Soil-pile Model Research Articles

Vertical vibration of a floating pile considering the incomplete bonding effect of the pile-soil interface

Based on the dynamic Winkler model and fictitious soil pile model to consider the relative sliding at the pile-soil interface and the propagation effect of soil beneath the pile toe, respectively, a more rigorous analytical model for the vertical vibration problems of a floating pile is established. The general solution for the displacement of soil is derived by combining the Laplace transform and variable separation methods. Then, the vertical dynamic solution of the floating pile is obtained by introducing the pile-soil interface condition and transfer function recursion method. Furthermore, extensive parametric analyses are performed to investigate the effects of the stiffness coefficient for the dynamic Winkler model, the length of the fictitious soil pile, and the inhomogeneity of the soils and pile on the vertical vibration for the floating pile. This indicates that the proposed analytical model provides a wider range and more rigorous application for the vertical vibration problems of floating piles embedded in layered and limited thickness soils. Moreover, the reduced form of the obtained solution can also be applied to investigate the corresponding vibration problems of end-bearing piles.

Vertical dynamic response of a floating pile in unsaturated poroelastic media based on the fictitious unsaturated soil pile model

Decades of high-speed railways operation around the world have revealed an increasing number of maintenance issues of pile-supported embankments subjected to long-term dynamic train loads in typical unsaturated soft soil areas. As the physical explanation of vibration characteristics of unsaturated soil piles is a theoretical basis for pile foundation design and pile integrity detection, a developed analytical solution of the vertical dynamic response of a floating pile in homogeneous unsaturated soils is proposed in this work. Based on the fictitious unsaturated soil pile (FUSP) model, the influence of saturation degree on soil shear modulus is considered in governing equations, uncoupled by using potential function and operator decomposition methods. The vertical dynamic response of a floating pile in a frequency domain is derived by employing the pile–soil compatibility conditions and the impedance function transfer method. Time-domain results are obtained by utilizing inverse Fourier transform and the convolution theorem. The correctness of the proposed model is verified by degradation to compare with the two existing solutions. Finally, a parameter study is undertaken to find the most sensitive parameters to the dynamic response of the floating pile in unsaturated soils.

New calculation method for the settlement of long-short-pile composite foundation based on virtual soil-pile model

New calculation method for the settlement of long-short-pile composite foundation based on virtual soil-pile model

Analytical model of large-diameter viscoelastic floating pile and application in pile low-strain integrity testing

Based on a three-dimensional fictitious soil pile model, a rigorous analytical model of a large-diameter viscoelastic floating pile is developed by considering the material damping of the pile and the soil. Subsequently, closed-form solutions of the pile dynamic response of this analytical problem are derived. The rationality and accuracy of the proposed model and its relevant theoretical solutions are verified by comparison with the results of previous studies. Finally, parametric analyses are further conducted to investigate the effects of the radial direction position, length of the fictitious soil pile, material damping coefficient of the pile, and soil-to-pile stiffness ratios on the dynamic response of the pile. Based on the obtained results, an appropriate method to eliminate high-frequency interferences and evaluate the material damping coefficient of a pile in actual pile low-strain integrity testing is established. Furthermore, the proposed methodology affords legible theoretical insights into the wave propagation in large-diameter viscoelastic floating piles.

Vertical Dynamic Impedance of a Viscoelastic Pile in Arbitrarily Layered Soil Based on the Fictitious Soil Pile Model

The vertical vibration of a viscoelastic pile immersed in arbitrarily layered soil is investigated by taking the interaction among pile, pile surrounding soil (PSS) and pile end soil (PES) into account. Firstly, considering both the stratification and stress wave effect of soil, a mathematical model of the pile–soil system is established based on the fictitious soil pile (FSP) model. Then, utilizing the impedance function transfer method and Laplace transform technique, the analytical solutions of the vertical dynamic impedance of pile are derived in the frequency domain. The analytical solutions are validated by comparing them with other existing solutions. Finally, a parametric study is put forward to investigate the properties of PES on the vertical dynamic impedance of pile. The results reveal that the properties of PES have a significant effect on the vertical dynamic impedance of pile, but there is a critical influence thickness for this effect. For the cases of the PES thickness exceeding the critical influence thickness, further increase of PES thickness will not affect the dynamic behavior of the pile–soil system.

