No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Characterization of reconsolidation volumetric strain of liquefied sand and validation by centrifuge model tests
Abstract
The results of consolidation tests under K0 condition and cyclic triaxial tests on Fujian sand at a given relative density are introduced, and the mechanism of the liquefaction-induced volumetric strain is revealed, which is composed of the re-sedimentation and the re-consolidation processes. The re-sedimentation is closely related to the cyclic shear strain history, especially that after liquefaction, and the more the accumulated shear strain ratio is, the more the re-sedimentation volumetric is. Besides, the re-consolidation behavior is significantly affected by the previous consolidation history and cyclic stress history, and the post-liquefaction reconsolidation will follow the trend of the previous normal consolidation curve, and the compression index is larger than that of the normal consolidation curve under the same conditions. A post-liquefaction volumetric strain model accounting for both the consolidation and cyclic shear strain histories is proposed, which treats the re-sedimentation as part of the re-consolidation by introducing the concept of assumed initial stress, and the estimation methods for re-compression index and the assumed initial stress are recommended accordingly. Then dynamic centrifuge model tests are performed, and the consolidation settlement and liquefaction responses are monitored, whereupon the mechanisms of liquefaction-induced volumetric strain are observed at the model scale, and the proposed model for post-liquefaction settlement estimation is preliminarily validated.
... Uzuoka et al. (2007) proposed a prediction model for liquefaction and post-liquefaction settlement using the minimum effective stress. Zhou et al. (2014) discovered that the compression index during reconsolidation is 1.3-1.5 times as great as that during consolidation and proposed a model for post-liquefaction settlement estimation based on an assumed initial stress. ...
... The reconsolidation process after liquefaction can be categorized into liquified and solidified portions (Florin and Ivanov, 1961). The liquified portion is known as resedimentation (Zhou et al., 2014). A consensus is achieved, i.e., volume contraction in the liquefied portion accounts for a substantial proportion of total volume change in post-liquefaction reconsolidation. ...
... In the solidified portion of reconsolidation, both the MCN and ′ v increase with decreasing void ratio, which indicates that a stable structure is gradually taking shape. It bears noting that owing to the measurement limitations, the boundary between the liquefied and solidified portion observed in previous laboratory tests (Uzuoka et al., 2007;Zhou et al., 2014) exceeds that evidenced herein (less than 1 kPa), and the phenomenon of void ratio decreasing at extremely slow rate in the solidified portion when the magnitude of ′ v below 10 0 kPa was not observed previously. ...
Ground reconsolidation following strong earthquakes can severely damage buildings and infrastructure. However, the nonlinear void ratio–effective stress relationship during reconsolidation remains poorly characterized owing to limitations of laboratory testing. The discrete element method is utilized to examine reconsolidation characteristics in K0-consolidated granular materials. The results demonstrate that the residual effective stress is the core indicator of the volumetric strain during reconsolidation. Particular focus is given to the post-liquefaction reconsolidation process, which can be divided into two stages: liquefied portion and subsequent solidified portion. The former stage accounts for a substantial proportion of total volumetric strain. For specimens with identical average size, particle size distribution has a major influence on volumetric strain in the liquefied portion while a negligible impact in the solidified portion. This study also finds a higher proportion of floaters in the polydisperse (PD) versus monodisperse (MD) specimen. Furthermore, pore uniformities in both specimens increase during undrained cyclic shear. During the reconsolidation process, pore uniformity rises in the MD specimen while the PD specimen exhibits variable changes.
