Shaft Capacity Research Articles

Investigation into Skin Friction of Bored Pile Including Influence of Soil Strength at Pile Base

Traditional pile design methods of separately assessing the shaft and base capacities have been doubted by some model pile tests and numerical analyses. In this study, two case histories in China are investigated to look through the influence of the material strength beneath the pile base on the shaft resistance mobilized under working load and at failure. The case studies involve a series of full-scale load tests on long bored piles instrumented with strain gauges along the shafts. Comparative parameters include the formation of end-bearing soils and the craftwork of base-grouting, all attributing to one unified factor of material strength at the pile base. The field observations reveal that an increase in the material strength at the pile base puts significant and positive impact on mobilizing the shaft resistance. A simple analysis procedure based on soil mechanics principles is likely to capture the mechanism that the slip displacement over the whole shaft and the radial stress acting on the shaft at deep depth are essentially responsible for the phenomena. This study is an attempt to initiate design approaches with optimistic evaluation on the shaft capacity given that special techniques of enhancing the pile base strength have been made.

Shaft Resistance of Drilled Shafts in Clay

Thecurrentstudyexaminesandquantifiesthelimitshaftresistanceofdrilledshaftsinstalledinclaythrough finite-elementanalyses that use an advanced two-surface-plasticity constitutive model for clay. The clay constitutive model used in this study reproduces the mechan- ical responseofclaysundermultiaxial loadingconditions,predictsbothdrainedandundrainedbehavioratsmallandlargestrains,andcaptures the drop in strength toward a residual value at very large shear strains. One-dimensional finite-element analyses are performed to simulate the essential stages of the installation and loading of drilled shafts in clay for different initial stresses, different overconsolidation ratios, and dif- ferent values of the difference between the critical-state and the minimum residual friction angles. On the basis of these simulations, equations for the shaft friction coefficient are proposed that can be used in the calculation of the shaft resistance of drilled shafts in clay. Predictions using theproposedequationscomparewellwiththedataderivedfromdifferent fieldpileloadtestsondrilledshafts.DOI:10.1061/(ASCE)GT.1943- 5606.0000803. © 2013 American Society of Civil Engineers. CE Database subject headings: Drilled shafts; Piles; Clays; Numerical models; Plasticity; Finite element method; Constitutive models. Author keywords: Drilled shaft; Nondisplacement piles; Shaft capacity; Clay; Numerical modeling and analysis; Two-surface plasticity.

Expansion of Cavities Embedded in Cohesionless Elastoplastic Half-Space and Subjected to Anisotropic Stress Field

Cavity expansion theories are employed in a wide range of geotechnical applications including interpretation of pressure meter tests, evaluation of shaft capacity of piles, and pulling forces for horizontal directional drilling. Most of these theories assume infinite medium and isotropic stress field, which may not be justified for many applications. The main objectives of this paper are two folds: to investigate the effects of the free surface, stress gradient, and in situ stress anisotropy on the displacements during the expansion phase of cavities embedded in dilatant sands; and to establish correction factors to account for these effects. The investigation was conducted using two-dimensional finite element analyses. It was found that the cavity expansion theory due to Yu and Houlsby (Geotechnique 41:173–183, 1991) can be used reliably for cases subjected to an initial isotropic stress and embedment depth to diameter ratio of 20 or higher. However, it becomes inaccurate for shallow embedment depth and/or stress anisotropy conditions. An analytical procedure to account for the effects of embedment and/or stress anisotropy was proposed. The applicability of the proposed procedure was demonstrated for a wide range of soil properties and geometrical configurations. The results obtained confirmed its ability to estimate the cavity pressures within 10 % of the values obtained using FEA calculations.

Behaviour of displacement piles in sand under cyclic axial loading

Field experiments have demonstrated that piles driven into sand can respond to axial cyclic loading in Stable, Unstable or Meta-Stable ways, depending on the combinations of mean and cyclic loads and the number of cycles. An understanding of the three styles of responses is provided by experiments involving a highly instrumented model displacement pile and an array of soil stress sensors installed in fine sand in a pressurised calibration chamber. The different patterns of effective stress developing on and around the shaft are reported, along with the results of static load tests that track the effects on shaft capacity. The interpretation links these observations to the sand's stress–strain behaviour. The interface-shear characteristics, the kinematic yielding, the local densification, the growth of a fractured interface-shear zone and the restrained dilatancy at the pile–soil interface are all found to be important. The model tests are shown to be compatible with the full-scale behaviour and to provide key information for improving the modelling and the design rules.

