Ultimate Shaft Resistance Research Articles

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.

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.

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.

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.

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.

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.

Experimental study on axial response of different pile materials in organic soil

Sixty four tests were performed in a steel tank to investigate the axial responses of piles driven into organic soil prepared at two different densities using a drop hammer. Four different pile materials were used: wood, steel, smooth concrete, and rough concrete, with different length to diameter ratios. The results of the load tests showed that the shaft load capacity of rough concrete piles continuously increased with pile settlement. In contrast, the others pile types reached the ultimate shaft resistance at a settlement equal to about 10% of the pile diameter. The ratios of base to shaft capacities of the piles were found to vary with the length to diameter ratio, surface roughness, and the density of the organic soil. The ultimate unit shaft resistance of the rough concrete pile was always greater than that of other piles irrespective of soil condition and pile length. However, the ultimate base resistance of all piles was approximately close to each other.

Time-dependent uplift capacity of driven piles in low to medium density chalk

A series of load tests have been performed on instrumented 762 mm dia. tubular steel piles driven into low to medium density grade A/B chalk at St Nicholas at Wade, Kent, UK. This paper presents the results from the static axial uplift tests, which were performed on two piles 7, 50 and 120 days after installation in order to investigate the time-dependent variations in shaft resistance. The results show that the static ultimate shaft resistance of this type of chalk can increase by up to a factor of seven over this time period, as a consequence of ‘set-up’ effects. The test results also show that the ‘set-up’ effect is reduced if the pile is subject to lateral loads up to 50% of the ultimate lateral capacity before uplift loading, while the application of lateral loading up to 10% of ultimate lateral capacity had negligible influence on axial capacity. The measured load distribution from strain gauges suggests a mobilisation of larger unit shaft resistance in the lower half of the pile. This paper also describes the geotechnical site conditions, the pile instrumentation and the effects of pile driving on the chalk.

Growth-rate-dependent prediction of pile set-up and its application in driven pile foundation construction

ABSTRACTIn this paper research was presented on the development of a growth-rate-dependent model for pile set-up prediction using the restrike and static/statnamic load testing data collected from different projects. The data included: a) restrike records from ninety-five production piles and restrike and load test results of nine instrumented piles driven in soft clays from the relocation project of Highway No. 1 in Louisiana (LA-1); and b) restrike and static load testing data of five fully instrumented square PPC piles driven at four different bridge sites in various soil layers from sands to clays in Florida. Research effort was focused on the prediction of the ultimate shaft resistances with pile set-up formulated using the pile resistance growth rate-dependent model. The timeframe of interest was studied for a practical set-up magnitude such as 90% of the ultimate shaft resistance (Q90). As an application of the rate-dependent model, it was found that piles at the LA-1 relocation project, in general, reached about 95% of the ultimate shaft resistances at the time of 2 weeks after pile installation. The strategy of incorporation of pile set-up in adjusting pile driving criteria or/and design during pile construction, such as the experience-based plan of a two-week waiting period adopted by Louisiana DOTD, was investigated and justified.

Bidirectional cell tests on non-grouted and grouted large-diameter bored piles

Abstract The 37-storey apartment buildings of the Everrich II project in HoChiMinh City, Vietnam was designed to be supported on a piled foundation consisting of bored piles assigned a 22-MN working load per pile. The foundation design included performing bidirectional-cell, static loading tests on four test piles. The soil profile consisted of organic soft clay to about 28 m depth followed by a thick deposit of sandy silt and silty sand with a density that gradually increased with depth from compact to dense, becoming very dense at 65 m depth. In March 2010, the test piles, one 1.5-m diameter pile and three 2.0-m diameter piles, were installed to 80 m through 85 m depth and constructed using bucket drill technique with bentonite slurry and a casing advanced ahead of the hole. The bidirectional-cell assemblies were installed at 10 m through 20 m above the pile toes. The piles were instrumented with pairs of diametrically opposed vibrating wire strain-gages at three to four levels below and six to seven levels above the respective cell levels. After completed concreting, the shaft grouting was carried out throughout a 20 m length above the pile toe for the 1.5-m diameter pile and for one of the 2.0-m diameter piles. The static loading tests were performed about 34 through 44 days after the piles had been concreted. The analysis of strain-gage records indicated an average Young’s modulus value of about 25 GPa for the nominal crosssections of the piles. The average unit grouted shaft resistances on the nominal pile diameters were about two to three times larger than the resistance along the non-grouted lengths. The measured load distribution of maximum mobilized shaft resistances corresponded to effective stress proportionality coefficients, ß, of about 0.2 through 0.3. The ultimate shaft resistance for the pile lengths below the bidirectional cells reached an ultimate value after about 8 to 10 mm movement, whereafter the load-movement was plastic. The pile toe stress-movement responses to toe stiffness were soft with no tendency toward an ultimate value.

