Dynamic Analysis of Large Diameter Piles Statnamic Load Test
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Dynamic analysis of large diameter piles Statnamic load test K.J. Bakker Delft University of Technology, Delft, and WAD32 bv, IJsselstein, The Netherlands F.J.M. Hoefsloot Fugro Ingenieursbureau, Leidschendam, The Netherlands E. de Jong Volker Wessels Stevin Geotechniek bv, Woerden, The Netherlands ABSTRACT: To check for the bearing capacity of a newly introduced large diameter casing pile, both common type CPT analysis as well as static load testing was used. In addition to that two piles where tested using the Statnamic load testing technique. For the latter, in addition to the standard Unloading Point method by Middendorp (1992), a dynamic analysis with Plaxis was done. Further the static load test results were used to calibrate the soil parameters for the analysis. The numerical results were used to corroborate the physical test results. Comparing the Dynamic load testing results according to Middendorp with the Numerical results; it comes forward that some additional mass below the pile tip, more or less moving with the pile, needs to be taken into account. Further after Mullins (2002), the damping is calculated for the stationary part of track 4. This gives overall a better agreement with static numerical analysis for the bearing capacity. Based on an extrapolation of the physical test up to deformation required by NEN 6743-1 (2006), an ultimate load bearing capacity of 15.5MN was established. 1 INTRODUCTION Related to the city extension Leidsche Rijn near Utrecht, The Netherlands, and the extension of the railway between Utrecht and Gouda to four railway tracks, a total of 20 new viaducts need to be build. In order to improve the foundation concept of the classic multi pile pier foundation, an alternative was proposed that includes the use of bored casing piles with a diameter of 1.65m. This innovation reduces the original design of 48 prefab concrete piles to 6 large diameter casing piles only. Since the bridges are intended to support the railway track, the design requirements are very strict. On the one hand the piles should be capable to bear a design load of at the least 12,000 kN, on the other hand the deformation during train passing’s should be less than 10 mm. Since bored piles show a relatively less favourable load-displacement curve compared to driven piles, to improve the stiffness, the introduction of pile tip grouting device was adopted. Both the large diameter casing pile in combination with the grout injection device at the tip Figure 1 Stages in statnamic testing was a first time application in the Netherlands. For total of three piles were instrumented and tested on the evaluation of the bearing capacity, as a start site by means of a static load test and further the CPT’s after prestressing the grouting device, were deformation of all piles were monitored during taken and evaluated as a first step in checking the construction and the actual passing of the first trains influence of the installation procedure. In addition a in December 2007. Details are given in de Jong et al (2010). In addition, after that it became clear that a deformation rate was above 0.3mm/h up to a suitable testing device would be available, two maximum of 4 hours. At a load of 6,000kN the piles additional test piles were tested with a 16MN stat- were unloaded to 4,000kN and reloaded to 6,000kN namic device. in steps of 1,000kN. Only one of the piles was In addition to the standard procedure of checking loaded up to 8,000kN. these tests using the UPM (Unloading Point More details and results of the Static Pile test can Method), by Middendorp (1992), a dynamic axi- be found in de Jong et al (2010). symmetric Finite Element analysis of the load tests was performed. The result of the static load tests After that the Static load tests were done, the both with respect to load and deformation was used introduction of a 16MN Statnamic device in the to calibrate the soil parameters for the dynamic Netherlands by Fugro made it possible to test the numerical analysis. Further the dynamic response as piles to loads that would approach the ultimate calculated with Plaxis was put into the same UPM bearing capacity of the piles. evaluation procedure for further corroboration of the In addition to the field test it was decided to try method. for a dynamic finite element analysis to get a better understanding and to create added value to the field In this paper a comparison between the UPM test evaluation procedure by Middendorp and the The Statnamic load test were done not too far Numerical analysis with Plaxis will be described. from the site were the