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Lateral stress changes and shaft friction for model displacement piles in sand Barry M. Lehane and David J. White
Abstract: The paper describes a series of tests performed in a drum centrifuge on instrumented model displacement piles in normally consolidated sand. These tests examined the influence of the pile installation method, the stress level, and the pile aspect ratio on the increase in lateral effective stress on the pile shaft during static load testing to failure. A parallel series of constant normal load and constant normal stiffness (CNS) laboratory interface shear experiments was performed to assist interpretation of the centrifuge tests. It is shown that although the cycling associated with pile installation results in a progressive reduction in the stationary horizontal effective stress acting on a pile shaft and densification of the sand in a shear band close to the pile shaft, this sand dilates strongly during subsequent shearing to failure in a static load test. The dilation (the amount of which depends on the cyclic history) is constrained by the surrounding soil and therefore leads to large increases in lateral effective stresses and hence to large increases in mobilized shaft friction. The increase in lateral stress is shown to be related to the radial stiffness of the soil mass constraining dilation of the shear band and to be consistent with measurements made in appropriate CNS interface shear tests. The paper’s findings assist in the extrapolation of model-scale pile test results to full-scale conditions. Key words: sand, displacement pile, centrifuge tests, shaft friction. Résumé : Cet article décrit une série d’essais réalisés dans un centrifuge en forme de tambour sur des modèles de pieux à déplacement instrumentés placés dans un sable normalement consolidé. Ces essais examinent l’influence de la méthode d’installation des pieux, du niveau des contraintes et du rapport géométrique du pieu sur l’augmentation de la contrainte latérale effective sur le fût du pieu durant l’essai de chargement statique à la rupture. On a réalisé en parallèle une série d’expériences de cisaillement à l’interface en laboratoire, à charge et rigidité normales constantes, pour aider à l’interprétation des essais au centrifuge. Il est montré que, quoique la charge cyclique associée à l’installation du pieu résulte en une réduction progressive de la contrainte effective stationnaire horizontale agissant sur le fût du pieu et en une densification du sable dans une bande de cisaillement près du fût du pieu, ce sable se dilate fortement durant le cisaillement subséquent à la rupture dans un essai de chargement statique. La dilatation dont la quantité dépend de l’histoire des contraintes est confinée par le sol environnant, et en conséquence, conduit à de fortes augmentations des contraintes effectives latérales et de là, à de fortes augmentations du frottement mobilisé au fût. On montre que l’augmentation en frottement latéral est reliée à la rigidité radiale de la masse de sol contenant la dilatation de la bande de cisaillement et est cohérente avec les mesures faites dans des essais appropriés de cisaillement CNS sur l’interface. Les résultats de cet article aident à l’extrapolation des résultats d’essais de pieux, de l’échelle du modèle à des conditions de pleine échelle. Mots clés : sable, pieu à déplacement, essais au centrifuge, frottement du fût. [Traduit par la Rédaction]
Lehane and White
Introduction Laboratory-scale investigations into pile behaviour remain popular because of the high cost of field testing and the possibility of achieving specific soil characteristics in a laboratory environment. Model pile tests in sand have been performed in laboratory test chambers for many years (e.g., Kérisel 1964) and, more recently, at elevated g levels in the centrifuge (e.g., de Nicola 1997; Bruno 1999; Klotz 2000; Fioravante 2002). Extrapolation of these experimental results
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to full-scale conditions is hampered, however, by the uncertainty surrounding scale and size effects. For example, although Klotz and Coop (2001) argued that there is little evidence for the diameter dependence of the ultimate shaft shear stress (qs) that can develop on a pile in sand, Foray et al. (1998) and others showed that qs decreases as the pile diameter (D) increases and as the mean particle size (D50) decreases. Furthermore, as illustrated by Garnier and König (1998), the displacement required to mobilize ultimate shaft friction on model piles is often comparable to that required
Received 29 June 2004. Accepted 8 December 2004. Published on the NRC Research Press Web site at http://cgj.nrc.ca on 18 August 2005. B.M. Lehane.1 School of Civil and Resource Engineering, University of Western Australia, Crawley, Perth, WA 6009, Australia. D.J. White. University of Cambridge, The Schofield Centre, Cambridge University Engineering Department, Trumpington Street, Cambridge CB2 1PZ, UK. 1
Corresponding author (e-mail:
[email protected]).
