Ultimate Shaft Research Articles

The load-transfer method as a tool for determining the load-displacement curve of piles

The paper presents two applications (software packages) in which the load-transfer method is used for axially loaded Kelly drilled bored piles and displacement ductile iron piles. In the first, the ultimate friction is related to the effective stress via the so-called β method. The β method is refined into three stages to cover the variety of soils typical of Central Europe. For the driven piles, a different approach is presented in which the ultimate shaft friction is related to the reference hammering time. The recorded hammering time profile is fed directly into the software based on the load-transfer method. Analyses of five loading tests are presented proving that the load transfer method in combination with the β method or the recorded hammering time profile is able to compute the load-displacement curve of both replacement and displacement piles with a reasonable accuracy in various geological conditions.

Advancing Rock-Socketed Pile Design with a Unified Interface Shear Strength Framework for Soft Rocks

The shaft resistance of rock-socketed piles (RSPs) is primarily influenced by the interactions between the pile, rock and any soft interface materials. This study presents a fundamental experimental and numerical investigation to predict the shaft resistance of model RSPs in soft rocks through a comprehensive shear strength framework incorporating the major variables such as roughness and smear fabric. By calibrating the Discrete Element Method (DEM) results with the experimental outcomes, this study evaluates the load-resistance attributes of RSPs in three different soft rocks for a wide range of roughness and smear configurations. The interface roughness effect of the RSPs in terms of the friction coefficient was correlated with the ultimate shaft resistances, uniaxial compressive strength and Young’s modulus of the rock. The study then incorporated the effect of smear fabric (placement, thickness and area proportion) in the shear strength framework of clean shafts. Comprehensive review of the DEM results revealed that the socket roughness effect diminishes at a critical roughness factor (RF)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$({\ ext{RF}})$$\\end{document} of 0.4, beyond which the smear predominantly influences the shaft capacity. Following this, the effects of roughness and smear fabric were incorporated into a single equation representing the interface shear strength of RSPs, where the existence of smear results in a maximum reduction of up to 75% of the ultimate shaft resistance. The distinctive feature of this unified interface shear strength framework lies in its integration of the new smear fabric parameter and the linkage to the mechanical properties of soft rocks, which is the limitation of the earlier studies. It thereby sets a strong base for future studies aimed at advancing the pile-rock interface models.

Method for Calculating Vertical Compression Bearing Capacity of the Static Drill Rooted Nodular Pile

The static drill rooted nodular (SDRN) pile is a new kind of composite pile foundation made by inserting a precast nodular pile into the cemented soil. Based on the tests and analysis on the mechanism characteristics of these two kinds of interfaces, which are between the pile and cemented soil, and between the cemented soil and soil, this paper proposes a calculation method for the ultimate vertical bearing capability of the SDRN pile considering two failure modes. When the precast pile and surrounding cemented soil fails as a whole, the diameter of the cemented soil pile is taken to calculate the ultimate shaft resistance, and the shaft resistance of the SDRN pile is about 1.05∼1.10 times that of the bored pile in the same soil layer. When the core precast pile fails in penetration mode, the diameter of the core pile is taken. The pile shaft resistance takes that of displacement piles in the same soil layer. The lower nodular pile shaft resistance should consider the squeezing effect of nodular joints. Besides, the improvement of pile tip resistance due to the expanded cemented soil should also be taken into consideration. The result is the smaller value calculated according to these two failure modes. The ultimate bearing capacity of a SDRN pile calculated by the theoretical method is not only compared with the field test result, but also the simulation result of a 2D model pile built by PLAXIS finite element software. In addition, the finite element simulation is also confirmed to be an effective way to investigate the mechanism characteristics of SDRN piles more thoroughly.

