Dynamic Experiments and Analyses of a Pile-Group-Supported Structure

Experimental data on the seismic response of a pile-group-supported structure was obtained through dynamic centrifuge model tests, and then used to evaluate a dynamic beam on a nonlinear Winkler foundation (BNWF) analysis method. The centrifuge tests included a structure supported on a group of nine piles founded in soft clay overlying dense sand. This structure was subjected to nine earthquake events with peak accelerations ranging from 0.02 to 0.7g. The centrifuge tests and dynamic analysis methods are described. Good agreement was obtained between calculated and recorded structural responses, including superstructure acceleration and displacement, pile cap acceleration and displacement, pile bending moment and axial load, and pile cap rotation. Representative examples of recorded and calculated behavior for the structure and soil profile are presented. Sensitivity of the dynamic BNWF analyses to the numerical model parameters and site response calculations are evaluated. These results provide experimental support for the use of dynamic BNWF analysis methods in seismic soil-pile-structure interaction problems involving pile-group systems.     

Liquefaction Centrifuge Modeling of Sands of Different Permeability

This paper presents the results of six centrifuge model tests of liquefaction and earthquake-induced lateral spreading of fine Nevada sand using an inclined laminar box. The centrifuge experiments simulate a gently sloping, 10 m thick stratum of saturated homogeneous sand of infinite lateral extent and relative densities ranging from 45 to 75%. Such idealized models approach some field situations and they provide significant general insight into the basic mechanisms and parameters influencing the lateral spreading phenomenon. The layer was subjected to lateral base shaking with prototype peak acceleration ranging from 0.20 to 0.41 g, a frequency of 2 Hz, and duration of approximately 22 cycles. The simulated field slope angle was 5°. The model deposits were all saturated with a viscous fluid 50 times more viscous than water, so that testing under the increased gravitational field (50 g) produced a deposit with the prototype permeability of the same fine-grained sand saturated with water in the field. Detailed discussions and comparisons of the six centrifuge tests are included. The observed effects of relative density Dr and input peak acceleration amax on the following measured parameters are summarized: thickness of liquefied soil H1, permanent lateral displacement DH, and ground surface settlement S. Comparisons and discussions are also presented on the effect of permeability for a Dr = 45% deposit. This is done by comparing the results reported herein using a viscous pore fluid, with other published centrifuge tests where a similar deposit using the same model soil, also tested at 50 g and shaken with the same input motion, was saturated with water, thus simulating a prototype sand having 50 times the permeability of the fine sand reported in this paper.     

Modeling of Shallowly Embedded Offshore Pipelines in Calcareous Sand

The results of centrifuge modeling of pipe–soil interaction for shallowly embedded offshore pipelines are presented. A non-associated bounding surface model is constructed in vertical–horizontal (V–H) load space on the basis of test data and the theory of plasticity to simulate the response of a pipeline embedded in sandy soil under combined (vertical and horizontal) monotonic loading, taking into account possible pre-loading effects. The model needs nine parameters that can be back calculated from model or field tests and in some cases estimated theoretically. It provides a suitable basis for modeling the load-displacement response of shallowly embedded offshore pipelines. The model reproduces the key features of the load-displacement response of pipelines observed in centrifuge model tests. In particular, the adoption of bounding surface plasticity allows a gradual transition from elastic to plastic response to be simulated and the introduction of a non-associated flow rule allows the model to predict the strain-softening behavior of pipes under horizontal loading. The lateral breakout resistance predicted by the model agrees very well with experimental data.     

Centrifugal and Numerical Modeling of a Reinforced Lime-Stabilized Soil Embankment on Soft Clay with Wick Drains

A centrifuge test was performed on a reinforced embankment backfilled with lime-stabilized soil on soft clay installed with wick drains. The finite-element program PLAXIS was employed to simulate the centrifuge test on the basis of the test dimensions. Verifications of the numerical model were taken by comparing the numerical results with the measurements obtained from the centrifuge test, and it was found that both were in good agreement. Three cases of unreinforced, one-layer reinforced, and two-layer reinforced embankments were simulated and compared on the basis of the numerical model. A centrifuge similarity drainage relationship was also proposed in this paper on the basis of the equal degree of consolidation between the model and prototype drains.     

