Cyclic Lateral Load Behavior of a Pile Cap and Backfill

A series of static cyclic lateral load tests were performed on a full-scale 4×3 pile group driven into a cohesive soil profile. Twelve 324-mm steel pipe piles were attached to a concrete pile cap 5.18×3.05?m in plan and 1.12?m in height. Pile–soil–pile interaction and passive earth pressure provided lateral resistance. Seven lateral load tests were conducted in total; four tests with backfill compacted in front of the pile cap; two tests without backfill; and one test with a narrow trench between the pile cap and backfill soil. The formation of gaps around the piles at larger deflections reduced the pile–soil–pile interaction resulting in a degraded linear load versus deflection response that was very similar for the two tests without backfill and the trenched test. A typical nonlinear backbone curve was observed for the backfill tests. However, for deflections greater than 5 mm, the load-deflection behavior significantly changed from a concave down shape for the first cycle to a concave up shape for the second and subsequent cycles. The concave up shape continued to degrade with additional cycles past the second and typically became relatively constant after five to seven cycles. A gap formed between the backfill soil and the pile cap, which contributed to the load-deflection degradation. Crack patterns and sliding surfaces were consistent with that predicted by the log spiral theory. The results from this study indicate that passive resistance contributes considerably to the lateral resistance. However, with cyclic loading the passive force degrades significantly for deflections greater than 0.5% of the pile cap height.     

Large-Scale Passive Earth Pressure Load-Displacement Tests and Numerical Simulation

Passive earth pressure is recorded in two different tests, using a 6.7-m long, 2.9-m wide soil container. In these tests, sand with 7% silt content is densely compacted behind a moveable test wall to a supported height of 1.68 m (5.5 ft). Lateral load is applied to the vertical reinforced concrete wall section, which displaces freely along with the adjacent backfill in the horizontal and vertical directions. The recorded passive resistance is found to increase until a peak is reached at a horizontal displacement of 2.7–3% of the supported backfill height, decreasing thereafter to a residual level. In this test configuration, a triangular failure wedge shape is observed, due to the low mobilized wall-soil friction. Backfill strength parameters are estimated based on this observed failure mechanism. From these estimates, along with triaxial and direct shear test data, theoretical predictions are compared with the measured passive resistance. Using the test data, a calibrated finite-element model is employed to produce additional load-displacement curves for a wider range of practical applications (e.g., potential bridge deck displacement during a strong earthquake). Hyperbolic model approximations of the load-displacement curves are also provided.     

Lateral Resistance of Full-Scale Pile Cap with Gravel Backfill

A static lateral load test was performed on a full-scale 3×3 pile group driven in saturated low-plasticity silts and clays. The steel pipe piles were attached to a concrete pile cap which created a “fixed-head” end constraint. A gravel backfill was compacted in place on the backside of the cap. Lateral resistance was therefore provided by pile–soil–pile interaction, as well as base friction and passive pressure on the cap. In this case, passive resistance contributed about 40% of the total resistance. The log–spiral method provided the best agreement with measured resistance. Estimates of passive pressure computed using the Rankine method significantly underestimated the resistance while the Coulomb method overestimated resistance. The cap movement required to fully mobilize passive resistance in the gravel backfill was about 6% of the cap height. This is somewhat larger than reported in other studies likely due to the underlying clay layer. The p-multipliers developed for the free-head pile group provided reasonable estimates of the pile–soil–pile resistance for the fixed-head pile group once gaps adjacent to the pile were considered.     

Rotational Restraint of Pile Caps during Lateral Loading

A pure fixed-head (zero-rotation) condition at the top of a group of laterally loaded piles is seldom achievable in the field, even when piles are installed in a group that is “rigidly” constrained by a stiff concrete pile cap. Assuming complete fixity during design (zero rotation at the pile head) can result in underestimated values of pile-head deflection, and incorrect estimates of the magnitude and the location of maximum bending moments. A simple and practical approach is presented for estimating the moment restraint that is provided by the pile cap at the top of a pile group. The moment restraint, represented by the rotational restraint coefficient (KMθ), serves as a boundary condition for analyzing groups of laterally loaded piles. Full-scale field tests performed on two pile groups with concrete pile caps show that the proposed method for estimating rotational restraint provides results that are in good agreement with measured field performance.     

