Pile Head Displacement Research Articles

Model scale tests on the behaviour of single piles in cohesive soils under combined loads

ABSTRACT Model scale tests were conducted to study the influence of combined loads (uplift + compressive, uplift + lateral, lateral + compressive and lateral + uplift) on stress distribution, ultimate capacities and deformations of single piles in Indian black cotton soils, in comparison to pure uplift and lateral loads. Numerical analysis was performed in Plaxis 3D to examine the effect of pile slenderness ratio (L p /d = 15, 20 and 25) on ultimate capacity and pile head displacements. The test results showed that uplift capacity and pile head displacements increased by 35–49% and 30–50% with an increase in staged compressive loads, respectively. Therefore, piles subjected to combined uplift and compressive loads should be designed for pile head displacements rather than uplift capacity. The compressive loads did not affect the stress distribution along the pile–soil interface. However, lateral loads altered stress distribution, resulting in a curved failure plane and a substantial increase of 80–150% in the uplift capacity of piles under combined uplift + lateral loads. The lateral load–deflection curve for 60% and 80% of staged uplift loads showed an elastic response, indicating that the pile and surrounding soil did not yield under the applied lateral loads.

Cyclic loading experiments and numerical analysis of laterally loaded piles constructed in marine soil

This study aims to thoroughly analyze the lateral loads that impact tubular steel piles, which are extensively employed in the construction of coastal structures. The ASTM D 3966 standard was followed for conducting field tests on test piles installed at the Mersin International Port. The “time-displacement” and “load-displacement” curves were obtained from the cyclic loading test of the laterally loaded piles at the construction site. The finite elements models of the tests conducted on the construction site with the same parameters was built to perform numerical analysis. At the end of the analysis, it was determined that the numerical model's results were highly consistent with those obtained from the field test. After verifying the field experiments with numerical models, a parametric study was conducted. Parametric studies were conducted to compare the lateral displacements of tubular steel piles with variations in pile diameter, wall thickness, and load application height effect. The relationship between pile head displacements and these parameters appears to be almost linear. It is noteworthy that a change of approximately 10 percent in these parameters shows a correlation with changes of up to 3 percent in deformations.

Kinematic response of floating single piles in saturated and nearly-saturated soil to P-waves

This paper presents closed-form expressions for the analysis of floating single piles subjected to vertically propagating harmonic P-waves. The foundation soil is treated as poroelastic medium, and can be either fully-saturated or nearly-saturated. We show that the existence of non-continuous air phase in the form of air bubbles dissolved in the pore water will have noteworthy effect on the vertical kinematic response of piles. Through parametric analyses we demonstrate that hydromechanical analysis techniques that consider soil as fully saturated may significantly over-estimate pile head displacements for frequencies relevant to seismic P-waves. In addition, we quantify differences in the response between end-bearing and floating piles, and show that these differences are more prominent when soil is not fully saturated. Although the proposed expressions have been derived while considering elastic soil and pile response, findings relevant to the effect of the degree of soil saturation to the vertical kinematic response of piles are generally applicable to any coupled stress-flow analysis method.

Ultimate Lateral Resistance of Single Piles in Sand

The p-y curve method is currently the most widely utilized analytical approach for horizontally loaded piles. Precisely determining the ultimate soil resistance is essential to enhance the accuracy of the p-y curve. Despite various scholars proposing methods to establish the law governing ultimate soil resistance with soil depth in the sand, significant discrepancies exist among these approaches. This paper posits that the three-dimensional failure mode of the shallow soil in front of the pile manifests as a cone when it reaches the limit state. The expression for the ultimate soil resistance of a single pile in shallow soil is derived through limit equilibrium theory, and subsequently, the modified p-y curve is obtained. The accuracy was validated through a comparison between the corrected p-y curve and results obtained from centrifugation experiments and field studies. In contrast to the API specification method, the refined hyperbolic function p-y curve demonstrates enhanced precision, facilitating precise predictions of pile head displacement and pile bending moments under horizontal loads.