Bayesian Model Calibration Using Geotechnical Centrifuge Tests

The predicted performance using a geotechnical prediction model is expected to deviate from reality. A practical approach to assess the model error is through calibration with observed performances in physical model tests. In this paper, a Bayesian framework of model calibration using centrifuge modeling tests is proposed and the procedure of model calibration is illustrated. Two centrifuge tests conducted to investigate the performance of soil slopes under rainfall conditions are used to calibrate a coupled hydromechanical analysis model. It is found that for centrifuge tests with different levels of soil variability, the test with a smaller variability of soil properties is more efficient for model calibration. According to the concept of random field, a centrifuge model with a larger model size and accelerated to a lower acceleration is better for model calibration. When the discrepancy between the performance interpreted from the centrifuge model and the field performance is small, the improvement of the reliability estimation for a new slope is significant. However, when there is little information about the discrepancy, the reliability estimation cannot be significantly improved by the information from centrifuge modeling. The proposed procedure is shown to be able to quantify the calibration effects of centrifuge tests and may be used to achieve a more reliable calibration.     

Centrifuge Modeling of Earthquake Effects on Buried High-Density Polyethylene (HDPE) Pipelines Crossing Fault Zones

Permanent ground deformation is a severe hazard for continuous buried pipelines. This technical paper presents results from four centrifuge tests designed to investigate the influence of pipe-fault orientation on pipe behavior under earthquake faulting. The experimental setup and procedures are described, and the test results are presented. The test results show that, as expected, pipe axial strain is strongly influenced by the pipe-fault orientation angle, whereas the influence of pipe-fault orientation angle on pipe bending strain is minor. The measured pipe strains were shown to follow the trend predicted by the Kennedy model. Also, through a parametric study using the Kennedy model, the experimental data were extrapolated for cases of pipeline with longer unanchored length. By combing the data from strain gauges and tactile pressure sensors, transverse force–deformation relations or p–y relations for the pipe were determined. The data indicates that the underlying p–y relationship varies along the length of the pipe with a stiffer p–y relationship at points closer to the fault and a softer p–y relationship at points farther away. The stiffer p–y relationship, appropriate for locations moderately close to the fault, was compared with the ASCE Guidelines in 1984 and Turner’s recommendation in 2004 for moist sand. It was found that the force level for the plastic p–y behavior in the centrifuge tests compared favorably with that in the ASCE Guidelines (1984).     

Model for Large Strain Consolidation by Centrifuge

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 curves with experimental measurements for Singapore marine clay and a parametric study illustrating the effects of nonuniform N distribution on centrifuge consolidation behavior. Simulations 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.     

Earth Dam on Liquefiable Foundation and Remediation: Numerical Simulation of Centrifuge Experiments

A series of four dynamic centrifuge model tests was performed to investigate the effect of foundation densification on the seismic performance of a zoned earth dam with a saturated sand foundation. In these experiments, thickness of the densified foundation layer was systematically increased, resulting in a comprehensive set of dam-foundation response data. Herein, Class-A and Class-B numerical simulations of these experiments are conducted using a two-phase (solid and fluid) fully coupled finite element code. This code incorporates a plasticity-based soil stress–strain model with the modeling parameters partially calibrated based on earlier studies. The physical and numerical models both indicate reduced deformations and increased crest accelerations with the increase in densified layer thickness. Overall, the differences between the computed and recorded dam displacements are under 50%. At most locations, the computed excess pore pressure and acceleration match the recorded counterparts reasonably well. Based on this study, directions for further improvement of the numerical model are suggested.     

