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Due to insufficient backfilling and compaction, voids existing around the bottom of a buried pipe can have a negative effect on the mechanical behavior of the pipe. Expanded polystyrene (EPS) has been widely utilized in civil engineering as fill, backfill, moisture cutoff, and impact mitigation materials for falling rocks because of its light weight and workability. In this study, centrifuge model tests were conducted in order to investigate the possibility of pipe failure when voids existed around the bottom of the buried pipe, and the effect of installing EPS blocks as a countermeasure. In the test, a model polyvinyl chloride (PVC) pipe having a diameter of 140 mm was buried in the dry sand and loaded at the ground surface in a centrifugal field of 21G. Cases with voids around the bottom of the buried pipe and with EPS blocks were studied, and the differences in the pipe mechanical behavior under standard backfill conditions were discussed. The loading conditions and the shape of the EPS were varied. The test results showed that the loss of soil around the bottom of the pipe caused a stress concentration by a factor of more than 2.5 compared to the typical burial condition, and could directly cause pipe failure. In addition, it is clear that EPS installation was an effective countermeasure to avoid the risk of insufficient compaction, because it did not significantly affect pipe deformation and strain distribution, although the earth pressure tended to concentrate slightly at the bottom of the pipe. Practical Applications: Pipelines that are buried in the ground usually need to be backfilled uniformly with soil around the pipe. However, due to difficulties in construction, the backfilling and compaction may be insufficient on the bottom sides of the pipe, resulting in voids or localized low-density areas. In this study, centrifuge model tests were conducted to investigate the behavior of pipes buried in such nonuniform ground and the effectiveness of a countermeasure to install expanded polystyrene foam (EPS) blocks as a foundation instead of backfilling with soil. In the tests, a 150-mm-diameter polyvinyl chloride (PVC) pipe was buried in a container, and the strain of the pipe was measured in detail when a vertical load of up to 140 kPa was applied to the ground surface in a 20G centrifugal force field. As a result, when the soil in the lower part of the pipe is not uniform, a large strain is concentrated at the bottom of the pipe, which may lead to the pipe failure. Then, when EPS was installed, no large strains were measured, indicating that EPS installations are effective as a countermeasure. [ABSTRACT FROM AUTHOR]

Monopiles are the favored foundation type for offshore wind turbines due to a relatively simple design and installation process. This paper investigates using coupled three-dimensional finite-element analysis the load-deformation behavior of extra-large (XL) and extra-extra-large (XXL) monopiles in low-shear strength overconsolidated kaolin clay subjected to monotonic lateral loading at varying load eccentricities. The subloading tij constitutive model, robustly calibrated using data from centrifuge tests carried out by the authors, has been adopted in these analyses. For the 16 cases analyzed, a detailed discussion is provided on the behavior of XL/XXL monopiles, including their mode of deformation, lateral capacity, deflection, rotational stiffness, bending moment and shear force, and the response of the clay in terms of soil resistance and excess pore pressure. It is also demonstrated that the p-y method is inaccurate in designing monopiles in this size range. [ABSTRACT FROM AUTHOR]

This paper presents a probabilistic model for evaluating the liquefaction-triggering hazard in level, layered, and saturated granular soil profiles improved with dense granular columns (DGCs). The model is developed using the results of a comprehensive numerical parametric study, validated with a dynamic centrifuge test, and subsequently tested with the available case histories involving DGCs as a liquefaction countermeasure. The numerical database includes a total of 30,000, three-dimensional (3D), fully coupled, nonlinear, dynamic finite-element simulations with a statistically determined range of layer-, profile-, DGC-, and ground motion–specific input parameters. The criteria for the predicted degree of liquefaction (i.e., full, marginal, and no liquefaction) are based on the peak values of excess pore pressure ratio and shear strain anticipated within each soil layer. A machine learning approach that performs multinomial logistic regression along with variable selection and regularization is used to develop a set of functional forms for estimating the probabilities of full-, marginal-, and no-liquefaction in sites improved with DGCs. The proposed probabilistic model is the first of its kind that explicitly considers variations in the area replacement ratio (Ar), stiffness, and drainage capacity of the DGC; the thickness, depth, relative density, and hydraulic conductivity range of each layer; the evolutionary characteristics of ground motions; and the underlying uncertainty in the prediction of pore pressures and shear strains within each layer. [ABSTRACT FROM AUTHOR]