Incorporating Setup Effects into the Reliability Analysis of Driven Piles

Driven-pile setup is referred to a phenomenon in which the bearing capacity of driven piles increases with time after the end of driving (EOD). The setup effect can significantly improve the bearing capacity (ultimate resistance) of driven piles after initial installation, especially the ultimate shaft resistance. Based on the reliability theory and considering the setup effects of driven piles, this article presents an increase factor (Msetup) for the ultimate resistance of driven piles to modify the reliability index calculation formula. At the same time, the correlation between R0 and Rsetup is comprehensively considered in the reliability index calculation. Next, the uncertainty analysis of load and resistance is conducted to determine the ranges of relevant parameters. Meanwhile, the influence of four critical parameters (factor of safety FOS, the ratio of dead load to live load ρ = QD/QL, Msetup, the correlation coefficient between R0 and Rsetup, and ρR0,Rsetup) on reliability index are analyzed. This parametric study indicates that ρ has a slight influence on the reliability index. However, the reliability index is significantly influenced by FOS, Msetup, and ρR0,Rsetup. Finally, by comparisons with the existing results, it is concluded that the formula proposed in this study is reasonable, and more uncertainties are considered to make the calculated reliability index closer to a practical engineering application. The presented formula clearly expresses the incorporation of the pile setup effect into reliability index calculation, and it is conducive to improving the prediction accuracy of the design capacity of driven piles. Therefore, the reliability analysis of driven piles considering setup effects will present a theoretical basis for the application of driven piles in engineering practice.

Model test study of bending moment and negative skin friction for batter rock-socketed piles under surface load

Batter rock-socketed piles (BRSP) foundation is one of common foundations, such as port engineering or cross-sea bridge, while there are few studies on negative skin friction for BRSP. A series of model tests are conducted to explore negative skin friction of BRSP which are embedded in thick soft clay. The effects of the inclined angle of piles and soil consolidation time to negative friction resistance and the bending moment of BRSP are analyzed. The test results show that: the development of negative friction and bending moment BRSP have pronounced time effect; the longer the consolidation time is, the slower the axial force and bending moment intensify. The ultimate pile shaft axial force and bending moment increases nonlinearly concerning the inclined angle of piles. And the “neutral point” position and peak point of bending moment is always located at 0.9~1.0 times soil depth.

Numerical modelling of the long-term effects of XCC piling in fine-grained soil

Although the development and utilization of X-section cast-in-place concrete (XCC) pile have been reported for some time, the long-term effects of XCC piling in fine-grained soil, in particular the set-up effects, have received little attention. This paper reports a coupled effective analysis of XCC piling using the dual-stage Eulerian-Lagrangian (DSEL) technique. The pile installation and subsequent soil consolidation was explicitly and consecutively modelled. The generation of excess pore pressure and alteration of stress states during the pile installation were explored. The distribution and magnitude of effective normal stress at pile/soil interface, in particular its evolution with time, were investigated. The influence of set-up effects on the XCC pile shaft resistance was assessed and quantified. It was found out, although the shaft resistances of both XCC and circular pile develops substantially with time, the consolidation in the wake of XCC pile installation can bring in more capacity enhancements and therefore practical benefits. The ultimate shaft resistance of XCC pile is 45% higher than that of the circular pile of the same cross-sectional area. Additionally, practical advice was given on how to optimize of the cross-sectional shape of XCC piles to take full advantage of set-up effects and achieve economical designs.

Forecasting bearing capacity performance with semi-empirical and theoretical methods applied to precast concrete piles founded on sandy clay in the region of Uberlândia-MG, Brazil

This article presents a study on the performance of semi-empirical methods based on the Standard Penetration Test (SPT) for the prediction of bearing capacity already disseminated in the practice of Brazilian Foundation Engineering (Aoki and Velloso, 1975; Décourt and Quaresma, 1978, 1996; Teixeira,1996), together with the recent method proposed by Pereira (2020), and the theoretical method known as a Method (disseminated around the world). These were applied to precast concrete piles based on sandy clay in the Uberlândia-MG region. In the performance analysis, the ultimate shaft and tip resistances and the bearing capacity values, which were utilized as references, were mobilized in the dynamic loading tests performed on ten piles. The results of these tests were compared with those obtained using the mentioned methods. In general, both methods yielded robust results, with relatively accurate predictions, except the a Method, which was found to perform below the semi-empirical methods considered. The research still highlights the need for discretion in the application of semi-empirical/theoretical methodologies because there are certain scenarios wherein these methods can yield inaccurate results.

Relating shaft friction of buried piles and CPT resistance in clayey sands

This paper examines factors controlling the ultimate shaft friction developed on buried piles in normally consolidated clay–sand mixtures and explores the relationship between this friction and the cone penetration test (CPT) end resistance. Results from tension load tests on buried piles in a laboratory pressure chamber and geotechnical centrifuge are presented. Variable rate CPTs combined with Rowe cell, undrained simple shear and interface direct shear tests are used to characterise the mixtures. It is shown that the measured drained CPT end resistances could be predicted using parameters assessed from the laboratory tests and a simple expression derived from cavity expansion theory in an elastic Mohr–Coulomb soil. The tension pile tests indicate how changes in radial effective stresses during loading can be inferred from interface shear tests and these combined with the cavity expansion analysis allow derivation of a relationship between shaft friction (τf) and CPT end resistance (qt). This expression reveals the significant effects on the qt/τf(= βc) ratio of soil stiffness, stress level, dilation and drainage during cone penetration and is shown to be consistent with a recently proposed empirical formula for bored piles.