Analyzing Dynamic Behavior of Geosynthetic-Reinforced Soil Retaining Walls

An advanced generalized plasticity soil model and bounding surface geosynthetic model, in conjunction with a dynamic finite element procedure, are used to analyze the behavior of geosynthetic-reinforced soil retaining walls. The construction behavior of a full-scale wall is first analyzed followed by a series of five shaking table tests conducted in a centrifuge. The parameters for the sandy backfill soils are calibrated through the results of monotonic and cyclic triaxial tests. The wall facing deformations, strains in the geogrid reinforcement layers, lateral earth pressures acting at the facing blocks, and vertical stresses at the foundation are presented. In the centrifugal shaking table tests, the response of the walls subject to 20 cycles of sinusoidal wave having a frequency of 2 Hz and of acceleration amplitude of 0.2g are compared with the results of analysis. The acceleration in the backfill, strain in the geogrid layers, and facing deformation are computed and compared to the test results. The results of analysis for both static and dynamic tests compared reasonably well with the experimental results.     

Liquefaction-Induced Softening of Load Transfer between Pile Groups and Laterally Spreading Crusts

Laterally spreading nonliquefied crusts can exert large loads on pile foundations causing major damage to structures. While monotonic load tests of pile caps indicate that full passive resistance may be mobilized by displacements on the order of 1–7% of the pile cap height, dynamic centrifuge model tests show that much larger relative displacements may be required to mobilize the full passive load from a laterally spreading crust onto a pile group. The centrifuge models contained six-pile groups embedded in a gently sloping soil profile with a nonliquefied crust over liquefiable loose sand over dense sand. The nonliquefied crust layer spread downslope on top of the liquefied sand layer, and failed in the passive mode against the pile foundations. The dynamic trace of lateral load versus relative displacement between the “free-field” crust and pile cap is nonlinear and hysteretic, and depends on the cyclic mobility of the underlying liquefiable sand, ground motion characteristics, and cyclic degradation and cracking of the nonliquefied crust. Analytical models are derived to explain a mechanism by which liquefaction of the underlying sand layer causes the soil-to-pile-cap interaction stresses to be distributed through a larger zone of influence in the crust, thereby contributing to the softer load transfer behavior. The analytical models distinguish between structural loading and lateral spreading conditions. Load transfer relations obtained from the two analytical models reasonably envelope the responses observed in the centrifuge tests.     

Effects of Tip Location and Shielding on Piles in Consolidating Ground

Pile foundations located within consolidating ground are commonly subjected to negative skin friction (NSF) and failures of pile foundations related to dragload (compressive force) and downdrag (pile settlement) have been reported in the literature. This paper reports the results of four centrifuge model tests, which were undertaken to achieve two objectives: first, to investigate the response of a single pile subjected to NSF with different pile tip location with respect to the end-bearing stratum layer; and second, to study the behavior of floating piles subjected to NSF with and without shielding by sacrificing piles. In addition, three-dimensional numerical analyses of the centrifuge model tests were carried out with elastoplastic slip considered at the pile-soil interface. The measured maximum β value at unprotected single end-bearing and floating pile was similar and slightly smaller than 0.3. On the contrary, smaller β values of 0.1 and 0.2 were mobilized at the shielded center piles for pile spacings of 5.0 d and 6.0 d, respectively. The measured maximum dragload of the center pile in the group at 5.0 d and 6.0 d spacing was only 53% and 75% of the measured maximum dragload of an isolated single pile, respectively. Correspondingly, the measured downdrag of the center pile was reduced to about 57% and 80% of the isolated single pile. Based on the numerical analyses, it is revealed that sacrificing piles “hang up” the soil between the piles in the group and, thus, the vertical effective stress in the soil so reduced, as is the horizontal effective stress acting on the center pile. This “hang-up” effect reduces with an increase in pile spacing. For a given pile spacing, shielding effect on dragload is larger than that on downdrag.     

