Fixed-head Pile Research Articles

Soil–pile interaction of laterally loaded fixed-head piles and pile groups in unsaturated sand

This study investigates the lateral behavior of piles in unsaturated cohesionless sandy soil using centrifuge modelling. The lateral loading was conducted on a single fixed-head pile and 2 × 2 fixed-head pile groups with 3D and 5D spacings. The results showed that the single fixed-head pile demonstrated greater magnitudes of lateral resistance response compared to the leading and trailing piles of the pile groups. Moreover, the piles experienced greater lateral loads when the water table was about a quarter of the embedded pile depth in the soil layer for the single fixed-head pile and leading piles in the group. The group efficiencies of the pile increased as the water table reduced up to half of the embedded pile length for the 3D pile spacing. However, this effect was negligible for 5D pile spacing. Considering the mixed unsaturated–saturated ground condition can lead to a more resilient foundation design under climate-induced and seasonal fluctuations of water table level and also pave the path for the inclusion of future climatic scenarios.

Investigating the P–delta effect on displacement ductility capacity of scoured fixed-head piles in cohesive soil

Investigating the P–delta effect on displacement ductility capacity of scoured fixed-head piles in cohesive soil

Influence of the head fixity adjustment of existing piles on the lateral load response of new piles

The reuse of existing piles is a significant path to achieving low-carbon-dioxide, resilient and sustainable urban construction. However, some key issues in this area remain unresolved, especially with regard to the lateral interaction of new and existing piles. In this study, centrifuge tests and numerical simulations were used to investigate the influence of the head fixity adjustment of existing piles on the lateral load response of new piles. The lateral load responses of new piles were compared for four conditions of the heads of existing piles, and the contributions of existing piles to new piles were examined. An extensive parametric study was conducted to quantify the relative contributions of the existing pile head fixity to the lateral load response of new piles for different variables, including new pile–existing pile spacings, displacements of new piles, restraints of heads of new piles and soil properties. Finally, a practical method was developed for the design of laterally loaded new piles that considers the head fixity of existing piles. The results of this study provide a feasible way to enhance the contributions of existing piles to the lateral load responses of new piles through the adjustment of the fixities of existing pile heads.

An analytical solution for the filtering effect of piles in two-layer soil

The aim of this work is to investigate the kinematic response of a floating pile (either free- or fixed-head) embedded in a two-layer soil subjected to upward-propagating seismic waves. Closed form analytical solutions are provided for the kinematic interaction factors, based on a Beam-on-Dynamic-Winkler-foundation (BDWF) model properly adjusted for the problem at hand. For the fixed-head pile case, a simplified solution to assess the pile-to-soil acceleration ratio is provided adopting the concept of average soil motion over an effective pile length. The proposed solutions compare well with rigorous elastodynamic Finite-Element analysis. Insight is given into the interpretation of the mechanisms regulating the reduction of design spectra as function of structural period. Finally, as an outcome of a wide parametric study, ready-to-use formulae are proposed for a rapid assessment of the pile-induced spectral reduction.

P-y curves from in-situ ROBOCONE tests: a similarity approach for laterally loaded piles in clay

p-y curves from in-situ ROBOCONE tests: a similarity approach for laterally loaded piles in clay

Theoretical investigation of the displacement ductility capacity of scoured fixed-head piles in cohesive soil

Scouring causes the exposure of structure foundations, which may influence their seismic performance. In this study, we develop a theoretical model to investigate the influence of foundation exposure caused by scouring on the displacement ductility capacity of fixed-head piles. The proposed theoretical model is based on consideration of a long pile in cohesive soil. The analytical solutions of the displacement ductility capacity and system overstrength ratio of scoured piles in linear soil are derived. The ultimate state of the pile is set as the state in which the pile-head moment reaches its ultimate capacity or when in-ground plastic hinging begins to occur. The proposed solutions are validated through numerical pushover analyses. A pile with a large scour depth is prone to in-ground plastic hinging. If no in-ground plastic hinging occurs, the displacement ductility capacity increases until an upper limit as the scour depth increases; however, the displacement ductility capacity decreases with increasing scour depth when in-ground plastic hinging occurs. The proposed model for linear soil provides a lower-bound estimate of the displacement ductility capacity. Soil nonlinearity can increase a pile’s ductility; however, this effect becomes negligible for large scour depths. The solutions for linear soil are modified for nonlinear soil by using an equivalent linear soil model.

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.

