Simplified Approach for the Seismic Response of a Pile Foundation

Pseudostatic approaches for the seismic analysis of pile foundations are attractive for practicing engineers because they are simple when compared to difficult and more complex dynamic analyses. To evaluate the internal response of piles subjected to earthquake loading, a simplified approach based on the “p-y” subgrade reaction method has been developed. The method involves two main steps: first, a site response analysis is carried out to obtain the free-field ground displacements along the pile. Next, a static load analysis is carried out for the pile, subjected to the computed free-field ground displacements and the static loading at the pile head. A pseudostatic push over analysis is adopted to simulate the behavior of piles subjected to both lateral soil movements and static loadings at the pile head. The single pile or the pile group interact with the surrounding soil by means of hyperbolic p-y curves. The solution derived first for the single pile, was extended to the case of a pile group by empirical multipliers, which account for reduced resistance and stiffness due to pile-soil-pile interaction. Numerical results obtained by the proposed simplified approach were compared with experimental and numerical results reported in literature. It has been shown that this procedure can be used successfully for determining the response of a pile foundation to “inertial” loading caused by the lateral forces imposed on the superstructure and “kinematic” loading caused by the ground movements developed during an earthquake.     

Pile Responses Caused by Tunneling

In this paper, a two-stage approach is used to analyze the lateral and axial responses of piles caused by tunneling. First, free-field soil movements are estimated based on an analytical method, and, second, these estimated soil movements are imposed on the pile in simplified boundary element analyses to compute the pile responses. Through a parametric study, it is shown that the influence of tunneling on pile response depends on a number of factors, including tunnel geometry, ground loss ratio, soil strength, pile diameter, and ratio of pile length to tunnel cover depth. Simple design charts are presented for estimating maximum pile responses and may be used in practice to assess the behavior of existing piles adjacent to tunneling operations. A published case history has been studied in which the measured lateral pile deflections are compared with those computed using the present method and fair agreement is found.     

Behavior of Slender Piles Subject to Free-Field Lateral Soil Movement

Soil movements associated with slope instability induce shear forces and bending moments in stabilizing piles that vary with the buildup of passive pile resistance. For such free-field lateral soil movements, stress development along the pile element is a function of the relative displacement between the soil and the pile. To investigate the effects of relative soil-pile displacement on pile response, large-scale load tests were performed on relatively slender, drilled, composite pile elements (cementitious grout with centered steel reinforcing bar). The piles were installed through a shear box into stable soil and then loaded by lateral translation of the shear box. The load tests included two pile diameters (nominal 115 and 178?mm) and three cohesive soil types (loess, glacial till, and weathered shale). Instrumentation indicated the relative soil-pile displacements and the pile response to the loads that developed along the piles. Using the experimental results, an analysis approach was evaluated using soil p-y curves derived from laboratory undrained shear strength tests. The test piles and analyses helped characterize behavioral stages of the composite pile elements at loads up to pile section failure and also provided a unique dataset to evaluate the lateral response analysis method for its applicability to slender piles.     

Combined Finite- and Boundary-Element Analysis of the Effects of Tunneling on Single Piles

An efficient and practical method of analysis to predict the effects of tunneling on existing single pile foundations is described. The method involves a combination of the finite- and boundary-element (FAB) methods, with free-field ground movements predicted by the finite-element method and the response of an embedded pile to these ground movements predicted by the boundary-element method. The method allows prediction of the full three-dimensional (3D) response of the pile as tunnel excavation proceeds towards the pile and away from it. Very good agreement is obtained between predictions of the pile response obtained by the FAB method and a 3D finite-element analysis which specifically includes the pile in the finite-element mesh. The vastly superior computational efficiency of the FAB method over the full 3D finite element approach is also illustrated.     

Soil-Pile Response to Blast-Induced Lateral Spreading. II: Analysis and Assessment of the p–y Method

This paper presents an assessment of the potential of using the p–y analysis method for single piles and pile groups subjected to lateral spreading. The computed responses were compared with the results from the full-scale lateral spreading tests in Japan as presented in the Part I companion paper. The responses of the single piles subjected to lateral spreading were determined by imposing the known free-field soil movement profile to the Winkler spring model. The soil springs of nonliquefied soils used in this study were based upon standard p–y springs whereas zero spring stiffness was used for liquefied soils. For the case of pile groups, they were modeled as an equivalent single pile with a rotational spring at the pile head to simulate effect of pile head restraint. A decrease of soil spring stiffnesses using the p-multiplier approach was used to account for pile group effects. Based on the results of analyses, the computed responses of all sets of the test piles using a single set of baseline soil properties were in good agreement with the measured responses. These results suggest that the p–y analysis method may be used to estimate the behavior of piles subjected to lateral spreading.     

