ULTIMATE STRENGTH OF STEEL SCREW PILES IN SAND

Transcription

1 ULTIMATE STRENGTH OF STEEL SCREW PILES IN SAND P.J. Yttrup 1 & G. Abramsson 2 1 P.J. Yttrup & Associates Pty Ltd, 2 Civil Engineer Denmark ABSTRACT Steel screw piles have been in use for about a decade in Australia, with design strengths up to about 15kN. Screw piles resist applied loads by shaft friction and end bearing at the helix. However, the end resistance is often limited by the structural capacity of the steel helix plate, resulting in design strength that can be significantly less than the available geotechnical strength of the foundation. The paper presents a method for estimating the base resistance of steel screw piles in sand, which compares well with pile load test data. 1 INTRODUCTION The steel screw pile considered in this paper consists of a shaft of circular hollow section steel with a helical plate welded to the shaft at its end. The pile is installed by rotation of the pile with some downthrust also applied. The form of the pile is shown in Figure 1. Figure 1: Steel Screw Pile Screw piles have been used for many years and across the world. Screw piles were used in England in the 183 s and by the Dutch in Indonesia. The use of screw piles in Russia is described by Trofimenkov and Mariupolskii (1964). The Constitution Docks in Hobart, Tasmania were apparently documented with cast iron helicals fixed to timber shafts, however, the actual piling constructed was driven timber. Screw anchors for landslide stabilisation is described by Yttrup & Miner (1998) The behaviour of screw piles during static load testing is similar to other piles. However, the mobilised end bearing strength is generally less than would be expected from CPT or other site investigation data. The observation that pile helices are often bent when piles are exhumed after load testing suggests that structural failure of the helix is involved and limits the base resistance that can be mobilised. For strong ground conditions the mobilised base resistance may be significantly less than the available geotechnical resistance. In this paper, an analysis method is presented which has been calibrated against eight static load tests. The method is also compared to a number of commercially conducted load tests of steel screw piles in sand with very good agreement. 2 BEHAVIOUR OF STEEL SCREW PILES UNDER LOAD Steel screw piles are classified as preformed displacement piles in AS2159 Piling Design and Installation (1995). This may be misleading because steel screw piles do not compact or improve the ground below the base, as do driven piles. This is due to the relatively low displacement caused by the thin helix plate and also because the pile is pulled into the foundation by the helix reacting upwards. The action of the helix during pile installation unloads the ground ahead of the helix, creating a condition similar to bored piers or non-displacement piles. Therefore, the ultimate base pressure, f b, is typically.3 times the CPT cone resistance q c rather than.5 typical for driven piles in sand. However, Australian Geomechanics Vol 38 No 1 March 23 17

2 for strong ground or thin helices the helix plate will fail in bending by plastic deformation before the ultimate geotechnical base resistance can be reached. The ultimate base resistance of steel screw piles is governed by a simultaneous geotechnical and structural failure at the helix. Ground displacement occurs simultaneously with bending of the helix plate as seen in Figure 2 and for piles exhumed after load testing in Figure 3. Figure 2: Helix Bending Figure 3: Helix Bending in Load Test A comparison between the theoretical ultimate base pressure f b and the actual ultimate base pressure realised from load test is shown in Figure 4. The theoretical base pressure is taken as.3q c. The base pressure realised is significantly less than the theoretical base pressure particularly for stronger ground conditions. The ultimate base pressure for the design of steel screw piles has to take into account the structural capacity of the steel helix plate as well as the geotechnical strength of the ground. Analysis of load test data suggests that the shaft friction component of the ultimate geotechnical strength of the pile can be estimated using the same methods as are applicable for normal displacement piles. The following sections of this paper deal with the ultimate base resistance of steel screw piles. 18 Australian Geomechanics Vol 38 No 1 March 23

3 fb from geotech vs fb from load tests f b from load tests kpa f b from geotechnical data kpa Figure 4: Predicted Versus Observed Base Pressure for Steel Screw Piles 3 ULTIMATE BASE RESISTANCE OF STEEL SCREW PILES The screw pile can be modelled as an axisymetric problem for small deformations. At large deformations the hoop compression induced in the large base plate does not model the helix behaviour, which can not develop significant hoop compression due to the discontinuity at the leading and trailing edges of the helix. None the less, finite element analysis suggests a bearing stress distribution similar to that shown in Figure 5. The pressure under the shaft, which is typically 1mm to 2mm below the helix, is about twice that at the inside of the helix which reduces to zero towards the outer edge of the helix. As the thickness of the helix is reduced the pressure distribution contracts inwards towards the shaft, leaving the outer part of the helix at close to zero bearing pressure. "Weak" Helix "Strong" Helix f 2f Where f =.3 x q c Figure 5: Base Pressure Distributions Australian Geomechanics Vol 38 No 1 March 23 19

