Negative skin friction pile concepts with soil–structure interaction

Several full-scale, long-term tests on instrumented piles performed since the 1960s and through the 1990s are presented. The results of the tests show that a large drag load will develop in piles installed in soft and loose soils. The test cases are from Norway, Sweden, Japan, Canada, Australia, United States, and Singapore and involve driven steel piles and precast concrete piles. The test results show that the transfer of load from the soil to the pile through negative skin friction, and from the pile back to the soil through positive shaft resistance, is governed by effective stress and that already a very small movement will result in mobilization of ultimate values of shaft shear. The pile toe resistance, on the other hand, is determined by downdrag of the pile and the resulting pile toe penetration. Reconsolidation after the pile installation with associated dissipation of pore pressures will result in significant drag load. An equilibrium of force in the pile will develop, where the sustained loads on the pile head and the drag load are equal to the positive shaft resistance plus the pile toe resistance. The location of the force equilibrium, the neutral plane, is also where the pile and the soil move equally. The drag load is of importance mostly for very long piles (longer than 100 pile diameters) for which the pile structural strength could be exceeded. Downdrag, i.e., settlement of the piled foundation, is a very important issue, however, particularly for low-capacity short piles. Soil settlement at the neutral plane will result in a downdrag of the pile. The magnitude of the downdrag will determine the magnitude of the pile toe penetration into the soil, which will determine the pile toe resistance and affect the location of the neutral plane. Nature's iteration of force and soil settlement will decide the final location of the neutral plane.Key words: piles, negative skin friction, drag load, downdrag, neutral plane, pile settlement.

Sandy soils may undergo compression during liquefaction. A review of published design manuals, including the 2004 AASHTO LRFD Bridge Design Specifications, indicates that some recommendations for pile design may not represent the pile response in a manner consistent with the actual axial response of the pile during liquefaction. The actual response is discussed in light of the unified pile design method and separated between liquefaction occurring above and below the static nonliquefied neutral plane location before the liquefaction event. In the former case, the effect on the pile is minor regardless of the magnitude of liquefaction-induced settlement of the surrounding soil. In the latter case, the axial compressive load in the pile increases and additional pile settlement (downdrag) will occur when the force equilibrium is reestablished through the necessary mobilization of additional toe resistance. This means that the magnitude of the downdrag is governed by the pile toe load-movement response to the downward shift of the neutral plane. While there is a reduction in shaft resistance due to the reduction in strength within the liquefied layers, this reduction will only influence the pile design length where the liquefying layer is very thick.

Numerical modeling using finite element method (FEM) is well-recognized as a powerful method for both engineers and researchers to solve boundary value problems. In the modeling of geotechnical problems, the analyses are often limited to simple static problems with either steady-state effective or total stress approach while the transient response (development and dissipation of excess pore water pressure, uex) is seldom considered. Besides, infinitesimal small soil deformation is usually assumed. The simulation is further complicated when the soil-structure interaction problems involve significant soil displacements; like a pile subject to negative skin friction (NSF) and a cone/pile penetration. However, conventional FEM analysis prematurely terminates due primarily to excessive mesh distortion. One could see that simulating a transient problem with large deformation and distortion remains a great challenge. In this study, advanced FE simulations are performed to give new insights into the problems of (1) a pile subject to NSF; and (2) a cone penetration. The transient response of the NSF problem is modeled with the fluid-coupled consolidation technique and geometric nonlinearity. The fluid-coupled cone penetration problem is modeled with a newly developed adaptive approach.
\n
\nThe NSF and cone penetration simulations involve complex soil-structure interface modeling. Two types of modified interface responses are developed and verified which consider fluid coupling. The developed algorithm is applied to back analyze a case history of a pile subject to NSF induced by surcharge loading. Promising results were shown. Development of dragload and neutral plane (NP) with time is studied. NP locates at 75% of the pile embedded length (D) in long-term. Next, a parametric study is performed to investigate the influences of pile geometries, ground compressibility and loading conditions towards the pile responses. The long-term NP locates at around 0.55D to 0.65D in the studied engineering scenarios. The maximum downdrag can be up to 10% of the pile diameter. NP shifts upward when the head load increases. A simple design chart is proposed which helps engineers to estimate the long-term axial load distribution. An illustrative example is given to demonstrate the application and performance of the chart.
\n
\nThe study is extended to investigate the cone penetration problem. An advanced adaptive method is developed and implemented into the FE package ABAQUS to resolve the problems of numerical instability, excessive mesh distortion and premature termination. The proposed method is verified by modeling a ground consolidation problem. Next, total stress back analysis of cone penetration is conducted with the proposed method. The development of cone factor predicted by the proposed method gives a better match with the laboratory result when comparing with the built-in ALE method. Next, the development and dissipation of uex during cone advancing with the proposed method and fluid-coupled technique is investigated. uex develops dramatically around the cone tip. The soil permeability is back calculated from the dissipation test and agrees well with the input value. It is believed that the construction effects of a press-in pile and the subsequence NSF on that pile can be modeled by utilizing the finding of this study.

