Axial Pile Response Research Articles

Analytical 3D failure envelopes for RC pile groups under combined loading: A generalised design approach

The paper presents a generalised design approach for pile groups subjected to combined (VHM) loading. The Limit Equilibrium method is extended to pile groups under general planar loading, considering the axial load dependency of bending stiffness and strength of reinforced concrete (RC) piles. 3D failure envelopes are analytically derived for different layouts. The key simplifying assumptions are critically discussed, and the failure envelopes are verified against finite element (FE) analysis results. Key features of pile group response, such as plunging vs. non-plunging axial pile response and the role of transverse reinforcement, are identified and quantified. A key novelty of the proposed method lies on the consideration of coupling between the lateral–bending resistance of the piles. The method further incorporates well-recognised features of pile group response regarding the axial load dependency of bending moment capacity and stiffness of the RC piles. The latter results in redistribution of bending moments and shear forces along the piles of the group. These features allow the reproduction of key aspects observed in demanding FE simulations and centrifuge tests, regarding the ultimate resistance under VHM and in addition the load path dependent failure modes. The proposed method offers a practical engineering tool for preliminary design and optimisation.

Variation in axial load distribution of piles in liquefiable slope by centrifuge test

During seismic loading, the axial pile response is significantly affected, due to the development and dissipation of excess pore pressure in the foundation soil. Positive skin friction is reduced by liquefaction during shaking, while negative skin friction is caused by reconsolidation after the end of the shaking. In particular, the negative skin friction causes a large drag load, resulting in pile compression and settlement. Thus, this study investigated the axial pile response under seismic loading by performing dynamic centrifuge tests on liquefied sand with the slopes of 15° and 27°. The centrifuge tests were conducted to simulate the end-bearing single and 2 × 2 group piles embedded in a liquefied slope. During the centrifuge spinning, the obtained drag load due to the negative skin friction agreed with the value calculated with the beta method for shaft resistance. The model was shaken with sinusoidal base motion with peak amplitudes of 0.1 and 0.2 g, and it was found that the liquefaction caused a loss of spin-induced drag load. The dissipation of excess pore pressure resulted in reconsolidation settlement, leading to large drag loads acting on the pile. In addition, the negative skin friction was found to be larger than the liquefied residual strength recommended in the current practice codes. As a result, it is seen that the consideration of the strength reduction is not necessary for the calculation of negative skin friction induced by reconsolidation, since the liquefied ground regains skin friction after the dissipation.

Lateral response of pile due to combined load under free and fixed conditions

Pile foundations are used to support both vertical and horizontal loads in many geotechnical projects, such as coastal and offshore engineering. In this project, the Finite Difference Method is proposed to solve the differential equation governing the lateral and axial pile response. Initially, the behaviour of the pile subjected to lateral load will be analysed. The effect of various parameters like pile head fixity, the cohesion of surrounding soil, pile diameter, and length of the pile on lateral pile response will be analysed. Finally with these conditions, the deflections profile of the pile subjected to both lateral and axial load is investigated. By using python code we can easily find out the increase in diameter of pile, cohesion of surrounding soil effect on pile head and effect of increase in combined load will be studied. The above stated parameters will be studied for combined loading also under the free and fixed head conditions.

Axially loaded piles in inundated collapsible soils under compression and tension forces

The paper studies the behavior of axially loaded piles driven into inundated collapsible soils under compression and tension forces. Collapsible soils exhibit a considerable drop in the void ratio (i.e., significant changes in dry unit weight, friction angle, and shear strength) with the increase of water content. Several sandy and silty specimens of collapsible soils with different initial conditions are utilized to develop a correlation that evaluates the post-inundation soil properties and stress–strain relationship (as obtained from the conventional triaxial test). The developed model is integrated into a pile–soil model to predict the axial response of piles loaded after the inundation of the collapsible soils (i.e., no negative skin friction). The developed technique allows the assessment of the axial load transfer (t-z) and pile-head load–displacement curves. Comparisons between predicted and measured stress–strain curves of inundated collapsible soils and responses of axially loaded piles in such soils are presented for validation.

