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The rocking foundation mechanism has proven to be an effective means to dissipate seismic energy, and thereby reduce the seismic demand on the superstructure. Its use has been broadly applied in both the building and bridge domains. Nonetheless, a common concern remains regarding its global behavior in a building system. Specifically designers must understand how the rocking foundation dynamically interacts with other inelastic components, particularly those designed into the structural components. To advance understanding, this paper presents and compares the seismic response of two extreme structural configurations, namely, a structural hinging-dominated (SHD) system and a foundation rocking-dominated (FRD) system. These two structural systems were recently tested in a 9-m-radius centrifuge-based shake table at UC Davis. Results indicate that the SHD system is more prone to localize the seismic energy at the structural fuse element, with relatively little energy transferred to the rocking footing. As a result, the structural fuses have a fairly large demand, which in turn cause significant transient and residual deformation in the superstructure. The FRD system, on the other hand, with broad moment-rotation hysteretic response of the foundation effectively minimizes the superstructure seismic demands in terms of roof drift and base shear, while the structural fuse element remains linear-elastic, even under intense base shaking.

Current research has shown the capabilities and improved seismic performance of rocking shallow foundations for bridges, and much work has been done for implementation of such a mechanism in industry. By properly reducing the size of the footing, rocking behavior due to seismic loading can occur about the footing base. It has been shown experimentally that rocking foundations on competent soils can reduce seismic ductility demand on bridge columns and improve bridge performance. One concern of rocking systems is the potential for significant settlement accumulation in poor soil conditions. This research explores soil, footing, and structural response of similar bridge structures on poor, liquefiable, and saturated soil conditions in addition to the effectiveness and practicality of using unconnected piles as a potential settlement mitigation technique. A centrifuge test series showed rocking foundations in saturated sand over a liquefiable soil layer not only experience the traditional deformations from liquefaction, but are also susceptible to additional deformations such as suction and erosion around the footing.

Rocking foundations have shown performance advantages that may benefit the seismic performance of structures. Previous centrifuge tests have well characterized the behavior of rectangular rocking foundations on sand, but few results are available for clayey ground. In this study a series of centrifuge model tests of shear walls and single-degree-of-freedom (SDOF) structures with rocking foundations supported on clay of medium strength are conducted. Vertical bearing failure, slow cyclic loading, and dynamic shaking tests were performed at three different levels of centrifugal acceleration. The moment capacity, settlement, and rotation of each loaded model were tracked and then compared to previous published results. The measured moment capacity is found to match the theoretical values. The settlement vs. rotation behavior was characterized, and it is found that settlement is somewhat smaller than previous results for similar models tested on sand.

Dynamic centrifuge modeling was used to study the behavior of two mildly sloping ground models subjected to earthquake loading and liquefaction. The first model consisted of a uniform layer of saturated sand, and the second consisted of a layer of sand overlain by a relatively impermeable layer of nonplastic silt; both model slopes were constructed at an angle of 2.6°. Time histories of acceleration, excess pore‐water pressure, and displacement are presented with photographs to illustrate the mechanisms of failure. In both tests approximately 0.8 m of prototype lateral displacement was measured at the surface. In the uniform sand model this lateral displacement was distributed throughout much of the layer; however, in the layered model displacement was concentrated along the interface between soil layers. The concentration of displacement is consistent with a reduction in the sliding resistance between the silt and the sand layers due to the redistribution of voids at the interface. This possibility of re...

A constitutive model based on the critical state concept and viscoplasticity is developed. The model is capable of predicting primary and secondary compression, including the effects of over-consolidation ratio and load increment ratio on these phenomena. The model can also predict the effect of strain rate during undrained shear tests, drained and undrained creep as "veil as typical stress paths followed in conventional triaxial tests. All rate effects are introduced by a single parameter-the coefficient of secondary compression. The traditional concepts of primary consolidation, secondary compression and overconsolidation ratio are discussed. Un modèle constitutif a été développé sur la base du concept de l'état critique et la viscoplasticité. Ce modèle peut prédire la compression primaire et secondaire, y compris les effects exercés sur ces phénomènes par le rapport de surconsolidation et le rapport de l'accroissement de chargement. Le modèle peut aussi prédire les effets exercés par la vitesse de déformation pendant des essais de cisaillement non-drainé, le fluage drainé et non-drainé et les chemins typicques de contrainte suivis dans les essais triaxiaux conventionnels. Tous les effets de vitesse sont introduits grâce à un seul paramètre, le coefficient de compression secondaire. L'article discute les concepts traditionnels de la consolidation primaire, de la compression secondaire et du rapport de surconsolidation.

