Free-head Pile Research Articles

Kinematic bending of piles in made ground

This work explores earthquake-induced kinematic bending in the so far unresolved case of a pile embedded in a two-layer soil with a thin surface layer. The problem is treated analytically by means of a generalised Winkler model, which in addition considers the effect of boundary conditions at the pile head over earlier contributions on the subject. Novel analytical closed-form expressions of the kinematic bending moment at the pile head and at the interface between the two soil layers are provided for both fixed- and free-head piles. The analytical solutions are validated through a rigorous finite-element model, which proves a remarkable accuracy in the static regime. While for interface bending the past literature indications for the dynamic coefficient make the proposed formula accurate for both shallow and deep interface, a novel dynamic interaction factor, describing dynamic effects for pile-head bending, is introduced. A numerical example provides guidance on application of the formulae in real design scenarios.

Impact of Suction on the Near Surface Lateral Soil Response using Centrifuge Modeling

Recent studies have shown that unsaturated soils lead to greater lateral pile capacity. This study aims to experimentally assess how suction stress affects the lateral response of piles in unsaturated cohesionless soil. Two centrifuge tests were performed at 50 g to evaluate the effect of suction stress in the soil. Lateral loads were applied monotonically on a single free-head pile in a displacement-controlledmanner to a maximum pile head displacement of 0.44 m. The first test was conducted on fully saturated cohesionless soil, while the second test was performed in an unsaturated state with a mixed unsaturatedsaturated soil layer. The water table was lowered to about 0.12 times the embedded pile depth to ensure an unsaturated condition in the zone closer to the surface of the soil. Lateral response assessment indicates that the unsaturated soil influenced the pile head response, leading to larger applied lateral loads for similar pile displacements in comparison to the fully saturated soil test. Experimental findings reveal that suction stress played a meaningful role in magnitudes of pile bending moments and lateral resistances for unsaturated cohesionless soils.

Seismic Analysis of Free Head Pile Under Combined Vertical and Lateral Load Embedded in Virgin and Improved Clayey Deposit

Seismic Analysis of Free Head Pile Under Combined Vertical and Lateral Load Embedded in Virgin and Improved Clayey Deposit

Predicting Lateral Resistance of Piles in Cohesive Soils

The ultimate lateral resistance of free- and fixed-headed piles in cohesive soil is examined in this paper using the three-dimensional finite element limit analysis with upper and lower bound theorems. A special concern, and that is the novelty of this study, is devoted to the combined effect of the three important dimensionless parameters; namely, the overburden stress factor (n), the pile length-diameter ratio (L/D), and the ratio of eccentric length to diameter (e/D). Numerical results are expressed by using Broms’s horizontal load factor, and comparisons are made with several published solutions. In addition, the associated failure mechanisms are investigated with respect to the three parametric effects. The adopted new technique has been successfully used to study a number of different geo-stability problems. It is thus the aim of this paper to produce accurate and practical results with design equations and charts that can be used by practitioners to predict the undrained lateral capacity of fixed- and free-headed piles.

Discussion of “Numerical Analysis of Laterally Loaded Single Free-Headed Piles within Mechanically Stabilized Earth Walls” by Saif Jawad and Jie Han

Discussion of “Numerical Analysis of Laterally Loaded Single Free-Headed Piles within Mechanically Stabilized Earth Walls” by Saif Jawad and Jie Han

Closure to “Numerical Analysis of Laterally Loaded Single Free-Headed Piles within Mechanically Stabilized Earth Walls” by Saif Jawad and Jie Han

Closure to “Numerical Analysis of Laterally Loaded Single Free-Headed Piles within Mechanically Stabilized Earth Walls” by Saif Jawad and Jie Han

Predicting the behaviour of laterally loaded flexible free head pile in layered soil using different AI (EPR, ANN and GP) techniques

Predicting the behaviour of laterally loaded flexible free head pile in layered soil using different AI (EPR, ANN and GP) techniques

Investigation of soil–pile–structure interaction induced by vertical loads and tunnelling

The response of pile groups and piled structures to vertical and tunnelling-induced loads is studied. A two-stage model is adopted that can efficiently consider external actions, greenfield tunnelling movements, superstructure stiffness, ultimate pile shaft and base stresses, pile-soil interactions in uniform or layered soils, and local soil behaviour (as either linear elastic, elastic perfectly-plastic, or nonlinear). Several scenarios are analysed: namely, piles subjected to vertical loads; piles and piled structures that are affected by tunnelling induced ground movements. Model results for piles under vertical loads compare well with field and other analytical models, confirming the robustness of the model. For tunnelling adjacent to or beneath single piles and pile groups, the impact of layered soils, soil yielding, and hyperbolic transfer mechanisms are shown to be significant, indicating that these aspects should be considered in risk assessments when using simplified models. Analyses of tunnelling beneath free-head piles and piled equivalent beams (describing flexible slabs or stiff buildings) confirm that pile-foundation connections and superstructures decrease tunnelling-induced displacements and deformations at the surface level; however, their action can also worsen the foundation distress with respect to force-moment structural capacity. Considering that the envelopes of fully-flexible and perfectly rigid superstructures will not always be conservative, soil-pile-structure interaction models are recommended for design.