Passive Earth Pressure with Critical State Concept

This paper presents experimental data of earth pressure acting against a vertical rigid wall, which moved toward a mass of dry sand. The backfill had been placed in lifts to achieve relative densities of 38, 63, and 80%. The instrumented retaining-wall facility at National Chiao Tung University in Taiwan was used to investigate the effects of soil density on the development of earth pressure. Based on the experimental data, it has been found that the Coulomb and Terzaghi solutions calculated with the peak internal friction angle significantly overestimated the ultimate passive thrust for the retaining wall filled with dense sand. As the wall movement S exceeded 12% of the wall height H, the passive earth thrust would reach a constant value, regardless of the initial density of backfill. Under such a large wall movement, soils along the rupture surface had reached the critical state, and the shearing strength on the surface could be properly represented with the residual internal-friction angle. The ultimate passive earth pressure was successfully estimated by adopting the critical state concept to either Terzaghi or Coulomb theory.     

Increasing the Resistance of Piles Subject to Cyclic Lateral Loading

Small-scale tests were carried out on a monopile and fin piles to determine the effect the length of fins had upon the lateral displacement of cyclically loaded piles. A variety of loading conditions were applied to model piles in a dense sand by using a mechanical loading system. Ten thousand cycles were used in each test to represent 20 years of environmental loading on offshore structures. Variables included the magnitude, frequency, and direction of the load; the type of pile tip; and the length of the fins. The reduction in pile head displacement was used as a measure of the efficiency of the fins. The tests show that the fins reduced the lateral displacement by at least 50% after 10,000 cycles.     

Lateral Behavior of Vertical Pile Group Embedded in Stabilized Earth Slope

An experimental study of the lateral behavior of vertical pile groups embedded in reinforced and nonreinforced sandy earth slopes was carried out. The model tests include studies of group configurations, pile spacing, embedment length of pile, relative densities of sand, and location of pile groups relative to the slope crest. Several configurations of geogrid reinforcement with different lengths, widths, and number of layers were used to reinforce a sandy slope of 1 (V): 1.5 (H). Pile groups of 2×2 and 3×3 along with center-to-center pile spacing of 2D, 3D, and 4.5D and piles with embedment length to diameter ratios of L/D = 12 and 22 were considered. Based on test results, geogrid parameters that give the maximum lateral capacity improvement are presented and discussed.     

Nonlinear Efficiency of Bored Pile Group under Lateral Loading

A 3×3 bored pile group consisting of nine cast-in-drilled-hole reinforced concrete shafts and a comparable single-shaft were subjected to reversed cyclic, lateral head loading to investigate group interaction effects across a wide range of lateral displacements. The piles had the same diameter of d = 0.61?m and similar soil conditions; however, various equipment constraints led to two differences: (1) a fixed head (zero rotation) boundary condition for the single pile versus minor pile cap rotation in the vertical plane for the group and (2) shaft longitudinal reinforcement ratios of 1.8% for the single pile and 1% for the group piles. To enable comparisons between the test results, a calibrated model of the single pile (1.8% reinforcement) was developed and used to simulate the response of a single shaft with 1% reinforcement. Additional simulations of the pile group were performed to evaluate the effects of cap rotation on group response. By comparing the simulated responses for common conditions, i.e., 1% reinforcing ratio and zero head rotation, group efficiencies were found to range from unity at lateral displacements <0.004×d to 0.8 at small displacements ～ 0.01–0.02×d and up to 0.9 at failure (displacements >0.04×d). Hence, we find that group efficiency depends on the level of nonlinearity in the foundation system. The general group efficiency, although not its displacement-dependence, is captured by p-multipliers in the literature for reinforced concrete, fixed-head piles.     