Lateral Loaded Pile Response in Liquefiable Soil

This paper provides a new analysis procedure for assessing the lateral response of an isolated pile in saturated sands as liquefaction develops in response to dynamic loading such as that generated during earthquake shaking. This new procedure predicts the degradation in pile response and soil resistance due to the free-field excess porewater pressure generated by the earthquake, along with the near-field excess porewater pressure generated by lateral loading from the superstructure. The new procedure involves the integration of the developing (free- and near-field) porewater pressure in the strain wedge (SW) model analysis. The current SW model, developed to evaluate drained response (a nonlinear three-dimensional model) of a flexible pile in soil, has been extended in this paper to incorporate the undrained response of a laterally loaded pile in liquefied sand. This new procedure has the capability of predicting the response of a laterally loaded isolated pile and the associated modulus of subgrade reaction (i.e., the p–y curve) in a mobilized fashion as a result of developing liquefaction in the sand. Current design procedures assume slight or no resistance for the lateral movement of the pile in the liquefied soil which is a conservative practice. Alternatively, if liquefaction is assessed not to occur, some practitioners take no account of the increased free-field porewater pressure, and none consider the additional near-field porewater pressure due to inertial interaction loading from the superstructure; a practice that is unsafe in loose sands.     

Pile Response Adjacent to Braced Excavation

The excavation of soil for the construction of basements or cut-and-cover tunnels results in ground movements. One particular concern is that the excavation-induced lateral soil movements may adversely affect any nearby pile foundation. The lateral loads imposed by the soil movements induce bending moments and deflections in the pile, which may lead to structural distress and failure. This paper presents the results of an actual full-scale instrumented study that was carried to examine the behavior of an existing pile due to nearby excavation activities resulting from the construction of a 16 m deep cut-and-cover tunnel. The pile was located 3 m behind a 0.8 m thick diaphragm wall. Excavation to the formation level that was 16 m below the ground surface resulted in a maximum lateral pile movement of 28 mm. A simplified numerical procedure based on the finite-element method was used to analyze the pile response. Generally, the theoretical predictions were in reasonable agreement with the measured results.     

Three-Dimensional Nonlinear Seismic Analysis of Single Piles Using Finite Element Model: Effects of Plasticity of Soil

Much of the reported research on the dynamic analysis of pile foundations assumes linear behavior of soil that may not be valid for strong excitations. In this paper, material nonlinearity of the soil caused by plasticity and work hardening is considered in the dynamic analysis of pile foundations. An advanced plasticity based soil model, HiSS, is incorporated in a finite element technique. To simulate radiation effects, proper boundary conditions are used. The model and algorithm are verified with analytical results that are available for elastic and elastoplastic soil models. Analyses are carried out for free-field response and pile head response of end-bearing single piles. Both harmonic and transient excitations are considered in the analyses. Effects of frequency of excitation and stiffness of soil are investigated. It was found that the nonlinearity of soil has significant effects on the pile response for lower and moderate frequencies of excitations (a0<0.6) while at higher frequencies its effects are not as significant.     

Soil–Pile Response to Blast-Induced Lateral Spreading. I: Field Test

Two full-scale experiments using controlled blasting were conducted in the Port of Tokachi on Hokkaido Island, Japan, to assess the behavior of a single pile, a four-pile group, and a nine-pile group subjected to lateral spreading. The test piles were extensively instrumented with strain gauges to measure the distribution of bending moment during lateral spreading which allowed the backcalculation of the loading conditions, as well as the assessment of damage and performance of the piles. Based on the test results, it was concluded that using controlled blasting successfully liquefied the soil, and subsequently induced lateral spreading in the 4–6% surface slope test beds. The free-field soil displacements in the vicinity of the test piles were over 40 cm for both tests. When compared with the results from the single pile case, the effect of pile head restraint from the pile cap improved overall pile performance by decreasing the displacement of the pile groups and lowering the maximum moments in individual piles within each group. Finally, backcalculated soil reactions indicated that the liquefied soil layer imparted insignificant force to the piles. In the companion to this paper (Part II), an assessment of the potential of using the p–y analysis method for single piles and pile groups subjected to lateral spreading is presented.     