Preliminary results from a study to develop innovative bridge foundations to optimize bridge performance under earthquake events are presented. Numerical models of bridges with both flexible and stiff footings are compared to demonstrate the potential importance of soil-footing stiffness on the performance of a soil-footing-column-deck-abutment system. Detailed numerical models were developed to capture the inelastic behavior of soil-footing systems. This article summarizes two types of foundation models developed using the OpenSees computational platform. One model uses a contact interface model, soilFootingSection2D, to simulate the foundation and has been proven to be effective in analyzing the cyclic response and displacement of soil-foundation-bridge system. The other uses 2-D nonlinear Winkler foundation consisting of various constitutive springs. Although the former model is more detailed and fundamentally sound in modeling the footings, in its current state it is limited to 2-D use only. The latter model, once verified via comparison to experimental data, can be extended to 3-D. Future work will use the 3-D nonlinear footing models to more accurately simulate the 3-D response of a bridge system to 3-D ground motions.

Procedures for the seismic design of pile foundations for liquefaction effects are presented with emphasis on the conditions relevant to bridges. Two local subsystems for a bridge are discussed in detail: (1) pile groups in laterally spreading ground away from the abutments and (2) pile groups at the abutments where the restraining or “pinning” effects of the piles and bridge superstructure can be advantageous. The recommended design procedures involve equivalent static analyses using beam on nonlinear Winkler foundation models. Guidance for these design procedures was derived from a combination of dynamic centrifuge model tests and associated nonlinear dynamic finite element studies. The design procedures, their basis, and other issues for design of bridges for liquefaction effects are discussed.

Eight dynamic model tests were performed on a 9 m radius centrifuge to study the behavior of single piles and pile groups in liquefiable and laterally spreading ground. Pile diameters ranged from 0.36 to 1.45 m for single piles, and from 0.73 to 1.17 m for pile groups. The soil profile consisted of a gently sloping nonliquefied crust over liquefiable loose sand over dense sand. Each model was tested with a series of realistic earthquake motions with peak base accelerations ranging from 0.13 g to 1.00 g. Representative data that characterize the important aspects of soil-pile interaction in liquefiable ground are presented. Dynamic soil-pile and soil-pile cap forces are backcalculated. Directions of lateral loading from the different soil layers are shown to depend on the mode of pile deflection relative to the soil, which depends on the deformed shape of the soil profile, the pile foundation stiffness, and the magnitude of loads imposed by the nonliquefied crust. Procedures for estimating the total horizontal loads on embedded piles and pile caps (i.e., passive loads plus friction along the base and sides) are evaluated. Due to liquefaction of the sand layer beneath the crust, the relative displacement between the pile cap and free-field crust required to mobilize the peak horizontal loads is much larger than expected based on static pile cap load tests in nonliquefied soils. Journal of Geotechnical and Geoenvironmental Engineering © ASCE.

This paper addresses the development of an electrical resistivity needle probe to be deployed during centrifuge model testing to assess with high -resolution the spatial variability of soil electrical resistivity. The probe is able to detect thin layers and accurately resolve interfaces between soil layers. The prototype probes are made from thin, stainless steel needles, with an insulated wire inserted into the needle, and bonded to it with epoxy resin, to form a coaxial probe (Cho et al. 2004). Different tip shapes including single -wedge, double-wedge and cone have been developed to optimize the spatial resolution of porosity, soil interfaces and layering. The calibration and testing of the needle probe has been conducted both at Georgia Tech and UC Davis. This paper presents results of resistance versus depth and porosity versus depth, and compares the porosity based on measurement of mass and

Nonlinear dynamic analyses using the finite element (FE) method are compared to the results of dynamic centrifuge model tests of pile-supported structures in liquefying sand profiles. The FE models utilized soil spring elements that connect pile elements to one- or two-dimensional meshes of a soil profile. Development and implementation of the soil spring materials models in the OpenSees FE platform are described. Single element examples are used to illustrate the behavior of the soil and soil spring material models. Comparisons of FE analysis to centrifuge test data of pile-supported structures in liquefying sand profiles suggest that these FE methods can reasonably approximate the essential features of soil and structural response.

We present a system that visualizes displacement, acceleration, and strain that were measured during an earthquake simulation experiment in a geotechnical centrifuge. Our visualization tool starts by reading the data describing experiment set-up and displaying this data along with icons for the sensors used during data acquisition. Different sensor types (measuring acceleration, displacement and strain) are indicated by different icons. One general experiment set-up is used in a sequence of simulated earthquake events. Once a user has selected a particular event, measured data can be displayed as a two-dimensional (2D) graph/plot by clicking the corresponding sensors. Multiple sensors can be animated to obtain a three-dimensional (3D) visualization of measured data.

