Numerical modeling of centrifuge cyclic lateral pile load experiments

To gain insight into the inelastic behavior of piles, the response of a vertical pile embedded in dry sand and subjected to cyclic lateral loading was studied experimentally in centrifuge tests conducted in Laboratoire Central des Ponts et Chaussées. Three types of cyclic loading were applied, two asymmetric and one symmetric with respect to the unloaded pile. An approximately square-root variation of soil stiffness with depth was obtained from indirect in-flight density measurements, laboratory tests on re...     

Interpretation from large-scale shake table tests on piles undergoing lateral spreading in liquefied soils

Results from a benchmark test on full-scale piles are used to investigate the response of piles to lateral spreading. In the experiment, two single piles, a relatively flexible pile that moves together with the surrounding soil and a relatively stiff pile that does not follow the ground movement have been subjected to large post-liquefaction ground displacement simulating piles in laterally spreading soils. The observed response of the piles is first presented and then the results are used to examine the lateral loads on the pile from a non-liquefied soil at the ground surface and to evaluate the stiffness characteristics of the spreading soils. The measured ultimate lateral pressure from the crust soil on the stiff pile was about 4.5 times the Rankine passive pressure. The back-calculated stiffness of the liquefied soil was found to be in the range between 1/30 and 1/80 of the initial stiffness of the soil showing gradual decrease in the course of lateral spreading.     

Property estimation of free-field sand in 1-g shaking table tests

Experimental data taken from free-field soil in 1-g shaking table tests are valuable for seismic studies on soil-structure interaction. But the available data from medium-to large-scale shaking table tests were not abundant enough to cover a large variety of types and conditions of the soil. In the study, 1-g shaking table tests of a 3-m-height sand column were conducted to provide seismic experimental data about sand. The sand was directly collected in-situ, with the largest grain diameter bein...     

Pile response in submerged lateral spreads: Common pitfalls of numerical and physical modeling techniques

A three dimensional dynamic numerical methodology is developed and used to back-analyze experimental data on the seismic response of single piles in laterally spreading slopes. The aim of the paper is not to seek successful a-priori (Type A) predictions, but to explore the potential of currently available numerical techniques, and also to get feedback on modeling issues and assumptions which are not yet resolved in the international literature. It is illustrated that accurate simulation of the physical pile–soil interaction mechanisms is not a routine task, as it requires the incorporation of advanced numerical features, such as an effective stress constitutive soil model that can capture cyclic response and shear-induced dilation, interface elements to simulate the flow of liquefied ground around the pile and proper calibration of soil permeability to model excess pore pressure dissipation during shaking. In addition, the “conventional tied node” formulation, commonly used to simulate lateral boundary conditions during shaking, has to be modified in order to take into account the effects of the hydrostatic pore pressure surplus that is created at the down slope free field boundary of submerged slopes. A comparative analysis with the two different lateral boundary formulations reveals that “conventional tied nodes”, which also reflect the kinematic conditions imposed by laminar box containers in centrifuge and shaking table experiments, may underestimate seismic demands along the upper part of the pile foundation.     

Physical modeling of lateral spreading induced by inclined sandy foundation in the state of zero effective stress

The state of zero effective stress is a situation at which the effective stress of saturated sand decreases to zero in the process of liquefaction. In the state of zero effective stress, sand particles suspend in water and the foundation is vulnerable to much large lateral deformation. The state of zero effective stress can be achieved through dynamic loading tests, but the obtained state is difficult to sustain a steady situation. To simulate the suspended sand in water under fully liquefied condition, plastic sand, characterized by small specific gravity, is used instead of quartz sand to build an inclined foundation. Salt water with similar density is used to pass in slowly near bottom of the foundation. As observed in tests, the plastic sand is able to suspend in sodium chloride solution (salt water) of a specific density and thus this model can be used to simulate the lateral spreading of a foundation under zero effective stress state. Lateral deformation occurs within a certain depth beneath the ground and the magnitude increases from the bottom up, showing nonlinear behaviors. This paper presents a physical modeling approach for achieving the state of zero effective stress under static laboratory condition.     