A series of numerical tests is performed using the three-dimensional discrete element method to study the volume contraction characteristics of granular material during the reconsolidation process following undrained cyclic shear. Results show that post-liquefaction reconsolidation can be categorized into a liquefied and a solidified portion. The decrease in the void ratio in the liquefied portion is not accompanied by an increase in the mean effective stress, while the opposite is true for the solidified part. The residual mean effective stress affects the volumetric strain significantly during reconsolidation. The decrease in void ratio during the reconsolidation beginning from effective stress reduction ratios of 0.1, 0.5, and 0.9 is 86%, 37%, and 7% of that beginning from the initial liquefaction state, respectively. In addition, the volumetric strain during reconsolidation is associated with the change in pore uniformity.KeywordsVolumetric strainUndrained cyclic shearResidual mean effective stress3D DEM
The 2011 M W ¼ 9.0 Tohoku-oki earthquake generated a large number of unique soil liquefaction case histories, including cases with strong ground motion recordings on liquefiable or potentially liquefiable soils. We have compiled a list of 22 strong motion stations (SMS) where surface evidence of liquefaction was observed and 16 SMS underlain by geologically recent sediments or fills where surface evidence of liquefaction was not observed. Pre-earthquake standard pene-tration test data and borehole shear wave velocity (V s) profiles are available for some stations, but critical information, such as grain size distribution and fines plasticity, are often lacking. In the heavily damaged city of Urayasu, we performed post-earthquake cone penetration testing at seven SMS and V s profiles, using surface wave methods at 28 additional locations to supplement existing geotech-nical data. We describe the liquefaction effects in Urayasu, the available site char-acterization data, and our initial data interpretations. [
Seismically induced settlement of buildings with shallow foundations on liquefiable soils has resulted in significant damage in recent earthquakes. Engineers still largely estimate seismic building settlement using procedures developed to calculate postliquefaction reconsolidation settlement in the free-field. A series of centrifuge experiments involving buildings situated atop a layered soil deposit have been performed to identify the mechanisms involved in liquefaction-induced building settlement. Previous studies of this problem have identified important factors including shaking intensity, the liquefiable soil's relative density and thickness, and the building's weight and width. Centrifuge test results indicate that building settlement is not proportional to the thickness of the liquefiable layer and that most of this settlement occurs during earthquake strong shaking. Building-induced shear deformations combined with localized volumetric strains during partially drained cyclic loading are the dominant mechanisms. The development of high excess pore pressures, localized drainage in response to the high transient hydraulic gradients, and earthquake-induced ratcheting of the buildings into the softened soil are important effects that should be captured in design procedures that estimate liquefaction-induced building settlement.
A new approach for estimating the susceptibility of a layered, liquefiable, infinite slope to shear deformations associated with void redistribution is presented. The excess pore water pressures associated with liquefaction produce upward seepage within the slope. The lower portion of a liquefied layer expels a certain volume of pore water, V-con, as it contracts (densifies). If the liquefied layer is overlain by a lower permeability soil, then the pore water expelled from the lower contracting zones can become trapped causing void ratio increase in a dilating sublayer near the interface, reducing its undrained shear strength. The volume of water that can be absorbed by the dilating sublayer prior to slope instability is termed the dilation capacity, V-dil. The ratio V-con/Y-dil is a measure of the potential for localization due to strength losses from void redistribution. The localization potential is shown to strongly depend on relative density, slope angle, and thickness of the liquefiable layer. The thickness of the dilating sublayer, a critical parameter, is significantly greater than the thickness of a shear band (which may be only tens of grain diameters). A detailed example is presented to show how the procedure can be applied. The results of the analysis are shown to be consistent with observed deformations and localizations in centrifuge model tests.
Immediately following the 11th March 2011 Mw 9.0 Tohoku (Japan) earthquake, a field investigation was carried out around the Tokyo Bay area. This paper provides first-hand observations (before or just at the onset of repair) of widespread liquefaction and the associated effects. Observations related to uplift of manholes, settlement of ground, performance of buildings and bridges and the effects of ground improvements are also presented. Recorded ground motions near the Tokyo Bay area were analysed to understand their key characteristics (large amplitude and long duration). Lessons learnt are also presented.Highlights► Geotechnical field investigations of the 2011 Mw 9.0 Tohoku earthquake are reported. ► First-hand observations of widespread liquefaction around the Tokyo Bay area are presented. ► The effects of long-duration ground motions critical to liquefaction-related damage are analysed. ► Successful cases of ground improvement for reclaimed/filled land are discussed.
Subsidence of a sandy ground in a nuclear power plant, Japan, is calculated using two kinds of simple methods. Only the data of SPT N-value is needed in calculation. The calculation results indicate that the ground will subside largely in a L2 level earthquake due to liquefaction. The two methods show the same trend of ground deformation because the basic principles of the two methods are the same. However, the subsidence value is much different, which is mainly caused by the different basic experimental data and the different ways for evaluating the volumetric strain of ground.
Based on experimental observations and cause analysis of formation, an intrinsic relationship was revealed between the large post-liquefaction shear deformation of saturated sand under undrained cyclic loading and three types of volumetric strain components (i.e., a reversible component due to dilatancy, an irreversible component due to dilatancy and a component due to change in mean effective stress). It was found that the development of the large post-liquefaction shear deformation is governed by coupling variation of the three volumetric strain components and is accompanied with three physical states of soil particles (i.e., the frictional contact state, the critical contact state and the suspension state) that appear alternately. The above new knowledge provides a rational explanation why unstable flow slides and large reconsolidation deformation may take place after the initial liquefaction and also a rational approach to the establishment of an elasto-plastic constitutive model used to describe the large post-liquefaction deformation.