Can. Geotech. J. 42: 1039–1052 (2005)
doi: 10.1139/T05-023
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for full-scale piles (i.e., 2–10 mm); in model tests, such relative movements may even be greater than those required to mobilize full base capacity. Scaling laws also seem to vary with the pile installation method, as shown, for example, by Al-Mhaidib and Edil (1998), who indicated that qs for “buried” piles was over double that for a driven pile with D = 48 mm, whereas little influence of the method of installation was inferred for piles with D = 178 mm. Experimental investigations of pile shaft friction in sand generally use strain gauges or load cells to measure the axial pile load distribution, from which the distribution of local shaft friction can be calculated. A more complete understanding of the mechanisms controlling the shaft friction may be obtained if the lateral stresses acting on the pile shaft are also measured, as illustrated, for example, in tests involving the 100 mm diameter Imperial College London instrumented field pile; see Lehane et al. (1993) and Chow (1997). Klotz and Coop (2001) were the first to report lateral stress measurements on (16 mm diameter) centrifuge displacement piles in sand, but they provided no information concerning lateral stress changes that occur as a pile is loaded to failure from an at-rest position. The investigation presented here examines the lateral stress changes that take place during installation and load testing of 9 mm square, closed-ended centrifuge model piles installed into sand. Such lateral stress changes were measured under a variety of testing conditions and are used, together with data from constant normal load (CNL) and constant normal stiffness (CNS) direct shear interface tests, to shed light on factors that can assist in extrapolation from model-scale to full-scale conditions.
Fig. 1. Analogy between a pile–soil interface and a CNS test.
Model for pile shaft response Previous model testing has shown that a thin shear zone exists adjacent to a loaded pile shaft, whereas the more distant soil remains largely undeformed (Robinsky and Morrison 1964; White and Bolton 2004). Therefore, as shown in Fig. 1, the load transfer behaviour at the pile shaft can be idealized as the shearing of a thin interface zone surrounded by a soil mass that undergoes minimal deformation and restrains the shear-induced volume changes in the interface zone. Changes in lateral stress on the pile shaft arise from changes in volume of the shear zone and will increase with increasing levels of confinement provided by the surrounding soil. This behaviour is analogous to that seen in CNS interface shear box tests (e.g., see Airey et al. 1992). The change in lateral stress, ∆σ h, on a cylindrical pile shaft of diameter D, due to a change in shear band thickness, ∆t, can be compared with the response in a CNS interface shear box test by considering elastic cavity expansion (Boulon and Foray 1986), as follows: [1]
∆σ h = (4G∆t)/D = kn ∆t
where G is the operational shear modulus of the soil around the pile; and kn is the spring stiffness in the CNS test. Lehane and Jardine (1994) verified the form of eq. [1] by using a database of measured or estimated values of ∆σ h adjacent to displacement piles and showing that ∆σ h increased with sand density and stress level and decreased strongly with pile diameter. Values for ∆t and G cannot, however,
currently be estimated to provide realistic predictions for ∆σ h. The soil deformation imposed by pile installation influences G, which, even for small ∆t, may differ appreciably from the in situ very small strain value, G0. The thickness of and the volumetric strain within the pile–soil interface zone govern ∆t. Few observations of this zone have been made, although some insight can be gained from interface tests in transparent-sided shear boxes (e.g., Uesugi et al. 1988). Such uncertainty prompted the investigation, described in the following, of the link between ∆σ h measured in a load test on a displacement pile in sand with that inferred from a CNS interface test.
Drum centrifuge experiments Pile details The pile experiments were performed in the drum centrifuge at the University of Western Australia (UWA). The ring channel of this machine has an outer diameter of 1.2 m, an inner diameter of 0.8 m, and a channel height (⬅ sample width) of 0.3 m. This centrifuge was selected in preference to the UWA beam centrifuge, as it offered the possibility of conducting multiple pile installations within the same sample without the need to halt the centrifuge (and hence cycle the soil self-weight). The independently rotating central shaft and tool table can be driven relative to the ring channel by a hollow stepper motor and brought to a halt, independ© 2005 NRC Canada
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Fig. 2. UWA geotechnical drum centrifuge.