Nonlinear analysis of single pile settlement based on stress bubble fictitious soil pile model

AbstractTaking both pile end stress diffusion effect and pile‐soil interface softening effect into account, a new nonlinear method for the vertical settlement of pile is proposed in this paper. In detail, a new model, namely stress bubble fictitious soil pile model, is proposed to simulate the support from the pile bottom soil. The nonlinear soil softening model is introduced to establish the relationship between unit pile shaft friction and pile‐soil relative displacement. Then, a nonlinear calculation method for load‐settlement analysis in multilayered soil is developed with the integration of these two models. The validity and accuracy of the proposed method are verified through the comparisons against experimental and existing simplified methods. At last, a detailed parametric study is conducted to assess the influence of the parameters related to the proposed model and pile‐soil system on the load‐displacement behavior of single pile. The promotion of this method could hugely boost the prediction of the single pile settlement for the preliminary design of pile foundation in layered soil.

A new approach for estimating the vertical elastic settlement of a single pile based on the fictitious soil pile model

The development of settlement at pile heads due to vertical loads is one of the most important factors in pile design. For large-scale pile-supported infrastructure and elevated roads, the cumulative plastic deformation caused by cyclic loads could be so dangerous that only elastic deformation is permitted. However, existing methods rely heavily on measurements from field tests, while only a few analytical approaches have been developed based on the Winkler model or load transfer functions, which are mostly empirical. Moreover, present theories can hardly unify the computation of end-bearing piles and floating piles, since the classification between these two types of piles is often ambiguous. In this study, a new approach based on the fictitious soil pile model is proposed, and a semi-analytical is derived. The efficacy of the solution is demonstrated via comparisons against measured curves in the field and rigorous numerical results. Finally, a parametric study is performed to investigate the influence of pile and soil properties and the existence of a strong or weak soil interlayer on the vertical settlement of a single pile. On this basis, a new pile design factor referred as ‘optimal aspect ratio’ is proposed to acquire suitable pile dimensions.

Simplified model of defective pile-soil interaction considering three-dimensional effect and application to integrity testing

A new simplified model for defective pile-soil interaction (DPSI) is developed to account for the three-dimensional (3D) effect in the pile integrity test (PIT) for large diameter piles. To account for the 3D effect, the defective pile is treated as linear continuum rather than the conventional 1D rod model. The surrounding soil is simulated employing the classical plane strain model and the soil resistance at the pile base is represented by the fictitious soil pile model. Theoretical solutions for the velocity field of the defective pile under vertical half-sine excitation are derived by integral transformation technique. The developed DPSI model and solutions are then used to investigate the 3D effect in the PIT for defective pile considering four common pile defect types. The obtained results provide important insights into the high-frequency (HF) interferences on the velocity-time history of the defective pile top caused by the 3D effect. Recommendations to minimize the HF interferences in the actual PIT are consequently provided. Application of the DPSI model in interpreting PIT measurements can improve accuracy of pile defect diagnosis.