An Analytical Method Evaluating the Evolution of Group Effect for Vertically Loaded Pile Groups Subjected to Tunnel Excavation

Tunnel excavations near existing vertically loaded pile groups are frequently encountered in urban areas, and most available studies have focused on the additional deformation and stress induced in the pile group, lacking in consideration of the variation in the pile-soil interaction (PSI) in the process, which plays an important role in pile group behavior with close pile spacing. In addition, for vertically loaded pile groups subjected to excavations in their proximity, the combined actions of the overlapping stress and shielding effect can lead to complicated variations in the group effect, which in turn results in difficulty in evaluating the PSI relationship and the pile group response. Thus, the modified Poulos method is extended in this paper to account for the effects of tunneling, and the variations in the group effect, load redistributions and pile settlements are also investigated. The validity of the proposed method is firstly verified by an available centrifuge model test with tunneling near a 2 × 2 fixed-head pile group. Further, the influences of the pile spacing and relative distance between the pile group and the tunnel are analyzed, and the most unfavorable working condition could be a tunnel excavated near the soil surface, close to a pile group with a small pile spacing.

Analysis and intrinsic correlations of partially head-restrained piles under lateral loading: virgin sites and new-existing pile sites

Piles are commonly used under partial head-restraint conditions. Currently, quantification of the lateral capacity of partially head-restrained piles is an open problem. This paper reports the results of a comparative study on the partially head-restrained behaviours of single piles in virgin sites and new piles in sites with existing piles. The lateral performance of new piles affected by pile-head restraints was examined and compared with that of single piles in virgin sites, and the characteristics of new and single pile capacities with respect to pile-head restraints were evaluated. An intrinsic H–δδ correlation (the lateral pile capacity and the factor linking partially head-restrained piles to pure free- and fixed-head piles) is discussed, which enables the calculation of capacities of partially head-restrained piles based on those of pure free- and fixed-head piles. In addition, analytical models of the δδ–λ correlation (λ represents the pile-head restraint percentage) are deduced. By combining the δδ–λ and H–δδ correlations, the lateral pile capacities for arbitrary pile-head restraints in virgin sites and sites with existing piles are calculated. The proposed method is comprehensively validated by numerical and literature examples, showing significant advantages in the determination of the lateral pile capacity considering head restraints.

Fictitious pile method for fixed-head pile groups with dissimilar piles subjected to horizontal loading

Pile groups are traditionally designed with similar piles in length, diameter, and property. However dissimilar piles may occur in the cases of foundation reuse and economical building design. This paper develops a fictitious pile method for the interaction factors between two dissimilar piles in a pile group that have different dimensions and properties. The effects of different pile lengths/properties on the horizontal load distribution, group efficiency factor and group reduction factor are studied here by using the fictitious pile method. Based on the principle of superposition and the concept of interaction factor, the proposed approach avoids the full analysis on pile groups and thus significantly improves the calculation efficiency. The horizontal load distributions for the fixed-head pile group with dissimilar piles by the proposed method match well with the results by the finite element method. The parametric study shows that, for stiffer pile group, the corner pile length is an important parameter affecting the fixed-head pile group behavior under horizontal loading.

Strategies for Modeling Partial Pile Head Fixity in Laterally Loaded Pier Foundations

When designing pile groups subjected to lateral loading, modeling and analysis of the connections between piles and the overlying pile caps require careful consideration. For a given pier foundation, characterization of the pile-to-cap connections typically corresponds to one of: pinned (no moment transfer); fixed (moment transfer up to pile rotational capacity); or, partially fixed pile head conditions. The present study aims to provide bridge engineers with practical resources for modeling laterally loaded pile group behaviors, in association with partially fixed pile head conditions. Modeling strategies are presented for both relatively simple (i.e., highly idealized) configurations, as well as for an array of conceivable pier foundations. In particular, an analytical approach is presented, which is suitable for preliminary design of configurations analyzed in a linear elastic manner, and on a representative-pile basis. For nonlinear analysis of pile groups (including soil–structure interaction), a search algorithm is developed that converges on a given target level of partial pile head fixity and associated pile group response (e.g., group lateral displacements, pile head moments). The search algorithm is utilized to investigate 20 unique pier foundation configurations under various lateral load intensities and across target levels of partial fixity. Pairing of the computed foundation responses with the levels of partial fixity, in the ensemble, is intended to serve as a resource to bridge engineers when making initial estimates to model partially fixed pile head conditions.