This research investigates the influence of pile spacing on the dynamic behavior of pile groups by performing a series of specifically designed dynamic centrifuge experiments on pile foundations embedded in a two-layered soil profile. A single pile and two 3×1 row pile groups with different pile spacing were used as model pile foundations, and the soil models consisted of a soft clay underlain by dense sand. The influence of earthquake frequency on the dynamic behavior of two-layered soils is discussed using the centrifuge data and 1D site response analysis from DEEPSOIL. Further, the results of these centrifuge tests agreed with the conviction that the group effects will be diminished with the increase in pile-to-pile spacing in a pile group due to reduced pile-soil-pile interaction. However, these reduced pile group effects can lead to larger kinematic pile bending moments in the widely spaced pile group compared with a closely spaced pile group. Moreover, the single pile always has larger bending moments than both the tested pile groups—an exception to this is when there is a significant phase difference between the kinematic and inertial loads for a single pile but not for the widely spaced pile group. The influence of pile spacing on the shadowing effects and location of peak bending moments in the piles of a group are also discussed in this paper. Lastly, an attempt is made to evaluate the individual contribution of inertial and kinematic loads for the seismic design of pile foundations considering soil-pile-structure interaction effects. [ABSTRACT FROM AUTHOR]

Tunneling-induced ground movements can affect the equilibrium state of an existing pile, causing uneven settlement among pile groups and damage to connected structures. This paper presents results from five centrifuge tests aimed at evaluating the load redistribution mechanisms that occur within piles located close to tunnel excavation. Two main mechanisms are studied: first, those related to ground displacements and stress relief related to tunneling and, second, those related to pile head load changes caused by connected superstructures (accomplished using a hybrid centrifuge-numerical modeling method). A novel fiber Bragg grating sensor system was used to measure shaft shear stresses along model piles. Results are used to quantify the relative impact that these two mechanisms have on pile load redistribution during tunnel volume loss. In addition, posttunneling pile loading tests were performed, with results indicating that tunneling-induced ground volumetric strains could influence the posttunneling loading response of piles. [ABSTRACT FROM AUTHOR]

The need to tunnel closely beneath piles is increasing due to the development of urban areas. This poses a risk to the stability and serviceability of overlying structures (e.g., buildings, piers, and piled embankments). The impact of tunneling on piles is usually assessed using a displacement threshold, yet this provides no information about the posttunneling pile safety factor. Knowledge of a pile's safety factor under serviceability or extreme loading conditions is important, especially if future repurposing of the associated superstructure is a possibility. Tunneling can reduce the safety factor of a pile up to the point of geotechnical failure (i.e., when the pile capacity reduces to that of the applied load), yet little guidance is available to enable a straightforward means of assessing the posttunneling safety factor of a pile. This paper aims to address this shortcoming by providing design charts based on an analytical tunnel-single pile interaction approach that provides a means of determining a posttunneling pile safety factor. The methodology and design charts are applicable to drained soil conditions and include for the effects of the initial pile safety factor, the pile installation method [displacement (driven and jacked), nondisplacement (bored) with only the shaft capacity, and nondisplacement with base and shaft capacity] and varying water table depths. In the paper, as a validation exercise, analytical predictions are compared against data from geotechnical centrifuge tests designed to model both displacement and nondisplacement piles in sands, including a variety of tunnel–pile relative locations and initial pile safety factors. For a specified design value of a posttunneling pile safety factor, the design charts enable a quick assessment of the safe location of a pile or a tolerable tunnel volume loss considering ground parameters, water table position, pile installation method, and initial safety factor. [ABSTRACT FROM AUTHOR]

Earthquake-induced liquefaction is a major threat to the safety of earth embankments. This paper presents an experiment-based safety assessment of antiliquefaction performance of a constructed reservoir embankment. This reservoir is a major project constructed in the middle of a tidal estuary in 2007 and supplies nearly half of the raw water for the Shanghai, China. The embankment was directly constructed on liquefiable foundation without reinforcement. Hence, it is essential to evaluate present antiliquefaction performance of this constructed embankment. The concept of performance-based design was introduced to the safety assessment. Dynamic centrifuge model tests are successfully applied to predict the seismic performances of the embankment. In order to reflect the real situation of the embankment, the materials and structures involved in the embankment were well replicated according to a scale rule. Moreover, the seismic wave used in model tests was determined according to the report of seismic safety of the site. Finally, the safety of antiliquefaction performance of the embankment was evaluated and discussed in light of the performance-based design concept. [ABSTRACT FROM AUTHOR]

The results of fully coupled, three-dimensional (3D), nonlinear finite-element analyses of structures founded on liquefiable soils are compared with centrifuge experiments. The goal is to provide insight into the numerical model's capabilities in predicting the key engineering demand parameters that control building performance on softened ground for a range of structures, soil profiles, and ground motions. Experimental and numerical observations will also guide future analyses and mitigation decisions. The numerical model captured excess pore pressures and accelerations, the dominant displacement mechanisms under the foundation, and therefore building's settlement, tilt, and interstory drift. Both experimental and numerical results showed that increasing the structure's contact pressure and height=width (H=B) ratio generally reduces net excess pore pressure ratios in soil but amplifies the structure's tilting tendencies and total drift. The settlement response of a structure with a greater pressure and H=B ratio was also more sensitive to soil-structure-interaction induced forces, which could at times amplify on a denser soil with less softening. A denser soil profile also increased building's flexural drift in all cases by reducing excess pore pressures and rocking drift, while amplifying foundation accelerations and total drift. Numerical simulations captured these trends well. These experimental and numerical results point to the importance of taking into account a building's dynamic properties and overall performance in mitigation design. [ABSTRACT FROM AUTHOR]