Pile Behavior due to Excavation-Induced Soil Movement in Clay. I: Stable Wall

A series of centrifuge model tests has been conducted to investigate the behavior of a single pile subjected to excavation-induced soil movements behind a stable retaining wall in clay. The results reveal that after the completion of soil excavation, the wall and the soil continue to move and such movement induces further bending moment and deflection on an adjacent pile. For a pile located within 3?m behind the wall where the soil experiences large shear strain (>2%) due to stress relief as a result of the excavation, the induced pile bending moment and deflection reach their maximum values sometime after soil excavation and thereafter decrease slightly with time. For a pile located 3?m beyond the wall, the induced pile bending moment and deflection continue to increase slightly with time after excavation until the end of the test. A numerical model developed at the National University of Singapore is used to back-analyze the centrifuge test data. The method gives a reasonably good prediction of the induced bending moment and deflection on a pile located at 3?m or beyond the wall. For a pile located at 1?m behind the wall where the soil experiences large shear strain (>2%) due to stress relief resulting from the excavation, the calculated pile response is in good agreement with the measured data if the correct soil shear strength obtained from postexcavation is used in the analysis. However, if the original soil shear strength prior to excavation is used in the analysis, this leads to an overestimation of the maximum bending moment of about 25%. The practical implications of the findings are also discussed in this paper.     

Prediction of Ground Movements due to Pile Driving in Clay

This paper evaluates theoretical predictions of ground movements caused by the installation of driven (or jacked) piles in clay. The predictions are based on an approximate analysis framework referred to as the shallow strain path method that simulates undrained pile penetration from the stress-free ground surface. Large strain conditions close to the pile are solved numerically, and closed-form analytical expressions are obtained from small strain approximations at points further away. These results show that, for closed-ended cylindrical piles of radius R and embedment L, the normalized displacements δL∕R2 are functions of their dimensionless position x∕L. In contrast, for a planar sheet pile or unplugged open-ended pile, the far-field soil displacements at x∕L depend only on the wall thickness w; i.e., δ∕w = f[x∕L]. The proposed analyses show favorable agreement with data from a variety of available sources including field measurements of (1) building movements caused by installation of large pile groups; (2) uplift of a pile caused by driving of an adjacent pile within a group; and (3) spatial distributions of ground movements caused by installation of a single pile (both cylindrical closed-ended and sheet pile wall), including a particularly detailed set of measurements in a large laboratory calibration chamber. The comparisons show that the proposed analysis is capable of reliably predicting the deformations within the soil mass but generally underestimates the vertical heave measured at the ground surface. Further investigation suggests that this discrepancy may be related to the occurrence of radial cracks observed at the ground surface during pile installation and is consistent with tensile horizontal strains computed in the shallow strain path method analyses.     

Kinematic Pile Response to Vertical P-wave Seismic Excitation

An analytical solution based on a rod-on-dynamic-Winkler-foundation model is developed for the response of piles in a soil layer subjected to vertical seismic excitation consisting of harmonic compressional waves. Closed-form solutions are derived for: (1) the motion of the pile head; (2) the peak normal strain in the pile, and (3) the group effect between neighboring piles. The solutions are expressed in terms of a dimensionless kinematic response factor Iv, relating pile-head motion and free-field soil surface motion, a dimensionless strain transmissibility factor Iε, relating pile and soil peak normal strains, and a pile-to-pile interaction factor α measuring group effects. It is shown that a pile foundation may significantly reduce the vertical seismic excitation transmitted to the base of a structure.     

Interaction of Shallow Foundations with Reverse Faults

A series of centrifuge model tests is reported that investigated the effects of foundation position on the interaction of reverse, dip-slip faults with shallow foundations resting on sand. The model tests have allowed careful examination of both the soil and foundation deformation as a shear localization (fault) propagates through a 15?m thick sand layer for fault throws up to 5?m. By comparing results of the tests with foundations present with those from a “free-field” test, the effect of the foundation on the faulting pattern has been observed directly. The response of the foundation is very sensitive to the exact position of the fault and even when the fault emerged remotely from the foundation it sometimes caused significant foundation movements. Detailed results are presented for the tests and it is suggested that these results are used as: (1) indications of likely foundation–soil–fault interaction mechanisms; and (2) to allow future validation of numerical models for similar problems. Finally, foundation rotations measured during the fault–foundation interaction tests are compared to those predicted using a simple method based on free-field soil displacements. This simple method makes surprisingly good prediction of maximum fault rotations for different throws.     

Predicting RC Frame Response to Excavation-Induced Settlement

In many tunneling and excavation projects, free-field vertical ground movements have been used to predict subsidence, and empirical limits have been employed to evaluate risk. Validity of such approaches is largely unknown given that ground movements are in fact not one-dimensional and that adjacent applied loads are known to have an impact. This paper employed analytical and large-scale experimental efforts to quantify these issues, in the case of excavation adjacent to a reinforced concrete frame with tieback anchors and a sheetpile wall in dry sand. With this flexible system, a disproportionate amount of the soil and building movements occurred prior to installation of the first tieback, even when conservative construction practices were applied. Furthermore, free-field data generated a trough as little as one-half the size of that recorded near the building frames. Empirically based relative gradient limits generally matched the extent and distribution of the damage, while the application of various structural limits did not fully identify local damage distribution but did generally reflect global response. The use of fully free-field data or a failure to include lateral soil displacements both underpredicted building displacements by as much as 50% for low-rise concrete frames without grade beams on sand.     