Sway, sliding and back-rotation of piles subjected to pulse-like soil movement

Back-rotation and hinges were detected in in situ piles during lateral spreading, but they were not reported in model tests. This paper presents, for the first time, model tests on vertically loaded capped piles subjected to pulse-like soil movement. The study reveals (i) modes of pile–soil interaction, including sway at a sliding depth (SD) of 0.286l (l = pile embedment), back-rotation at SD = 0.57l and sliding of the piles at either SD; (ii) the evolution rule of yielding moments at the ground level (Moy) and maximum bending moment (Mm) of the pile body for each mode. Expressions are synthesised from the tests to estimate Mm, to define the ultimate state as a rotation angle exceeding 0.8–1.2 times that of free-head piles, to gain limiting soil movement wf* (that incurs hinges) and residual displacement of piles. The simple expressions offer a good estimation of limiting soil movement and permanent displacement for the piles underpinning bridges in Christchurch, New Zealand. They are useful to complement the three-layer modelling of the response of piles incurred by sliding, sway and back-rotation.

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.

Numerical Analysis of Laterally Loaded Single Free-Headed Piles within Mechanically Stabilized Earth Walls

Abstract A three-dimensional numerical analysis was conducted to evaluate the performance of mechanically stabilized earth (MSE) walls with free-headed piles installed within the geosynthetic-reinf...

Non-linear performance analysis of free headed piles in consolidating soil subjected to lateral loads

The analysis of laterally loaded pile is complex due to its site specific behaviour. Whereas most of the analytical methods provides only a generalized solution to it. Behaviour of piles in clay depends on site specific factors like subsoil condition, type of super-structure and the load transfer from it, construction duration and ground improvement process. The present study deals with the assessment of lateral load behaviour of pile based on the results of conventional and instrumented field load tests. The pile was designed based on the results of laboratory and field geotechnical investigations. The diameter and depth of bored cast in-situ pile were fixed as 600 mm and 16 m from the natural ground level respectively. The deflection profile obtained using inclinometer for various lateral loads on piles tested during different phases of site development were analyzed. The pile bending moment, shear force and soil reaction profile were established from the inclinometer data. The results obtained from the experimental data were found to be comparable with that of analytical method. The non-linear lateral load-displacement curves obtained from the load test conducted at different period of consolidation induced by site development process were analyzed. The curves thus obtained were transferred into a non-dimensional kh/khmax versus shear strain curve for the formation of equation explicit to the specific site. A site specific equation was proposed for the estimation of lateral load-displacement curve with respect to degree of consolidation of the surrounding soil subjected to surcharge load induced by the site development process.

Finite element analyses of energy piles using different constitutive models

Energy piles are foundation elements having the double scope of transferring structural loads from the structure to the ground and of exchanging heat with the surrounding soil. It follows that pile state of stress and settlement are altered by the time-dependent temperature change in both pile and soil. This work is aimed at investigating the effect of thermal cycles on the behaviour of a single energy pile. To this end, fully coupled thermo-hydro-mechanical analyses have been carried out using the Finite Element code ABAQUS. The single pile is installed in a normally consolidated clay behaving according to different constitutive models involving Mohr-Coulomb, Modified Cam Clay and Hypoplastic. The latter is employed with and without the thermal formulation capable of accounting for the thermal collapse of NC clays during heating. A single free-head pile is considered and the results are presented in terms of pile axial force and settlement developed cycle by cycle.

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.

A field study on pile response to blast-induced ground motion

A series of field tests using controlled blasting was conducted at a location in the North Western part of Singapore to assess the behaviour of pile foundations subjected to ground excitation. The field tests involved three piles with different pile head fixity conditions. The piles were instrumented with strain gauges to evaluate the bending moments and axial force along the piles. For the fixed-head piles, the maximum bending moment occurred at the pile head level. For free-head pile exhibited higher bending moments close to the mid-height of the pile and zero bending moment at the pile head due to the absence of restraints at the top. For all cases, the axial force was maximum at the pile head.