Determination of depth-of-fixity point for laterally loaded vertical offshore piles: A new approach

Offshore piles are used as foundations for various marine structures such as quay walls, dolphins, and offshore platforms. Offshore piles are primarily subjected to lateral loads from wind, waves and currents, which is particularly challenging for piles with substantial free length above the seabed. This paper presents a new approach to determine the depth-to-fixity and the depth of the apparent plastic hinge of offshore piles considering the pile head condition, type of soil below, and the soil nonlinearity employing the p-y curve method. A comprehensive numerical investigation was conducted covering a wide range of key parameters governing the lateral response of offshore piles. The results of the parametric analysis were used to establish numerical equations to determine the depth-to-fixity and the depth of the apparent plastic hinge for free and fixed head piles embedded either in sand or in clay. In addition, a closed form solution has been developed utilizing the equivalent cantilever fixed-base model to estimate the lateral capacity of an offshore pile at specified lateral deflection and the corresponding maximum bending moment. The new approach was verified by field load test results conducted on partially embedded piles in sand and clay.

Slope effect on dynamic p–y backbone curve in dry sand

This study aims to evaluate the slope effect on the dynamic p–y backbone curve, which is a worldwide simplified tool in the seismic design of pile foundations. Data of dynamic centrifuge tests on single and group piles in sloping sandy ground were adopted to derive soil resistance and pile displacement. Meanwhile, the free-field soil displacement was computed by performing three-dimensional numerical simulations of centrifuge models excluding the pile-supported structure. Dynamic p–y loops of soil resistance and pile displacement relative to the free-field soil displacement showed an irregular shape with the occurrence of large residual displacement. The soil resistance mobilized at the free-head pile was smaller than that at the fixed-head pile because of the difference in the phase responses. An empirical factor was introduced to account for the slope effect on the dynamic p–y backbone curve, which was formed by fitting peak points of the dynamic p–y loops with a hyperbolic function. Pseudo-static analyses applying the suggested dynamic p–y backbone curve showed good agreement with the centrifuge test data.

Effect of spudcan penetration on laterally loaded pile groups

Engineering cases, where jack-up rigs with large-diameter spudcan foundations operate near a jacket platform supported by pile groups, are increasing. Therefore, the group interaction of lateral-loaded pile groups should be re-evaluated. A pile group analysis method that is able to evaluate the effect of spudcan penetration on the pile group interaction is proposed in this study. This method is based on the modified Poulos method and is universally used in current ocean engineering practices. The pile group interaction-induced additional pile head deflection of a pile group, when subjected to both spudcan penetration and the influence of another pile group, is initially determined by elastic analysis. Next, the nonlinear foundation beam (NFB) method is adopted to calculate the head deflection of an isolated pile, which we utilise to calculate the head deflection of the pile group after considering the effects of spudcan penetration. Finally, the Y multiplier is obtained by several pilot modifications of p-y curves of the isolated pile in the NFB model, whose variation during spudcan penetration indicates the effect that such penetration has on the pile group interaction. Predicted results were compared with those of the centrifuge model test, and further with numerical simulation results, in order to prove the validity of the method in analysing a free-head and a fixed-head pile group.

Transient analysis of a fixed-head pile group in multi-layered transversely isotropic media due to horizontal loadings

This study presents a FEM-BEM coupled method to research the dynamic response of a fixed-head pile group in multi-layered transversely isotropic media subjected to transient horizontal loadings. Piles are discretized into finite elements on the basis of Euler-Bernoulli model. Fundamental transient solutions of the media derived by the analytical element method are taken as kernel boundary element functions. Then pile-soil interaction solutions are obtained using the FEM-BEM coupled method. Numerical examples are carried out to confirm the correctness of the theoretical derivation and to discuss further the influence of length-diameter ratio, transversely isotropic characteristics and stratification of the media on the transient horizontal response of the pile group.

Fictitious pile method for fixed-head pile groups subjected to horizontal loading

A novel fictitious pile method based on the elastic theory is presented for the analysis of horizontally loaded pile groups. By considering the strengthening effect of intervening piles, the interaction between the piles and the soil is decomposed into the extended soil and fictitious piles. The proposed approach is based on the superposition of the horizontal displacement of individual piles. It enables the avoidance of a full analysis of pile groups, and thus, improves the calculation efficiency. The pile-to-pile interaction factors, horizontal load distributions, and group efficiency factors produced by the proposed method are very close to those calculated by the finite element method. Several examples are then presented to discuss the influence of nondimensionalized piles and soil parameters on the behavior of pile groups.

