Centrifuge experiments on laterally loaded piles with wings

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1 Centrifuge experiments on laterally loaded piles with wings J. Dührkop & J. Grabe Hamburg University of Technology, Hamburg, Germany B. Bienen, D.J. White & M.F. Randolph Centre for Offshore Foundation Systems, University of Western Australia, Perth, Australia ABSTRACT: Foundation piles of offshore wind farms and other engineering structures, including dolphins as well as anchor piles of floating offshore facilities, are predominantly laterally loaded. Near the pile head the lateral response is weak and compliant owing to the low soil strength and stress level close to the seabed. The lateral bearing capacity can be improved by enlarging the pile near the pile head by adding wings. This cross-sectional expansion increases the overall resistance from the soil, with the added benefit of enabling a reduction in required pile length. This paper presents experimental results from laterally loaded piles in sand with and without enlargement at the pile head and compares the observed behaviour. The tests were conducted in the beam centrifuge at the University of Western Australia. 1 INTRODUCTION Many offshore facilities are founded on, or are anchored with, piles to which the structure transfers large horizontal loads relative to the imposed vertical load when compared to onshore installations. Examples include offshore wind farms or floating offshore facilities (such as FPSOs). Due to the often low soil strength near the mudline large pile dimensions are required in order to mobilise the required lateral resistance. The recently developed concept of expanded cross-sections or 'wings' near the pile head (Deutsches Gebrauchsmuster 2005) has been shown to increase the lateral capacity and the lateral stiffness of the pile response. Evidence includes results from 1g laboratory tests (Dührkop & Grabe 2008) and small-scale onshore field trials (Dührkop 2009; Dührkop & Grabe 2009). The improved lateral capacity results in significant cost savings through quicker installation and material savings as the required pile length (and/or diameter) is reduced. The shorter pile length implies the added benefit of rendering refusal during pile driving less likely. This paper further explores the validity of the winged pile concept through experiments of model scale large diameter piles in a geotechnical centrifuge. The performance of piles in sand with and without wings was investigated in both the installation and lateral loading phases. 2 EXPERIMENTAL SET-UP AND PROCEDURE 2.1 UWA beam centrifuge and set-up A series of lateral pile load tests were performed using the beam geotechnical centrifuge facility at the University of Western Australia (Randolph et al. 1991). The soil sample was contained in a rectangular strongbox of 650 mm length and 390 mm internal width, representing a prototype test bed with plan dimensions of 130 m by 78 m (at a centrifuge acceleration of 200g). Figure 1. Model pile sections. Threaded connection to load cell Depth of embedment The tests featured a short solid (i.e. closed-ended) aluminium pile section that was rigidly connected to

2 the motion actuator. The piles, shown in Figure 1, had a prototype diameter of 2.4 m and an embedment length of 9.6 m. One reference pile and one pile with wings were tested. The wings were located just below the soil surface of the embedded pile. The conical widening of the pile above the embedded length was necessary to accommodate the threaded connection to the load cell, but remained above the soil surface throughout the testing. Unlike the prototype piles, which are made from steel, the model piles were fabricated from aluminium. For the problem under consideration direct scaling of the wing dimensions and the pile wall thickness were of secondary importance to correct scaling of the stiffness. The short pile sections were connected to the actuator via a stiff loading arm through a load cell as shown in Figure 2. This arrangement does not represent the usual field conditions which correspond to a free-headed pile. However, it was intended to provide a simpler boundary condition for the backanalysis of these tests. The two-dimensional actuator, which allows movement in the vertical and horizontal planes, was mounted across the strongbox. The experimental set-up is shown schematically in Figure 3. The dimensions and other properties of the model piles are given in Table 1. The pile wings contributed an additional 44% to the lateral projected area of the piles. All tests were conducted at a centrifuge acceleration level of 200 g. Figure 3. Model pile and loading arm. Table 1. Model pile characteristics. Property Value Model scale Prototype scale Diameter (of body), D 12 mm 2.4 m Diameter (across wings) 28 mm 5.6 m Embedded length, L 48 mm 9.6 mm Wing length 16 mm 3.2 m Wing width 8 mm 1.6 m Wing thickness 1 mm 0.2 m Flexural stiffness (of body), EI 70.2 Nm GNm 2 Flexural stiffness (of wings), EI w Nm MNm 2 Bending strain gauges Axial load cell Fixation points to actuator 2.2 Instrumentation The piles were un-instrumented. However, the vertical loads during installation as well as the horizontal load and moment loads in the lateral loading phase were measured with a VHM (vertical, horizontal, moment) load cell that connected the short pile section to the actuator. The vertical and horizontal movement of the actuator was recorded from the motor encoders. However, in addition the displacement of the pile head was measured directly by a linear displacement transducer (LDT) that was placed against the pile head in line with the horizontal movement. This additional instrument provided an indication of any rotation that occurred at the pile head due to flexure of the actuator and load cell arrangement. Figure 2. Model pile and loading arm. Winged pile 2.3 Soil characteristics The experiments were carried out using commercially available superfine silica sand, the properties of which are summarized in Table 2 (Cheong 2002).

