High strain dynamic pile testing - Independent Geoscience Pty Ltd

1. Case study on the application of high strain dynamic pile testing to non- uniform bored piles J.G.Cannon Independent Geoscience Pty Ltd, Melbourne, Australia ABSTRACT: Dynamic pile testing is used frequently to prove the performance of driven pre-formed piles in Australia. It is either specified by the designer or is offered as an alternative to conventional static testing by piling contractors. However, it’s use on cast in place piles is much less frequent. This paper describes a project at Noosa Heads in Queensland, Australia where an excellent “Grade A” correlation has demonstrated that dynamic testing can provide a good prediction of the load vs displacement behaviour of cast in place piles even when the pile shaft is not as designed. The static test pile had the highest over-consumption of concrete at the site, with more than 2 times the design quantity of concrete used during construction. A non-uniform shaft is known to make dynamic testing more difficult and probably less accurate. Nevertheless a very good correlation was obtained between the static test and the dynamic test results for both overall mobilized resistance and the stiffness response of the pile. The static test included measurements that demonstrate potential problems with static testing and these will also be discussed. 1THE SITE constructed using a continuous flight auger (cfa) that is drilled into the ground to a pre-selected level or depth and then a cement grout or, as in this case, high slump concrete is injected down the hollow core of the auger as the auger is withdrawn without rotation. The rig used on this project allowed for monitoring of concrete volume and pressure throughout construction of each pile. This permits the pile constructor to assess whether the pile is “consuming” more or less concrete than would be expected for the nominal shaft diameter. When the pile concreting is completed a reinforcing cage is lowered with vibration into the high slump concrete. The piling contractor adopted a high “geotech- nical reduction factor” (ie low factor of safety) in the design and in accordance with the Australian Piling Code (see references) it was necessary to demon- strate the load vs resistance behaviour of the piles. He decided to adopt a single static load test and 4 high strain dynamic tests. One of the dynamic tests was conducted on the static test pile to establish that the dynamic testing would provide a good prediction of a static load test. Many of the piles for this project “consumed” more concrete than would be expected for the nomi- nal shaft diameter but the greatest “over- consumption” was during construction of pile 68 and this was selected for both static and dynamic testing. The over-consumption on this pile was 105%, ie The site is on Hastings St, which is the commer- cial/tourist centre of Noosa Heads in Queensland, Australia. This is located only a few metres behind a popular surf beach and is immediately adjacent to a tidal inlet and small river. A geotechnical investi- gation of the site had been conducted and 4 borehole logs were provided that described subsoil conditions. The logs suggest that the site is underlain by coastal sand dune or “beach sand” material to a depth of about 10m and this sand is underlain by very stiff to hard clays with SPT results generally 35 but some as low as 18. The sand is loose to moderately dense with standard penetration test results generally in the range 10 to 20 but with some higher and some lower measurements. The SPT results in the sand did not necessarily increase steadily with depth. 2THE PILES The foundation contract was let as a design and con- struct package and the contractor adopted 600mm nominal diameter “cfa” cast in place concrete piles founded at a depth of about 10m (ie entirely within the near surface sand with the toe being influenced by the underlying hard clays). These piles were

2. Figure 1 - Construction record for dynamic/static test pile 4DYNAMIC PILE TESTING more than 2 times the required volume of concrete was used during construction. The contractor’s equipment provided for measuring pressure and vol- ume throughout construction of the pile so the pile profile could be estimated. The contractors record for pile 68 is shown in Figure 1. Most of the extra concrete consumption is shown as a cone between 5 and 10m depth. The test piles were cast above ground level inside a steel sleeve of about 4mm wall thickness and about the same diameter as the pile for about 2.5 pile di- ameters above the surrounding ground. This was done at the same time as casting the pile or as soon as possible after casting the pile in order to keep the concrete for the extension of the same strength and age as the remainder of the shaft. After the concrete had hardened the bottom half of the steel sleeve was removed. This leaves a substantial steel collar at the top to reinforce the pile top during impacts of a drop weight and allows the test equipment to be attached at a level where there is a regular smooth surface with no additional impedance that might interfere with strain measurements. The location of the test gauges had a diameter close to that of the pile shaft and had similar reinforcing. The Author uses the PAK model Pile Driving Analyzer from Pile Dynamics Inc. together with the associated CAPWAP signal matching software. The method and current “state-of-the-art” has been de- scribed in Goble et al (1996). The option to test with 4 strain gauges was not adopted for piles of this size. 3STATIC PILE TESTING A static load test had been conducted on pile 68 prior to the dynamic pile testing. The contractor was careful to avoid the dynamic testing consultant be- coming aware of the static test results before the dy- namic test results were reported. The static test was conducted with several cycles in accordance with the Australian Piling Code. Applied load was measured using the jack pressure only. There are shortcom- ings to this system that are described below. Dis- placement was measured using 3 dial gauges. A check of pile displacement was also taken using a level survey. Load was applied by jacking against a reaction beam and displacement of this beam was also measured by level survey.