Seismic design of geotechnical structures often requires estimates of how shearing resistance of liquefiable soil is reduced to its minimum value (residual strength, sr) as pore pressures build up and is subsequently regained as pore pressures dissipate. It was envisioned that the shear strength of liquefying sands could be measured in-flight in a seismic geotechnical centrifuge model by pulling thin coupons (plates) horizontally through the sand models before, during, and after shaking to simulate the large strains and strain rates associated with liquefaction flow failures. This paper presents the results of seismic centrifuge tests that were used to make such measurements. Companion ring shear (RS) tests also are described. Although centrifuge and RS residual strengths were generally similar and increased with relative density, the centrifuge sr and sr=σ1 vo (sr divided by preshaking effective vertical stress) increased significantly with small changes in relative density, while the RS test sr and sr=σ 1 vo increased only slightly with changes in relative density. Furthermore, many measured sr and sr=σ 1 vo values fell below penetration test-based design curves used in practice. Dilative tendency during shearing is believed to have resulted in a dramatic increase in sr and sr/=σ1 vo in the centrifuge tests that consisted of soil dense of the critical state. In contrast, RS tests likely resulted in shear band development, where intense shearing and particle damage occurs, leading to suppressed soil dilative tendency and divergence from the centrifuge test results at higher relative densities. The centrifuge tests provided evidence that, as expected, postliquefaction strength recovery is linearly proportional to effective stress as excess pore pressures dissipate. [ABSTRACT FROM AUTHOR]

This paper presents the earthquake response of three low-rise frame-wall-foundation centrifuge building models, including a structural hinging-dominated, foundation rocking-dominated, and balanced design model. The models were designed such that the capacities of the dominant inelastic elements varied; however, each was subjected to a similar sequence of earthquake shaking in a 30-g centrifuge environment to facilitate comparison. Experimental results reveal that the traditional frame-wall building with a strong foundation (i.e., a hinging-dominated system) has the least potential to recover from displacements induced during earthquake shaking. As a result, this system suffered the most significant residual deformation. In contrast, systems with rocking foundations sustained a smaller peak roof acceleration, residual drift, and peak base shear despite the relatively larger peak transient drift demand. Consistent with slow cyclic test results, dissipated hysteretic energy was reasonably distributed amongst superstructure and substructure inelastic components in the balanced model during moderate and high intensity shaking. [ABSTRACT FROM AUTHOR]

By appropriately reducing the size of the footing, rocking behavior caused by seismic loading can occur about the footing base. Recent research has shown how rocking shallow foundations can result in improved seismic performance of bridges. One concern of rocking systems is the potential for significant settlement accumulation in poor soil conditions. This research explores the effectiveness and practicality of using unattached piles to mitigate settlement while still allowing rocking. Two series of centrifuge tests investigated the performance of individual footing-column-mass structural models in these soil conditions on unattached piles. The lateral load connection between the rocking foundation and piles is an important consideration; a variety of possible shear key configurations capable of transferring lateral and dead loads to the piles while allowing separation associated with rocking are discussed. Unique design issues associated with rocking on unattached piles and methods to address these issues are described. Concepts are presented to support the evaluation of the effect of excess pore pressure on the pile capacity. Methods of evaluating effects of pile capacity, number of piles, and arrangement of piles on the residual settlement and base shear coefficient to initiate rocking on piles are also described. [ABSTRACT FROM AUTHOR]

The author has discovered several errors that are not typographical in a famous governing differential equation and its general solution. The purpose of the current study is to reveal the errors, correct them, and present some numerical results. [ABSTRACT FROM AUTHOR]

This paper presents the results from a series of centrifuge tests and three-dimensional finite-element (FE) analyses, which examined the benefits of combining a footing with a monopile as a solution for foundations that are subjected to large moment loading, such as those used for towers and wind turbines. The experiments were carried out in silica sand and involved monotonic application of lateral loads at an equivalent prototype height of 26 m above the foundations. Tests were conducted on piled footings, monopiles, and unpiled footings. These experimental results together with the findings from the FE analyses show that the footing interacts positively with the piled foundation and that both the rotational stiffness and capacity of the combined piled footing system are greater than the sum of the individual contributions. Increased capacity arises as the footing causes a significant reduction in moment loading on the pile (hence facilitating the application of larger loads), primarily owing to an increased footing effective area arising from the tension capacity of the pile. [ABSTRACT FROM AUTHOR]