Instability and Complete Failure of Steel Columns Subjected to Cyclic Loading

In a testing system design for large deformations, structural columns were loaded to complete failure, defined as either complete separation of the column or inability to sustain the prescribed axial load. The test system consists of very large stroke quasistatic jacks, digital displacement transducers that can ensure accurate measurement of large deformations, hydraulic pump units capable of controlling the oil flow, controllers that control the jack motion, and separate personal computers for operating the jack controllers and for supervising and measuring data. These components are connected on-line for data and signal operations, which enables automatic and accurate load control for tests that lead specimens to complete failure. Six columns having a square tube cross section are tested in cyclic loading condition, with the axial load and column length as major parameters. The load–deformation relationships obtained from the tests are presented in detail, and relationships among the deformation capacity, failure mode, slenderness, and axial load are discussed. Intermediate axial load (30% of the yield axial load) is effective in retarding the occurrence and growth of cracks, resulting in larger deformation capacity to complete failure. Finite element analysis accurately duplicates the experimental behavior up to a large inelastic range including material yielding, strain hardening, and local buckling. It fails to simulate the experimental behavior in a very large deformation range where the column surfaces crashed and contacted each other. More experimental data is strongly needed for the behavior of structural systems and elements at and near complete failure.     

Contact Interface Model for Shallow Foundations Subjected to Combined Cyclic Loading

It has been recognized that the ductility demands on a superstructure might be reduced by allowing rocking behavior and mobilization of the ultimate capacity of shallow foundations during seismic loading. However, the absence of practical reliable foundation modeling techniques to accurately design foundations with the desired capacity and energy dissipation characteristics and concerns about permanent deformations have hindered the use of nonlinear soil–foundation–structure interaction as a designed mechanism for improving performance of structural systems. This paper presents a new “contact interface model” that has been developed to provide nonlinear relations between cyclic loads and displacements of the footing–soil system during combined cyclic loading (vertical, shear, and moment). The rigid footing and the soil beneath the footing in the zone of influence, considered as a macroelement, are modeled by keeping track of the geometry of the soil surface beneath the footing, along with the kinematics of the footing–soil system, interaction diagrams in vertical, shear, and moment space, and the introduction of a parameter, critical contact area ratio (A/Ac); the ratio of footing area (A) to the footing contact area required to support vertical and shear loads (Ac). Several contact interface model simulations were carried out and the model simulations are compared with centrifuge model test results. Using only six user-defined model input parameters, the contact interface model is capable of capturing the essential features (load capacities, stiffness degradation, energy dissipation, and deformations) of shallow foundations subjected to combined cyclic loading.     

Pore Pressure Generation Models for Sands and Silty Soils Subjected to Cyclic Loading

This paper discusses the applicability of two simple models for predicting pore water pressure generation in nonplastic silty soil during cyclic loading. The first model was developed by Seed et al. in the 1970s and relates the pore pressure generated to the cycle ratio, which is the ratio of the number of applied cycles of loading to the number of cycles required to cause liquefaction. The second model is the Green-Mitchell-Polito model proposed by Green et al. in 2000, which relates pore pressure generation to the energy dissipated within the soil. Based upon the results of approximately 150 cyclic triaxial tests, the writers show that both models are applicable to silty soils. A nonlinear mixed effects model was used for regression analyses to develop correlations for the necessary calibration parameters. The results show that the trends in both α and pseudoenergy capacity calibration parameters for the Seed et al. and Green et al. pore pressure generation models, respectively, differ significantly for soils containing less than and greater than ～ 35% fines, consistent with the limiting fines content concept.     

Behavior of Axially Loaded Pile Groups Driven in Clayey Silt

This paper presents a case history describing measurements made during the installation and load testing of groups of five, closely spaced, precast concrete piles in a soft clay-silt. The test results extend the presently limited set of reported high-quality data for pile groups at field scale and allow assessment of the reliability of existing numerical and analytical predictive approaches. Full scale maintained compression and tension load tests on groups as well as tests on single (reference) piles and an individual test on a pile within a pile group enable the effects of multiple pile installations and interaction between piles under load to be assessed. The results are compared with existing simple methods of pile group analysis and with other case histories reporting results on small pile groups. A simple expression to evaluate pile group stiffness efficiency is proposed.     