An Experimental Investigation for Soil-pile Interaction Under Harmonic Load

In the present paper, the soil-pile model is tested on a shake table subjected harmonic load. A reduced-scale model (1:20) based on the similitude rule for a group of piles with a pile cap is used in the present study. The sinusoidal base excitation with varying frequencies in the range 3 Hz to 12 Hz and base acceleration ranging from 0.05 to 0.3 g are used as input base excitations for shake table testing. The amplifications at various depths of a cohesionless soil deposit without and with a group of piles are studied using shake table testing. Also, behaviour of group of piles embedded in soil is investigated in terms of variation in the strain levels and pile bending moment along the depth without and with pile cap mass with floating tip condition. It is observed from test results that the amplification of acceleration near top of the soil column is up to 4.266 times the base acceleration in the soil only cases. However, the amplification of acceleration near top of the soil column is up to 4.00 times the base acceleration when the model piles are placed inside the soil and is up to 4.66 times the base acceleration at the pile cap level. It is also observed that, when the model piles along with pile cap and pile cap mass are placed inside the soil, the amplification on base acceleration near top of the soil column and at the pile cap level are up to 3.986 and 4.67 times, respectively. The pile bending moments are evaluated by mounting the strain gauges at different depths along the pile surface. These are also evaluated using analytical methods, and the comparison of the results shows reasonable agreement. It is also observed that the pile bending moment at centre pile is lesser compared to pile bending moment of corner piles which may be attributed to effects of confinement. The present study will be useful guide for pile designers to obtain pile response under dynamic loads.

Analytical solution for longitudinal vibration of a floating pile in saturated porous media based on a fictitious saturated soil pile model

Based on Biot’s theory of wave propagation in saturated porous media, a new analytical approach for the vertical vibration of a floating pile in saturated viscoelastic soil is proposed, by combining the differential operator method with a fictitious saturated soil pile (FSSP) model. As an extended fictitious soil pile model, the FSSP model can fully consider the wave propagation in saturated soil with limit thickness beneath pile toe. The analytical solution for the velocity admittance of pile head is derived by introducing the completely coupled conditions at the pile-soil interface. The accuracy of the proposed approach and relevant solutions are degenerated and subsequently verified via comparisons with previous solutions. In addition, comparative analyses are further carried out to examine the effects of the porosity and shear modulus of soil as well as the layer thickness of the saturated soil layer beneath pile toe on the dynamic behavior of the floating pile. It is indicated that the presented approach and relevant solutions provide a more wide-ranging application for vertical vibration problems of a floating pile in saturated soil, which can be degenerated for the vertical vibration problems of a floating pile in single-phase soil or an end-bearing pile in saturated soil.

Torsional dynamic response of tapered pile considering compaction effect and stress diffusion effect

Considering both the compaction effect of pile surrounding soil and the stress diffusion effect of pile end soil, this paper theoretically investigates the torsional vibration characteristics of tapered pile. Utilizing the complex stiffness transfer model to simulate compaction effect and tapered fictitious soil pile model to simulate stress diffusion, the analytical solution for the torsional impedance at tapered pile top is obtained by virtue of Laplace transform technique and impedance transfer method. Based on the present solution, a parametric study is conducted to investigate the rationality of the present solution and the influence of soil and pile properties on the torsional vibration characteristics of tapered pile embedded in layered soil. The results show that, both the compaction effect and stress diffusion effect have significant influence on the torsional vibration characteristics of tapered pile, and these two factors should be considered during the dynamic design of pile foundation.

Cross-section piles in transparent soil under different dimensional conditions subjected to vertical load: an experimental study

In this study, soil pile models using transparent soil were used to study the displacement distribution law under different dimensional and loading conditions (vertical load ranges from 3 (30 N) to 50 kg (500 N)). Experiments were conducted to obtain displacement contour maps from model piles with variable diameters and lengths. Overall, the results show that side resistance increases monotonically with the pile diameter and that the pile displacement trend under the same load increases on shorter piles as well as the angle between the soil displacement direction and vertical direction. The final results were utilized to obtain displacement contour maps of rock mass and identify failure zones under different loads. It is found that the side resistance enhancement effect increases monotonically with pile diameter. The angle between the soil displacement direction and vertical direction between model piles of different lengths under identical loading conditions increases with decreasing soil volume, and the side frictional resistance and order variation resistance play a more substantial role than pile tip resistance. The findings provide a basis for constructing highways and also bridge pile foundations.