Shaft Capacity of Displacement Piles in Clay Using the Cone Penetration Test

AbstractThe prediction of the shaft capacity of displacement piles in clay still relies wholly on empirical or semiempirical approaches. This paper examines the predictive abilities of five well-known methods against an extended version of an existing database of pile tests. It is seen that coefficients of variation (COVs) of the ratio of calculated to measured capacities (Qc/Qm) are significantly higher than expected. The paper then uses the database of shaft capacities to examine potential relationships between local shaft friction and the cone penetration test end resistance (qt). Predictions are subsequently compared with capacities measured in a series of centrifuge tests and with the distributions of peak frictions observed in two well-documented field tests. Widely different formulations adopted by each of the existing empirical methods give broadly similar COV values for Qc/Qm ratios, and it is concluded that the database of high-quality pile tests needs to be expanded significantly if the reliabi...

Resistance factor contour plot analyses of load and resistance factor design of axially-loaded driven piles in clays

This paper presents a framework for calculating the resistance factors of load and resistance factor design (LRFD) for axially-loaded driven piles in clays that can fully account for all necessary reliability-related parameters. The Imperial College Pile (ICP) design method was adopted to determine the resistance factors. The ICP design method was selected because it has been widely verified and has produced close matches to measured pile load capacities. A high-quality database, originally employed to develop the ICP design method, was used to assess the uncertainties of base and shaft capacities. The uncertainties of dead and live loads were determined from previously reported results. The challenge of this paper was to identify the effect of base-to-shaft capacity ratios on resistance factors, which has not been considered in previous methodologies. The resistance factors, compatible with the load factors given in the American Association of State Highway and Transportation Officials (AASHTO) LRFD design specifications and those provided in the American Petroleum Institute (API) LRFD recommended practice, were calculated and proposed for different levels of target reliability index. The effect of base-to-shaft capacity ratios on resistance factors was noticeable, while the effect of dead-to-live load ratios on resistance factors was relatively small.

Insights gained from instrumented centrifuge displacement piles in kaolin

The paper describes the results from a series of centrifuge and 1<i>g</i> model pile tests in kaolin clay. The piles, which were equipped with total pressure sensors located at different levels on the pile shaft, were jacked into samples of reconstituted kaolin clay. The kaolin clay samples were prepared to range of undrained strengths (<i>s_u</i>) and overconsolidation ratios (OCRs). These pile tests were used to investigate lateral stress changes around piles during pile installation and subsequent equalization. A specific investigation of the influence of strong soil layering on the development of lateral stresses is also presented. The experiments demonstrate a strong dependence of the lateral stress developed on a displacement pile on the Cone Penetration Test end resistance (<i>q_t</i>) and the relative depth of the pile tip (<i>h</i>). It is shown that a field-test based empirical method that allows for a dependence of shaft friction on <i>q_t</i> and <i>h</i> provides good estimates of the shaft capacities measured in the centrifuge experiments.

Pile-clayey soil interaction analysis by boundary element method

This paper is an attempt to solve the soil-pile interaction problems using the boundary element method (BEM). A computer package called PGroupN, which deals mainly with the analysis of the pile group problem, is employed in this study. Parametric studies are carried out to assess the impacts of the pile diameter, pile length, ratio of spacing to diameter and the thickness of soil stratum. The external load is applied incrementally and, at each increment, a check is made that the stress state at the pile-soil interfaces does not violate the yield criteria. This is achieved by specifying the limited stresses of the soil for the axial pile shaft capacity and end-bearing resistance. The elements of the pile-soil interface yielded can take no additional load, and any increase in load is therefore redistributed between the remaining elements until all elements have failed. Thus, by successive application of loading increments, the entire load-displacement relationship for the pile group is determined. It is found that as the applied load reaches the ultimate bearing capacity of the pile group, all the piles will share the same amount of load. An exception to this case is for the center pile in a group of 9 piles embedded in clay, which is not consistent with the behaviors of the other piles in the group even if the load reaches the ultimate state. For the 4 piles group embedded in clay, the maximum load carried by the base does not exceed 8% of the load carried by each pile with different diameters. This low percentage ascertains that the piles embedded in cohesive soils carry most of the load throughout their shafts.