Stress Deformation and Fluid Pressure of Bone Specimens under Cyclic Loading

Cyclic loading has been known to induce fluid flow and thus mechanotransduction in bones. In the past, four-point bending tests have been used exclusively in studying fluid flow in bones. In order to better understand the mechanism of deformation and fluid flow under loading, compression tests were done on trabecular bone specimens under drained and undrained conditions. In the drained tests, the volume change was observed, whereas in the undrained tests, excess pore fluid pressure was measured. Cyclic loading tests were conducted in addition to monotonic loading tests to observe the permanent volume change or excess pore fluid pressure with loading cycles. A fast loading rate gave a sharp rise in the excess fluid pressure compared to a slow loading rate. The strength and stiffness of the specimens appeared to deteriorate with an increased speed of loadings, but there was no appreciable difference between the results obtained from drained and undrained tests. The drained and undrained tests as described allowed a better understanding of bone behavior under loadings for a coupled stress-flow analysis.     

Lateral Resistance of Short Single Piles and Pile Groups Located Near Slopes

Many transmission towers, high-rise buildings, and bridges are constructed near steep slopes and are supported by large-diameter piles. These structures may be subjected to large lateral loads, such as violent winds and earthquakes. Widely used types of foundations for these structures are pier foundations, which have large diameter with high stiffness. The behavior of a pier foundation subjected to lateral loads is similar to that of a short rigid pile, because both elements seem to fail by rotation developing passive resistance on opposite faces above and below the rotation point, unlike the behavior of a long flexible pile. This paper describes the results of several numerical studies performed with a three-dimensional finite-element method (FEM) of model tests and a prototype test of a laterally loaded short pile and pier foundation located near slopes, respectively. Initially, in this paper, the results of model tests of single piles and pile groups subjected to lateral loading, in homogeneous sand with 30° slopes and horizontal ground were analyzed by the three- dimensional (3D) finite-element (FE) analyses. Furthermore, field tests of a prototype pier foundation subjected to lateral loading on a 30° slope was reported. The FE analyses were conducted to simulate these results. The main purpose of this paper is the validation of the 3D elasto–plastic FEM by comparisons with the experimental data.     

Modeling the Stress-Strain Relations of Sand in Cyclic Plane Strain Loading

A modeling procedure to simulate stress-strain relations of sand subjected to cyclic loading is proposed. Results from drained plane strain compression, extension, and cyclic loading tests on Toyoura sand are analyzed. The monotonic loading behavior is simulated by the generalized hyperbolic equation to use as the skeleton curves in the simulation of cyclic behavior. To construct hysteretic stress-strain curves based on the skeleton curves, the Masing’s rule is generalized to the proportional rule consisting of the internal and external rules. The drag rule is then introduced to simulate cyclic stress-strain behavior in which the stress amplitude increases at a decreasing rate during cyclic loading with a constant strain amplitude. It is assumed that any plastic shear strain increment taking place in a certain direction drags the whole skeleton curve for loading in the opposite direction towards the direction of the concerned shear strain increment. The measured cyclic stress-strain behavior is well simulated by the proposed method.     

Data Reduction of Horizontal Load Full-Scale Tests on Bored Concrete Piles and Pile Groups

This paper proposes a new approach for data reduction of horizontal load full-scale tests on piles and pile groups. This approach has been developed on results from tests run on bored concrete piles embedded in homogeneous and nonhomogeneous ground. Due to nonlinear response of pile material and also to nonhomogeneous embedding ground, the problem of fitting reliable curves for representing strains along shafts is increased. It is suggested that B-splines fixed by a weighted least-squares algorithm should be used to overcome that problem. Taking advantage of the mathematical properties of B-splines, an algorithm for computing the internal force distribution amongst pile heads direct from test results is also proposed for pile groups. It is shown that the integration of the curvatures to compute pile movements should be done using natural boundary conditions instead of pile head measurements whenever possible. Despite the concrete crack, the distribution of bending moments can be computed from curvatures provided a reliable reinforced concrete model is used. Finally, it is proposed to compute the soil reactions by the integration of bending moments, solving an integral equation by again using B-spline functions.     