Numerical Assessment of Squeezed Branch Pile in Silty Clay Soil

Squeezed Branch Pile is derived on the basis of caste in place concrete pile. It has one or more branches along the pile shaft at design depth. Squeezed branch piles are often used in high rise building, transmission tower and in other pile foundations where anticipated uplift or vertical load may cause failure. This pile is one of the excellent options of pile foundation for soft soil and silty soil. The behaviour of Squeezed Branch pile is difficult to explain using simple pile-soil theories or two dimensional numerical analyses because of complicated geometry of pile.In the present numerical analysis, a 3D pile-soil model of conventional circular pile and squeezed branch pile foundations are analysed using MIDAS GTS NX finite element software to find out effectiveness of squeezed branch pile over conventional pile. The aim is to study the performance of Squeezed Branch Pile foundation in silty clay with respect to various parameters such as types of loading, branch diameter, branch spacing and number of branches. Analysis shows that the squeezed branch pile has higher vertical, lateral and uplift load capacity as compared to conventional pile.

Vertical dynamic response of a pile embedded in radially inhomogeneous soil based on fictitious soil pile model

The vertical dynamic response of a pile embedded in radially inhomogeneous soil is investigated while allowing for the vertical wave effect of soil. For most engineering applications, the soil adjacent to pile may be hardened or softened due to the construction disturbance effect of pile, leading to the radial inhomogeneity of soil which has a significant influence on the vibration characteristics of pile. To consider this problem, this paper introduced the vertical shear complex stiffness transfer model to simulate the radial inhomogeneity of the surrounding soil and underlying soil of pile. Then, the governing equations of pile-soil system subjected to vertical dynamic loading are established based on the fictitious soil pile model. By means of Laplace transfer method, vertical shear complex stiffness transfer method and impedance function transfer method, the analytical solution of the vertical dynamic impedance of pile head is derived in the frequency domain. The present solution is verified by comparing it with existing solution. Based on the present solution, a parametric study is conducted to study the influence of the construction disturbance effect of pile surrounding soil and underlying soil on the vertical dynamic response of pile head. The results show that, even if the surrounding soil of pile is hardened or softened within a very small range, the construction disturbance effect still has a remarkable influence on the vertical dynamic response of pile head.

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.

A hybrid analytical-numerical solution for a circular pile under lateral load in multilayered soil

A hybrid analytical-numerical solution is proposed to solve the problem of a laterally loaded pile with a circular cross-section in multilayered soils. In the pile-soil model, the lateral load is located at the pile head including both lateral force and bending moment. The single pile is considered as a beam on elastic foundation while shear beams model the soil column below the pile toe. The differential equations governing pile deflections are derived based on the energy principles and variational approaches. The differential equations are solved iteratively by using the finite element method that provides results of pile deflection, rotation angle, shear force, and bending moment along the pile and equivalent stiffness of the pile-soil system. The modulus reduction equation is also developed to match the proposed results well to the three-dimensional finite element analyses. Several examples are conducted to validate the proposed method by comparing the analysis results with those of existing analytical solutions, the three-dimensional finite element solutions.
 Keywords:
 beam on elastic foundation; finite element method; pile; energy principle; lateral load.

Undisturbed Soil Test on the Effects of the Slope Angle of Plate for Anti-Overturning Resistance of Rigid CEP Pile Used for Oceanographic Engineering

