Shaft Capacity Of Piles Research Articles

Posttemperature Effects on Shaft Capacity of a Full-Scale Geothermal Energy Pile

AbstractShallow geothermal heat exchangers integrated in structural pile foundations have the capability of being an efficient and cost-effective solution to cater for the energy demand for heating and cooling of built structures. However, limited information is available on the effects of temperature on the geothermal energy pile load capacity. This paper discusses a field pile test aimed at assessing the impact of thermomechanical loads on the shaft capacity of a geothermal energy pile. The full-scale in situ geothermal energy pile equipped with ground loops for heating/cooling and multilevel Osterberg cells for static load testing was installed at Monash University, Melbourne, Australia in a sandy profile. Strain gauges, thermistors, and displacement transducers were also installed to study the behavior of the energy pile during the thermal and mechanical loading periods. It has been found that the pile shaft capacity increased after the pile was heated and returned to the initial capacity (i.e., initi...

A numerical approach to estimate shaft friction of bored piles in sands

A new approach to estimate shaft capacity of bored piles in sandy soils, based on numerical analysis, is presented. The topic is relevant as current design methods often largely underestimate the shaft capacity of piles in sands, thus resulting in an over-conservative design. The proposed approach is based on explicitly modelling the thin cylinder of soil surrounding the pile, where strain localization concentrates (shear band), and on the fundamental mechanic behaviour of sandy soils (e.g. dilatancy, softening). This approach is both simple and easy to apply. Results of a broad parametric study involving axially loaded single piles embedded in different sandy soils are presented, highlighting that relative density and grain size distribution mainly affect the shaft capacity. The capability of the procedure to predict shaft friction is checked against data from a well-documented full-scale axial load test on instrumented pile. Some suggestions for calibration and application of the method are also reported.

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.

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...

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

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 & Sons, Ltd.

Shaft Capacity of Open-Ended Piles in Sand

Shaft Capacity of Open-Ended Piles in Sand

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.

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.

Influence of Particle Properties and Initial Specimen State on One-Dimensional Compression and Hydraulic Conductivity

Particle crushing can adversely affect geotechnical system performance; examples include clogging in wells, pile shaft capacity degradation, and postconstruction settlements. The generation of fines results in volumetric compression and a reduction hydraulic conductivity, which is important for geotechnical systems whose performance is directly dependent on pore pressure dissipation, groundwater flow, or hydraulic pumping. Knowledge of hydraulic conductivity change is poorly understood due to limited experimental data, and an ability to predict this change is lacking. The role of single particle properties, initial specimen state conditions, and loading conditions on the evolution of hydraulic conductivity with particle crushing was examined experimentally. Specimen response exhibited an overshoot behavior and the convergence to a unique condition independent of initial relative density, gradation, and particle shape. The hydraulic conductivity decreased by 2–3 times before specimen yield, and by 2–3 orde...

Shaft Capacity of Continuous Flight Auger Piles in Sand

This paper presents the results of a series of field experiments performed to study the development of shaft resistance on continuous flight auger piles installed in sand. The test piles were instrumented in order to separate the shaft and base resistance, and to allow the determination of the distribution of shaft resistance along the pile shaft. The tests highlighted the importance of accurate calculation of the shaft resistance for nondisplacement piles. At a typical maximum allowable pile head settlement of 25mm, more than 71% of the pile resistance was provided by shaft friction. Conventional methods of estimating shaft resistance were assessed. It was found that methods which incorporated parameters directly interpreted from in situ test results provided the most consistent estimates. In the final section, differences between the shaft resistances mobilized on displacement and nondisplacement piles are considered.

Piles shaft capacity from CPT and CPTu data by polynomial neural networks and genetic algorithms

Cone penetration test (CPT) is one of the most common in situ tests which is used for pile design because it can be realized as a model pile. The measured cone resistance ( q c) and sleeve friction ( f s) usually are employed for estimation of pile unit toe and shaft resistances, respectively. Thirty three pile case histories have been compiled including static loading tests performed in uplift, or in push with separation of shaft and toe resistances at sites which comprise CPT or CPTu sounding. Group method of data handling (GMDH) type neural networks optimized using genetic algorithms (GAs) are used to model the effects of effective cone point resistance ( q E) and cone sleeve friction ( f s) as input parameters on pile unit shaft resistance, applying some experimentally obtained training and test data. Sensitivity analysis of the obtained model has been carried out to study the influence of input parameters on model output. Some graphs have been derived from sensitivity analysis to estimate pile unit shaft resistance based on q E and f s. The performance of the proposed method has been compared with the other CPT and CPTu direct methods and referenced to measured piles shaft capacity. The results demonstrate that appreciable improvement in prediction of pile shaft capacity has been achieved.