This symposium, sponsored by the ASTM Committee on Soil and Rock, had as its primary goal the identification of both established and innovative tests for determining cyclic and dynamic properties of soils. A secondary goal was to provide a forum for a discussion of testing which has been conducted using standard methods. Papers are divided into the following sections: Field testing; Laboratory methods; and Centrifuge testing. Eight of the papers have been selected for inclusion on the data base.

A novel technique for monitoring movement of buried objects using an electrode switching system is developed. The idea is to establish electromagnetic fields in a conductive medium by injecting low-frequency alternating currents through boundary electrodes. The movement of a buried object is related to the electrical potential measured on an electrode attached to it. A closed-form expression for potential distribution in a rectangular container is developed; boundary conditions are accounted for using a method of images. The resolution of spatial location using the proposed electro-location scheme is limited by the electrical noise and the accuracy of potential measurement, which depends on the excitation frequency, measurements per cycle of excitation, and samples per measurement. The tradeoff between spatial and temporal resolution is investigated. With appropriate sampling parameters, spatial and temporal resolutions of 1 mm and 1 ms have been demonstrated. The viability of the electro-location technique is demonstrated by measuring the liquefaction-induced settlements of heavy objects in the geotechnical centrifuge model test using an electrode switching system. The obtained positions agree remarkably with independent measurements. The electro-location method is considered to be a useful technique for monitoring subsurface movements in physical centrifuge models and has practical application potentials in the field.

A series of undrained and drained constant p triaxial compression and extension tests were conducted on Nevada sand at a relative density of about 70%. Most drained tests exhibited a peak followed by a drop of deviatoric stress. The drop in stress corresponded to a drop in dilatancy rate and the formation of a shear band. In most undrained tests, negative pore pressures developed and deviatoric stress increased during the constant volume phase of the test. Eventually, cavitation of the pore fluid occurred and the deviatoric stress stabilized. The maximum stress ratios (and maximum mobilized friction angles) achieved in drained, constant p′ tests were significantly larger than in the constant volume portions of the undrained tests. Rowe (1969) and Bolton (1986) proposed that the peak friction angle has a component due to the critical state friction angle and a component due to the dilation rate. In undrained shear, there is no net dilation, yet the peak mobilized friction angle exceeds the critical state friction angle. The friction angle mobilized in undrained shear appears to be a function of the plastic dilation rate. The plastic dilation rate was estimated in the undrained tests by setting the sum of elastic and plastic dilation rates to zero. The difference between drained and undrained friction angles is consistent with the difference between plastic dilation rates in drained and undrained tests.

A model study of deformations of kaolin embankments during earthquakes has been carried out using a ‘Bumpy Road’ actuator to impose earthquake-like base accelerations on centrifuge models. This paper begins by briefly describing the earthquake simulating apparatus. The results of the model tests indicate that predictions of displacement using sliding block type calculations need to take account of softening of the soil during the earthquake. A simple mathematical model for degradation of strength with increasing block displacement is shown to satisfactorily account for softening. In one model test, strain softening and pore pressures caused by an earthquake apparently induced a collapse of the model a few seconds after the earthquake. Delayed failure produced deformations of an order of magnitude larger than any measured during actual base shaking.

The results of centrifuge tests for bearing capacity of circular footings are used to verify the bearing capacity predictions using Hansen's equations with interpretations by Meyerhof and De Beer. The correlation with De Beer's method, which accounts for the variation of friction angle ϕ with normal pressure, is very good. An alternate proposal, using a constant ϕ and an intercept c at zero normal pressure that best fit the failure envelope in the range of interest, also gives a good correlation with the data. The proposed method does not require the iteration procedure that is required by Meyerhof and De Beer. The results are applicable to shallow circular foundations in dense sand where general shear failure is the dominant mechanism. Modeling of models tests were conducted to verify the centrifuge scaling laws over a range in scale factors. The good agreement of modeling of model tests shows that the centrifuge is a useful tool for modeling bearing capacity problems.

Some initial results of centrifuge model tests to examine the mechanism causing flow failure of a less permeable layer resting on a more permeable layer, subjected to seismic loading conditions, is presented. The centrifuge model that was subjected to a base motion using a servohydraulic shaker, consisted of a clay layer overlying a sand embankment submerged in water. During shaking the clay layer almost completely flowed off the sand and collected at the toes of the embankment. Settlement of the sand was also observed during shaking. As the soil overlying the sand is relatively impervious before flow failure, sand particles rearrange without any global change in volume. Locally, however, it is possible that loosening of sand close to the interface between sand and clay did occur due to the densification of the sand beneath it. This may leave a liquid film or a layer of very loose sand at the interface thus creating an unstable situation which will result in the creation of a flow slide of the impermeable...