Reduction Factor for the Unloading Point Method at Clay Soil Sites

Full-scale testing can be an integral component of quality control/quality assurance for projects involving construction of deep foundations. Rapid load tests are being used in the deep foundation industry as a method for assessing the axial static behavior of deep foundations. Since rapid load tests involve dynamics, inertial and damping forces must be considered in analyzing measured pile response to estimate the static pile response. The unloading point method (UPM) is typically used for this purpose. Generally considered a consequence of load rate effects in clays, results from the UPM must be further modified by a reduction factor to obtain a reasonable estimate of the static pile response. A reduction factor of 0.65 applied to the UPM for clay soil sites has been recommended by others. However, a review and analysis of readily available literature reporting static and rapid pile load test results at sites predominantly consisting of clay soils indicate that an average reduction factor of 0.47 is more appropriate. Rapid load testing should be used judiciously. When using the UPM to estimate static pile capacity from rapid load tests in clay, static load tests should be performed to validate the reduction factor used to interpret rapid load tests.     

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.     

Pile-Soil Interaction in Expansive Soil Foundation: Analytical Solution and Numerical Simulation

Pile-soil interaction in an expansive soil foundation has been primarily analyzed by an analytical solution in our previous study. As a complementary work, this paper briefly reviews the derivation process in more detail and extends the capability of the solution in dealing with the pile-soil interaction issue under various conditions. The results show that (1)?increasing pile length is able to effectively decrease the upward pile movements, but meanwhile, the tensile stresses along the pile shaft increase as well; (2)?a critical pile diameter exists (d ≈ 0.04L); (3)?when the diameter is less than this value, the upward pile movements can be reasonably reduced, but for the pile with a larger diameter, it has little effect on reducing the upward pile movements, and as irrigation proceeds, both the displacements and tensile stresses increase and both tend to be much less sensitive to the water’s incoming over time; and (4)?a critical applied pressure for preventing the upward pile movements that is induced by soil’s swelling pressure is found to be 2.5?times of the maximum tensile stress of the free pile only under swelling pressure in expansive soil foundation.     

Seismic Earth Pressure on Retaining Structures

A simple kinematic method to predict the seismic earth pressure against retaining structures is developed. The fundamental solution to the free-field seismic response considering nonlinear, plastic behavior of soil is included in the retaining wall analysis for the first time. Perturbation to the free-field response caused by soil-structure interaction effects for different types of wall movement is considered. Results from this kinematic method are compared with those obtained from finite-element analysis and observed from laboratory shaking table tests performed on model retaining walls.     

Free-Field Measurements to Disclose Lateral Reaction Mechanism of Piles Subjected to Soil Movements

Soil-pile interaction remains to be the most ambiguous yet one of the most crucial aspects in the design of laterally loaded soil-pile systems subjected to embankment-induced movements. This paper proposes a new method that is capable of producing soil stiffness degradation curves, which are the outcome of real field behavior through free-field measurements. Soil-pile interaction mechanism can be solved with the proposed method for any possible case either the piles are constructed before the embankment construction or during and after. For any time considered, the method enables the computation of resultant stress effects on the pile cross section and the accompanying deflections. To provide a basis of comparison, an example problem has been solved with the proposed method and with two well-known commercial finite-element softwares. Obtained results indicated the capability of the proposed method to disclose real field behavior, which can be attributed to its inherent property of being also an observational method.     

Observed Seismic Lateral Resistance of Liquefying Sand

This paper presents results from a study of the dynamic response of pile foundations in liquefying sand during seismic loading. The study included a series of dynamic centrifuge tests of pile-supported structures and the back-calculation of time histories for the lateral resistance p and relative displacement y between a pile and the free-field soil. Details of the centrifuge experiments and the procedures used to back-calculate p and y time histories are described. The back-calculated p-y time histories provide a concise representation of the experimental results and can be compared to the equivalent p-y behavior predicted by soil-pile interaction analysis methods. The observed p-y behavior provides insight into the mechanisms of soil-pile interaction in liquefying sand, showing characteristics that are consistent with the undrained cyclic loading behavior of saturated sand, including the effects of relative density, cyclic degradation, pore-pressure generation, prior displacement (strain) history, and phase transformation behavior.