ently of the channel, to allow instruments to be changed. An actuator is mounted on the tool table, on which the pile is attached and controlled. A full technical description of this centrifuge is presented by Stewart et al. (1998), and the centrifuge is shown, with the safety cover removed, in Fig. 2. The tests used a 9 mm square, 185 mm long, steel, closed-ended pile, shown in Fig. 3. Lateral soil stresses (σ h) acting on the pile were measured with four 6 mm diameter earth pressure sensors, which were located at distances above the pile base (h) of 9, 27, 54, and 108 mm, that is, at h/B values of 1, 3, 6, and 12, where B is the pile breadth (9 mm). Two further sensors were located at the opposite pile face, at h/B = 1 and 3, to provide a check on the repeatability of lateral stress measurements at these relative tip depths. The sensors were bonded in slots machined into the pile face, which were then filled with clear epoxy. The sensor cables passed through a central void in the shaft and emerged 165 mm above the pile tip, for connection to the data acquisition system. The model pile design also allowed for full-length extension pieces to be added to each of the uninstrumented pile faces so that pile width (W) to breadth (B) ratios of 2 and 6 could also be tested. The centreline average roughness (Rcla) of the steel pile and extension pieces was 0.6 ± 0.1 µm. Test programme Fine silica sand was used in the experiments. This sand was deposited in flight (at 20g) by dry pluviation in the 300 mm wide drum channel to the full channel depth of 200 mm. Water was introduced into the sand to induce a suction pressure that allowed the sample to be levelled to a uniform full depth of 180 mm after the channel was brought to a halt. The channel was then accelerated to 50g and remained spinning at either 50g or 150g over the 9 day duration of the pile-testing programme. Cone penetration tests
(CPTs) at 50g were the first tests conducted, and these commenced 24 h after “spin-up”, when no further drainage of water from the base of the sample was observed. Emptying of the channel at the end of the test series revealed evidence of slightly moist sand (with a water content of 2.5 ± 0.5%) in a 50 mm thick layer adjacent to the sample base, indicating that completely dry conditions had not been achieved. The testing schedule, which is summarized in Table 1, included 18 separate pile installations in the sand, 14 of which had a final embedment depth of 120 mm. The investigation examined the effects of three modes of installation explicitly, as follows: (i) Monotonic installation: The piles were pushed into the sand continuously at a rate of 0.2 mm/s, with a brief pause at a tip depth of 60 mm to allow a static load test to be conducted. (ii) Jacked installation: The piles were installed in a series of jacking strokes. For each stroke the piles were pushed at 0.2 mm/s for a distance of 2 mm, followed by extraction at 0.005 mm/s until the pile head load was zero. Each “jacking stroke” resulted in a net penetration of between 1 and 1.5 mm (with the lower net penetration being typical toward the end of installation, because of the higher rebound on unloading). (iii) Pseudo-dynamic installation: The piles were installed in a series of downward jacking increments of 2 mm at 0.2 mm/s, followed by extraction by 1.5 mm at 0.2 mm/s; the extraction component simulated the rebound experienced by driven piles as the reflected tension wave approaches the pile head. Each cycle resulted in a net penetration of 0.5 mm. Static compression load tests (performed at 0.005 mm/s) were conducted at embedded pile lengths of 60 and 120 mm for the 50g tests and at an embedment of 60 mm for the 150g tests. The maximum embedment (of 120 mm) ensured © 2005 NRC Canada
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Fig. 3. Schematic drawing and photo of instrumented centrifuge model pile.
that the piles were at least 60 mm (≈7B) above the base of the drum channel. Tension tests followed the compression tests on the 120 mm long piles. It is noteworthy that a preliminary experiment indicated no difference in penetration resistance when the monotonic installation rate was varied between 0.005 and 0.2 mm/s. All pile installations and load tests could therefore be considered fully drained. Sand properties Properties of the sand used in the experiments are summarized in Table 2. This sand is produced commercially by Imdex Ltd., in Australia, and is a natural, subrounded, fine–medium silica sand. The interface friction angle (δ) between the sand and the model pile (where tan δ = ratio of shear to normal effective stress) was of importance to the test interpretation, and therefore 16 direct shear, CNL interface tests were performed, using a steel interface with the same roughness as that of the pile (i.e., Rcla ≈ 0.6 µm) for a range of initial sand relative densities (Dr) and normal effective stresses (σ ′n). Typical plots of mobilized interface friction angle and vertical displacement against horizontal displacement are shown in Fig. 4a; all peak and constant volume interface friction angles measured (δ p and δ cv) are plotted in Fig. 4b. No systematic variation of δ with σ ′n was observed for the investigated σ ′n range of 50–150 kPa. The mean measured δ p values (for Dr > 0.5) and δ cv values of 16° and 12.5°, respectively, are comparable to δ p and δ cv values of 18.4° and 11°, respectively, reported by Frost et al. (2002) in similar experiments with dense, but coarser, sand (D50 = 0.72 mm). De-
spite the relatively smooth interface, all samples with Dr > 0.5 dilated during shear and attained a maximum dilation of 5–10 µm before reaching constant volume conditions. This small measured change in sample height is over 10 times the Rcla value and cannot therefore be associated with the soil particles lifting as a rigid body out of the troughs on the steel surface. Peak interface friction angles were developed at horizontal shear box displacements of