Vertical seismic response of end-bearing piles in nearly saturated soil

This paper presents a study on the effect of the degree of saturation of the foundation soil on the vertical seismic response of end-bearing piles subjected to P-waves. The research focuses on nearly saturated soil, where the air phase is not continuous and air bubbles are dissolved in the pore water, and thus can be treated as two-phase material. The response of the two-phase soil–pile system is quantified by means of a rigorous coupled hydromechanical model, which is based on Biot’s theory for poroelastic media and treats the air bubbles–pore water mixture as a homogeneous fluid obeying Boyle’s law. Numerical results are used to illustrate the influence of the degree of saturation of the soil layer on the seismic strong motion transferred to the pile head, i.e., the capacity of piles to filter seismic wave energy. This work bridges the gap between single-phase and two-phase saturated soil models, which predict profoundly different pile head displacements at incident wave frequencies of practical interest, and elucidates the mechanisms that lead to these differences.

Model test and finite element analysis results of a monopile in very dense sand under unidirectional horizontal cyclic loading

This study is carried out to understand the response of a monopile for a large (8–9 MW) offshore wind turbine located at a 45m water depth under lateral cyclic loading. Firstly, a 1g large-scale model pile test was performed in saturated very dense sand under numerous cycles of lateral loading, N=2000. The test results show that the lateral pile head displacement and rotation increase while the secant stiffness of pile decreases with increasing N. The numerical model was then validated by comparing the pile head displacement results of the test and the finite element (FE) model analysis. The soil was modelled using the UBC3D-PLM constitutive model to understand the development of the excess pore pressure ratio (EPPR) around the monopile. The FE analysis was performed cycle by cycle up to N=5 for a large diameter “XL monopile” with diameter D=9m and embedded length L=50m. The results showed that the EPPR around the monopile is insignificant. Furthermore, the effect of the number of loading cycles on the evolution of the EPPR, pile head deformation and pile head rotation are insignificant. In addition, a relatively high maximum pile head rotation accumulation θmax≅0.13° was obtained during cyclic loading.

Longitudinal Seismic Response of a Pipe Pile Embedded in Saturated Thermoelastic Ground Subjected to Rayleigh Waves

This work investigates the longitudinal vibration characteristics of the pipe pile in the liquid-saturated ground due to Rayleigh waves propagation with considering the influence of the nonisothermal condition on the wave motion. Firstly, the Rayleigh wavefield in the liquid-saturated thermoelastic ground is deduced theoretically with taking advantage of the Biot theory and the non-Fourier law of heat conduction. Secondly, the longitudinal frequency domain solution of the pipe pile under Rayleigh waves is derived analytically. Finally, the dependence of the pipe pile response on the physical parameter is illustrated by utilizing several numerical examples. The result reveals that increase in the thermal expansion coefficient of saturated soil significantly augments the longitudinal displacement and side friction of pile body. Increase in the permeability of saturated soil will reduce the pile head displacement. When the pile length is less than its effective length, the rise of the pile length will reduce the pile head displacement. Augment of both the outer radius and wall thickness of the pipe pile will help to reduce the pile head displacement. However, the lateral inertia effect of the pipe pile has slight influence on its longitudinal behavior under Rayleigh waves.

Experimental study on cyclic lateral response of piles in sandy sloping ground

This study aims to experimentally investigate the cyclic lateral responses of piles in sloping ground. The effects of slope angle θ, number of cycles n, and loading pattern (static or dynamic) on the pile responses (inc. unloading stiffness and residual displacement) were revealed by a series of laboratory (1-g) model tests considering the combined effects of slope and cyclic lateral loading. The measured responses indicated that all the pile head displacement, bending moment, lateral earth pressure, and unloading stiffness increase non-linearly with n. Thereinto, the pile head displacement x can be fitted by the function x = A(θ)n0.1 where the coefficient A(θ) is linearly related to tanθ; the residual displacement decays exponentially with the number of cycles. In addition, the slope effect would exacerbate the pile responses under cyclic loading, for instance, the maximum bending moment is linearly related to tanθ, and the depth of contraflexure point increases by 0.14 times the pile diameter when the slope angle increases by 1°.