The physico-chemical processes of advection, dispersion, adsorption, and degradation that control pollutant transport processes in groundwater are described. Dimensional analysis is presented, and the scaling requirements for centrifuge modeling of these processes is developed. The validity of these scaling laws is examined by conducting modeling of models tests using several types of soils in idealized models of one-dimensional flow situations. The importance of scaling gravity (by using a centrifuge) is highlighted. The centrifuge permits simulation of prototype stress levels and thus makes it possible to obtain identical soil properties such as permeability in a small-scale model as compared to a full-scale soil deposit. Simulation of capillary effects and a phreatic surface is also possible in a centrifuge model, but not in a 1-g scale model. It is concluded that the scaling laws are valid for adsorption and advection in the reported model tests, but in coarse-grained soils where the Peclet number is often greater than one, the dispersion process cannot be directly scaled from model to prototype. Even if direct scaling is not possible, the centrifuge is a useful tool for providing data to test or verify numerical models, since full-scale test data are not abundant, and the full-scale boundary and site conditions are poorly defined.

Centrifuge model tests were conducted on models of flexible shallow tunnels in dry sand subject to blast loading from nearby high-energy explosives. Experiments were conducted with varying centrifugal accelerations (from 1 g to 97 g on 2.5 – 5-cm-diameter model tunnels made of thin brass and aluminum sheet. Buried, 1/2-g and 1/16-g pentaerythrito tetranitrate (PEIN) explosive charges were detonated at a distance of 0 – 2 diameters from the tunnels during centrifugation. It appears that for flexible tunnels which are exposed or nearly exposed by the crater, significant damage to the tunnel occurs during crater excavation. The intense flow which occurs in the soil near the crater appears to be the major cause of distortion of the tunnel and the shock wave itself is often less significant. The distance from the tunnel to the crater appears to be a more significant factor than the distance from the tunnel to the charge in causing damage to flexible tunnels. Since gravity influences the cratering process, and consequently influences the soil-structure interaction, it is important to conduct scale-model tests of this type of event on a centrifuge. The centrifuge scaling laws were verified by conducting modeling of models tests using 1/97- and 1/48.5-scale tests. Cratering efficiency is also compared to that measured by other researchers with reasonable agreement.

A modified sliding block analysis, which allows the sliding resistance to be nonlinear visco‐elastoplastic, has been developed to predict deformations of embankments during earthquakes. Contrary to the assumption (of the original Newmark sliding block analysis) that a distinct slip surface forms, the centrifuge models were observed to deform along relatively wide shear bands during base shaking. The sliding resistance provided by a wide shear band is better characterized by visco‐elastoplastic than rigid‐plastic behavior. A detailed analysis concerning rational parameter selection is included. The modified analysis is used to predict the deformation of clay embankment centrifuge models during simulated earthquakes, with good agreement between experiment and theory. The centrifuge modeling technique is shown to be a useful tool for development, tuning, and verification of analytical models, especially in the subject of earthquake engineering where detailed field evidence is in acute shortage.

Geocentrifuge experiments require rapid event sampling. Electrical Resistivity Tomography is a method well suited for geocentrifuge experiments, however, commercially available systems can not perform at the speeds required to sample these experiments without temporal aliasing occurring. A novel ERT system was designed, and two of these systems were constructed – one for research at the INEEL, and one for research at UC Davis. System performance is comparable to that of existing commercial system, while the maximum system speed is close to four orders of magnitudes higher than that of commercial systems. This system should thus be able to adequately sample processes occurring on the geocentrifuge.

As part of the VELACS (VErification of Liquefaction Analysis Using Centrifuge Studies) Project, a series of dynamic centrifuge model tests were duplicated at different Universities (U.C. Davis, UC Berkeley, Caltech, Cambridge, Colorado, MIT, Princeton, and RPI) to assess the repeatability of the test results and to provide a data base for a Class A (before the event) type prediction of the test results. This paper will present the results obtained on different centrifuges, the evaluation of the repeatability of the results, and discussions on the use fulness of the results for the validation of constitutive models and numerical procedures. The experience and lessons learnt from this practice show that standardization of dynamic centrifuge modeling is very important in order to achieve repeatability of test results. The results resented here are based on the materials published in a Proceedings of the VELACS Symposium.