Plasticity Model for Cyclic Axial Pile Response with Back-Analyses of West Delta Pile Tests

A two-surface bounding surface plasticity model for the undrained axial pile response in fine-grained soils is described and used in the back-analysis of full-scale pile tests using the empirical one-dimensional t-z spring method to represent the axial soil resistance. A damage mechanics component incorporates degradation of resistance as a result of accumulated plastic displacement. The computed pile shaft-resistance damage along the pile allows estimation of its evolving capacity. The model produces evolving local cyclic and average resistances mobilized along the pile that are consistent with the observed test responses. The model is used to investigate the cyclic capacity of the same pile conditions under combinations of mean and variable loads. The implications of the test data and modeling results on the design of tension piles are discussed.

Axial pile response of bidirectional O-cell loading from modified analytical elastic solution and downhole shear wave velocity

The Osterberg cell (O-cell) type of bidirectional pile load testing is a modern full-scale proofing method in the realm of performance-based pile design. It is done at considerable cost, not possible on small- to medium-size projects. An economical approach of utilizing the flexible and approximate analytical solution proposed by Randolph has frequently been adopted in the past for evaluating pile settlements under static, unidirectional, top-down axial compression loading. To extend this solution for O-cell loadings, the following adaptations are warranted: (i) appropriate modifications to handle the loadings in two directions and (ii) development of a nonlinear stiffness reduction model, derived from the back-analysis of O-cell pile load tests. Accordingly, a modified analytical solution is presented for the two common cases of O-cell loading arrangements. Using these modified sets of solutions and a well-documented database of O-cell load tests on drilled shaft foundations from different sites, two stiffness reduction models have been developed. The shear wave velocity readings obtained from the hybrid geophysical–geotechnical seismic piezocone tests afford the evaluation of fundamental shear stiffness modulus (Gmax) profiles. These profiles together with the re-arranged modified solution were applied to the axial loads versus displacements (Q–w) from the database of load tests to back-calculate the applicable operational shear stiffness (G) values. Additional sensitivity analyses indicate that pile geometry and soil stiffness profile are the two most significant factors affecting the outcome of this solution. A comprehensive set of step-by-step example calculations is included to explain the procedure for implementing the solution.

Numerical evaluation of effects of nonlinear lateral pile vibrations on nonlinear axial response of pile shaft

Axial and lateral dynamic pile analyses are generally handled separately; and consequently, dynamic soil reactions are assumed to be uncoupled. However, pure loading is rarely encountered as combined loading occurs in many situations (offshore piles, pile driving as well as pile groups and pile rafts). In this study, the effects of nonlinear lateral pile vibrations on the in-phase nonlinear axial pile response of a pile shaft are studied. New approximate nonlinear solutions for both axial and lateral pile behavior, developed from general elastodynamic equations, are presented. The solutions are obtained by extending the elastodynamic solution for plane strain cases with a view to model soil nonlinearity. Since axial soil resistance depends on the confining stress around the pile shaft, the effect of the lateral soil behavior on the confining pressure of the pile circumference is investigated and the axial soil reaction from coupled in-phase vibrations is derived. It is concluded that the axial unit shear strength significantly increases when lateral soil vibrations involve plasticity, which in turn results in an increase in the axial dynamic resistance of the pile shaft.

Response of piles to inclined uplift loads

This paper presents a three-dimensional finite element analysis of the response of piles to inclined uplift loads. It deals with the influence of the soil-pile interface on this response. Calculations are carried out for several load’s inclinations and different conditions at the soil-pile interface. The concept of the “critical depth” at the soil-pile interface is investigated. Analyses show that the lateral behaviour of the pile is independent from both the load’s inclination and the interface condition, while both the load’s inclination and the soil-pile interface largely affect the axial response of piles.

Response of piles to inclined uplift loads Influence of the soil-pile interface

This paper presents a three-dimensional finite element analysis of the response of piles to inclined uplift loads. It deals with the influence of the soil-pile interface on this response. Calculations are carried out for several load’s inclinations and different conditions at the soil-pile interface. The concept of the “critical depth” at the soil-pile interface is investigated. Analyses show that the lateral behaviour of the pile is independent from both the load’s inclination and the interface condition, while both the load’s inclination and the soil-pile interface largely affect the axial response of piles.

