studied for certain in situ conditions.  相似文献   

20.
This paper is a systematic effort to clarify why field liquefaction charts based on Seed and Idriss׳ Simplified Procedure work so well. This is a necessary step toward integrating the states of the art (SOA) and practice (SOP) for evaluating liquefaction and its effects. The SOA relies mostly on laboratory measurements and correlations with void ratio and relative density of the sand. The SOP is based on field measurements of penetration resistance and shear wave velocity coupled with empirical or semi-empirical correlations. This gap slows down further progress in both SOP and SOA. The paper accomplishes its objective through: a literature review of relevant aspects of the SOA including factors influencing threshold shear strain and pore pressure buildup during cyclic strain-controlled tests; a discussion of factors influencing field penetration resistance and shear wave velocity; and a discussion of the meaning of the curves in the liquefaction charts separating liquefaction from no liquefaction, helped by recent full-scale and centrifuge results. It is concluded that the charts are curves of constant cyclic strain at the lower end (Vs1<160 m/s), with this strain being about 0.03–0.05% for earthquake magnitude, Mw≈7. It is also concluded, in a more speculative way, that the curves at the upper end probably correspond to a variable increasing cyclic strain and Ko, with this upper end controlled by overconsolidated and preshaken sands, and with cyclic strains needed to cause liquefaction being as high as 0.1–0.3%. These conclusions are validated by application to case histories corresponding to Mw≈7, mostly in the San Francisco Bay Area of California during the 1989 Loma Prieta earthquake.  相似文献   

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