The effective design of earthquake-resistant structures and liquefaction mitigation techniques requires an improved understanding of the development and consequences of liquefaction. In this paper, the results from centrifuge experiments of structures with shallow foundations on liquefiable sand were used to evaluate the predictive capabilities of a state-of-the-practice numerical tool. Fully-coupled numerical simulations with the UBCSAND model implemented in FLAC-2D captured building settlements measured in these experiments reasonably well for one scaled input motion, mostly within factors of 0.7 and 1.8. The soil model captured the overall contribution of deviatoric displacement mechanisms and localized volumetric strains during partially drained cyclic loading. The primary limitation of the model became evident for slower rates of earthquake energy buildup, when the extent of soil softening and building displacement was overestimated by up to a factor of 4. The observations from recent case histories, the results of the experiments, and the insights gained from the numerical analyses are combined to provide guidance on the evaluation of building response on liquefiable sand and the performance of liquefaction remediation strategies. [ABSTRACT FROM AUTHOR]

Centrifuge modeling of pore pressure buildup in a sand deposit as a result of shaking is evaluated by comparison with a large-scale experiment. In large-scale Test SG-1, a 5.6-m-thick, mildly sloping deposit of hydraulic fill clean Ottawa sand of , was subjected to 5 s of low-intensity base shaking () that induced excess pore pressures short of liquefaction. Three centrifuge experiments using various soil deposits and saturation fluids were conducted and compared with the large-scale test. One of these centrifuge simulations used the same Ottawa sand and of the prototype, a viscous pore fluid, and dry pluviation deposition, which created a soil fabric stiffer than the prototype. The other two centrifuge simulations used silty sand saturated with water. The pore pressure buildup in one of the silty sand tests was in good agreement with the prototype, while the other two centrifuge deposits did not develop any excess pore pressure. The various responses in the four tests are explained by various levels of cyclic soil shear strain using a cyclic strain approach. The cyclic strains in the two tests with no pore pressure were smaller than the threshold strain needed to start pore pressure generation in sands (), while the cyclic strains in the two tests that built up pore pressure were greater than the threshold (). In addition, two more experiments using Ottawa sand were also conducted: a large-scale level-ground test and a centrifuge sloping test subjected to a greater excitation. The results of the six experiments including the two additional tests are very consistent, further verifying the validity of the strain approach and threshold strain for both level-ground and mildly sloping-ground sites. [ABSTRACT FROM AUTHOR]

Numerical simulations of a centrifuge test, in which dissipation patterns, lateral spreading, and shear strain localization were measured and recorded, are performed for validation of the numerical modeling approach and insight on the deformation mechanisms. The constitutive model calibration process is performed based on available laboratory data and is described in detail. The model container and construction sequence are simulated to provide good approximations of the boundary conditions and initial stress distributions. Two approaches for simulating successive shaking events are presented and compared. The key observations and mechanisms from the centrifuge test, namely (1) the dynamic response and onset of liquefaction, (2) the amount and pattern of surface deformation, (3) the patterns of pore pressure dissipation and void ratio redistribution, and (4) the difference in response between a sand profile treated and not treated with liquefaction drains, are all reasonably captured and bounded by the simulations. [ABSTRACT FROM AUTHOR]

Negative skin friction (NSF) can induce an increased compressive force on piles, called dragload, and additional pile settlement caused by the downward pull of the soil, called downdrag. To investigate the efficiency of shielding effects on dragload and downdrag of piles, centrifuge tests have been carried out to study the shielding mechanisms created by installing sheet piles sleeves around an existing pile in consolidating ground. The effects of various shielded lengths have also been investigated. Comparisons between centrifuge test results and finite-element (FE) analyses are made and discussed. Based on centrifuge tests, it is clear that the shielding effect on the dragload decreases only gently with a decrease in the shielded length, whereas the shielding effect on the downdrag decreases exponentially with a decrease in the shielded length. Numerical simulations of the centrifuge model tests on the sleeved center piles reveal that the observed shielding effects on the center pile are attributed to the stress transfer from the consolidating soft soil to the sheet pile sleeve. As consolidation proceeds, the relatively stiff sheet piles sleeve hangs up the soil, leading to a significant reduction in the vertical and horizontal effective stresses in the soil and in the NSF on the center piles. The deeper the depth, the greater the hang-up effects. Thus, the shielding effect increases with the shielded length of the center pile. The reduction in the NSF on the center piles protected by the sheet pile sleeve is more significant than the reduction in the NSF from the sacrificial piles with the same shielded length. [ABSTRACT FROM AUTHOR]