A review of research on the shaft resistance of rock-socketed piles

Shaft resistance generally dominates at the service loads of rock-socketed piles and therefore is always a topic of large research interest. This paper reviews the research progress that has been made in the last four decades in understanding the shear mechanism of the pile–rock interface and in calculating the shaft resistance. First, particular attention is given to notable previous studies of the shear mechanism and the method for calculating the shear strength at the pile–rock interface. Next, some commonly used design methods and many empirical correlations between the ultimate shaft resistance fsu and the unconfined compressive strength σc of the intact rock are summarized, and the factors considered in these design methods (e.g., roughness, joints, discontinuities, smear, construction, and disturbance) are compared. Also, the factors that influence the shaft resistance of rock-socketed piles are summarized. Then, by evaluating briefly the existing theoretical methods, the limitations of elastic normal stiffness and the two-dimensional shear model are discussed. Finally, combined with a comparison between the elastic and elastoplastic solutions of the normal stress increment with the radial displacement increases and an analysis of paths of radial and tangential stress and possible crack formation of the bore wall during expansion, three modification methods using the elastoplastic solution to calculating the increment of normal stress are proposed to calculate shear strength at the pile–rock interface and some suggestions are also made for future research to optimize the calculation of shaft resistance.

Bored piles in tropical soils and rocks: shaft and base resistances, t–z and q–w models

Analyses of load tests on 100 instrumented bored piles in different weathering grades of different tropical geological formations of peninsular Malaysia enabled correlations of ultimate shaft and base resistance with standard penetration test (SPT) results and unconfined compression strengths. The data also enabled development of shaft resistance (t–z) and base resistance (q–w) models that are related to SPT, unconfined compressive strength and rock types. The t–z models can be used for strain softening and strain hardening while the q–w models are for strain hardening and stiffening behaviour. The models thus developed were applied for analysis of 35 piles in the database that were loaded until the load–settlement curves were significantly non-linear. Most of the analyses resulted in a reasonable match with the measured load–settlement and load-transfer curves up to 1.5 times the pile working load, regardless of whether the q–w function was strain hardening or stiffening. Accurate matching with measured load–settlement and load-transfer curves for 1.5–3 times working load was conditional on the correct choice of the q–w function. The models were further tested against 27 published load test results from across the world.

A new method for predicting the ultimate shaft resistance of rock-socketed drilled shafts

Data from 187 loading tests on rock-socketed shafts are used to develop a new relationship between the ultimate shaft resistance fsu and unconfined compressive strength σc of intact rock. The adhesion factor αq of the shaft resistance for different types of rock and the relationship between αq and the embedment ratio are discussed. Furthermore, because the shafts are socketed in the natural rock mass, as well as the aforementioned correlation with intact rock, three new relationships between fsu and the unconfined compressive strength σcm of the rock mass are proposed by considering the rock quality designation, which is related to discontinuity of the rock mass. Statistical methods are used to evaluate and verify the existing methods and newly proposed relationships. Results obtained using the new relationship based on σc are found to be more accurate than those obtained using existing methods, and when the rock quality designation is available, results obtained using σcm are closer to measured values than those based on σc. The value of αq of the discussed rocks decreases with σc. Mudstone and shale have relatively high values of αq with more dispersion, whereas αq values for gneiss, basalt and granite are small overall with less dispersion.

Interface shear strength characteristics of steel piles in frozen clay under varying exposure temperature

The ultimate shaft capacity of pile foundations in frozen grounds has long been correlated to the long-term shear strength of the surrounding frozen soils using a surficial roughness factor “m”. This roughness factor is different for different pile materials (e.g., steel, concrete, and timber), but is often assumed to be constant for any soil type, ground temperature, or stress condition. The current study evaluates the validity of the proposed roughness factor “m” for steel piles embedded in frozen clay and exposed to different scenarios of ground temperatures and normal stress levels. Interface element tests were utilized to characterize the shear strength of frozen Leda clay and the adfreeze strength of the pile-frozen clay interface and to investigate the proposed roughness factor “m” for steel piles exposed to various temperatures and normal stress conditions. The experiments were carried out in a walk-in temperature-controlled environmental chamber. Roughness factor “m” was found not to be a constant number for a given pile material, but rather to decrease with an increase in the freezing temperature. A frictional factor “n”, analogous to roughness factor “m”, was also introduced to correlate the frictional resistance of frozen soil to the frictional resistance of the pile-soil interface. A temperature-dependent empirical equation was also proposed for predicting the shaft capacity of steel piles based on the shear strength parameters of the surrounding ice-rich clay soil.