Kinematic response of floating piles in a saturated soil layer subjected to seismic S-wave propagation

A mathematical formulation for the seismic analysis of floating piles embedded in a saturated soil layer to vertically propagating S waves is presented in this paper. Pivotal to the formulation is the establishment of a closed-form expression for the lateral soil reaction to pile displacement, and results from expressions for the free-field and scattered soil displacements obtained from Biot’s governing equations. Subsequently the pile is analysed as Euler-Bernoulli beam and the soil column between the pile tip and the bedrock is analysed as one-dimensional shear beam. This results in expressions for the kinematic response factors, that provide the frequency-dependent pile head displacement and rotation as function of the free-field soil movement. Validation of the developed model is followed by a discussion on the sensitivity of pile response to the two-phase nature of soil, focused on informing researchers and practitioners on the uncertainties introduced in conventional seismic analysis, where the foundation soil is modelled as single-phase medium.

An energy-based method for predicting the peak displacement of steel casing composite pile under lateral impact loading

Effective and convenient prediction of the peak displacement of piles under collision conditions facilitates the design framework preparation and pile damage evaluation. Hence, an energy-based method was proposed to predict the peak displacement of the widely used steel casing composite (SCC) piles under impact loading. The pile deformation energy was first calculated based on its deformation characteristics and the relationship between the sectional bending moment and curvature. The pile deformation energy and pile–soil energy distribution ratio were then employed to determine the relationship between the total absorbed impact energy and pile head displacement, which was represented by curves. The peak pile displacement under lateral impact loading was finally obtained by interpolating the absorbed impact energy curves. By conducting sectional analysis of the pile considering the strain rate effects and using a structural pushover procedure, the peak pile head displacements for various collision cases were predicted with reasonable accuracy. The effectiveness and reliability of the proposed method were verified by comparing its results with those of previous reduced-scale impact tests and validated full-scale finite element models.

EVALUATION OF LATERAL RESISTANCE OF STEEL PIPE PILE WITH WINGS IN COHESIVE SOIL BASED ON THREE-DIMENSIONAL FINITE ELEMENT ANALYSIS

Simulation analyses for in-situ cyclic loading tests in cohesive soil were conducted to indicate that finite element models considering nonlinearity of soil material , pile-soil separation , and a loosened soil layer were valid for evaluating lateral resistance of steel pipe piles with wings driven by rotational penetration. The results of analytical studies demonstrated that pile head load of piles with a loosened soil layer approached that of piles without a loosened soil layer when pile head displacement progressed. Linear finite element models were proposed to evaluate reduction of lateral resistance of piles due to a loosened soil layer.

Assessment of Several Interpretation Criteria for Tensile Axial Load Tests on Deep Foundations

Axial tensile load tests on deep foundations are increasingly significant because of the expansion in their usage for a myriad of projects requiring support of tensile loads as well as for being a less costly alternative to compression load tests. A variety of interpretation criteria for compression load tests is available, which are often also used for tensile tests, but a few have been proposed specifically for tension load tests. In this study, the performance of seven of the available widely used interpretation criteria was evaluated using a data set of 80 tensile load tests. These criteria are (i) original Davisson, (ii) AASHTO, (iii) New York City building code, (iv) French building code, (v) 5% diameter, (vi) 10% diameter, and (vii) New York University (NYU) compression criteria. Their performance was evaluated for applicability, accuracy, precision, and the diameter and length effects. The AASHTO criterion exhibited the best accuracy and a moderate precision, which is promising. The study also presents efforts to identify a better performing criterion, which yields the introduction of the NYU tension criterion where capacity is defined as that corresponding to the smallest displacement of (i) half the elastic elongation of the pile plus 0.25 in.; (ii) pile head displacement corresponding to the first incidence of pull-out or strain-softening resulting in loss of more than 5% of capacity; and (iii) pile head displacement of 0.5 in., unless modified by the structural engineer of record. The NYU tension criterion achieved a similar accuracy to the AASHTO criterion and a better precision, which is optimal.