The liquefaction behaviors in the great Wenchuan 8.0 Earthquake in 2008 are quite notable, and the liquefaction of gravel soil is significant. Considering the wide distribution of gravel soil in some places in China, the liquefaction prediction methods should be developed. The existing methods for evaluating the liquefaction of sand soil result from the sand liquefaction cases, however, SPT technique can not be conducted in the gravel soil layers, and as a result, the existing code is not suitable for the liquefaction assessment of gravel soil. After the investigation for the liquefaction-induced damages in the great Wenchuan Earthquake and in-situ tests for the liquefied and non-liquefied sites, the liquefaction prediction method of gravel soil based on DPT, i. e., the dynamic penetration tests, is presented, and the corresponding model and formula are obtained. The analytical results indicate the liquefaction discrimination of the gravel soil can be divided into two steps, the initial discrimination and the second discrimination. In the initial step, the impossible liquefaction cases are selected, and in the second, the calculation model is adopted by using N 120 as the basic index from DPT. The geological ages, buried condition of the gravel soil layer and gravel contents of gravel soils are considered in the initial discrimination. In the second discrimination, five parameters including the reference value of N 120, gravel content of gravel soils, depth of gravel soils, water table and seismic intensity are concerned. Considering the wide range of liquefied soil depths and its water levels, the reference value of N 120 is deduced by the normalization method and the influence coefficients of the gravel soil depths and the water levels are obtained by the optimal method. The effect of various factors on liquefaction possibility of gravel soil is considered in the present method and the advantages of the model and formula for the second discrimination are noticed by clear expression, high success ratio of regression discrimination, good connection with the past work and convenience in engineering application.
Based on the constitutive equations for large deformation of sand given by Yang and Elgamal, et al, the constitutive equations are drived in details, and the harden rule of model is advanced based on the harden rule of memorial nested yield surface. The model is extended to 3-D equations. This advanced model is implemented by using FORTRAN subroutine program and is implanted into ABAQUS finite-element software. By using ABAQUS software with the advanced model, the dynamic response of sand soil samples was tested by dynamic triaxial machine, which was simulated by 3-D model. The time-histories of vertical displacement, the stress-strain curves and the time-histories of super pore pressure are given by the advanced model. The feasibility of the model is verified firstly.
The preparation, saturation and uniformity checking methods for clayey sand ground model are proposed. Then liquefaction model tests on clayey sand with clay content of 10% are performed by means of ZJU-400 geotechnical centrifuge shaking table, and the other model for clean sand with the same relative density is also performed for comparison. Seismic liquefaction phenomena of level ground are reproduced and the liquefaction characteristics of clayey sand ground are revealed. Measuring of bender elements shows that both models are uniformly prepared, and the shear wave velocity of the clayey sand is lower than that of the clean sand under the same condition. The permeability coefficient of the clayey sand is nearly an order of magnitude lower than that of the clean sand according to the test observations, which affects the responses of the excess pore water pressure (PWP) and stress-strain relationship during liquefaction. The difference of permeability results in slower buildup of PWP in the clayey sand at shallow depth but faster one at deep depth, while the post-liquefaction dissipation is 15 times longer than that of the clean sand. Response of shear stress-strain of the clayey sand is more pronounced than that of the clean sand at the liquefaction stage, and its secant modulus is smaller and the damping ratio is larger under post-liquefaction cyclic loadings. Model settlement mainly occurs during the process of PWP dissipation, and the clayey sand has longer dissipation time and doubles the settlement of the clean sand. This study presents scientific evidence for seismic response analysis and liquefaction evaluation of clayey sand ground.
The real-time liquefaction monitoring and warning techniques are novel ways to mitigate liquefaction hazard. The key point for the techniques is to establish a reverse liquefaction detection method based on seismic records. However, the reliability of the existing liquefaction detection methods is lack of verification by earthquake records. There were serious liquefaction damages in Christchurch in New Zealand Earthquake in 2011 and creates a wonderful chance to exam the existing liquefaction detection methods. The acceleration records in the earthquake are employed to perform the blind detection of liquefaction situation by frequency decrease rate (FDR) method presented by the authors before. The results indicate that in all 27 sites of which the distances from epicenter are less than 50 km; there are 9 liquefaction sites and others are identified as non-liquefaction sites. Two liquefaction sites among the 9 liquefaction sites; have been confirmed by the news reports; and it means the detection results by FDR method are correct. The identified liquefaction sites by method are mainly located near the Avon River in the eastern of Christchurch, which are agreeable to the corresponding news reports. From the analysis of the paper, two earthquake records are in doubt because of the un-normal phenomena with significant high frequency components in the middle or beginning part of the waves, separately; and the revised results for the two seismic waves are given after filtering the records.