Performance of Transportation Infrastructure

Performance of Transportation Infrastructure

Load and Resistance Factor Design of Drilled Shafts in Sand

Resistance factors are developed for drilled shafts for a design method based on soil variables. The uncertainties associated with the design variables and equations were systematically quantified, and Monte-Carlo simulations were performed to obtain the distributions of the shaft and base capacities. Both the base and shaft capacities were found to follow normal distributions, and the applied dead and live loads were assumed to follow normal and lognormal distributions, respectively. Reliability analysis was then performed to obtain the limit state and nominal resistances and loads for a variety of soil profiles and pile dimensions. The optimal dead- and live-load factors were subsequently obtained from the analysis. The optimal resistance factors were then adjusted for use with the load factors recommended by the Federal Highway Administration.

Shaft Friction from Instrumented Displacement Piles in an Uncemented Calcareous Sand

AbstractThe behavior of displacement piles in uncemented calcareous sand is investigated using field piles instrumented with a sensor that simultaneously records the radial stress and shear stress at specific locations on the pile shafts. These tests are interpreted with the assistance of data from adjacent self-boring pressuremeter tests and from monotonic and cyclic direct shear interface tests performed on reconstituted samples. The existence of extremely low radial stresses on the pile shafts is verified. Although dilation during shear is seen to compensate for such low radial stresses, short-term shaft capacities are much lower than capacities of equivalent piles in siliceous sands. The development of a bonded or welded crust to the pile shaft was seen to be the primary contributor to the setup observed at the test site; this crust forced failure to take place at a sand-sand rather than a sand-steel interface and also gave rise to higher levels of dilation during monotonic loading. The welded sand cr...

Prediction of the shaft resistance of nondisplacement piles in sand

SUMMARYUsing pile segment analysis, the mobilized shaft resistance of axially loaded nondisplacement piles in sand is investigated here. It is accepted that the shaft capacity of piles constructed in granular soils is highly influenced by the mechanical behavior of soil–structure interfaces forming adjacent the piles skin. Adopting the thin interface layer as a load transfer mechanism, a simple but accurate critical state compatible interface constitutive model is introduced. After evaluation, the interface model in conjunction with the pile segment analysis is applied for the prediction of the shaft resistance mobilized in nondisplacement piles. The proposed approach takes into account the influences of pile diameter and surface roughness together with the effects of the surrounding soil density and stiffness on the mobilized shaft resistance. The performance of the proposed method is verified by comparing its predictions with the experimental data of various model piles covering wide ranges of length, diameter, roughness, and surrounding soil properties. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Evaluation of uplift interpretation criteria for drilled shafts in gravelly soils

Representative interpretation criteria are examined in this paper to evaluate the capacity of drilled shaft foundations under axial uplift loading in gravelly soils. A large number of uplift shaft load tests for gravelly soils are used for analysis, and the interpretation criteria are applied to these load test data to establish a consistent uplift interpretation criterion. The statistical results show that the smaller the uplift displacement, the higher the coefficient of variation. In general, the displacements required to mobilize shaft failure load in gravelly soils are larger than those in non-gravelly soils. Based on these analyses, the relative merits and interrelationships of these criteria are established. Specific design recommendations for the evaluation of uplift drilled shaft capacity are given.

Shaft Capacity of Open-Ended Piles in Sand

Shaft Capacity of Open-Ended Piles in Sand

Improved Evaluation of Interface Friction on Steel Pipe Pile in Sand

AbstractOpen-ended steel pipe piles are now widely used as the foundations for offshore structures. The pile-soil interface behavior is of particular interest in sands where shaft friction plays an important role in resisting the applied load. The rational design of the shaft capacity depends on a good understanding of the mechanisms of interface friction during pile installation and static loading. There are two new methods on the basis of the cone penetration test results that take into account the effect of friction fatigue arising from pile installation. An improvement is made in this study to account for the influence of plugging degree, which is a key issue for open-ended piles. The significance of the modified design framework lies in that it allows for the role of plugging in a more rational way by using the soil-squeezing ratio that is closely related to the radial effective stress and, consequently, the shaft capacity. The performance of the improved method is assessed against the existing metho...