Horizontal dynamic response of offshore large-diameter pipe piles

The dynamic response of an offshore large-diameter pipe pile-soil-water system under horizontal dynamic loads is analytically studied in this paper. The hydrodynamic pressure of the outer and inner seawater are obtained by the inviscid compressible fluid theory, and the lateral resistances of the outer and inner soil are obtained based on the viscoelastic dynamic continuum models. The pile response is obtained by utilizing the continuity conditions at the soil-pile and seawater-pile interfaces along the pile shaft. Results of the proposed solution are validated against FEM numerical results and against existing analytical and experimental results. The effects of hydrodynamic pressure from seawater on the pile head displacement and on the natural frequency are subsequently investigated for different problem parameters. Finally, the sensitivity of horizontal dynamic responses of the pipe pile-soil-water system in the frequency range to certain key parameters, such as water depth, pile length above the seawater, thickness of the soil, outer and inner radius of the pipe pile is analyzed.

Group performance of multiple series-connected energy piles under thermal loading

Thermomechanical behaviors of energy piles in rows, which are connected by a branch or branches, are investigated through a series of in-situ field tests. Thermally induced group effect influenced by thermal interference was measured and discussed. The interaction factor was extended to reflect the group effect of the pile branches which had differential head displacements. Results indicated that the heat exchange efficiency and thermal expansion coefficients were reduced, while the degree of freedom and pile head displacement were enlarged with the thermal interference. The pile temperatures, thermally induced stresses, and head displacement are decreased linearly with the direction of the flowing water. The heat exchange efficiency of the thermally affected branch was 93.6% and 91.8% of that of the isolated branch (1 × 3) as the thermal interference was on one side (2 × 3) and both sides (3 × 3), respectively. Additionally, the thermal interference of one adding pile branch caused the maximum thermally induced stress of the pile in the adjacent branch to increase by 49.0%, which is −1.46 MPa. The interaction factor on the branch effected by both-side thermal interference is 52.7%, which is about five times that of the branch effected by one-side thermal interference (10.6%).

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.

Laboratory investigations on the thermo-mechanical behaviour of an energy pile group operated in heat release mode

Compared to the single energy pile, the thermo-mechanical behavior of pile group become more complicated due to mutual influences of different piles. A model test device of pile group with 2 × 2 layout was built to study the thermo-mechanical characteristics of pile group with different number of energy piles, inlet temperatures and connection forms of buried pipes. The results show that compared to single energy pile, the energy pile group mode will lead to the decrease of heat exchange performance and the increase of pile tip soil pressure and pile head displacement. But, the side friction and axial force of pile will decrease as a result of pile group effect. Meanwhile, increasing the inlet temperature can enhance heat exchange characteristics of pile group, but it will also result in the increase of the soil temperature, pile head displacement, pile tip soil pressure, pile shaft axial force and pile side friction. Additionally, reducing the number of energy piles in pile group can improve the heat exchange rate per unit pile depth, but the total heat transfer amount of energy pile group will decrease. With the increase of the number of energy pile in pile group, the temperature change and pile head displacement of the pile increase, but the pile side friction and pile body axial force decrease. Under current experimental conditions, the pile head displacement and pile body axial force in parallel connection mode are larger than those in series connection modes, and the pile body excess temperature is the smallest in parallel mode of four energy piles and the largest in series mode of four energy piles. For the series connection mode, the heat exchange performance, pile body excess temperature, pile head displacement and pile body axial force gradually decrease along the series direction due to the decrease of temperature change.