Static tests were done, and Attention shall be given to the effect of drained had a similar soil stratigraphy. Therefore the static bearing capacity for design and undrained behaviour test were used as an additional source of information during the test. for calibration of the soil parameters for the dynamic Some typical modelling issues such as mesh analysis. The static analysis was back-analysed and finesse at pile tip and dynamic effects in interface soil parameters were calibrated and applied for the elements will be discussed. first prediction of the Statnamic test. In addition to understanding the dynamic behaviour of the pile, the calibrated numerical model opens up the possibility to evaluate the equivalent 3 STATNAMIC TESTING static load bearing capacity, which can be compared to the Dutch code for static load capacity NEN 6743 In November 2008 two Statnamic load tests were (2007). carried out on sacrificial test piles that had been installed in 2005. Originally it was intended to test these piles to refusal by means of a static load test. 2 STATIC LOAD TESTING AND SOIL Such a test would have required a load of PARAMETER ESTIMATION approximately 20,000kN, even though the piles have a limited length of 13m. Before it was decided to do statnamic testing a static In a Statnamic load test, pile loading is achieved pile test was done; although it was known in by launching a reaction mass. Due to the high advance that the limit load with respect to bearing acceleration of the reaction mass, the effectiveness capacity could not be reached.. of the load is increased by a factor of about twenty. At bridge number 6 a row of three, 17.85m long, The loading is perfectly axial, the pile and the soil 1.65m diameter piles at a centre to centre distance of are compressed as a single unit and the static load- 3.75m where individually tested. The goal of these displacement behaviour of the pile can be tests was verification of the load displacement determined if a series of tests is performed. behaviour up to 8,000 kN, i.e. the pile stiffness. Given the fact that the information of the static Strains were measured at three levels with load test up to a load of 8,000kN was already vibrating wire strain gauges, and a load cell was available, the two test piles were only tested once installed at the top. Further displacements were with the maximum Statnamic load of approximately measured at the pile head by high accuracy 16MN. The general description of the statnamic test levelling. In addition to the deadweight of 1,000kN has been given by Bermingham et al (1992). The test the reaction frame was anchored to the ground by 8 procedure itself is according to the draft European Gewi-anchors with a capacity of 1,250kN each. guideline (Hölsher and van Tol, 2009). Prior to pile installation one CPT was carried out at the centre of each pile. Based on the 3 CPT’s before pile installation the ultimate bearing capacity 4 EMPIRICAL EVALUATION OF BEARING of the piles was estimated to be about 20,000kN. CAPACITIY During the static test, the piles were loaded in steps of 1,000kN. The load was then kept constant According to Middendorp’s (1992) simplified for 1 hour, a time that was extended if the evaluation procedure, that dynamic force balance may be written as: displacement. It is customary to back analyse the damping for FtFtFtFtstat() u () v () a () (1) Where Fu (t) = static soil reaction (point and shaft) Fv(t) = damping force (depends on pile velocity) Fa(t) = Inertia force, depending on mass and acceleration Both Fstat(t) and Fa(t) have been measured or can be inferred from the measuring data. Whereas Fu(t) is the unknown soil resistance we want to establish and further Fv(t) is also relatively unknown and depends on the known pile velocity. However, as the loading time is relatively long compared to the velocity of pressure waves in the pile, which is in the order of 3,800m/s, whereas the loading time is in the order of 0,08s, pressure waves may travel up and down the pile, 12 times within this period. For that reason the load may be judged Figure 2 Soil resistance as back-analyzed from the Statnamic test, assuming averaged damping in track 4 of to act semi-static. The effect will be that the pile will the test; concave loading curve is found displace more or less as a rigid mass. For that reason it is assumed that the time of maximum this whole trajectory and taking the average value. displacement equals zero velocity of the pile. According to this standard interpretation method, the Realising that at this point; i.e. referred to as the Soil resistance can be back analysed and will result “Unloading Point” the damping is zero, the pile in the curve given in Figure 2.