Liquefaction of sand is the main cause of failure for earth structures during earthquakes. Some past research showed that the fabric anisotropy of soils in the ground can cause different responses to both static and dynamic loading. The inherent fabric anisotropy of sand due to the deposition history and pattern may also affect the liquefaction potential of the ground. In this study, a group of earthquake centrifuge tests was performed on models of a level ground to investigate its seismic response with the soil deposited by different angles. A special rigid container was designed and used to prepare the models. It was found that the fabric anisotropy greatly influenced the response of the ground in displacement, acceleration, and excess pore pressure. The models with larger deposition angles had higher liquefaction potential. The results show that liquefaction evaluation of a sand layer should take into account the influence of fabric anisotropy caused by its deposition history and pattern. [ABSTRACT FROM AUTHOR]

Earthquake-induced deformation of piled bridge abutments in approach embankments underlain by liquefied soils may be reduced by the restraining forces provided by the piles and bridge superstructure. Design methods accounting for these pinning effects have been applied in practice but lack significant comparison with actual physical data. Three dynamic centrifuge model tests were performed to investigate pile-pinning effects, and the results are presented in this paper. Each centrifuge model was composed of two identical embankments underlain by liquefiable soil: one with a pile group and the other without. The tests demonstrated that the embankment with a pile group experienced less lateral movement and settlement than the embankment without a pile group. To better understand and extend the results of these centrifuge tests, nonlinear deformation analyses of the three centrifuge tests were made using a fully coupled effective stress model. The ability of the analyses to model the centrifuge tests is assessed primarily through comparison of dynamic histories of acceleration, pore water pressure, displacement, and pile bending moments. The usefulness of the modeling and dynamic analysis procedures is evaluated, and key lessons are described. [ABSTRACT FROM AUTHOR]

Evaluation of the impact of tunnel construction on existing buried structures is an important problem. This paper presents an analytical method for estimating the effect of tunnel construction on end-bearing piles located above the tunnel. The method can be used to estimate the safe relative distance between existing piles and newly constructed tunnels. Spherical and cylindrical cavity expansion/contraction analyses are used to evaluate pile end-bearing capacity and the reduction of confining pressure at the pile tip that results from tunnel volume loss. Pile end-bearing capacity is then reevaluated based on the reduced confining pressure at the pile tip. A modified shear modulus is used to account for the effect of pile installation on soil stiffness. The method is used to analyze centrifuge experiments conducted to study this problem. For the experimental data, where the service load applied to the piles during tunnel volume loss ranged between 50 and 60% of the maximum jacking force, the analytical method showed that pile failure occurred when the load-carrying capacity was reduced below 80% of its original value. A parametric analysis is included that highlights the effect of key soil properties on results. [ABSTRACT FROM AUTHOR]

This study focused on landslides resulting from heavy precipitation, such as rainfall from hurricanes. A series of centrifuge model simulations were performed with reference to a landslide that occurred during Typhoon Nabi in Japan in 2005. The procedures of simulation on the slope using a sand-clay soil mixture were illustrated. The rainfall event was simulated by applying precipitation in increments to the slope surface until it exceeded that of the field measurements. The instability was examined using an infinite slope analysis, and the mechanism of rainfall-induced failure was discussed. For this particular study, the results showed that incremental rainfall of less than 200 mm led to local failures, whereas total accumulation of 400 mm resulted in a global slope failure. A reduction in apparent cohesion in the soil combined with an increase in pore pressure because of infiltration was responsible for the slope instability during rainfall. [ABSTRACT FROM AUTHOR]

The aim of this study was to evaluate the uplift capacity of buried pipes to propose a new anchoring system using geogrids as an alternative and to improve the uplift resistance and inhibit the upward movement of pipes, which can compromise pipeline operation. The simulation was performed in a general scale model of using the geotechnical centrifuge of the State University of Norte Fluminense (UENF) at 10 times earth gravity acceleration to keep the similitude between the prototype and the model. The tests were carried out for two different sand densities, two geogrid widths, and three burial depths. The geogrid was manufactured in a laboratory to mimic the behavior of a real-scale geogrid when subjected to upward displacements. Therefore, an amount of 24 pullout tests were conducted in small-scale models at 10 g. A pipe of outer diameter was anchored to a geogrid with a width of in six tests and to a geogrid with a width of in six other tests. No anchoring system was used in 12 tests; the pipe was simply buried in sand. Standard sand at two different relative densities (23 and 70%) was used as backfill material. The results revealed improvement in the uplift normalized resistance for pipes anchored to the geogrid, especially for those at shallow burial depths. The postpeak (large displacement) behavior was highly improved, which will allow the pipe to withstand considerable displacements without loss of resistance. This is of particular importance when the pipeline is installed in sites where the groundwater table is high. [ABSTRACT FROM AUTHOR]