Simplified assessment of pile-head kinematic demand in layered soil

Soil-pile kinematic interaction develops as a consequence of soil movements during seismic excitations. Severe bending strains exceeding the strain limits of the pile material can then develop. The highest kinematic bending moments occur at the interface of layered soil, characterized by consecutive layers with a high Vs2/Vs1 ratio between their shear wave velocities, or at the pile-head of fixed-head piles. Seismic design codes are generally worried about the kinematic bending developing at the interface of two layers with different stiffnesses and suggest evaluating kinematic effects on strategic buildings in seismic prone areas. Recent research has shown that the kinematic demand at the pile-head is important in the design of reinforced concrete piles in soft deposits, and relatively simple formulas exist for its assessment, both in homogenous and inhomogeneous soil conditions. However, there are no simplified solutions for the assessment of the kinematic demand at the pile-head in two-layered soil with a relatively shallow interface depth. In this study a recently developed BEM-based code (KIN SP) is used for this, and the relevance of the threshold values of the average shear wave velocity parameter suggested in seismic codes is discussed. The influence of pile and soil nonlinearities is also investigated with KIN SP, which has been enhanced to account for these nonlinearities.

Evaluation of Dynamic Soil-Pile-Structure Interactive Behavior in Dry Sand by 3D Numerical Simulation

A 3D numerical model based on finite-difference approximation was formulated to predict the dynamic soil-pile-structure interaction (SPSI) in dry sand. A non-linear elastic, Mohr–Coulomb plastic soil-constitutive model was adopted for the proposed methodology with a hysteretic damping model which can simulate nonlinear behavior of soil and an interface model which can predict separation and slippage between soil and pile according to the external load condition. Simplified continuum model was used to properly simulate the semi-infinite boundary and improve analysis efficiency. The proposed numerical model was validated by comparison with experimental results performed by Yoo (2013). Thereafter, a parametric study was also carried out to investigate the complex dynamic behavior of pile foundation under varying conditions. It was demonstrated that inertial force induced by superstructure is dominant for dynamic SPSI in dry sand whereas the kinematic force induced by soil deformation is relatively insignificant. Pile peak bending moment occurs at 30% of the pile length when pile length is no longer than 5 T and at about 30% of 5 T (1.6 T) when the pile length is longer than 5 T. The pile head fixity governed the peak bending moment profile of pile and affected the dynamic responses of the system in conjunction with other factors, such as pile rigidity.

Theoretical solutions of laterally loaded fixed-head piles in elastoplastic soil considering pile-head flexural yielding

Group piles with a cap under lateral loading behave like fixed-head piles because of the rotational restraint of the pile cap. They are susceptible to flexural yielding at the pile head due to high bending strains. In addition to soil nonlinearity, the pile-head flexural nonlinearity also significantly contributes to nonlinear responses of the piles, which is not covered in most existing analytical solutions. For a fixed-head pile buried in uniform elastoplastic soils, this study derives analytical solutions for the lateral responses to account for soil yielding, and flexural yielding and failure of the pile section at the pile head. The model assumes elastoplastic Winkler-type soil with constant subgrade stiffness and yield strength and a nonlinear moment–curvature curve for the pile section. Examples are provided to apply the solutions to determine the complete capacity curves (moment and horizontal load) of a fixed-head pile and the derived analytical capacity curves are in good agreement with those from numerical analyses that use nonlinear beam elements to reflect the nonlinear flexural behavior of the pile section. The solutions are also applied to evaluate the influences of the yield displacement of the soil and different forms of simplified nonlinear moment–curvature curves of the pile section on the lateral load–displacement curves.

Heating and cooling induced stresses and displacements in heat exchanger piles in sand

Heating and cooling of heat exchanger piles causes changes in the axial stresses in the pile and vertical pile displacements. Changes in axial stress (thermal stresses) are more significant in the case of fixed head piles, while vertical displacements are more important in the case of free head piles. This paper presents a series of finite element analyses that was performed in order to investigate the effect of the heating or cooling duration, the surface thermal boundary condition, the mechanical and thermal parameters of the soil and the temperature change in the pile, on the thermal stresses that develop in fixed head piles and the thermal vertical displacements of free head piles in sand. The numerical modelling procedure is first validated through the simulation of a set of published centrifuge experiments of heat exchanger model piles in sand. The results of an extensive parametric numerical investigation are then presented and the effect of the abovementioned potentially influencing factors is presented and discussed. Based on the numerical results, the most important parameters that affect the mechanical response of heat exchanger piles to heating or cooling are identified. Finally, design charts are proposed for the calculation of the maximum thermal stresses that develop in fixed head piles and the pile head thermal vertical displacement of free head piles.