This paper presents finite element analyses of negative skin friction on a single pile under various conditions. Negative skin friction is a common problem if a pile is designed in a highly compressible soil. There are two most important parameters in estimating the load caused by negative skin friction: (1) the distribution and magnitude of skin friction and (2) the location of the neutral plane. The neutral plane is the location where the pile and soil settle the same amount or have no relative displacement. Negative skin friction is a very complex phenomenon influenced by many factors. In this paper, a two-dimensional axisymmetric model is built in the finite element program, ABAQUS. The model is first verified with a known case history. A systematic parametric analysis is performed to investigate the influence on both the neutral plane and the magnitude and distribution of negative skin friction along the pile length of various influencing factors, including the consolidation time, the properties of pile/soil interface, the lateral earth pressure coefficient, pile-soil limiting displacement, the intensity of surcharge, and soil stiffness. Based on the analyses, it is found that the location of the neutral plane is significantly influenced by the consolidation time and the stiffness of bearing layer. The distribution and magnitude of negative skin friction is influenced mainly by the pile/soil interface, soil compressibility, and the surcharge intensity. Based on the field measurements from literature and this investigation, a simple design procedure is proposed for estimating the pile load caused by negative skin friction.

Current views indicate that negative skin friction on piles can be mobilized at small relative deformations and should be considered in all designs, primarily for serviceability conditions. An elastic-plastic closed-form solution is presented that permits an estimate of down-drag forces and the location of the neutral plane. It is shown that the conventional rigid-plastic solution may overestimate down-drag forces by as much as 50% and may also overestimate the depth of the neutral plane. Key words : piles, negative skin friction, neutral plane, capacity.

A centrifuge model study is carried out to investigate the behavior of pile subject to negative skin friction induced by pile installation, ground water drawdown and surcharge loading. A single end-bearing pile is examined as the induced negative skin friction would induce the most severe stress on the pile structural material as compared to friction piles. In addition, the behavior of the pile under simultaneous negative skin friction and dead/live loads is examined. To facilitate detailed interpretations of the test results, the model setup is extensively instrumented and involves elaborate test control schemes. To further examine the phenomenon of negative skin friction on an end-bearing pile, finite element analyses were conducted. The numerical analysis is first validated against the centrifuge test data and subsequently extended to examine the effects of pile slenderness ratio, surcharge intensity and pile-soil stiffness ratio on the degree of mobilization of negative skin friction induced on the pile. Finally experimental and numerical studies are conducted to examine the effect of applied transient live load on pile subject to negative skin friction.