Effect of a single vibrating building on free-field ground motion: numerical and experimental evidences

The influence of vibrating buildings on the free-field ground motion could affect the earthquake recordings collected inside or nearby the buildings. Some evidences are known for large structures, but also small buildings could adversely affect the quality of the recordings. An example is given for a station of the Italian Accelerometric Network whose recordings show a clear mark of the frequency of the host building. To tackle this problem in a more general way, we performed numerical simulations whose first aim was to validate existing empirical evidence from a test site. Gallipoli et al. (Bull Seismol Soc Am 96:2457–2464, 2006) monitored a release test on a 2-storey R.C. building in Bagnoli (Italy), showing that a single vibrating building may affect the “free-field” motion with an influence that reaches 20% of peak ground acceleration. We re-analysed the data of that experiment following the Safak (Soil Dyn Earthq Eng 17:509–517, 1998) approach to building-soil motion, described as propagation of up- and down-going S-waves. The numerical model is a chain of single degree of freedom oscillators, whose dynamic behaviour depends on mass, stiffness and damping. The agreement between the synthetic and real data encouraged us to use this model to reproduce generalised structures as systems with a single degree of freedom. We run multiple tests varying the distance, between building and station, and the building-soil coupling, obtaining a statistical distribution of the influence of a single vibrating building on free-field ground motion taking into account the distance.     

Boundary effects of a laminar container in centrifuge shaking table tests

Two dynamic centrifuge model tests were performed to simulate dry or saturated sand deposits subjected to 1 Hz base shaking. This experimental study investigated the boundary effects of a laminar container on the seismic response acquired from accelerometers and from pore pressure transducers, both of which were embedded in the sand bed at various depths and distances from the end walls. Under the tested configurations and the employed input motion used in the study, the test results revealed minimal boundary effects on the seismic responses. The measured maximum amplitude, main frequencies, phase lags of acceleration, and the profiles of the calculated RMS acceleration amplification factor were not affected by the boundaries if the instruments were positioned at a distance of more than one-twentieth of the model length from the end walls and were not positioned on the ground surface. No obvious discrepancies were observed in the time histories of excess pore water pressure, measured at a distance of one-fourth of the model length from the end walls. These results infer that variations in the seismic response at the end walls were minimal; hence the laminar container used in the study may be used effectively to simulate 1D shear wave propagation in centrifuge shaking table tests. However, for other testing configurations, a similar study should be undertaken for evaluating the boundary effect of the laminar container on the seismic responses.     

Seismic behaviors of earth-core and concrete-faced rock-fill dams by dynamic centrifuge tests

The investigation on the seismic behavior of dams becomes crucial but is limited to lack of experimental or field data. This paper aims to experimentally simulate two major dam types of earth-core rock-fill dam and concrete-faced rock-fill dam by dynamic centrifuge tests to investigate the seismic response of the dam. A series of staged centrifuge tests was performed by applying real earthquake records from 0.05 to 0.5g. The distributions of amplification ratio differed depending on the magnitude of earthquake loading and the zoning condition. The amplification ratio at the crest increased in the bedrock acceleration that exceeds 0.3g and strongly influenced by the loosening behavior of the upper part. The residual settlements and horizontal displacement at the dam crest were small. Shallow surface sliding was dominant failure. The maximum tensile stress on the face slab by dynamic loading occurred at a height of around 4/5 near the upstream water level. Finally, two-dimensional numerical simulations were performed in an effort to simulate the centrifuge models. The centrifuge tests and numerical analysis obtained mostly comparable results, thus confirming that centrifuge modeling reasonably simulates the seismic behavior of dams.     

俯冲带耦合作用对苏门答腊地区应变场影响的三维数值模拟

采用有限元方法模拟了俯冲带耦合作用对巽他弧及其邻区的影响.根据模拟结果,对比GPS、地震和地质学观测数据,定量分析了苏门答腊及其周边地区的应变强度和主应变方向的分布特征,据此探讨了该区构造特征、地震发生模式与耦合面积之间的关系.模型由具有黏弹性性质的岩石圈和软流圈上地幔组成,其中岩石圈包括了大陆岩石圈和大洋岩石圈以及俯冲至上地幔中的俯冲板片.研究结果如下:(1) 通过对不同俯冲带耦合面积模拟,发现苏门答腊前弧伴随耦合面积的增加应变强度逐渐增大,而增大的应变强度又影响了其周边地区的应变分布特征,因此整个苏门答腊前弧呈现出明显的分段性,这与该区地震破裂模式有较好的对应.(2)苏门答腊北部地区主应变方向与南部相比存在一定的差异,该差异是俯冲带的俯冲方向、俯冲速度、俯冲形态以及不同区域间耦合面积共同作用的结果.(3)虽然苏门答腊2004年地震主震区处于弱耦合状态,但从本文模拟的结果中可以看到,在俯冲作用下该区依然存在垂直向下的位移,这为地震激发海啸提供了有利的构造环境.     