4 A simple design model based on the above observations is shown in Figure 6(a) and 6(b) for strong and weak helices respectively. For the strong helix, no yielding of the steel helix plate occurs, for the weak helix a plastic yieldline failure mechanism develops in the helix plate. f 2f r f =.3 x q c Where "r" is the distance to plastic hinge location Figure 6(a): Simplified Base Pressure, Strong Helix Plastic Hinge Location f 2f a r Figure 6(b): Simplified Base Pressure, Weak Helix 2 Australian Geomechanics Vol 38 No 1 March 23

5 For a weak helix, the components of base resistance are shown in Figure 7(a). These components can be related using virtual work, the virtual displacement diagram is shown as Figure 7(b). Of the four components of the ultimate base resistance there are three geotechnical and one structural component. The component of the ultimate base resistance are helix plate bending R UMP, helix plate bearing beyond the plastic hinge R UHB, helix plate bearing inside the plastic hinge R UA and bearing at the end of the shaft R UP. R u r R Plastic Hinge m d m R UHP Helix bending R UHB Bearing beyond hinge R UA Bearing inside hinge R UP Bearing on shaft Figure 7(a): Component of Base Resistance Australian Geomechanics Vol 38 No 1 March 23 21

6 Figure 7(b): Virtual displacements * 3.1 HELIX PLATE BENDING The location of the plastic hinge around the shaft is unknown. Using simple bending tests as shown in Figure 8, the plastic hinge is between two and three plate thicknesses out from the shaft surface. The fillet weld between shaft and helix has a size of half the plate thickness. For a 1mm plate, the hinge was three times plate thickness and for 2mm plate, two times. A load-deflection plot for a 12mm specimen is shown in Figure 9. The yield strength of the plate used for laboratory and field strength of the plate used for laboratory and field tests are shown in Table 1, from mill certificates and from bending tests. Figure 8: Bending Test 22 Australian Geomechanics Vol 38 No 1 March 23

7 Sample 6, 12 mm plate Load [N] Deflection [mm] Figure 9: Load Deflection for 12mm Plate Table 1: Yield Strength of Steel Plate Plate Thickness mm Yield Strength MPa Mill Certificate Bending Test The yield line failure mechanism for a helical plate is difficult to describe and to analyse. It was assumed that a flat circular plate would give a reasonable estimate of the behaviour of a helix with a reduction factor to allow for the free edges at leading and trailing edge of the helix not absorbing plastic energy. The internal virtual work for the helix plate is: R UHP. * =. 2πR. m. * Eq 1 (R-r) Where: = correction factor plate to helix, assumed to be.8 m = yield moment per unit length corrected for strain hardening m =β.m m= yield moment per unit length m=f sy.t 2 /4 f sy = yield strength of plate β= strain hardening factor = (.25 (D-d) +.75) 2t t= plate thickness The strain hardening is significant at large deformations as seen in Figure 9. The expression for β assumes strain hardening is related to the helix plate outside divided by thickness. The values of and β were set from observations with the bending tests and the back analysis of load test data described in Section 4 below. Australian Geomechanics Vol 38 No 1 March 23 23

8 3.2 HELIX PLATE BEARING The virtual work due to the bearing pressure under the helix plate outside of the plastic hinge is: R UHP. * = 2π.f. * (a 4 a 3 r + ar 3 r 4 ) (a-r)(r-r) For the plate bending and bearing outside the plastic hinge only the internal virtual work is the sum of Eq 1 and Eq 2. With vertical equilibrium the additional equation is available: (R UHP +R UHB ) = π.f (2r+a)(a-r) Eq 3 3 Equations 2 and 3 can be solved for the distance a when f, R, r, m, and β are known, the authors used an iterative spreadsheet procedure. The distance a defines the area over which base pressure can act and is governed by the plate strength, which depends on thickness and yield strength. Eq2 3.3 HELIX PLATE BEARING INSIDE PLASTIC HINGE AND AT PILE SHAFT These base resistance components are simply pressures times areas, and can be added to the above components of resistance. 4 STATIC LOAD TESTING OF STEEL SCREW PILES Eight controlled pile load tests were conducted in sand at Ocean Grove, Victoria. Four piles with a shaft diameter of 89mm and helix diameter of 4mm, and four piles with a shaft diameter of 139mm and helix diameter of 6mm were used. The helix plate thickness was 1, 12, 16 and 2mm. All piles were installed to 4m depth. The test site has deep sands, the results of CPT s are shown in Figure 1. Qc for CPT tests at Ocean Grove Qc [MPa] Depth [m] 3 4 CPT 1 CPT 2 CPT 3 CPT Figure 1: CPT Profile at Test Site 24 Australian Geomechanics Vol 38 No 1 March 23