The finite element method capable of simulating the behavior of deep foundations subjected to negative skin friction in Basrah soil is investigated. Single piles under drag forces are analyzed using the PLAXIS program with an axisymmetric model. Linear elastic, Soft Soil and Mohr-Coulomb constitutive relations are adopted, where higher order triangular element is chosen for pile and soil clusters. Both pile and soil are modeled using (15)-node triangular elements. Three sites in Basrah province (Umm Qasr Port, Khor Al-Zubair, and Shatt AlArab Hotel) were selected to perform this study. The soil profile and layer characteristics are obtained from the soil investigation reports. Where the negative skin friction is evaluated due to filling loads. It is Conclusion thatSmall relative displacements are necessary to activate the negative skin friction. The elastic shorting for pile effect negative skin friction, due to increase relative displacement. The elastic shorting of the driven pile is more than that of the bored pile due to the less cross-sectional area of the driven pile. The results revealed proportional relation between the developed drag forces and pile section dimensions, interface friction factor, and fill height, with a maximum effect on the section dimension and minimum effect on the interface factor. The locations of neutral points are not sensitive to the above-mentioned factors.

Collapsible soil exhibits considerable strength when it is dry, but when it is inundated, it loses its strength and experiences significant volume reduction. Foundations on collapsible soils subjected to inundation may experience sudden settlement without any increase in the in situ stress level. Pile foundations are often used to penetrate layers of collapsible soil to transfer the load to a lower, stronger soil layer. Nevertheless, because of indentation, these piles may experience negative skin friction, which may lead to significant reduction in the pile capacity, or perhaps separation of the pile from its cap. In the literature, although several theories can be found dealing with the design of pile foundations and the prediction of negative skin friction on these piles, little to none was found to estimate the negative skin friction because of the sudden collapse of the surrounding soils. This is mainly because of the difficulties associated in modeling collapsible soil experimentally or numerically. This paper presents a numerical model, which is capable of incorporating the effects of inundation of collapsible soil, and the negative skin friction on axially loaded vertical piles. The model uses the theories of the unsaturated soil and soil-water characteristic curve to estimate the effect of soil suction because of progressive inundation. The procedure takes place by estimating the change in the soil properties and the irrecoverable soil-volume change during inundation. The paper also presents the results of laboratory tests conducted on a single pile driven in collapsible soil subjected to inundation. The measured values of the negative skin friction compared well with the predicted values from the numerical model presented. The pile design guideline is also presented to assist designers in predicting negative skin friction because of inundation. Furthermore, the theory developed can be used to evaluate the condition of an existing pile foundation driven in collapsible soil and to establish the remedies needed to avoid catastrophic failure.

45 - Neutral plane concept and negative skin friction

Earthquake-induced soil liquefaction can cause settlement around piles, which can translate to negative skin friction, resulting in the development of drag load and settlement of the piles. Despite significant research progress on the effects of liquefaction on structures and the seismic response of piles, there is still a knowledge gap in the assessment of liquefaction-induced downdrag loads on piles. The interrelationships between mechanisms affecting negative skin friction are not accounted for in current practice, typically leading to over-conservative or unsafely designed piles. A series of centrifuge model tests were performed to assess liquefaction-induced downdrag and understand the interplay of pile embedment, pile-head load, excess pore pressure generation, dissipation, reconsolidation, and ground settlement on the axial load profile during and post shaking. The two tests featured instrumented piles, loaded with a range of axial loads and embedded at a range of depths into the bearing layer, in a uniform and a layered liquefiable profile respectively. The tests showed that most of the settlement of the piles occurred during shaking. The mechanisms behind the development of liquefaction-induced drag load on piles and settlements will be described and ramifications concerning the design of piles in liquefiable soils will be discussed.

Negative skin friction (NSF) of piles in recent filling or soft area is an important effect factor of pile bearing capacity. Since field experiments on NSF are time consuming and it is difficult to large surcharge loads in experimental research, a unified calculation method of pile positive/negative skin friction was established based on the effective stress method for investigation. The closed-form analytical solutions for calculating the pile skin friction corresponding to the plastic and elastic state were derived respectively. Meanwhile, the axial load of a single pile under different distribution forms of the pile skin friction was deduced. The calculation method was verified by comparing with an in-situ test. Furthermore, a computer model, which was established by the finite element method, was used to study the effect of the friction coefficient, consolidation time, consolidation pressure, drainage condition, and pressure ratio on the distribution of NSF and the location of neutral point. The results show that the effect of the friction coefficient, consolidation time, and pressure ratio on the NSF were significant. The friction coefficient increased from 0.05 to 0.4, the position of the neutral point rose by 22%, and the drag load of pile shaft was obviously increased. The effect of consolidation pressure and drainage conditions on the neutral point were relatively less, but they had a great influence on the distribution and magnitude of NSF. Furthermore, under different consolidation pressures, the normalized maximum axial load, Fmax/p, of the pile shaft had a good linear relationship with the pressure ratio, n, and the slopes were the same.