This paper describes a method to compute the mobilized shear modulus, G, and damping ratio, D, using the nonlinear fit of experimental transfer functions obtained at different depths in centrifuge models, with the analytical expression of the amplification function for a viscoelastic soil layer on a rigid base. The corresponding shear strain, ?, is computed as a function of the particle velocity and shear wave velocity. The sources of potential error in the determination of G, D, and ? embedded in the proposed method are identified and discussed in comparison with two other methods that have been proposed in the literature, based either on the determination of the time lag of accelerations between two accelerometers or on the evaluation of the shear stress-strain cycles from acceleration time histories recorded at different depths in the model. The performance of the three methods is evaluated using the experimental data obtained from nine centrifuge tests on dry sand. The values of G obtained by the proposed method compare well with the results of laboratory and literature data; D values are more dispersed and slightly higher than the literature data. [ABSTRACT FROM AUTHOR]

The paper examines the problem of tunneling beneath buried pipelines and the relationship between soil strains and pipeline bending behavior. Data are presented from centrifuge tests in which tunnel volume loss was induced in sand beneath pipelines of varying stiffness properties. The model tunnel and pipelines were all placed at a Perspex wall of the centrifuge strong box such that image-based deformation analyses could be performed. The method provided detailed data of subsurface soil and pipe displacements and illustrated the soil-pipe interaction mechanisms that occurred during tunnel volume loss, including the formation of a gap beneath the pipes. The relationship between tunnel volume loss, soil strain, and pipe bending behavior is illustrated. Experimental results of pipe bending moments are compared against predictions: (1) assuming the pipe simply follows greenfield displacements; (2) using an elastic continuum solution; and (3) using a new method in which an 'out-of-plane' shear argument, due to soil-pipe interaction, is introduced into the elastic continuum solution. It is shown that the new method gives the best prediction of experimental pipe bending moments. [ABSTRACT FROM AUTHOR]

Two series of centrifuge model tests were conducted using Nevada sand. Four saturated models placed in a mildly inclined laminar box and simulating a 6-m-thick deposit were shaken inducing liquefaction effects and lateral spreading. The sand was deposited at a relative density, Dr=45 or 75%; two of the 45% models were subjected to overconsolidation or preshaking. The second series involved in-flight measurements of static cone tip penetration resistance, qc, simulating the standard cone penetration test (CPT) 36-mm cone. Values of qc increased with Dr, overconsolidation, and preshaking. A normalized resistance, qc1N, was assigned to each of the four liquefaction/lateral spreading models. Increases in Dr, overconsolidation, and preshaking decreased liquefaction and ground deformation, but relative density alone captured these effects rather poorly. Conversely, qc1N predicted extremely well the liquefaction and lateral spreading response of the four models, confirming Seed's hypothesis to explain the success of penetration-based seismic liquefaction charts. The depth to liquefaction measured in the four centrifuge models is consistent with the field CPT liquefaction chart. [ABSTRACT FROM AUTHOR]

The effective application of liquefaction mitigation techniques requires an improved understanding of the development and consequences of liquefaction. Centrifuge experiments were performed to study the dominant mechanisms of seismically induced settlement of buildings with rigid mat foundations on thin deposits of liquefiable sand. The relative importance of key settlement mechanisms was evaluated by using mitigation techniques to minimize some of their respective contributions. The relative importance of settlement mechanisms was shown to depend on the characteristics of the earthquake motion, liquefiable soil, and building. The initiation, rate, and amount of liquefaction-induced building settlement depended greatly on the rate of ground shaking. Engineering design procedures should incorporate this important feature of earthquake shaking, which may be represented by the time rate of Arias intensity (i.e., the shaking intensity rate). In these experiments, installation of an independent, in-ground, perimetrical, stiff structural wall minimized deviatoric soil deformations under the building and reduced total building settlements by approximately 50%. Use of a flexible impermeable barrier that inhibited horizontal water flow without preventing shear deformation also reduced permanent building settlements but less significantly. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Hurricane Katrina was one of the worst natural disasters in U.S. history. The effects of the hurricane were particularly devastating in the city of New Orleans. Most of the damage was due to the failure of the levee system that surrounds the city to protect it from flooding. This paper presents the results of centrifuge models conducted at Rensselaer Polytechnic Institute and the U.S. Army Corps of Engineers simulating the behavior of the levees at London Avenue North and South that failed during Hurricane Katrina. Those levees failed without being overtopped by the storm surge. Also included are the results of a centrifuge model of one levee section at Orleans Canal South, which did not fail during the hurricane. The key factor of the failure mechanism of the London Avenue levees was the formation of a gap between the flooded side of the levee and the sheetpile. This gap triggered a reduction of the strength at the foundation of the protected side of the levee. The results are fully consistent with field observations. [ABSTRACT FROM AUTHOR]

