Steady state vibration testing of model pile foundations

Khan, Abdul Samad (1984) Steady state vibration testing of model pile foundations. University of Southampton, Doctoral Thesis.

Abstract

Working load static and dynamic experiments with small model single piles and 2x2 groups of piles were conducted in a soil bed of remoulded clay, compacted in a soil pit 2x2x2m, to simulate a complete soil balf-space without bin wall restraint. The static load-deflection characteristics of the systems were determined using a constant rate of penetration technique while their steady state vibration response was used to measure their dynamic characteristics.Much of the previously published experimental work on the dynamic testingof piles has been carried out using rotating mass vibrators in which the force amplitude is strongly dependent on frequency. In the present project, in addition to using such a mechanical vibrator, an attempt has been made to devise simpler testing techniques using an electrodynamic vibrator by which the piles can be excited vertically and horizontally over a wide range of frequencies at constant force-amplitude. Objectives of the experimental measuring techniques used are enumerated and the difficulties associated with achieving them critically examined to identify those which offer the greatest potential for use in the field. To predict the response of piled footings to dynamic loads, through elastic continuum theory, it is crucial to determine a suitable value for the soil half-space shear modulus. Such a shear modulus has been determined for the clay bed by a variety of methods, interpreted analytically and critically reviewed. The main experimental topics explored were (a) Static plate loading tests - to obtain the shear modulus of the soil from elasticity theory. These tests incorporated square rigid plates of four different sizes (450mm, 300mm, 198mm and 150mm). A second series of tests consisted of corresponding vibratory plate loading tests, using a number of different mass ratios. Lumped-parameter and elastic half-space analyses were used to obtain a dynamic shear modulus for the bed and a correlation between the shear moduli calculated by two analyses established. System damping was also measured and compared with theoretical predictions from the Hsieh (1962) and Lysmer (1965) analyses. (b) vertical static stiffness of single and 2x2 pile groups with L/D=50, 35, 25 and 15 were measured. The measured stiffnesses were compared with results obtained from elastic analysis (PGROOP) and a very satisfactory interpretation obtained using a Gibson soil model.The dynamic stiffness of the second series of piles was determined from their vertical dynamic responses at different frequencies. An attempt was then made to correlate the static and dynamic stiffnesses for each system tested. Those for single piles were compared with theoretical prediction based on Novak's (1977) approximate solution and the findings critically examined. (c) The lateral dynamic response of a few single and 2x2 pile groups. Measured values were compared with a simplified single-degree-of-freedom theoretical model. The testing procedure developed for this investigation may well provide an attractive practical means for determining the dynamicstiffness of such systems. An overall objective was to investigate the correlation between static and dynamic shear modulus measurements with a view to using the latter to predict the former.

This record has no associated files available for download.

Contributors

Download statistics