Centrifugal Modeling of Seismic Behavior of Large-Diameter Pipe in Liquefiable Soil

This study focused on the behavior of a large-diameter burial pipe with special reference to its stability against flotation subject to soil liquefaction. Centrifugal modeling technique was used where the results are presented for a total of eight shaking table tests conducted on the burial pipe in a laminar box under 30g gravitational field. The ground was prepared with Nevada sand at a relative density of 38% and shaken with a sinusoidal wave at an amplitude of 0.5g. The use of a viscous fluid in a saturated soil deposit satisfied the time scaling relationships of both dynamic and dissipation phenomena. The centrifugal modeling technique simulated flotation of pipeline as the soil liquefied. A technique that used gravels and geosynthetic material was used to mitigate flotation. The response of the soil deposit, in terms of acceleration and excess pore pressure, was investigated. The uplifting of the pipe, earth pressure response and ground surface deformation were also presented. Based on the test results, a design procedure was proposed for the burial pipe in resisting flotation due to soil liquefaction. The deadweight and stiffness of the gravel unit, which was confined by geosynthetic, were important items in design.     

Rotation Response of a Rigid Body under Seismic Excitation

After the 1994 Sanriku-Haruka-Oki, Japan, earthquake, rotation of tombstones along the vertical axis occurred in a graveyard about 34?m from the Japan Meteorological Agency Hachinohe Observatory where strong motion was recorded. The properties of seismic motion that make a rigid rectangular solid body rotate are discussed. Shaking table tests were conducted to reproduce the rotation response of Japanese-style tombstones which typically consist of several stone blocks whose shapes are rectangular solids. Data obtained from those tests were used to calibrate a numerical model by the three-dimensional distinct element method. Results of the shaking tests and numerical analyses showed that rotation of a rigid rectangular solid body may be caused by the combination of the rocking of the body and particle motion of the input acceleration. Rotation behavior of an actual tombstone was simulated based on the observed accelerogram. Findings show that one or two cycles of particle motion near peak acceleration caused the rotation.     

Centrifuge Simulations of Large-Scale Shaking Table Tests: Case Studies

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.     

Numerical Modeling of Nonhomogeneous Behavior of Structured Soils during Triaxial Tests

The nonhomogeneous behavior of structured soils during triaxial tests has been studied using a finite element model based on the Structured Cam Clay constitutive model with Biot-type consolidation. The effect of inhomogeneities caused by the end restraint is studied by simulating drained triaxial tests for samples with a height to diameter ratio of 2. It was discovered that with the increase in degree of soil structure with respect to the same soil at the reconstituted state, the inhomogeineities caused by the end restraint will increase. By loading the sample at different strain rates and assuming different hydraulic boundary conditions, inhomogeneities caused by partial drainage were investigated. It was found that if drainage is allowed from all faces of the specimen, fully drained tests can be carried out at strain rates about ten times higher than those required when the drainage is allowed only in the vertical direction at the top and bottom of the specimen, confirming the findings of previous studies. Both end restraint and partial drainage can cause bulging of the triaxial specimen around mid-height. Inhomogeneities due to partial drainage influence the stress–strain behavior during destructuring, a characteristic feature of a structured soil. With an increase in the strain rate, the change in voids ratio during destructuration reduces, but, in contrast, the mean effective stress at which destructuration commences was found to increase. It is shown that the stress–strain behavior of the soil calculated for a triaxial specimen with inhomogeneities, based on global measurements of the triaxial response, does not represent the true constitutive behavior of the soil inside the test specimen. For most soils analyzed, the deviatoric stress based on the global measurements is about 25% less than that for the soil inside the test specimen, when the applied axial strain is about 30%. Therefore it can be concluded that the conventional global measurements of the sample response may not accurately reflect the true stress–strain behavior of a structured soil. This finding has major implications for the interpretation of laboratory triaxial tests on structured soils.     

Effects of Layer Thickness and Density on Settlement and Lateral Spreading

This paper presents the results of six large-scale centrifuge model tests that were performed to study the effect of relative density and thickness of sand layers on the amount of settlement and lateral spreading. The models included a “river” channel with clay flood banks underlain by layers of loose and dense sand of variable thickness, and a bridge abutment surcharge on one of the banks. The model container was tilted to provide an overall slope to the model. Each model was subjected to three or four significant ground motion events, which were obtained by scaling the amplitude of recordings of the Kobe (1995) and Loma Prieta (1989) earthquakes. Several measurements of acceleration, pore water pressure, settlement, and lateral movement are presented. The liquefaction potential index and a deformation index, which combine the influences of depth, density, and layer thickness, were found to correlate reasonably well with liquefaction induced settlements and lateral deformations for the range of models tested and indicate that centrifuge results are consistent with field observations.     