A series of centrifuge tests were conducted on instrumented model suction caissons in normally consolidated, lightly overconsolidated, and sensitive clays, to investigate the uplift capacity and external radial stress changes for sealed caissons subjected to sustained loading and cyclic loading. The external shaft friction ratio during vertical pullout for these two types of loading was analyzed from the radial stress measured at failure, then the corresponding reverse end-bearing capacity factor was derived from the pullout capacity. Tests results were compared with those under monotonic undrained pullout. For caissons under sustained loading, the holding capacity was found to be 72–85% of that under monotonic undrained loading. Radial stress reduction around the caisson shaft reduced the external shaft friction ratio to 0.67–0.75, while dissipation of the “passive” suction at the caisson tip reduced the reverse end-bearing capacity factor to 7.5–9.4. Under cyclic loading, the uplift capacity of the caisson was found to be 72–86% of the monotonic capacity. Repeated loading reduced the external shaft friction ratio to 0.65–0.80, while the reverse end-bearing capacity factor reduced to 6.4–9.0. [ABSTRACT FROM AUTHOR]

It is commonly assumed that within inundated sand the Skempton B value and P-wave velocity decrease with decrease in saturation. In centrifuge tests a common saturation procedure is to inundate the specimen with carbon dioxide while under a vacuum and then slowly introduce the viscous pore fluid. The B value and related saturation is difficult to measure in centrifuge models and P-wave velocity—saturation correlations have been used for this purpose. A laboratory emulation of centrifuge saturation procedures was made using a triaxial cell with top and bottom bender elements and a viscous methyl cellulose–water pore fluid. Contrary to expectations, the laboratory tests showed high P-wave velocities indicative of full saturation when B values were low. Numerical modeling of the laboratory tests indicated that if air bubbles within the pore fluid are numerous and closely spaced then there is a good correlation between saturation, B value, and P-wave velocity. However if the air bubbles are larger and only present in some of the pores then the P-wave velocity is not a good indicator of B value and average saturation. The laboratory tests also showed that placing the specimen under backpressure for several days increased saturation and related B values. It is suggested that this common laboratory procedure should be considered for saturating centrifuge test specimens. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study of the strength in anisotropic clays by means of centrifuge model, cone penetration, and vane shear tests. To understand the effects of void ratio, overconsolidation ratio, and testing rate on the undrained shear strength (Su) of anisotropic Speswhite clay, a new centrifugal testing technique is designed to obtain constant overconsolidation ratio (OCR) profiles with varying void ratios (e), called the “descending gravity test.” The parameters controlling the generation of peak shear strength are quantified. As a result of this function, a new material and rate-dependent surface is defined in the e-OCR-Su space, which is identified as a “structural state capacity surface” since it relates the anisotropic structure to structure inherent capacity and properties. A new function for the estimation of excess pore pressure (uex) generated by cone penetration is found. By combining the strength and pore pressure functions a new model is proposed, called the “CU model.” The CU model is a structure-based model that provides reliable estimates of shear strength for in situ saturated clays using the knowledge of void and overconsolidation ratios. Finally, by combining Su-e-OCR and uex-e-OCR relationships, it estimates the void ratio and OCR profiles of anisotropic clays from piezocone penetration test results. [ABSTRACT FROM AUTHOR]

Monotonic, static beam on nonlinear Winkler foundation (BNWF) methods are used to analyze a suite of dynamic centrifuge model tests involving pile group foundations embedded in a mildly sloping soil profile that develops liquefaction-induced lateral spreading during earthquake shaking. A single set of recommended design guidelines was used for a baseline set of analyses. When lateral spreading demands were modeled by imposing free-field soil displacements to the free ends of the soil springs (BNWF_SD), bending moments were predicted within -8% to +69 (16th to 84th percentile values) and pile cap displacements were predicted within -6 to +38%, with the accuracy being similar for small, medium, and large motions. When lateral spreading demands were modeled by imposing limit pressures directly to the pile nodes (BNWF_LP), bending moments and cap displacements were greatly overpredicted for small and medium motions where the lateral spreading displacements were not large enough to mobilize limit pressures, and pile cap displacements were greatly underpredicted for large motions. The effects of various parameter relations and alternative design guidelines on the accuracy of the BNWF analyses were evaluated. Sources of bias and dispersion in the BNWF predictions and the issues of greatest importance to foundation performance are discussed. The results of these comparisons indicate that certain guidelines and assumptions that are common in engineering design can produce significantly conservative or unconservative BNWF predictions, whereas the guidelines recommended herein can produce reasonably accurate predictions. [ABSTRACT FROM AUTHOR]