3. The contractor supplied a “Hydroquip” HQ5 hy- draulic piling hammer to strike the piles. Some re- building of the hammer’s hydraulic valving had been conducted to increase energy transfer efficiency. Highest energy transfer efficiency during this testing was 76%, which we consider to be high for a hy- draulic hammer with a 5tonne ram striking a solid concrete pile of this diameter. Testing generally commenced with one or two small blows (0.5m stroke) to ensure hammer align- ment was satisfactory and then two or three blows of high energy (1.2m stroke) were applied to gather test data for later analysis with CAPWAP. During CAPWAP analysis the pile was modeled using the construction record but some additional enlargement of the shaft was required near the top. The Author considers the additional pressure created by the shaft extension after the contractors monitor- ing record was completed justifies this. The model pile volume in the CAPWAP model was very close indeed to the recorded volume of 205% of the nomi- nal design of it’s travel. However the measurements of the re- action beam, which was also deflecting elastically, show the same behaviour, with almost no deflection during the last load application cycle. This is shown in figure 3. The Author considers that the maximum load applied to the pile did not exceed 1700kN. Load vs Displacement - Reaction Beam 2500 2000 Applied Load (kN) Measured Expected 1500 1000 500 0 0 5 10 15 20 25 30 35 40 Displacement (mm) Figure 3 - Hastings St Static Test Reaction Beam 5RESULTS As the static test was conducted on this pile prior to the dynamic test it is relevant to plot the dynamic test as an additional cycle to the static test. The re- sults of the other tests at this site show a lower “break-point” in the load vs deflection behaviour and the Author considers this to be related to the loading history of the piles. The “cfa” construction is a non-displacement construction method. Owing to the stress relief that occurs during construction it would appear that these piles deflect more during initial loading. Subsequent loading cycles appear to behave with increased stiffness up to the point of previous maximum loading. This behaviour has also been noted by the Author at other projects with simi- lar piles in sand ground conditions. If deflection is a critical acceptance criteria for this pile type in sand ground conditions then it may well be necessary to “preload” the piles by “driving” them after construc- tion. If the maximum applied load during the static load test was 1700kN this correlates well with the “break point’ at about 1800kN in the CAPWAP load vs deflection prediction for this pile. The unload/reload stiffness shown in each of the load cy- cles of the static test is also of interest as this corre- lates quite well with the initial loading stiffness shown in the dynamic test results. The stiffness of the static test on initial loading during each of the cycles also correlates reasonably well with the dynamic test prediction after the “break point.” The Author considers the dynamic test was the first time the pile experienced sufficient deflection to generate a resistance of more than The static results are summarized in Figure 2, below together with the CAPWAP load vs deflection prediction plotted on the same axes. Hastings St Pile 68B Load vs Displacement 2500 2000 Static Load (kN) Dynamic Load (kN) Load (kN) 1500 1000 500 0 0 5 10 15 20 25 Displacement (mm) Figure 2 - Hastings St Load vs Deflection It can be seen that in the static test when the pile reaches a displacement of slightly less than 15mm the inferred load increases but there is no corre- sponding deflection of the pile. The Author consid- ers that this is impossible and that there must have been an error in the test measurements. The most likely error was that the jack reached the end of its travel or jammed such that although there was an in- crease in jack pressure and hence inferred load on the pile, in reality load did not increase and the pile consequently did not deflect. The contractor’s per- sonnel that conducted the test claim this was not the case and consider that the jack did not reach the end

4. sand appears to be related to previous load history with initial loading to any level of load being less stiff than reloading. Consequently dynamic testing should be conducted with as few blows as possible if it is necessary to predict initial load stiffness. Designers should be aware of low initial load stiffness. If displacement of this pile type is critical then pre-loading either statically or by “driving” should be considered. 7 REFERENCES G Goble + G Likins (1996) “On the Application of PDA Dynamic Pile testing” “Proceedings of Fifth Interna- tional Conference on the Application of Stress Wave Theory to Piles” Orlando, Florida USA. September, Townsend, Hussein, McVay Editors. pp263-273 AS2195-1995 “Piling – Design and Installation. Stan- dards Association of Australia. The load-displacement behaviour of “cfa” piles in about 1700kN and so after this point the lower initial loading stiffness is valid. The load vs deflection behaviour shown in other dynamic tests at the site were similar, with similar initial loading stiffness and stiffness after the “break point” but they showed a much lower “break point” and a typical example is shown in Figure 4. The Author considers this is because these other piles Hastings St Piles 68/218 Load vs Displacement 3000 2500 2000 68 Static Load (kN) Pile 218 (kN) Load (kN) 1500 1000 500 0 0 5 10 15 20 Displacement (mm) Figure 4 - Comparison Pile 68 Static vs Pile 218 Dynamic have not experienced high loading and deflection be- fore the test as did the static test pile. However the initial stiffness in these other dynamictests is still maintained to higher loads than shown in the first cycle of the static test. The Author considers the small blows applied to the pile at the start of each test cause this and owing to the previous loading by the small blows the dynamic test results should be plotted some distance to the right. The pile with more preliminary blows prior to the “test” blow also showed a higher “break point” in the prediction of static load vs deflection, however there were insuffi- cient tests to say that this behaviour has been proven. Further analysis of the test data may pro- vide more information on this behaviour. In particu- lar it may be worthwhile analyzingseveral blows from the one test to assess the change in “break point.” It is suggested that if it is hoped to avoid this behaviour that the number of small blows applied before the full test blows should be minimized. This would appear to minimize the “preliminary”loading of the pile and thus provide the best prediction of the first loading deflection behaviour of a cast in place pile. 6 CONCLUSIONS Dynamic testing appears to be just as valid for bored “cfa” piles as for driven pre-formed piles. Accuracy of the results for bored piles appears simi- lar to driven pre-formed piles provided the pile shaft can be realistically modeled. This requires some knowledge of the shape of the pile shaft.