Seismic Soil-Pile-Structure Interaction Experiments and Analyses

A dynamic beam on a nonlinear Winkler foundation (or “dynamic p-y”) analysis method for analyzing seismic soil-pile-structure interaction was evaluated against the results of a series of dynamic centrifuge model tests. The centrifuge tests included two different single-pile-supported structures subjected to nine different earthquake events with peak accelerations ranging from 0.02 to 0.7g. The soil profile consisted of soft clay overlying dense sand. Site response and dynamic p-y analyses are described. Input parameters were selected based on existing engineering practices. Reasonably good agreement was obtained between calculated and recorded responses for both structural models in all earthquake events. Sensitivity of the results to dynamic p-y model parameters and site response calculations are evaluated. These results provide experimental support for the use of dynamic p-y analysis methods in seismic soil-pile-structure interaction problems.     

Behavior of Pile Groups Subject to Excavation-Induced Soil Movement in Very Soft Clay

A series of centrifuge model tests was conducted to investigate the behavior of pile groups of various sizes and configurations behind a retaining wall in very soft clay. With a 1.2-m excavation in front of the wall, which may simulate the initial stage of an excavation prior to strutting, the test results reveal that the induced bending moment on an individual pile in a free-head pile group is always smaller than that on a corresponding single pile located at the same distance behind the wall. This is attributed to the shadowing and reinforcing effects of other piles within the group. The degree of shadowing experienced by a pile depends on its relative position in the pile group. With a capped-head pile group, the individual piles are forced to interact in unison though subjected to different magnitudes of soil movement. Thus, despite being subjected to a larger soil movement, the induced bending moment on the front piles is moderated by the rear piles through the pile cap. A finite element program developed at the National University of Singapore is employed to back-analyze the centrifuge test data. The program gives a reasonably good prediction of the induced pile bending moments provided an appropriate modification factor is applied for the free-field soil movement and the amount of restraint provided by the pile cap is properly accounted for. The modification factor applied to the free-field soil movement accounts the reinforcing effect of the piles on the soil movement.     

Centrifuge Modeling of Large-Diameter Bored Pile Groups with Defects

In this research, centrifuge model pile-load tests were carried out to failure to investigate the behavior of large-diameter bored pile groups with defects. The model piles represented cast-in-place concrete piles 2.0?m in diameter and 15?m in length. Two series of static loading tests were performed. The first series of tests simulated the performance of a pile founded on rock and a pile with a soft toe. The second series of tests simulated the performance of three 2×2 pile groups: One reference group without defects, one group containing soft toes, and one group with two shorter piles not founded on rock. The presence of soft toes and shorter piles in the defective pile groups considerably reduced the pile group stiffness and capacity. As the defective piles were less stiff than the piles without defects, the settlements of the individual piles in the two defective pile groups were different. As a result, the applied load was largely shared by the piles without defects, and the defective pile groups tilted significantly. The rotation of the defective pile groups caused large bending moments to develop in the group piles and the pile caps. When the applied load was large, bending failure mechanisms were induced even though the applied load was vertical and concentric. The test results confirm findings from numerical analyses in the literature.     

Rotating Block Method for Seismic Displacement of Gravity Walls

A rotating block method is developed to calculate the rotational displacements of gravity retaining walls based on rigid foundations under seismic loading. The method is similar to the pseudostatic sliding block method of Newmark. When a threshold acceleration for rotation is exceeded, a rigid wall will start to rotate until the angular velocity for rotation is reduced to zero. The influence of ground motion characteristics on computed wall deformation was evaluated. The procedure was validated by data from centrifuge tests. This method is also applicable for the most complex cases when the sliding and rotation of a gravity wall are coupled.