Realistic predictions of dynamic soil–structure interaction problems require appropriate constitutive models for the characterization of soils and interfaces. This paper presents a unified model based on the disturbed state concept (DSC). The parameters for the models for the Nevada sand, and sand–metal interface are obtained based on available triaxial test data on the sand and interfaces. The predicted stress–strain–pore water pressure behavior for the sand using the DSC model is compared with the test data. In addition, a finite element procedure with the DSC model, based on the generalized Biot’s theory, is used to predict the measured responses for a pile (aluminum) sand foundation problem obtained by using the centrifuge test. The predictions compared very well with measured pore water pressures. The DSC model is used to identify microstructural instability leading to liquefaction. A procedure is proposed to apply the proposed method for analysis and design for dynamic response and liquefaction. [ABSTRACT FROM AUTHOR]

Correct evaluation of shear modulus and damping characteristics in soils under dynamic loading is key to both the fundamental understanding of soil behavior and the practical application of soil modeling programs. Dynamic centrifuge tests can contribute significant information about soil behavior, but great care must be taken over the signal processing techniques involved, and the test conditions are different from the laboratory experiments that form the database of existing knowledge. This paper outlines several factors that require careful consideration when deriving stiffness and damping parameters from centrifuge data. Shear modulus and damping degradation curves for a dry sand, saturated sand, soft clay and a model waste are then evaluated to explore some of the factors that are introduced during centrifuge tests. Stiffness is seen to be a more reliable parameter than damping ratio. Damping during centrifuge tests for certain materials appeared to differ from the expected values. [ABSTRACT FROM AUTHOR]

A numerical model, called CC1, is presented for one-dimensional large strain consolidation in a geotechnical centrifuge. The model includes all the capabilities of a previous large strain consolidation code, CS2, written for surcharge loading under normal gravity conditions. In addition, CC1 accounts for variation of acceleration factor N over the depth of a centrifuge test specimen. The development of CC1 is first presented, followed by a comparison of simulated time - settlement curve with experimental measurements for Singapore marine clay and a parametric study illustrating the effects of nonuniform N distribution on centrifuge consolidation behavior. Simulation indicate that the effect of spatially varying N is most strongly controlled by the ratio of specimen height to centrifuge arm length and that the error associated with the assumption of constant N is relatively small if this ratio is 0.2 or less. Finally, CC1 is used to calculate the optimal location within a centrifuge specimen of Singapore marine clay at which to match the desired N value and the error that results if N is matched at the initial midheight of the specimen. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Remediation schemes for the treatment of liquefiable ground, such as vertical drains, have evolved over the last 30 years largely as a result of consideration of uniform sand beds. However, many liquefiable sites also contain thin layers of silts and clays which will act as an obstacle to fluid dissipating from liquefying sands, such that the performance and requirements of a remediation scheme will be different. In particular, water films may be produced. This paper presents results from a series of dynamic centrifuge experiments on vertical drains in level liquefiable sand deposits. Centrifuge tests were conducted both with and without surface fine layers. The drains prevented sand boiling, and the thin surface fine layers did not significantly affect excess pore pressure response. A further centrifuge test on a thin layer of low-permeability fines separating two liquefiable sandy layers, showed evidence suggesting that material boiled from the lower sand to the upper sand in the absence of a drain through the fines. All of the experiments suggest that the presence of unblocked vertical drains prevent water film formation. [ABSTRACT FROM AUTHOR]

The role of void redistribution in the liquefaction behavior of saturated sand slopes with and without silt interlayers was investigated using a series of dynamic centrifuge model tests. Twelve centrifuge model tests are described that represent four different simple slope configurations, a range of initial relative densities (DR), and three different input motions with different sequences of application. These experimental results demonstrate that the potential for void redistribution induced shear localizations and slope instability depends on the sand’s initial DR, slope geometry (silt layer shape, sand layer thickness), and shaking characteristics (duration, intensity, and history). The archived experimental data set provides a good basis for assessing the ability of numerical modeling methods to distinguish between conditions leading to localization or not. Apparent residual shear strengths mobilized in the models were backcalculated using techniques common to practice. The experimental and analytical results demonstrate that the apparent residual shear strength is unlikely to correlate closely to pre-earthquake penetration resistance alone, but rather is a function of the initial shear stresses and numerous factors affecting the process of void redistribution and localization. [ABSTRACT FROM AUTHOR]

This paper documents three case studies that involve dynamic centrifuge tests that simulated large-scale shaking table tests on soil–pile-structure systems. The large-scale shaking table tests were performed using the world’s largest laminar shear box with depth of 6 m and plan dimensions of 11 m and 3.5 m. Life-size steel and prestressed concrete piles were used in these tests. The large-scale tests involved intense shaking that produced strong nonlinear stress–strain effects and degradation of soil stiffness due to liquefaction in the foundation soil models. The dynamic centrifuge tests treated the large-scale models as their prototypes. Only essential information about the large-scale test models and the testing conditions were available to design and perform the dynamic centrifuge tests. The three case studies showed that carefully designed performed centrifuge tests could reproduce the key features of the responses of the large-scale models. However, some differences were also found in the results from these two types of tests. [ABSTRACT FROM AUTHOR]