Qian, Y.; Li, M.; Jin, Y., and Wang, R., 2019. Undisturbed soil test on the effects of the slope angle of plate for anti-overturning resistance of rigid CEP pile used for oceanographic engineering. In: Li, L.; Wan, X., and Huang, X. (eds.), Recent Developments in Practices and Research on Coastal Regions: Transportation, Environment and Economy. Journal of Coastal Research, Special Issue No. 98, pp. 46–49. Coconut Creek (Florida), ISSN 0749-0208.The rigid concrete CEP pile has an important foundation in oceanographic engineering because of its higher anti-overturning resistance. This paper, through the undisturbed soil test, studies the effects of the slope angle of plate on the failure state of the soil surrounding the rigid CEP pile under horizontal force and designs different expanded plate slope angle models in the case of undisturbed soil. By preparing the pile-soil model and conducting the loading test, the test analyses the effects on the bearing capacity of the rigid CEP pile and the failure state of the soil and compares them with the results obtained by the ANSYS analysis model. Conducting the test on undisturbed soil is an improvement and innovation to the current buried soil-model testing method and test equipment. It achieves vertical and horizontal loading simultaneously and ensures the reliability of test results. This paper benefits the study of the CEP pile and large-scale application of it in future oceanographic engineering.

Torsional vibration solution of tapered pile considering stress diffusion effect of pile end soil

Based on the torsional plane strain model and tapered fictitious soil pile model, the torsional vibration characteristics of tapered pile considering stress diffusion effect of pile end soil is investigated theoretically. Considering the coupling conditions of pile-soil system, the torsional impedance at the tapered pile head is obtained by means of Laplace transform and impedance transfer method. A parametric study is performed, the rationality of the tapered fictitious soil model is verified and the influence of the cone angle of tapered fictitious soil pile on the torsional vibration characteristics of tapered pile embedded in layered soil is studied. The results show that: the cone angle of tapered fictitious soil pile has obvious effect on the torsional vibration characteristics of tapered pile with the shorter length.

Experimental and numerical study of pile foundations subjected to rotating machine-induced coupled excitations

ABSTRACT Dynamic response characteristics of a hollow steel single-pile and three-pile group are investigated under coupled vibration. Forced coupled vibration tests are conducted on the piles for static loads of 12 kN and 14 kN under different eccentric moments. The time-acceleration responses are measured for different frequencies for both horizontal and rocking modes and finally the frequency-amplitude responses are determined for horizontal and rocking components separately. The dynamic frequency-amplitude responses of piles are analysed by continuum approach method with two different soil-pile models: (i) a linear visco-elastic composed medium with the consideration of reduced shear modulus and (ii) a boundary zone model with non-reflective interface of shear modulus with parabolic variation. The variation of stiffness and damping of the soil-pile system are also determined. The analytical results are compared with the dynamic test results to verify the applicability of those soil models for predicting the non-linear response of soil-pile system under coupled vibration. It is observed from the comparison curves that the results obtained from the first soil model are found reasonably close to the dynamic test data. However in the case of second soil model, the resonant amplitude values are found much higher than the test data. Based on the analytical results, the boundary zone parameters like modulus reduction ratio, thickness ratio, weak zone soil damping and separation lengths of pile under different eccentric moments are predicted in present field condition. The variations of group efficiency ratio of the pile group with frequency are also determined form the analysis.

Analysis of Negative Skin-Friction on Single Piles by One-Dimensional Consolidation Model Test

The computer program pile negative skin friction (PileNSF) was developed by the authors to predict the bearing capacity of a pile embedded in a consolidating ground due to surcharge loading. The program uses a one-dimensional analytical soil-pile model, which was formulated based on the nonlinear load-transfer method and Mikasa’s generalized one-dimensional consolidation theory. To investigate the development of negative skin friction on single piles, as well as to validate the computer program (PileNSF), a laboratory model test was performed in this study. The clay layer was subjected to increasing surcharge loads to simulate actual field conditions. Results showed that as excess pore pressure decreases and as surcharge load increases, the dragload and downdrag on pile increases. The measured values of soil settlement, excess pore water pressure, and axial force on pile were compared with the predicted values obtained from the computer program. The results of the computer program (PileNSF) showed to be in good agreement with the measured data. Therefore, negative skin friction on single piles can be effectively predicted using the computer program, PileNSF, provided that reasonable parameters are used in the analysis. After validating the program, a parametric study was carried out to study the influence of various pile design parameters on negative skin friction.