Estimating Laterally Loaded Pile Response

A simplified and practical approach for estimating the laterally loadedpiles in an elastic homogeneous soil is presented. The formulation of theapproach is similar to the conventional subgrade reaction theory but theresponse of the soil mass is evaluated by a semi-analytical solution that isrelated to the actual soil properties and the pile geometry instead of theconventional subgrade reaction modulus. Modified closed-form solutionsfor a free-head pile embedded in an elastic homogeneous soil arepresented. The solutions computed by using the proposed simplifiedapproach and the more rigorous boundary element approach for free-headpiles embedded in homogeneous soils are in reasonably good agreement

Experimental and Numerical Study on the Behavior of Energy Piles Subjected to Thermal Cycles

A laboratory‐scale model test is conducted to improve the understanding of the effects of thermal cycles on the mechanical behavior of energy piles. The model pile is composed of cement mortar and dry sand with a relative density of 30% is used for the model ground. After applying the working load to the pile head, the pile is subjected to three thermal cycles with a magnitude of 15°C. The measured temperature response and mechanical behavior are analyzed and used to validate the proposed numerical approach. In the numerical analysis, the temperature variation due to thermal cycles is calculated using uncoupled heat transfer analysis. Then, the computed temperature field is used as the boundary condition in the sequence stress analysis. A series of numerical sensitivity analyses are carried out using the sequentially coupled method to investigate the long‐term performance of energy piles under different soil and pile head restraint conditions. The numerical results suggest that the restraint condition at the pile head plays an important role in the mechanical response of energy piles. The ultimate pile resistance after thermal cycles does not decrease significantly. The accumulation of settlement of the free head pile and the reduction in the axial force of the restrained head pile should be considered in the design.

EFFECT OF PILE CONNECTIONS ON THE PERFORMANCE OF THE NAILED SLAB SYSTEM ON THE EXPANSIVE SOIL

Expansive soils are clay that swells and shrinks with changing moisture content. The pavement that constructed on these soils is subjected to large uplifting forces caused by swelling. Hence, there is an imperative need to counteract the problem posed by these soils by devising innovative pavement technique. An attempt to develop a simple, easy to install and cost-effective alternative pavement system, nailed slab system was developed, wherein slab pavement will be connected to a reinforced concrete mini piles.This research examines the emerging role of mini piles in the context of reducing soil uplift movement and the nailed slab system (pile supporting slab pavement) a system for minimizing slab movement due to swelling in expansive soil by conducting small-scale experimental modeling in laboratories. The heave prediction also doing by using the correlation between change in moisture content and vertical strain from oedometer test data. The results of this study indicate that reinforcing the soil by using the mini piles can reduce heave of soil, and the nailed slab system experiencing smaller upward movement than an unsupportedslab. The connection between the pile and the slab has a significant effect on the system’s ability to withstand the upward movement of expansive soil. When pile and the slab were monolithically connected, the system shows the better performance than those the slabs with the free head pile. Thereafter a heave prediction analysis provided the amount of heave that slightly overestimates, but still good enough for a rough estimation.

Numerical analysis of the seismic inertial and kinematic effects on pile bending moment in liquefiable soils

In an effort to study the seismic pile moment induced by the combined structure–pile inertial and soil–pile kinematic effects for single piles and pile groups in liquefiable ground, an extensive series of 3D finite element simulations are conducted in this paper. The roles that lateral inertial and kinematic interactions play on the pile moment are found to differ in different soil–pile–structure systems. Inertial structure force and kinematic soil displacement of the same direction could cause pile head moments of the same or opposite directions depending on the rotational constraint at the pile head. Kinematic interaction has a dominating influence on the pile moment for pile foundations with pile head rotation constrained by the existence of a pile cap, while inertial interaction is strongly influential for free-head piles. The coupling of inertial and kinematic interactions depends on the soil–pile–structure system configuration and the magnitudes of the inertial structure force and the kinematic soil displacement. Many current pseudo-static methods for calculating the seismic pile moment through summing a percentage of the kinematic induced moment with another percentage of the inertial induced moment could produce very inaccurate results under certain conditions.

Response of Piles Subjected to Progressive Soil Movement

Abstract Model tests were conducted to investigate the behavior of vertically loaded, free head piles undergoing lateral soil movement using an experimental apparatus developed in house. This paper presents ten new tests on an instrumented model pile in dry sand, which provide the profiles of bending moment, shear force and pile deflection along the pile, the development of maximum bending moment Mmax, maximum shear force Tmax, and pile deflection y0 at the ground surface with soil movement. The tests reveal the effects of axial load P (at pile head), the distance between the tested pile and source of free soil movement Sb, sliding depths, and angle of soil movement (via loading angle) on the pile response. For instance, the axial loading P leads to extra bending moment and deflection in the passive pile; the Mmax reduces with increase in Sb; and the Mmax is proportional to the “angle” of soil movement. The elastic solution by Guo and Qin [Guo, W. D., Qin, H. Y., 2010, “Thrust and Bending Moment of Rigid Piles Subjected to Moving Soil,” Can. Geotech. J., Vol. 47, No. 2, pp. 180–196] was used to predict the development of Mmax and Tmax observed in the current tests, a boundary element analysis, and an in situ pile test, respectively. It provides satisfactory predictions for all cases against the measured data.