Improving the design of piles driven in chalk through the ALPACA research project

Chalk is present under large areas of NW Europe as a low-density, porous, weak carbonate rock. Large numbers of offshore wind turbines, bridges and port facilities rely on piles driven in chalk. Current European practice assumes ultimate shaft resistances that appear low in comparison with the Chalk’s unconfined compression strength and CPT cone resistance ranges and can impact very significantly on project economics. Little guidance is available on pile driveability, set-up or lateral resistance in chalk, or on how piles driven in chalk can sustain axial or lateral cyclic loading. This paper describes the ALPACA (Axial-Lateral Pile Analysis for Chalk Applying multi-scale field and laboratory testing) project funded by EPSRC and Industry that is developing new design guidance through comprehensive field testing at a well-characterised low-to-medium density test site, supported by analysis of other tests. Field experiments on 36 driven piles, sixteen of which employ high resolution fibre-optic strain gauges, is supported by advanced laboratory andin situtesting, as well as theoretical analysis. The field work commenced in October 2017 and was largely complete in May 2019.

Field Performance of Open-Ended Prestressed High-Strength Concrete Pipe Piles Jacked into Clay.

The behavior of open-ended pipe piles is different from that of closed-ended pipe piles due to the soil plugging effect. In this study, a series of field tests were conducted to investigate the behavior of open-ended prestressed high-strength concrete (PHC) pipe piles installed into clay. Two open-ended PHC pipe piles were instrumented with Fiber Bragg Grating (FBG) sensors and jacked into clay for subsequent static loading tests. Soil plug length of the test piles was continuously measured during installation, allowing for calculation of the incremental filling ratio. The recorded data in static loading test were reported and analyzed. The distribution of residual forces after installation and the effect on the bearing capacity were also discussed in detail. The test piles were observed to be in partially plugged condition during installation. The measured ultimate shaft resistance and total resistance of the test piles were 639 and 1180 kN, respectively. The residual forces locked in the test piles after installation significantly affected the evaluation of the axial forces, and thus the shaft and end resistances. It tended to underestimate the end resistances and overestimate the shaft resistances if the residual forces were not considered in the loading test. However, the residual forces did not affect the total bearing capacity of open-ended PHC pipe piles in this study.

Maintained load-test results for continuous flight auger piles in south-east England Chalk

This paper presents a case study of the results from Roger Bullivant Ltd's testing database from the last 4 years, consisting of eight working load tests and one preliminary test spread across five sites in south-east England. The paper covers the geology of each site and the design of the piles, which was carried out using current industry design practice. The results were analysed to predict the ultimate shaft friction of each pile. As the test results demonstrated that the piles behaved satisfactorily at working load, it is argued that the piles could be seen to be uneconomical. When piles in Chalk are tested, it is common that they rarely deflect to a point where any noticeable failure is deemed to occur. Through a comparison of available published data together with this new data set, a more efficient design is proposed for piles in Chalk with a higher limiting shaft friction value.

Resistance factor calculations for LRFD of driven piles based on setup effects

Abstract Setup effects, one of a large number of uncertainties in geotechnical engineering, can strongly enhance the bearing capacity (ultimate resistance) of driven piles after initial installation, especially for ultimate shaft resistance. Based on load and resistance factor design (LRFD) principle and the reliability theory, this paper incorporates the setup effects to propose a methodology for separately calculating the resistance factors for ultimate base and shaft resistance for reliability-based design (RBD) of driven piles. The proposed approach can clearly explain the different sources of uncertainties of the ultimate resistance, including those from ultimate base and shaft resistances. Meanwhile, the discussions on the uncertainties of resistance and load and the probability characteristics of setup effects are presented, and the effects of four relevant parameters (setup effect factor, ratio of dead to live loads ρ = LD/LL, target reliability index, and ratio of ultimate base resistance to load ψ = Rult,b/(LD + LL)) on resistance factors are analyzed. Parametric study indicates that, setup effects and target reliability index significantly influence on the optimal ultimate base and shaft resistance factors for RBD of driven piles, and ρ = LD/LL has the limited effect on the optimal ultimate base and shaft resistance factors. Varying ψ = Rult,b/(LD + LL) obviously causes the change of the optimal ultimate base resistance factor although it slightly influences the optimal ultimate shaft resistance factor. Therefore, the appropriate choice of target reliability index, and the accurate evaluations of setup effects and ψ = Rult,b/(LD + LL) have great roles on resistance factor calculations for LRFD for driven piles.