Experimental and Numerical Study of Railway Ballast Behavior under Cyclic Loading

This paper presents the results of the influence of frequency on the permanent deformation and degradation behavior of ballast during cyclic loading. The behavior of ballast under numerous cycles was investigated through a series of large-scale cyclic triaxial tests. The tests were conducted at frequencies ranging from 10–40 Hz, which is equivalent to a train traveling from 73 km/h to 291 km/h over standard gauge tracks in Australia. The results showed that permanent deformation and degradation of ballast increased with the frequency of loading and number of cycles. Much of breakage occurs during the initial cycle; however, there exists a frequency zone of 20?Hz ? f ? 30?Hz where cyclic densification takes place without much additional breakage. An empirical relationship among axial strain, frequency and number of cycles has been proposed based on the experimental data. In addition, discrete-element method (DEM) simulations were carried out using PFC2D on an assembly of irregular shaped particles. A novel approach was used to model a two-dimensional (2D) projection of real ballast particles. Clusters of bonded circular particles were used to model a 2D projection of angular ballast particles. Degradation of the bonds within a cluster was considered to represent particle breakage. The results of DEM simulations captured the ballast behavior under cyclic loading in accordance with the experimental observations. Moreover, the evolution of micromechanical parameters such as a distribution of the contact force and bond force developed during cyclic loading was presented to explain the mechanism of particle breakage. It has been revealed that particle breakage is mainly due to the tensile stress developed during cyclic loading and is located mainly in the direction of the movement of ballast particles.     

Employment of the P-Multiplier in Pile-Group Analysis

This paper discusses the variation of the P-multiplier (Pm) used with the p-y curve to assess the response of a pile group under lateral loads, which is a crucial topic for the design of bridge pile foundations. Pm is influenced by the site geotechnical conditions (i.e., soil profile, type and properties), pile front and side spacings, and pile-group deflection. The presented study shows the needs to incorporate these factors with the recommended sets of Pm to avoid any compromise or uncertainty when Pm is treated as a single (unique) value based only on pile spacings. The current study addresses these influential elements using the strain wedge (SW) model technique, suggested Pm values, and data collected from full-scale pile-group load tests. The experimental results show that Pm is not unique and must be assessed based on the site geotechnical conditions along with the pile-row front and side spacings. Because the employed Pm values must be a function of these influential factors, additional full- and model-scale load tests with different pile spacings and soil types might be required. The paper also emphasizes that using other techniques, such as the SW model, in addition to the P-multiplier could increase the confidence in the predicted pile-group lateral response.     

Earth Pressure due to Vibratory Compaction

This paper presents experimental data on the variation of lateral earth pressure against a nonyielding retaining wall due to soil filling and vibratory compaction. Air-dry Ottawa sand was placed in five lifts and each lift was compacted to achieve a relative density of 75%. Each compacted lift was 0.3?m thick. The instrumented nonyielding wall facility at National Chiao Tung University in Taiwan was used to investigate the effects of vibratory compaction on the change of stresses at the soil-wall interface. Based on the experimental data it has been found that, for a compacted backfill, the vertical overburden pressure can also be properly estimated with the traditional equation σv = γz. The effects of vibratory compaction on the vertical pressure in the backfill were insignificant. On the vertical nonyielding wall, extra horizontal earth pressure was induced by vibratory compaction. After compaction, the lateral earth pressure measured near the top of the wall was almost identical to the passive Rankine pressure. It is concluded that as the cyclic compacting stress applied on the surface of the backfill exceeded the ultimate bearing capacity of the foundation soil, a shear failure zone would develop in the uppermost layer of the backfill. For a soil element under lateral compression, the vertical overburden pressure remained unchanged, and the horizontal stress increased to the Rankine passive pressure. It was also found that the compaction-influenced zone rose with the rising compaction surface. The horizontal earth pressure measured below the compaction-influenced zone converged to the Jaky state of stress.     

Influence of Underlying Weak Soil on Passive Earth Pressure in Cohesionless Deposits

Finite-element simulations demonstrate the influence of underlying weak soil on mobilization of passive pressures in cohesionless deposits. Traditional passive earth pressure theories with typical angles of interface friction may overestimate passive forces in such cases. Simple analytical models that incorporate the underlying weak soil using traditional passive earth pressure concepts are shown to agree reasonably with the finite-element simulations. The studies presented herein are relevant for cases in which cohesionless soil deposits overlie soft clay, liquefiable sand, or other weak layers.