High strain dynamic modulus and damping of chemically grouted sand

Laboratory resonant column and cyclic triaxial tests are performed to determine the dynamic response (i.e. shear modulus and damping) of chemically grouted sand. The effect of chemical grouting is evaluated as a function of shearing-strain amplitude, confining stress, cycling prestrain, number of cycles, grout type, concentration, and curing time. The test results show that the shearing-strain amplitude, grout type and grout concentration have significant effects on the shear modulus and damping ratio of the test specimens. The increased addition of sodium silicate grout, which produces stiff gels, improved the shear modulus of the test sand. The acrylate (AC-400) and polyurethane (CG5610) grout, which produces flexible (rubber-like) gels, improved the damping capacity of the sand with increasing grout concentration. The addition of chemical grout greatly reduces the effect of cyclic prestraining over untreated sands. In the case of dense sands, the reduction of cyclic prestraining is less pronounced than in loose sands, which have a higher potential for particle movement and reorientation.     

Micro- and macro-observations of liquefaction of saturated sand around buried structures in centrifuge shaking table tests

A novel experimental method was designed to study the micro-behavior of saturated sand around a buried structure in centrifuge shaking table tests under strong simulated earthquake loading, in addition to the traditional macro-measurements. One free field test was first carried out as a reference, followed by one test with a deep buried structure and one with a shallowly buried structure. During the tests with the buried structure, high quality pictures of moving sand around the structure were recorded by a newly developed image acquisition system. By analyzing the interesting pictures at reasonable intervals using an image analysis software, the evolutions of microstructural features were obtained such as the orientations of the long axes of particles, the orientations of contact normals between particles and the average contact number of the interesting group of particles. The results showed that the evolutions of the micro-features were consistent with those of the macro-measurements such as excess pore pressures and accelerations, which help illuminate the mechanism of sand liquefaction.     

Evolution of the shear wave velocity during shaking modeled in centrifuge shaking table tests

Two in-flight shear wave velocity measurement systems were developed to perform the subsurface exploration of shear wave velocity in a centrifuge model. The bender elements test and the pre-shaking test used in the study provided reliable and consistent shear wave velocity profiles along the model depth before and after shaking in the centrifuge shaking table tests. In addition, the use of the bender elements measurement system particularly developed here allowed continuous examination of the evolution of shear wave velocity not only during and after the shaking periods in the small shaking events but also during the dissipation period of excess pore water pressure after liquefaction in the large shaking events. The test results showed that the shear wave velocity at different values of excess pore water pressure ratio varied as the effective mean stress to the power of 0.27, to a first approximation. Consequently, a relationship between the shear wave velocity evolution ratio and the excess pore water pressure ratio is proposed to evaluate the changes in shear wave velocity due to excess pore water generation and dissipation during shaking events. This relation will assist engineers in determining the shear stiffness reduction ratio at various r_u levels when a sand deposit is subjected to different levels of earthquake shaking.     

Observation and numerical modeling of tidal dune dynamics

Tidal sand dune dynamics is observed for two tidal cycles in the Arcachon tidal inlet, southwest France. An array of instruments is deployed to measure bathymetric and current variations along dune profiles. Based on the measurements, dune crest horizontal and vertical displacements are quantified and show important dynamics in phase with tidal currents. We observed superimposed ripples on the dune stoss side and front, migrating and changing polarity as tidal currents reverse. A 2D RANS numerical model is used to simulate the morphodynamic evolution of a flat non-cohesive sand bed submitted to a tidal current. The model reproduces the bed evolution until a field of sand bedforms is obtained that are comparable with observed superimposed ripples in terms of geometrical dimensions and dynamics. The model is then applied to simulate the dynamics of a field of large sand dunes of similar size as the dunes observed in situ. In both cases, simulation results compare well with measurements qualitatively and quantitatively. This research allows for a better understanding of tidal sand dune and superimposed ripple morphodynamics and opens new perspectives for the use of numerical models to predict their evolution.     