Cyclic shear stress degradation and post-cyclic behaviour from sand–steel interface direct shear tests

The sliding interaction between sand and structural materials is involved in many geotechnical applications and is particularly important for the derivation of the shaft capacity of piles. Such interaction develops principally at the interface between the sand mass and the structural surface, and the comprehension of such interaction can be analysed through soil–structure interface tests. In particular, by using a modified version of the interface direct shear apparatus, that is, the constant normal stiffness direct shear apparatus, the friction characteristics of the interface and the role of the soil deformability on the experimental results can be studied. This paper focuses on the stress degradation occurring in these types of tests when cyclic loading is applied on sand–steel interfaces. Also, the post-cyclic response is analysed and compared to the response under monotonic conditions.Key words: interface, sand, shear stress, cyclic loading, stress degradation.

Interface Load Transfer Degradation During Cyclic Loading: A Microscale Investigation

The shaft capacity of piles in sand subjected to cyclic (wave) loading has been observed to decrease significantly with loading cycles (Poulos, 1989). A number of researchers (Boulon and Foray, 1986; Tabucanon et al., 1995; Shahrour et al., 1999) have replicated the characteristics of the load transfer degradation behavior in the laboratory through cyclic interface shear testing with a constant normal stiffness confinement condition (Vesic, 1972). However, no consensus currently exists as to the primary microscale mechanisms that govern cyclic interface shear behavior and load transfer degradation. A research program was undertaken to quantify the contribution of soil properties, cementation, confinement condition, and displacement mode, in load transfer degradation. Monotonic and cyclic interface shear tests were performed using a modified interface direct shear device with a Perspex side window. The specimen particle displacement fields were quantified during selected cycles by capturing high resolution digital images (1600 x 1200 pixels) and using Particle Image Velocimetry (White et al., 2001a). Results indicate that the confinement condition, which is intended to replicate the elastic response of the far-field soil, is of primary importance as it allows for normal stress relaxation with soil contraction adjacent to the interface. The displacement magnitude, particle characteristics, and particle-particle cementation were also observed to affect the magnitude and rate of degradation. It is anticipated that these findings will provide a fundamental rationale to identify field conditions where shear stress degradation is likely to occur and a basis from which more rigorous models may be developed.

The shaft capacity of pipe piles in sand

This paper describes results from an experimental programme that investigated factors affecting the shaft capacity of open-ended (pipe) piles in sand. A number of jacked pile installations in a test chamber filled with loose sand were performed using both open- and closed-ended, 114 mm diameter piles. The test series was designed to investigate the effects of in situ stress level, pile end condition, and degree of plugging on the development of pile shaft resistance. The results indicate that the maximum local shaft resistance that can develop at a given location on a pipe pile may be expressed as a function of the incremental filling ratio of the soil plug during installation, the cone penetration test (CPT) qc value, and the relative position of the pile toe. The experimental results allowed a simple expression to be developed for the plug resistance during pile installation, and this is used in conjunction with a popular design method for closed-ended piles to provide a means of estimating the shaft capacity of open-ended piles. The new approach is shown to provide good estimates of overall shaft capacity and skin friction distribution.Key words: shaft capacity, pipe piles, sand.

Characteristics of Bored Piles Installed Through Jet Grout Layer

When bored piles are installed through a jet grout layer, significant interaction may take place between the piles and the jet grout. Field load tests have indicated that significant enhancement of the pile shaft capacity and axial stiffness was possible for both compression and tension piles. The influence of the jet grout layer was more pronounced for piles under compression loading compared with uplift loading. The effectiveness of the jet grout in transmitting load via shearing action was dependent on the thickness of the grouted zone, the strength of the interlocks between individual jet grout columns forming the grout slab, as well as the interface bond between the pile shafts and the grout. No apparent adverse effect on the performance of permanent foundations was envisaged as a result of the presence of the jet grout layer. However, interpretation of pile behavior from load tests was complicated by the interaction between the test pile and jet grout, resulting in overprediction of pile capacity and axial stiffness. The significance of the interaction has to be carefully evaluated so that a correct interpretation of the true pile capacity and axial stiffness can be made.

Discussion on “Direct Shear Interface Test for Shaft Capacity of Piles in Sand” by E. Saibaba Reddy, D. N. Chapman, and V. V. R. N. Sastry

The authors are to be congratulated for presenting some interesting results on the interfacial friction angle between sands and solid surfaces. The author's results are consistent with some of the findings of the writers, which they are presenting in this discussion.

Closure to “Discussion on ‘Direct Shear Interface Test for Shaft Capacity of Piles in Sand’ by K. S. Subba Rao, Mehter M. Allam, and Retnamony G. Robinson” by E. Saibaba Reddy, D. N. Chapman, and V. V. R. N. Sastry

Closure to “Discussion on ‘Direct Shear Interface Test for Shaft Capacity of Piles in Sand’ by K. S. Subba Rao, Mehter M. Allam, and Retnamony G. Robinson” by E. Saibaba Reddy, D. N. Chapman, and V. V. R. N. Sastry