3 Table 2. Soil characteristics. Property Value d mm φ cv 34 Minimum dry density 14.9 kn/m 3 Maximum dry density 18.0 kn/m 3 Additional penetrometer tests performed with a 2.4 mm diameter needle confirmed the sample homogeneity near the soil surface, which was important for the winged piles. The gradient of the needle resistance, q N, was consistent with that from the larger cone penetrometer (Fig. 5). The tests were performed on dry medium dense sand (γ 15.9 kn/m 3 ). All eight tests presented here were performed in the same sample. Profiles of cone resistance that were obtained from miniature cone penetrometer tests (CPT) performed in flight are shown in Figure 4. Figure 5: Needle and cone resistance profiles. Figure 4. Cone resistance profiles. As expected for sand of this relative density, the measured cone resistance, q c, increased approximately linearly with depth. However, the testing program involved starting and stopping the centrifuge several times between tests. This process leads to densification, which resulted in higher cone resistances in the later penetrometer tests. CPT1a and CPT1b were performed early on, after the first two pile tests, while CPT2a and CPT2b were carried out after all of the eight pile tests were completed. The following correlation (1) of cone resistance and relative density (Schneider & Lehane 2006), which is also plotted in Figure 4, suggests a relative density, D r, of 40 to 49 %. qc Q Q = Dr (1) σ 250 ' v Experimental procedure Both the pile installation and the lateral loading were achieved using a two-dimensional actuator that was mounted across the strongbox. The short pile sections were installed (jacked) in flight, i.e. at a centrifuge acceleration of 200g. All short piles were installed at a model rate of 0.5 mm/s. However, this is not believed to be critical as the tests were performed on dry sand. Following installation of the pile, the centrifuge was required to be stopped to position the LDT against the pile head in order to measure its lateral movement in the subsequent testing stage. The lateral loading phase could commence once the centrifuge had again reached the target acceleration. Table 3 provides an overview of the eight pile tests and the corresponding cone tests (CPT name) presented in this paper. The pile was installed to 9.6 m depth (prototype). In all of the tests the pile was monotonically pushed to a lateral displacement of at least 10% of the diameter of the reference pile (D) at a model rate of 0.01 mm/s. In six of the experiments, a cyclic loading stage preceded the lateral monotonic capacity test. The amplitudes of the one-way cyclic loading were (a) 0.16 MN, (b) 0.48 MN and (c) 0.8 MN, respectively at prototype scale. The cyclic loading was applied via a sinusoidal waveform at a model