Progress in the numerical modeling of mantle plumes

The mantle plume model, as an integral part of the Earth’s internal convection system, is complementary to the theory of plate tectonics. They together constitute the key configuration of material circulation and energy transport in the Earth’s interior. Seismology, high-temperature and high-pressure mineralogy, geology, and geodynamic numerical modeling have conducted comprehensive studies on the mantle plume model since it was proposed. In particular, numerical simulation, which investigates t...     

Progress in numerical modeling of subducting plate dynamics

The core concerns of plate tectonics theory are the dynamics of subducting plates, which can be studied by integrating multidisciplinary fields such as seismology, mineral physics, rock geochemistry, geological formation studies, sedimentology, and numerical simulations. By establishing a theoretical model and solving it with numerical methods, one can replicate the dynamic effects of a subducting plate, quantifying its evolution and the surface response. Simulations can also explain the observations and experimental results of other disciplines. Therefore, numerical models are among the most important tools for studying the dynamics of subducting plates. This paper provides a review on recent advances in the numerical modeling of subducting plate dynamics. It covers various aspects, namely, the origin of plate tectonics, the initiation process and thermal structure of subducting slab, and the main subduction slab dynamics in the upper mantle, mantle transition zone, and lower mantle. The results of numerical models are based on the theoretical equations of mass, momentum, and energy conservation. To better understand the dynamic progress of subducting plates, the simulation results must be verified in comparisons with the results from natural observations by geology, geophysics and geochemistry. With the substantial increase in computing power and continuous improvement of simulation methods, numerical models will become a more accurate and efficient means of studying the frontier issues of Earth sciences, including subducting plate dynamics.     

Magnetism of rocks and volumetric strain in uniaxial failure tests

Summary The relation between remanent magnetization and volumetric strain for gabbro samples stressed in uniaxial compression inside a near zero-field -metal shield has been examined. For samples with an induced IRM parallel to the axis of compression, remanent magnetization decreased linearly up to the onset of dilatancy. As increased stress produced additional dilatancy, the variation of remanent magnetization became nonlinear, and the stress dependence continually decreased until the rock failed. Stress cycling with the peak stress augmented for each cycle produced a continuous decrease in the zero stress value of the IRM although an appreciable amount of recovery was observed during unloading. When the sample was loaded in constant stress increments after the onset of dilatancy and held for several minutes at each level, time-dependent variations in remanent magnetization coincided with time-dependent increases in inelastic volumetric strain. In general as the inelastic creep rate increases, the rate of change in remanent magnetization increases. These results suggest that dilatancy related effects of the intensity of rock magnetization should be observed in magnetic rocks in epicentral regions prior to earthquakes and may serve as both long- and short-term precursors.     

Modeling of strain dependency of shear modulus and damping of clayey sand

A series of cyclic triaxial tests on clayey sands was carried out and attempts were made to evaluate the strain dependency of shear modulus and damping. Strain dependencies of shear modulus and damping were simply modeled. It was shown that the change in the effective confining stress with loading cycles in the undrained shear test needed to be considered particularly in the large strain range. The consideration could be made by normalizing G with G′₀=AF(e)(σ′_m/σ_mr)ⁿ, the initial shear modulus for the effective confining stress of that particular loading cycle, instead of using G₀. G/G′₀ was expressed by a function of γ as G/G′₀=1/(1+b_gγ) which was almost stress level independent for clayey sands used in this study. The damping ratio was not much affected by the confining stress. The strain dependency of the damping ratio was modeled by h=a_hγ/(1+b_hγ). Effects of load irregularity on the shear modulus were also investigated. The excess pore pressure and the residual strain were generated especially when the major peaks in the irregular loading were applied to the specimen. However, G/G′₀ for the irregular loading could be represented reasonably well by the average curve for the uniform cyclic loading, if the excess pore water pressure and the residual strain were taken into account.     

Numerical modelling of the effect of horizontal drains in centrifuge tests on soil-structure interaction in liquefiable soils

Bulletin of Earthquake Engineering - The research presented herein was carried out in the framework of the H2020 LIQUEFACT project ( http://www.LIQUEFACT.eu/ ). This paper presents the results of a...