9 The load test results are shown in Figure 11 and 12 for the 4mm and 6mm diameter helix piles respectively. The ultimate strength of the pile was defined as the load at a pile deflection of 1% of the helix diameter, the results are summarised in Table 2 with the predicted load from the analysis method from Section 3 above. Bottom Deflection for Test Pile 1-4 at Ocean Grove Load [kn] Deflection [mm] x4x1 89x4x2 89x4x16 89x4x Figure 11: Load Test Results for 4mm Screw Pile Bottom Deflection for Test Pile 5-8 at Ocean Grove Load [kn] Deflection [mm] x6x1 139x6x16 139x6x2 139x6x Figure 12: Load Test Results for 6mm Screw Pile Australian Geomechanics Vol 38 No 1 March 23 25

10 Table 2: Test Results and Predicted Pile Strength Test Pile Measured Ultimate Predicted Ultimate Ratio of Predicted Shaft Helix Strength kn Strength kn Measured 89x95CHS 4 x x95CHS 4 x x95CHS 4 x x95CHS 4 x x7.7CHS 6 x x7.7CHS 6 x x7.7CHS 6 x x7.7CHS 6 x Results from static load tests on screw piles in sand, from actual projects, are plotted in Figure 13 with the predicted strength from the analysis method from Section Estimated Ru [kn] 15 1 Ru Helix Design Linear (1:1 Line) Measured Ru [kn] Figure 13: Load Test Results v. Predicted Strengths 5 STEEL SCREW PILE DESIGN STRENGTHS The Piling Code AS2159 considers the structural and geotechnical strength of a pile as separate and unrelated issues, which is probably appropriate for most types of piles. However, for steel screw piles the bore strength is generated by geotechnical and structural strengths simultaneously. The design equations Eq 4 and Eq 5 below are from AS2159, for steel screw piles an additional equation, Eq 6, may be required when both geotechnical and structural strengths are mobilised together. S* φ s R US Eq 4 S* φ g R Ug Eq 5 S* φ g (Geotechnical Component of Resistance) + φ s (Structural Component of resistance) Eq 6 The geotechnical strength reduction factor φg being as normally defined in AS2159 and the structural reduction factor from AS41 Steel Structure (1998). Any shaft friction resistance can be calculated using normal methods and added to the geotechnical resistance component. 26 Australian Geomechanics Vol 38 No 1 March 23

11 6 CONCLUSIONS The strength of steel screw piles in sand is derived principally from the base resistance at the helix. Unlike other pile types, the base resistance that can be mobilised is limited by the bending strength of the helix plate, that is, the plate thickness, the outstands from the shaft and the yield strength of the steel. The simple design model presented in this paper is considered to be valid for screw piles with helix plate outstands less than 25 times the plate thickness. However, practical piles have helix outstands in the range of 1 to 15 times plate thickness. For large helix outstands, the deflection of the pile becomes a problem. Also, it is likely that the yield line pattern for large and thin helices may develop another plastic hinge and the assumptions made in this paper become invalid. Because of the extensive use of steel screw piles consideration should be given to changing AS2159 to account for simultaneous mobilisation of structural and geotechnical resistance. 7 ACKNOWLEDGMENTS The static test load data used in this paper was provided by Piletech Pty. Ltd., the authors are grateful for their support. 8 REFERENCES Trofimenkov, J.G and Mariupolakii, L.G. (1964) Screw Piles Used for Mast and Tower Foundations Proc. Int. 6 th Conference SMFE Vol 2, pp Yttrup, P.J and Miner, A.S (1998) Landslide Stabilisation Using Screw-in Soil Anchors in Heylasbury Marls 2 nd International Conference on Ground Stabilisation Technique, Singapore, pp Australian Standard AS2159 (1995) Piling Design and Installation Australian Standard AS41 (1998) Steel Structures. Australian Geomechanics Vol 38 No 1 March 23 27