Dynamic behavior of micropile systems subjected to sinusoidal ground motions: Centrifugal model studies

The behavior of micropile group systems under static loading has been investigated and design guidelines have been assessed in a state of practice review (Bruce and Juran, 1997). However, the application of micropiles to seismic retrofitting is facing the need for established and reliable design guidelines.; In order to investigate the seismic behavior of micropile group systems (isolated piles, groups and networks of reticulated micropiles) under axial, lateral and combined loading in selected types of engineering applications, a proposed Workplan for Laboratory Centrifugal and Numerical Model Studies on the Seismic Behavior of Micropile Systems has been undertaken by the Polytechnic University at New York in cooperation with the Ecole Nationale des Ponts et Chaussees Geotechnical Research Center - CERMES at Paris and the University of Canterburry at New Zealand. The prime objective is to provide the experimental data base to develop and evaluate seismic design methods for selected engineering applications, including new construction in earthquake zones and seismic retrofitting of bridge foundations, retaining systems and slope stabilization.; A series of centrifuge tests on instrumented micropile group systems were performed at the Rensselaer Polytechnic Institute (RPI) Geotechnical Centrifuge Research Center and several configurations of model micropile group systems were tested in loose to medium dry sand. Micropile bending moment, deflection, and acceleration were measured during testing. Dynamic p-y curves were derived from the measurements for low and high levels of shaking and were compared with the backbone p-y curves for sand recommended by API and other published data. Group and network effects were investigated for different configurations and at different loading levels.; The main objectives of this research were to investigate the effects of (i) acceleration shaking level; (ii )group and two-dimensional network configuration; (iii) superstructure loading level; and (iv) micropile inclination on the seismic response of selected micropile systems. The preliminary results illustrate that, due to their slenderness and flexibility (ductility), during the seismic induced ground motions, the micropile system remains integrated with the soil and appears to effectively transfer inertial forces acting on the superstructure to the foundations soils. For the selected frequency of excitation of 2 Hz, the results indicate a positive group effect increasing with the number of piles and the batter angle. This dissertation describes the experimental procedures used to carry out the centrifugal model tests, discusses the test interpretation methods, and summarizes the main findings.