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Downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE) INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING This paper was downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. Reliability of statnamic load testing of rock socketed end bearing bored piles Fiabilité d’un essai de charge Statnamic sur un pieu résistant à la pointe foré dans de la roche H. S. Thilakasiri Department of Civil Engineering, University of Moratuwa, Sri Lanka. ABSTRACT The pile load testing methods could be broadly classified into three categories: static, rapid and dynamic depending on the rate of loading. In this paper, the rapid load testing method referred to as the Statnamic test is discussed. The commonly used analysis method of the statnamic testing referred to as the Unloading Point (UP) method is used successfully for the floating piles but validity of some of the assumptions of the unloading point method to end bearing bored piles is questionable. Due to this problem, other analytical methods such as: Modified Unloading Point (MUP) method, Segmental Unloading Point (SUP) method and other signal matching techniques are introduced by some researches. Therefore, the validity of the unloading point method to rock socketed end bearing bored piles in Sri Lanka is investigated in this paper. This investigation is carried out using the commonly used wave number. Furthermore, the wave equation method, commonly used numerical procedure to model dynamic behavior of piles, is used by the author to investigate the validity of the assumptions associated with the unloading point method to rock socketed end bearing bored piles. RÉSUMÉ Les essais de charge sur pieu peuvent en général être classifiés en trois catégories principales : statique, rapide et dynamique, en fonction de la vitesse de chargement. Cet article se concentre sur la méthode d’essai de chargement rapide intitulée « Statnamic ». La méthode communément utilisée pour l’analyse des tests Statnamic est intitulée «Unloading point » (UP). Elle est utilisée avec succès pour le cas des pieux flottants mais la pertinence de certaines de ses hypothèses concernant les pieux résistants à la pointe est discutable. Par conséquent, d’autres méthodes d’analyse, telles que la « Modified Unloading Point » méthode (MUP) et la « Segmental Unloading Point » méthode (SUP) ont été utilisées par certaines études. Cet article examine la validité de l’application de la « Unloading Point » méthode aux pieux forés résistants à la pointe au Sri Lanka. Cette étude est réalisée en utilisant le couramment employé nombre d’onde. De plus, la méthode de l’équation d’onde, qui est communément utilisée dans les études numériques de modélisation du comportement dynamique des pieux, est aussi employée pour étudier la validité de la « Unloading Point » méthode pour les pieux forés résistants à la pointe. Keywords : Statnamic, bored piles, load testing, unloading point method. 1 INTRODUCTION Unloading Point Method (UPM) to analyze the statnamic load testing results of rock socketed end bearing bored piles in Sri The Sri Lankan piling industry is at a cross road at this point in Lanka is investigated. time due to the demand for high capacity foundation elements as a result of the recent interest shown by the property developers on high-rise structures. Therefore, economical design, construction 2 LOAD TESTING OF PILES USING TRANSIENT and testing of high capacity large diameter bored piles are very IMPULSE LOADING important for successful implementation of such projects. Meeting these challenges effectively require re-thinking on the way 2.1 Dynamic load testing foundation design, construction and testing done in Sri Lanka. In The pile load testing methods could be broadly classified into this endeavour, efficiency, economy and the effectiveness of the three categories: static, rapid and dynamic depending on the present practices should be investigated and appropriate and rate of loading. In the case of static load testing, load is relevant new methodologies should be introduced. applied on the pile very slowly whereas in the case of dynamic The aim of this article is to investigate the possibility of load testing an impact load is applied on the pile. Impact load using the pile load testing methodology commonly referred to as is applied by dropping a heavy hammer on the pile during ‘statnamic’ test to the rock socketed end bearing bored piles dynamic load testing. The impact applied at the top of the pile constructed in Sri Lanka. Even though this testing procedure is will generate a stress wave that propagates through the pile at new to Sri Lanka, most other countries in the world are using the wave speed (c). Generally, the wave speed through this method to load test drill