Some existing state-dependent mathematic models for compression curve of cohesionless soils are described and analyzed briefly. Isotropic compression tests on sand with different initial densities are performed. On basis of the results, a new state-dependent stress-strain model for cohesionless soil under hydrostatic compression is proposed. The model is able to describe well stress-strain behaviour which develops throughout first loading and can be adaptable to different freshly deposited cohesionless soils subjected to isotropic compression over a wide range of stresses and densities. The model parameters can be determined simply and conveniently from isotropic compression test. Comparison with experimental data of four types of sands indicates that the model can gives excellent predictions to the measured isotropic compressive behaviour; and it can be used to construct generalized state-dependent constitutive models.
Based on the unified hardening parameter, an extended critical state constitutive model for sand is proposed. One state parameter is used to adjust the hardening parameter and dilatancy equation, giving a better description of dilatancy and contraction behavior for sand. The other parameter is used to revise the yield function, predicting the plastic deformation more appropriately. The introduction of the parameters into the model emphasizes the dependency on both the density and the effective principal stress of sand. The linear isotropic consolidation and the critical state lines are applied in void ratio versus the power of the effective mean stress space. The combined effect of the density and the effective mean stress on sand can be described by the proposed model using a unique set of model parameters in a wide range of density and confining pressure. A comparison between model simulations and experimental results shows the validity of the proposed model.
The dynamic triaxial tests are the most conventional laboratory method to obtain the deformation and the strength properties for soils. A series of high cycle dynamic triaxial tests are conducted on a typical silty clay, the ground soil along high-speed lines. The comparisons among dynamic strain, pore pressure and the critical dynamic stress ratio of the test samples are made to study the effects of simulation difference between the in-situ and simplified test conditions, such as dynamic loading mode, drainage condition and duration of cyclic loading. The deformation, the pore pressure, the volumetric changes and the critical dynamic stress ratio under various test conditions are presented. According to the test results, to analyze the dynamic behavior of soft clay under high-speed train loading, the dynamic test with half sinusoidal waves under drainage condition is suggested to simplify the test procedure in laboratory studies.
A centrifuge testing study is conducted to investigate the effect of over-consolidation on liquefaction in clean saturated sand deposits. Thirty-four shaking tests on 11 level-ground models are performed. Soil models with Over-Consolidation Ratios (OCRs) of 1, 2, and 4 at relative densities of 35%, 50%, and 70% are tested. Model response to dynamic base shaking is monitored with accelerometers, pore pressure transducers, and displacement gauges. Test data show that the potential for liquefaction decreases with the increase in OCR, relative density, and prior shaking. The threshold peak acceleration needed to induce excess pore pressure increases as the OCR increases. Over-consolidated sand layers subjected to lower levels of excitation do not experience any excess pore pressure buildup even when shaken for a long duration. For acceleration marginally above the threshold, pore pressure buildup may be mild and liquefaction may be unlikely. As such, preloading can be a practical cost-effective liquefaction remedial technique in sandy soils under earthquake loading scenarios resulting in peak acceleration of up to about 0.15 g.
By examining a bulk of laboratory test data on sands obtained by using a simple shear apparatus, a family of curves was established in which the volumetric strain resulting from dissipation of pore water pressures is correlated with the density of sand and conventionally used factor of safety against liquefaction. Thus, given the factor of safety and the density in each layer of a sand deposit at a given site, the volumetric strain can be calculated and by integrating the volume changes throughout the depth, it becomes possible to estimate the amount of settlements on the ground surface produced by shaking during earthquakes. The outcome of the data arrangements as above was put in a framework of methodology to estimate the liquefaction-induced settlements of the ground. The proposed methodology was used to estimate the settlements at several sites devastated by liquefaction during the 1964 Niigata earthquake. The estimated values of the settlements were examined in the light of known performances of the sand deposits at respective site. It was shown that the proposed methodology may be used for predicting liquefaction-induced settlements with a level of accuracy suitable for many engineering purposes.
The great Wenchuan earthquake (M
s = 8.0) in 2008 caused severe damage in the western part of the Chengdu Plain. Soil liquefaction was one of the major causes
of damage in the plain areas, and proper evaluation of liquefaction potential is important in the definition of the seismic
hazard facing a given region and post-earthquake reconstruction. In this paper, a simplified procedure is proposed for liquefaction
assessment of sandy deposits using shear wave velocity (V
s), and soil liquefaction from the Banqiao School site was preliminarily investigated after the earthquake. Boreholes were
made at the site and shear wave velocities were measured both by SASW and down-hole methods. Based on the in-situ soil information
and V
s profiles, the liquefaction potential of this site was evaluated. The results are reasonably consistent with the actual field
behavior observed after the earthquake, indicating that the proposed procedure is effective. The possible effects of gravel
and fines contents on liquefaction of sandy soils were also briefly discussed.