Load sharing and behavior of single pile embedded in unsaturated swelling soil

This study presents a series of model experiments conducted on single pile embedded in saturated and unsaturated expansive soil. A soil composed of 70% bentonite and 30% sand which represents a very high expansive soil is employed in this study. Soil suction (total and matric) was measured using the filter paper technique and calibrated electrical resistance gauges to study the behavior of the soil water retention curve (SWRC) and initial suction and their influence on the results. The results showed that the ultimate skin resistance is increased about 49% when the initial degree of saturation decreases from 90 to 70%. However, the adhesion factor (α, used in calculating the skin friction) increases as the unconsolidated undrained cohesion decreases and as the initial matric suction decreases. The adhesion factor reached a value of 1.3 when the undrained cohesion is 36 kPa, initial degree of saturation of 100% and initial matric suction of 10 kPa. The sharing ratio of the ultimate shaft capacity to the ultimate pile capacity of a single pile changes from 91 to 81% when the degree of saturation decreases from 100 to 70%.

Determination of base and shaft resistance factors for reliability-based design of piles

This paper aims to propose a procedure for calculating separately the resistance factors for ultimate base and shaft resistances for the reliability-based design of piles. The proposed procedure can clearly explain the different sources of uncertainties of the bearing capacity, including those from ultimate base and shaft resistances. The study evaluates the convergence of the proposed procedure, and the effects of relevant parameters on resistance factors. Finally, two examples are used for comparison and application of the presented method for determining ultimate base and shaft resistance factors. Convergence analysis proves that the final resistance factors can be rapidly obtained, and maintain good stability using the iteration algorithm included in the proposed procedure. A parametric study indicates that the ratio of dead load to live load and initial values of the ultimate shaft (or base) resistance factor, have a limited effect on the final convergence values of ultimate shaft (and base) resistance factors. However, the target reliability index has significant influence on the ultimate shaft and base resistance factors. The validation example shows that the ultimate shaft and base resistance factors, as calculated in this paper, are conservative compared to the results by Kim et al (2011), due to the consideration of more uncertainties. The recommended ultimate shaft and base resistance factors for the reliability-based design of piles can be obtained conveniently using the proposed procedure in the application example.

Pile load test of jacked open-ended prestressed high-strength concrete pipe pile in clay

The performance of open-ended piles can be different from that of closed piles due to the effect of soil plugging. This paper presents the results of a large-scale field pile load test conducted on an open-ended pile in clay under static compressive loads. A prestressed high-strength concrete pipe pile with an outer diameter of 400 mm and a wall thickness of 75 mm was instrumented with strain sensors and installed in a marine deposit for static load testing. The measured load-transfer data during pile installation and the pile load test results are reported and analysed. The data show that the pile was in a partially plugged condition during jacking and behaved in a fully plugged mode in the static load test. The measured ultimate shaft resistance and total resistance of the test pile were 659 and 1180 kN, respectively, which were smaller than the values calculated using a previously proposed method. The back-calculated α and β values (as used in the α and β methods) for axial pile capacity in marine deposits were 0·42 and 0·56, respectively.

Characteristics of Gravelly Granite Residual Soil in Bored Pile Design: An In Situ Test in Shenzhen

Granite residual soil is widely distributed in south China and is treated as a special soil. Its design parameters in rotary drilling bored piles are a matter of debate due to lack of in‐situ pile load tests. Back‐analysis of test piles is a reliable means of studying the geotechnical capacity of granite residual soil for pile design. In this study, a series of in situ tests was conducted comprising six full‐scale instrumented test piles in gravelly granite residual soil in Shenzhen to consider the effects of different construction methods. The six piles were constructed with three different rotary drilling methods. Two commonly used design methods were investigated in the back‐analysis: the SPT and effective stress methods. The results of the loading tests and strain gauges were used to obtain the back‐analyzed parameters of the ultimate shaft resistance and ultimate base resistance for gravelly granite residual soil with these two design methods.