Analysis of Distribution Characteristics and Reliability for Bearing Capacity of Piles

This paper aims to study the distribution characteristics of the ratio of measured value and calculated value for ultimate bearing capacity, shaft resistance and tip resistance, and discuss the impact of shaft resistance and tip resistance on ultimate bearing capacity. A new performance function is proposed in terms of the three types of bearing capacity mentioned ahead. Take bored piles and driven piles for example, and the results from analysis indicate that the ratio of the measured value to calculated value of bored piles ranges from 0.75 to 1.45, and mostly is greater than 1.0; The ratio of the measured to predicted bearing capacity of driven piles lies between 0.8 and 1.5, and is larger than the corresponding ratio of bored piles. In addition, the reliability of tip resistance is lager than that of shaft resistance for bored pile, while the reliability of tip resistance is less than that of shaft resistance for driven piles. Meanwhile, the method presented in this paper can offer references to designers for revising and improving the technical code for pile foundations.

Discussion of “Design Methodology for Axially Loaded Auger Cast-in-Place and Drilled Displacement Piles” by Sungwon Park, Lance A. Roberts, and Anil Misra

The authors propose to employ t-z functions for estimating the load-movement response of a pile subjected to a static loading test. Judging by the authors’ Fig. 1, the proposed t-z functions are springslider relations, that is, they model the shaft and toe resistances as bilinear with an elastic initial part followed by a plastic part representing the ultimate resistance (capacity). The authors state that the capacity is mobilized at a pile-head movement of 5% of the pile diameter, which they also apply as the definition for the pile-toe load-movement response. First, the discusser would expect that themovement should be the movement at the pile element considered—a pile does not have to be very long before a substantial portion of the movement at the pile head is attributable to pile axial compression. It is not logical to assume that the length of the initial elastic response decreases with depth, all other factors being equal. Second, applying an elastic-plastic model for the pile-toe response is incorrect. The response of the pile toe—usually denoted as the q-z function to separate it from the t-z function expressing the shaft response—does not show an ultimate resistance but is a gently rising curve showing no kinks or explicit changes in curvature. Third, the pile diameter has nothing to do with the changeover from elastic to plastic response nor, indeed, anything to do with the ultimate shaft resistance. The magnitude of the movement at the point of transition from elastic to plastic response may differ in different soils. However, it is usually also much smaller than the authors’ mentioned range of 10–20 mm. Moreover, it is independent of the pile diameter, be the pile a small-diameter pile, say, 300mm, or a large one, say, 3,000mm, and be the pile shape circular or square or rectangular, such as a barrette, which can have one side about 1 m in length and the other between 3 and 8 m in length. Fourth, the elastic-plastic response is too simplified a model for the shaft resistance. Although it can occur, a strain-hardening response is more common, particularly in sand that includes silt and clay.Other soil types can exhibit strain-softening response, particularly in soft clays. The mathematical expressions for four common t-z functions are given in Eqs. (1)–(4) (Fellenius 2012). Eq. (1) presents the relation for the ratio function, so called because it states that the ratio between two values of unit shaft resistance (or load) is equal to the ratio between the movements produced by the same loads raised to an exponent