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Medio 2007 wordt een veldproef georganiseerd in Waddinxveen in het kader van de validatie van de snelle paaltest. Initiatiefnemers van de proef is de DC/CUR commissie ‘validatie snelle paaltest’. Tijdens deze veldproef worden twee prefab betonpalen beproefd op met een statische en een snelle paaltest. De snelle paaltest wordt uitgevoerd door de statnamische paaltest. Uitgangspunt is dat de proefpalen tot falen worden belast, hierbij zakt de paal 10% van haar equivalente diameter. Volgens de Nederlandse normen wordt gesteld dat de paal bij deze zakking niet meer voldoet aan haar gebruikerseisen. In het onderzoek ‘ontwerp veldproef snelle paaltest Waddinxveen’ zijn de proefopstelling en proefbelasting voor de statnamische paaltest bepaald. Voor de proefopstelling wordt geadviseerd om de statnamische test uit te voeren op een prefab betonpaal 350 * 350 mm² met een paalpuntniveau op NAP -15,25m. Het testapparaat moet een kracht van 3,3 MN uitoefenen om de paal tot falen te belasten.

Pile foundations are widely used, mainly to transmit structural load to an underlying stiffer soil or rock. This limit state load a certain pile can sustain without failure is known as pile ultimate bearing capacity. During design stage load-tests are performed in-situ on test piles to determine, among others, the value of the bearing capacity. Commonly static tests are performed as they provide the most reliable data. Dynamic tests are much more cost-effective but have a series of shortcomings, mainly the fact that they introduce stress-waves on the pile and that require calibration with the static values. To overcome both nature-kind problems, a new type of test in-between the previous ones, i.e. the pseudostatic test, has been developed. It is still a dynamic test but the loading pulse lasts longer (70-150ms), 20 times the dynamic pulse, emphasizing the static component. Hence, it is both an economical and reliable option as requires no calibration with the static load-displacement curves. Therefore, it is interesting to get more insight on it. Two main factors can influence the bearing capacity of a pile measured on the in-situ tests, namely, loading rate and excess pore pressures. In cases like The Netherlands, where end-bearing piles are driven into saturated sand, these two concepts may play an important role. A previous study had been carried out in dry sand and did not find a remarkable loading rate effect. However, for the case of saturated sand the soil response remains unknown. This research investigates the topic, the objective is to get more insight on the excess pore pressure generation and dissipation, evaluate the static-pseudostatic correlation and investigate the possibility of providing effective predictive tools. The research has been structured in three parts. First a series of experimental scaled tests have been carried out for three loading rates: a CPT (20 mm/s), a static test (1 mm/s) and a pseudostatic test (up to 250 mm/s). The sample consisted in saturated sand that was prepared by means of a fluidizaton-vibration system. Standard sounding rots with a piezocone acted as the pile; five values were recorded: force on the pile head, shaft friction, tip resistance, displacement and acceleration. Later on, the performed scaled tests have been modeled analytically and numerically. An analytical model based on the cone model of Wolf has been developed. Only the soil underneath the pile tip is considered and it is modeled as an elastoplastic material under static fully undrained loading followed by consolidation. PLAXIS is the program used for the numerical model.

Pile testing, which plays an importance role in the field of deep foundation design, is performed by static and non-static methods to provide information about the following issues: (Poulos, 1998) - The ultimate capacity of a single pile. - The load-displacement behavior of a pile. - The performance of a pile during the test conditions. - The integrity of a pile (pile integrity test). For the purposes of verification the design axial capacity and the static load – settlement behavior of piles, the static pile load test has long been considered as the most reliable method but because of its high cost and time consuming, non – static pile load tests are looked as efficient substitutions. The two non – static testing methods, i.e. dynamic and quasi – static pile load test are objects of this report. The non – static pile load tests are performed by means of exerting an impact force on the pile head while measuring and recording the responses of the pile, from which the test results are determined. Duration of the impact force (T), longitudinal wave velocity of tested pile (c) and pile length (L) are used as key factors to classify the testing methods.

Pile load tests are commonly used by engineers to determine its bearing capacity. At present, there are three methods of pile load tests: the static, the dynamic and the quasi-static test. The static pile load test is done by applying an axial load on the pile with a long duration. The dynamic and quasi-static tests are done with an impact load on pile head of very short duration. However, the required force pulse in the quasi-static test is longer than in the dynamic test. This research focuses on the comparison between quasi-static and static tests. An important aspect in order to verify the results of quasi-static application with respect to more widely used static loading. The results of quasi-static tests have both static and dynamic components. Then, in order to convert the results of a quasi-static test to static pile bearing capacity, the dynamic component (inertial and damping effects) in the soil responses have to be understood. The effect of generates pore water pressure and its dissipation during pile penetration are unclear and can limit the interpretation of the results of a quasi-static test.

When in-situ tests are performed to determine the ultimate capacity of a pile after the installation, quite a lot of types of tests are available. The following tests are generally used, in order from most used method to least used method: the static testing method, dynamic testing methods, pseudostatic and statnamic testing methods. During a pseudostatic (or statnamic) test the pile is loaded with a ~ 70-150 ms loading pulse. In comparison with the dynamic test this loading regime is about 20 times as long, in comparison with the static test this is very short lasting. The difference in loading regime is influencing the bearing capacity of the pile. In literature two main phenomena for the change in bearing capacity are recognized: the excess pore pressures and the loading rate. In the typical Dutch situation the end bearing piles are situated in the Pleistocene sand layer, so the influence of the loading rate in non-saturated and saturated sand are of interest. From literature is concluded that the effect of the loading rate on the strength and stiffness properties for dry sand are small, but for the rate effect on the shaft friction no conclusive answer could be found. Therefore a series of model scale tests are performed to investigate the influence of the loading rate on the bearing capacity.

In order to improve the interpretation of the quasi-static (e.g. Statnamic) pile load tests, a research project has been started to investigate effects of the loading rate on the bearing capacity of a pile in sand. A series of laboratory tests has been carried out. The testing program consists of a series of triaxial tests for sand and a series of load tests on a model pile embedded in sand in a large calibration chamber. The research pointed at answering two fundamental questions: - The effect of loading rate on the strength of sand and on the bearing resistance of a pile in sand; - The characteristics of excess pore pressure in sand and in the sand near the pile toe during a quasi-static load test. The results of the triaxial tests are: - In dry sand, a higher loading rate gives higher shear strength. In the range of applied loading rates, the angle of internal friction of the sand increases up to 2 degrees (strength increases 5-10%). - During high speed tests on dry sand an excess of pore air pressure is observed. So the dry sand is not in fully air drained condition during these tests. - The effects of loading rate in dry sand increase with the increase of relative density. - In saturated sand, the shear strength increases about 5% due to the rate effect. But, the true rate effect may be obscured by cavitation which occurs during the test. - Before cavitation occurs, the excess pore water pressure is independent of the loading rate. It depends on the relative density of the sand.