Shaft Capacity of Open-Ended Piles in Clay

This paper describes an experimental investigation designed to assess the impact of pile end condition on the capacity of piles installed in soft clay. A series of field tests are described in which instrumented open-ended and closed-ended model piles were jacked into soft clay. The radial stresses, pore pressures, and load distribution were recorded throughout installation, equalization, and load-testing. Although the total stress and pore pressure developed during installation were related to the degree of soil plugging, the radial effective stress that controls the shaft resistance was shown to be independent of the mode of penetration. The long-term shaft capacity of the open-ended pile was closely comparable to that developed by closed-ended piles, suggesting a limited influence of end condition on the fully equalized shaft resistance. In contrast to the shaft resistance, the base capacity was highly dependent on the degree of plugging.

The Shaft Capacity of Displacement Piles in Clay: A State of the Art Review

The rapid expansion of the offshore wind sector, coupled with increasing demand for high rise structures, has placed renewed demand on the driven piling market. In light of this industry growth, this paper reviews the evolution of design approaches for calculating the shaft capacity of displacement piles installed in cohesive soils. The transition from traditional total stress design towards effective stress methods is described. Complex stress–strain changes occur during pile installation, equalisation and load testing and as a consequence, the selection of parameters for use in conventional earth-pressure type effective stress approaches is not straight-forward. These problems have led to the development of empirical correlations between shaft resistance and in situ tests, such as the cone penetration tests. However, many of these approaches are limited because they were developed for specific geological conditions. Significant insight into pile behaviour has been obtained from recent model pile tests, which included reliable measurements of radial effective stresses. These tests have allowed factors such as friction fatigue and interface friction to be included explicitly in design methods. Whilst analytical methods have been developed to investigate pile response, these techniques cannot yet fully describe the complete stress–strain history experienced by driven piles. The use of analytical methods in examining features of pile behaviour, such as the development of pore pressure during installation and the effects of pile end geometry on pile capacity, is discussed.

Analysis of shaft resistance of jacked piles in sands

AbstractIn recent years, pile jacking has become a viable alternative installation method for displacement piles. Pile jacking produces minimal noise, vibration and air pollution during installation. In addition, it is possible, at the end of jacking, to have a good estimate of the ultimate static capacity of the pile. In this paper, the shaft resistance of piles jacked into sand is studied using one‐dimensional finite element analysis. The finite element simulations, using a two‐surface plasticity model, demonstrate the effects of relative density and confinement on the unit shaft resistance of piles jacked in sand. The impact of the number of jacking strokes on the unit shaft capacity is also assessed. Based on the numerical results, we developed equations for shaft resistance quantifying the effects of relative density, initial confinement and number of jacking strokes. Predictions using these equations are compared with data obtained from centrifuge tests and field tests. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Field investigation of the effect of installation method on the shaft resistance of piles in clay

This paper presents the results of a series of field experiments performed to study the effect of installation method on the shaft resistance developed by a pile installed in soft clayey silt. Tests were performed on piles that experienced different levels of cyclic loading during installation. The test results indicate that the radial total stress, pore-water pressure, and shear stress on the pile shaft during installation were strongly affected by the installation procedure; all three were found to increase when the jacking stroke length used during installation increased (or the number of cyclic load applications decreased). However, equalized radial effective stresses that control the long-term pile shaft capacity were found to be insensitive to the installation method. A simple expression that requires the results of a cone penetration test, laboratory measurements of the interface friction angle, and the pile geometry is proposed to calculate the shaft resistance.

Evaluation of Lateral Interpretation Criteria for Drilled Shaft Capacity

Representative interpretation criteria are examined to evaluate the capacity of nonrigid drilled shaft foundations under lateral loading. A wide variety of lateral load test data are used for analysis, and these data are divided into drained and undrained databases. The interpretation criteria are applied to these load test data to establish consistent lateral interpretation criteria. Among these criteria, the results are generally comparable for both drained and undrained loading. The statistical results show that the smaller the displacement or rotation is, the higher the coefficient of variation. Moreover, the undrained load test results present somehow less variability than the drained results. Based on these analyses, the relative merits and interrelationships of these criteria are established, and specific design recommendations for the interpretation of lateral drilled shaft load test are given in terms of capacity, displacement, and rotation.