shafts, driven steel and concrete concrete pile rages from 3500 m/sec to 4500 m/sec whereas piles. Unlike in traditions static load testing, where pile is loaded that for steel pile is about 5000 m/sec. The stress wave, that slowly under static conditions, in statnamic load testing pile is travels down a pile of length L, takes 2L/c time to reach the subjected to a transient impulse loading. Therefore, the data pile top after reflection at the toe. Therefore, for a 20m long from statnamic load test should be analyzed to obtain the pile the time taken for the reflected wave to reach the pile top behavior of the pile under static loading conditions. Eventhough is about 4 to 6 milliseconds. Compared to that time duration, there are number of interpretation techniques available, the the duration of the impact is very small in the order of 2 to 3 accuracy of the different methods depends on the duration of the msec. As the pile top is unloaded before the reflected wave transient impulse loading, pile properties (Middendrop & reaches the pile top, wave propagation through the pile is the Daniels 1996). In this paper applicability of the commonly used mode of stress transfer. Proceedings of the 17th International Conference on Soil Mechanics and Geotechnical Engineering 1185 M. Hamza et al. (Eds.) © 2009 IOS Press. doi:10.3233/978-1-60750-031-5-1185 1186 H.S. Thilakasiri / Reliability of Statnamic Load Testing 2.2 Rapid load testing obtain the static response. Analytical methods available to interpret statnamic load test results may be categorized into The statnamic test procedure,, developed by Bermindhammer following groups: equipment in 1988, applies a rapid load on the pile due to i. Intuitive interpretation methods; burning of solid fuel within a pressure chamber, which is ii. Analysis using concentrated mass models; directly in contact with the top of the pile as shown in Figure 1. iii.Signal matching using numerical simulation methods; Due to the high pressure generated by the combustion of the a. Analysis using one dimensional stress wave solid fuel, upward thrust is applied on set of reaction weights and equal and opposite force will be applied on the pile. The propagation theories; and sudden force that is applied on the set of reaction weights sets it b. Finite element methods. off in motion vertically upwards with a vertical acceleration of The most widely used method to estimate the static response a. If the mass of the reaction weights is equal to M, the force, of a pile from statnamic test is the concentrated mass model, commonly referred to as the Unloading Point (UP) method due to combustion FStanam, required to generate a vertical acceleration of a is Ma. If the acceleration of the reaction (Middendrop 1992). weights is 20g, FStanam, is twenty times the reaction weight and hence, the applied load on the pile is twenty times the reaction 3.1 Unloading Point (UP) method weights. As a result, the reaction load required is 5% of the In the UP method, pile-soil system is modeled using a simple reaction load required for static load testing. However, if the single degree of freedom system. The mass of the pile is lumped reaction weights are loosely placed, falling the mass on to the and is assumed to be resting on a nonlinear spring and a dashpot pile after the flight may damage the pile. To prevent this (Mullins et al. 1992). The nonlinear spring represents the elastic different catching mechanisms are in use. response of the pile and that of the surrounding soil while the linear dashpot represents the dynamic (or viscous) resistance of Ma the surrounding soil. Two main assumptions were used in the UP method of analyzing statnamic load test results: Reaction i. The pile plunges due to application of the weights statnamic force and the resistance during plunging is constant; and ii. The damping constant is constant during the entire test. FStatnamic Combustion chamber The equation of motion of the mass m can be represented by the following mathematical equation: FStatnamic F = ma + Cv + kx (1) Where F - Statnamic force a - Acceleration of the pile Pile v - Velocity of the mass m and k - Displacement of the mass m Figure 1 - Schematic diagram of the Statnamic load set up The duration of the load application is 10 to 100 times that of a hammer blow and the pile acceleration is 100 to 1000 times less than that due to a dynamic load (Research committee on rapid pile load test methods - Japanese Geotechnical Society, 1998). Moreover, the load duration is more than the wave reflection time and, as a result, the pile is loaded, even when the reflected wave reaches the pile top. Therefore, the entire pile is stressed during loading and the wave propagation through the pile is not predominant.
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