4 frequency of 0.25 Hz. Results of the cyclic load tests will be presented in a further publication. Table 3. Test overview. Test name CPT name Description of lateral test stage S1-1aM CPT1a, b Monotonic push S1-1aW CPT1a, b Monotonic push S1-2aM CPT1a, b Low amp. cyclic, monotonic push S1-2bM CPT2a, b Medium amp. cyclic, mon. push S1-2cM CPT2a, b Large amp. cyclic, monotonic push S1-2aW CPT1a, b Low amp. cyclic, monotonic push S1-2bW CPT1a, b Medium amp. cyclic, mon. push S1-2cW CPT1a, b Large amp. cyclic, monotonic push S1: sample 1; 1: monotonic; 2: cyclic; a, b, c: 1 st, 2 nd 3 rd test; M: reference pile; W: pile with wings 3 TEST RESULTS 3.1 Notation The vertical load is denoted V and the penetration z. The horizontal and moment loads at the pile head (level with the soil surface) are labelled H 0 and M 0, respectively, while the lateral pile head displacement is y. All results are presented in dimensionless form. Due to the densification of the sand sample the penetration resistance V/A tip (with A tip being the crosssectional area at the pile tip) in Figure 6 is related to the cone resistance, q c,z=l, that was measured in the most relevant cone test (Table 3) at a depth of 9.6 m. Over the first 6.4 m (or 0.66L) the winged piles mobilised a similar penetration resistance compared to the conventional piles without wings. However, additional resistance was activated when the wings touched down and penetrated into the soil as illustrated in Figure Lateral response The monotonic lateral response of the piles with and without wings was compared in eight tests (Table 3). Two of those piles were virgin-loaded. The remaining six were subjected to a cyclic load before being pushed to a maximum pile head displacement of 0.2D. Figure 7 shows the measured normalized load vs. displacement curves of the different piles. 3.2 Installation resistance The penetration resistance increased approximately linearly with depth as seen in Figure 6. Figure 7: Lateral response of short model piles. Figure 6: Penetration resistance of model piles. All piles with wings provided a considerably higher resistance and a stiffer behaviour, although there was no evidence of a limiting load being reached in any of the tests. Instead, the response gently strainhardened. Piles with a previous cyclic loading history (which is not shown in Figure 7 for clarity) exhibited a very stiff behaviour initially, which resulted from soil densification during the cyclic loading phase. As expected, the ultimate resistance of these piles was similar to those without the cyclic loading history (Fig. 7). The mobilised pile head moment was also measured and normalised by the cone resistance. This is depicted in Figure 8. Initially, the moment increases with increasing lateral load. The maximum moment was mobilised at a specific lateral load, which corresponds to a pile head displacement of about y/d = The winged piles were able to carry a higher moment than those without wings.

5 4 DISCUSSION 4.1 Installation resistance The penetration resistance profile of the reference pile is approximately linear with depth. The penetration resistance was dominated by the base resistance, due to the short aspect ratio and closed-ended tip condition of these piles. A value of V/A tip /q c,z=l = 1 at z/l = 1 in Figure 6 would describe a pile without shaft friction. On average, the normalised penetration resistance of the wingless piles (V/A tip ) was within 10% of q c,z=l at the final embedment depth (Fig. 6). It can be assumed that the unit base resistance followed the q c profile, with the shaft resistance making a negligible contribution to the total penetration resistance. As expected, the penetration resistance of the winged piles is similar to the reference piles over the first 6.4 m. A kink in the approximately linear resistance profiles marks the touch-down point of the wings in the test (Fig. 6). The additional penetration resistance from the wings can be principally linked to the end bearing on the wings, rather than shaft friction. Although the cross-sectional area of the wings is only a small fraction of the base area of the pile (14%), the additional penetration resistance as the wings embed to z/l = 0.33 is comparable to the penetration resistance of the pile body to the same depth. This suggests that the unit end bearing resistance on the wings is higher than on the pile body, by a ratio of 5 or more. This effect can be attributed to the high stresses created around the pile as the body is jacked into the soil, into which the wings must subsequently penetrate. Note that due to the difference in material the wing dimensions of the model piles are larger than is typical for a prototype steel pile (due to the preference for correct scaling of the flexural stiffness). However, prototype piles will typically be openended and therefore have a smaller tip area. The wing tip resistance in the field may be of the order of 40 to 50% of the pile tip resistance, at the same depth within the ground. Figure 8: Pile head moment vs. lateral load. 4.2 Lateral response The lateral response of the short winged piles showed an improvement of lateral bearing capacity of about 40% at the same pile head displacement, or a reduction of lateral displacement of about 50% at the same horizontal load. No peak in the lateral bearing capacity was visible in any of the tests. The capacity increased steadily with increasing deformation. In contrast the pile head moments had definite peak values, see Figure 8, which were mobilized at a pile head displacement of about y/d = This was similar in all tests. The existence of a peak pile head moment can be interpreted as a change in the mode of lateral response. At the beginning of the test, at small displacements, the pile moved as a rigid body. The subgrade reaction was activated in front of the pile (Fig. 9a). The pile head moment increased almost linearly with pile head load as the distance z R between the soil surface and the resultant reaction force R was constant. Figure 9: Schematic subgrade reaction due to (a) small and (b) larger pile head displacements. Beyond the peak value, the moment reduced due to increasing rotation of the pile head. Although the pile was rigidly connected to the loading arm, this did not impose infinite rotational stiffness at the pile head due to the free length of the loading arm extending to the fixed body of the actuator. This means that the imposed boundary conditions at the pile head are not controlled horizontal movement with zero rotation, but controlled horizontal movement coupled with a finite rotational stiffness. Softening of the p-y response near the soil surface may possibly also have contributed.