Experimental and numerical studies on the adjacent pile response to bucket spudcan penetration and extraction

ABSTRACT Jack-up rigs typically operate close to long-running jacket platforms. Unfortunately, penetration and extraction of the spudcan under jack-up rigs may impose an adverse influence on adjacent platform piles. In this study, the influence of bucket spudcan and inverted cone spudcan penetration and extraction on the lateral behaviour of adjacent pile were studied comparatively by conducting a series of model tests and numerical simulation in sand. The influence of spudcan-pile distance, pile diameter and pile head conditions on the spudcan-pile interaction were studied. Additionally, the soil flow patterns during penetration and extraction of two types of spudcans were analysed using the CEL. The results show that the pile bending moment and pile head displacement induced by bucket spudcan penetration and extraction are smaller than that of an inverted cone spudcan. It provides an engineering option for dealing with the adverse influence on adjacent piles induced by spudcan penetration and extraction.

Pile-soil interaction under cyclic lateral load in loose sand: Experimental and numerical evaluations

Pile foundations in loose sand are occasionally subjected to cyclic loading initiated by the influence of wind, wave, traffic loads, etc. Such load reversals alter the strength and stiffness of surrounding loose sand affecting the ultimate capacity and serviceability of the pile foundation. Although such cyclic loading may be under vertical, lateral or torsional modes or a combination, the lateral cyclic load dominates the other modes. To carry out an in-depth study on pile-soil interaction under lateral cyclic load in loose sand, a series of laboratory model tests were performed with 2 × 2 pile group, followed by developing two alternative numerical models, i.e., boundary element and finite element models (i.e., BEM and FEM). The BEM involved a p-multiplier technique to incorporate the group effect, while the FEM was developed by ABAQUS software incorporating 3D stress conditions. As observed, the BEM slightly over-predicts while the FEM marginally under-predicts the experimental observations. The lateral cyclic loading was found to produce stiffening effect on loose sand which increased the pile capacity and reduced the pile head displacement. Sand relative density is also found to affect the test and numerical results significantly. A set of important conclusions are drawn from the entire study.

Behavior of a vertical pile in loose sand under the effect of an inclined load

The paper investigates the behavior of single vertical steel pipe pile embedded in sand subjected to compressive inclined load through MIDAS GTS/NX 2019 FEM package. The effect of the loading inclination angle and pile length and diameter on the performance of the pile were investigated. Inclined load carrying capacity and pile stiffness were presented. The pile head displacement and pile stiffness are significantly affected by load inclination angle. It was observed that, p-y curves of the pile-soil system are independent of loading inclination angle. In addition, the pile length and diameter are markedly affecting the behavior of the pile.

Test results for a monopile in medium dense sand subjected to unidirectional lateral cyclic loading

It is necessary to understand the response of monopile foundations for large offshore wind turbines (OWTs) with rated powers of under cyclic loading. For this purpose, a monopile model test study was conducted in saturated medium dense sand under long-term unidirectional lateral sinusoidal cyclic loading for cycles. The model pile was constructed with displacement transducers on the pile head and strain gauges along the pile length. The experimentally obtained bending moment, displacement, soil reaction evaluation along the pile length, pile head displacement accumulation, and lateral soil-pile stiffness were substantially different from the results of the American Petroleum Institute (API)-recommended method of determination and were significantly affected by the number of loading cycles. The experimentally based soil reaction -pile displacement curves obtained considering the maximum soil reactions for particular cycles were compared with the - curves recommended by the API. The maximum soil reaction during the increase in pile displacement obtained experimentally was significantly higher than the soil reaction obtained with the API method. The p–y curves obtained experimentally show similarities with those determined with the API method at only small pile displacements in the initial stage of the curves.