Axial response of piles in electrically treated clay

Improvement of the shear strength of soft clayey soils around steel pipe piles using high-voltage electrokinetics is investigated in the present study. The experimental setup of a large-scale testing facility is described. Four model piles were installed in two identical cylinders filled with simulated marine sediment. Five electrically insulated electrodes were installed close to the piles to apply a high-voltage electric field in the test cylinder. Negative direct current voltages of -20, -30, and -10 kV were applied in three phases, respectively, for 33 days in the treatment cylinder. Axial compression and pullout pile load tests were performed and the results were compared for both cylinders after each phase of treatment. The pile response is presented in terms of the experimental load deflection curves. It is observed that the axial capacity was increased 30, 29, and 8% after the first, second, and third treatment phases, respectively. The pullout capacity was increased due to the treatment by 11, 23, and 12% after the first, second, and third treatment phases, respectively. Further development of this technique may provide potential solutions for the improvement of soft marine clays, and ultimately it could be applied in the field to rehabilitate existing offshore foundations.Key words: electrokinetics, piles, marine clays, soil improvement, bearing capacity, axial loading.

Axial response of piles in electrically treated clay

Axial response of piles in electrically treated clay

Approximate non-linear dynamic axial response of piles

The axial dynamic response of a single pile in clay is examined within the framework of a Winkler—type approach. Experimental data for the variation of soil shear modulus and hysteretic damping with the amplitude of shear strain and the soil 'plasticity' index are curve—fitted and utilized in simulating in a realistic way the non—linearity of the surrounding soil arising from the stresses induced by the pile. An equivalent radially—inhomogeneous but linearly—hysteretic continuum is established, and the 'springs' and 'dashpots' of the dynamic Winkler model are obtained at each depth by solving analytically the pertinent plane—strain (in z) and axisymmetric (in r) elastodynamic problem. Slippage at the pile–soil interface is also taken into consideration in a simple approximate way. Analytical results are presented to demonstrate the practical significance of soil non—linearity, soil 'plasticity' index and interface slippage in the dynamic stiffness and damping of the pile. Les auteurs examinent la réaction dynamique axiale d'un pieu dans Pargile dans le contexte d'une approche du type Winkler. Aprés I'ajustement de courbe, les données expérimentales obtenues pour la variation du module de cisaillement et de Pamortissement hystérétique en fonction de la déformation de cisaillement et de I'indice de plasticité du sol sont utilisées pour simuler de faccedil;on réaliste la non—linéarité du sol environnant sous I'effet des tensions engendrées par le pieu. Un continuum équivalent, non homogéne radialement, mais hystérétique liné airement, est établi, et les « ressorts» et « amortisseurs» du modéle dynamique de Winkler sont obtenus à chaque profondeur par résolution analytique du problème élastodynamique de la déformation plane (en z) et axisymétrique (en r). On dent également compte, de façon approximative et simple, du glissement à I'inter—face pieu—sol. Les résultats de I'analyse sont présentés pour démontrer les effets pratiques de la nonlinéarité du sol, de I'indice de plasticité du sol et du glissement interfacial sur la rigidity dynamique et I'amortissement du pieu.

Vertically loaded piles in non-homogeneous media

SUMMARY Analytical methods for the axial responses of piles can be classified under three broad categories of (1) simple but approximate analytical solutions, (2) one-dimensional numerical algorithms, (3) full axisymmetric analyses using boundary or finite element approaches. The first two categories rely on the so-called load transfer approach, with interaction between pile and soil determined by independent springs distributed along the pile shaft and at the pile base. The non-linear spring sti⁄ness is related to the elastic—plastic properties of the actual soil partly by empirically based correlations and partly by theoretical arguments based on simplified models of the pile—soil system. This paper presents new closed-form solutions for the axial response of piles in elastic—plastic, non-homogeneous, media. The solutions fall in the first of the three categories above, and have been verified through extensive parametric studies using more rigorous onedimensional and continuum analyses. The e⁄ect of non-homogeneity and partial slip on the load and displacement profiles along the pile shaft is explored, and comparisons are presented with experimental data. ( 1997 by John Wiley & Sons, Ltd.