6 With larger lateral loads, therefore, rotation at the pile head became more significant and also the pile started to bend. This was clearly visible during the test procedure. The pile head rotation can be deduced from the difference in the lateral displacement measured by the motor encoder on the actuator and the LDT located at the pile head, respectively, as shown in Figure 10. gradual change in the pattern of soil resistance, from that associated with rigid body motion to one reflecting increasing rotation of the pile, occurred at higher lateral load and pile head moment than in the reference piles. Although the focus here was the increased stiffness and capacity provided by the wings, they could also be used to reduce the pile length if no additional lateral pile capacity was required. Similar experiments as presented here have been conducted on long thin-walled free-headed piles. The p-y response that can be obtained from the experiments on the short pile sections investigated here will be used to predict the behaviour of those more realistic piles. 6 ACKNOWLEDGEMENTS Figure 10: Displacement measurements from encoder and LDT. The increasing inclination of the pile activated a subsoil reaction on the passive side of the pile toe, as shown schematically in Figure 9b. This change in lateral pile response took place gradually, which resulted in the soft peaks visible in Figure 8. Independent of the presence of wings this change in mechanism was characterized by a specific pile head deflection (or strain in the near surface soil). However, the piles with wings required a considerably higher load and pile head moment to reach this deflection. The pile load-deflection behaviour is planned to be investigated in more detail through p-y backanalyses. 5 CONCLUSION This paper presented a series of eight centrifuge model tests comparing the lateral capacity of piles with and without wings. The lateral response was demonstrated to be significantly improved by wings located in the softer soil close to the pile head. In the example shown, the lateral capacity of a winged pile was increased by 40% compared to a regular pile at the same pile head displacement. The The work described here forms part of the activities of the Centre for Offshore Foundation systems (COFS), established under the Australian Research Council s Research Centres Program and now supported by Centre of Excellence funding from the State Government of Western Australia. The authors acknowledge the contribution of Don Herley, who assisted with the centrifuge experiments, and Shane De Catania, who developed the control software. The collaboration was made possible through support from the Go8-DAAD research collaboration scheme. 7 REFERENCES Cheong, J Physical testing of jack-up footings on sand subjected to torsion. Honours Thesis, University of Western Australia. Deutsches Gebrauchsmuster Gründungspfahl DE Dührkop, J Zum Einfluss von Aufweitungen und zyklischen Lasten auf das Verformungsverhalten lateral beanspruchter Pfähle in Sand. thesis. Publications of the Institute for Geotechnical and Construction Engineering, Hamburg University of Technology, Vol. 20. Dührkop, J. & Grabe, J Laterally loaded pile with bulge. Journal of Offshore Mechanics and Arctic Engineering 130: Dührkop, J. & Grabe, J On the Response of Laterally Loaded Piles with Wings. Bautechnik, 86(12): Randolph, M.F., Jewell, R.J., Stone, K.J.L. & Brown, T.A Establishing a new centrifuge facility. Proc. Int. Conf. on Centrifuge Modelling, Centrifuge 91, Boulder, Colorado, 3-9. Schneider, J.A. & Lehane B.M Effects of width for square centrifuge displacement piles in sand. Proc. International Conference on Physical Modelling in Geotechnics. Hong Kong. Taylor & Francis. 2:

Numerical Investigation of the Effect of Recent Load History on the Behaviour of Steel Piles under Horizontal Loading

Numerical Investigation of the Effect of Recent Load History on the Behaviour of Steel Piles under Horizontal Loading K. Abdel-Rahman Dr.-Ing., Institute of Soil Mechanics, Foundation Engineering and Waterpower