This paper reports the results of four centrifuge model tests that were undertaken to investigate behavior of floating piles subjected to negative skin friction (NSF) and to study effects of axial load on the load-transfer mechanism along single floating piles and shielded center piles inside a group of sacrificing piles. In addition, three-dimensional numerical analyses of the centrifuge model tests were carried out with elasto-plastic slip considered at the pile–soil interface. Prior to applying load, the measured neutral plane position of the single floating piles was located at approximately the three-quarter depth level of the embedded pile length. The neutral plane elevation shifts lower down the pile shaft as the distance of pile tip above the bearing stratum decreases. Under the application of axial load, the dragload generated by excessive soil settlement decreases and is eventually eliminated. The amount of axial load for complete NSF elimination does not seem to be significantly affected by the presence of sacrificing piles, but it does increase with end-bearing stiffness of the pile. Numerical simulation revealed that the hang-up effect is not altered by the application of axial load.

Negative skin friction occurs when the settlement of the adjacent soil is larger than the pile settlement. Loading the surface next to the pile is one of the most common causes for negative friction to mobilize along the pile’s shaft. In this paper, a numerical simulation will be carried out using Matsui-Abe soil model to study the case of a single pile driven in soft clay layer. Matsui-Abe soil constitutive model represents the Elasto-plastic relation between stress and strain. Negative skin friction (NSF) will be produced due to the soil consolidation. The model will be validated by comparing the results obtained from the primary consolidation with different field tests. A parametric study of the surcharge load will be conducted then to examine the change in negative skin friction (NSF) distribution. Graphs will be obtained showing the relation between loading and changing in the neutral plane location (NPL).

This study conducted parametric analyses to investigate the influence of structural loading (pile-head loading) on the development of negative skin friction in friction single piles and friction-end-bearing single piles in consolidating ground. Numerical effective stress-based mechanical-flow analysis models were built in ABAQUS software. The appropriateness of the modeling approach was verified by applying it to simulate a field model test described in the literature. For small surcharges, pile-head loading considerably reduces the depth of the neutral plane and therefore drag load for the friction pile. Under the same pile-head loads, the reduction in total drag load for the friction-end-bearing pile is smaller than that for the friction pile. However, when the friction-end-bearing pile is subjected to larger pile-head loads, the degree of total drag load reduction is comparable to that of the friction pile. Considerable pile-head displacement occurs under the combined action of pile-head and surcharge loadings. In contrast to the friction pile, the friction-end-bearing pile, because of the greater stiffness and strength of its bearing stratum, exhibits a significant reduction in negative skin friction, no bearing failure, and a smaller pile-head displacement when it is subjected to large pile-head loading.

Residual stress exists in piles before load transferring from the upper structure to the pile head. The resulting mechanism of residual stress distribution in piles after unloading and the influencing factors are analyzed and compared with those of pile foundations with negative skin friction conditions. An instrumented bored pile was measured after a vertical compressive static loading test to obtain the distribution of residual stress and deformation. The results indicate that (1) the distribution of residual stress in the pile is similar to that in a pile foundation with negative skin friction on the piles due to the soil-constrained piles fully springing back, which is equivalent to prestress exerted by the surrounding soil on the pile; (2) compared with negative skin friction, the shaft resistance induced by residual stress is smaller, and the residual stress does not affect the settlement of the pile foundation; (3) the magnitude of residual stress is closely related to the pile size, properties of the soil, and stiffness of contact surface between the pile and the soil.