Nonlinear model for dynamic axial pile response : M.H. El Naggar /s& M. Novak, Journal of Geotechnical Engineering — ASCE, 120(2), 1994, pp 308–329

Nonlinear model for dynamic axial pile response : M.H. El Naggar /s& M. Novak, Journal of Geotechnical Engineering — ASCE, 120(2), 1994, pp 308–329

Effects of earthquakes on settlements and axial pile response in clays: Poulos, H G Trans Inst Engrs Aust Civ EngngVCE35, N1, March 1993, P43–48

Effects of earthquakes on settlements and axial pile response in clays: Poulos, H G Trans Inst Engrs Aust Civ EngngVCE35, N1, March 1993, P43–48

Non‐Linear Model for Dynamic Axial Pile Response

An analysis of pile axial response to transient dynamic loading and to harmonic loading is presented allowing for nonlinear soil behavior, energy dissipation through radiation damping, soil hysteresis, and the loading rate dependency of the soil resistance. Also, the postpeak soil resistance is modeled for certain soil types, and the effect of neighboring piles is taken into account for piles in a group. The approach is used to analyze the pile response to harmonic forces, pile driving, and Statnamic loading and to establish the bearing capacity and load‐deflection curves; nonlinear dynamic interaction factors are also presented.

Interaction of Axial and Lateral Pile Responses

A series of tests were carried out on instrumented model piles to investigate the effect of interaction on the axial and lateral responses of piles in clay. The effect of lateral loading proved to be significant on the axial pile response, while the effects of axial loading on the lateral pile response were rather limited. Lateral loads increased considerably the axial displacement of the pile head. The amount of this increase depended on the value of the applied lateral load and on the level of the axial load. At low load levels where the soil behavior is linearly elastic, the increase in pile settlement due to a lateral load is very small. The explanation of these interaction effects was attempted through a nonlinear finite‐element analysis, in which the yielded zone created, in the soil, by the combined axial and lateral loading is larger than that developed by pure axial loads.

Cyclic analysis of axially loaded piles in calcareous soils

This paper describes a simple nonlinear pile-soil interface model incorporated into a modified boundary element analysis to simulate the behaviour of piles in calcareous soils subjected to both static and cyclic loading. A shaft resistance degradation model and a cyclic secant soil modulus degradation model are proposed, and implemented in the nonlinear analysis. Parametric solutions are presented which examine the overall characteristics of axial pile response determined from the nonlinear analysis. Comparisons are made between the theoretical predictions and the measured results of laboratory model tests and published field tests of grouted piles in calcareous soils. These comparisons enable some conclusions to be drawn regarding the suitability of alternative nonlinear analyses. Key words : grouted piles, cyclic loading, calcareous soils, nonlinear analysis.

Load transfer model for piles in calcareous soil : Randolph, M F, Jewell, R J Proc 12th International Conference on Soil Mechanics and Foundation Engineering, Rio de Janeiro, 13–18 August 1989V1. P479–484. Publ Rotterdam: A A Balkema, 1989

The design of pile foundations to support offshore oil and gas production platforms has generally focused on the peak load that the piles must withstand, treating that load as a single (static) event. However, in calcareous deposits, more attention must be paid to the cyclic nature of loading on offshore platforms. Laboratory and field tests have shown that the axial pile response deteriorates under cyclic loading at load levels well below the static capacity. A model of the load transfer response along the shaft of piles installed in calcareous deposits, with particular emphasis on modelling the degradation under cyclic loading, has been implemented in a computer code, RATZ. The model has been used to match experimental data and to assess the likely response of prototype piles. The paper outlines the principal features of the load transfer model, and discusses the main assumptions incorporated in the model. Comparisons with experimental data are presented and a typical prototype situation is analysed and used to provide guidelines for the design of piles in calcareous soil (A).

Axial response analysis of piles in vertically and horizontally non-homogeneous soils

This paper presents a modified procedure for the analysis of the axial response of piles embedded in multi-layered soils. The results obtained by this procedure are compared with those computed by some previous methods and with a limited number of field test measurements. In the determination of the group settlement interaction between piles embedded in multi-layered soils, an additional simple soil mass stiffness model is developed in order to include the horizontal non-homogeneity of the soil due to soil disturbance cause by pile installation. The predictions by this model agree more closely with the observed field test group performance than do predictions by the conventional method which assumes lateral homogeneity of the soil.