An Evaluation of the Shaft Resistance of Piles Embedded in Gneissic Rock

Abstract

In the design of piles drilled in rock, the following questions arise: (i) at what point of the soil/weathered rock/sound rock profile should the pile socket be designed; (ii) is the contribution of residual soil to be disregarded; (iii) how much consideration should be given to the pile boring method (rotary or hammering). Furthermore, usual design methods consider only the side shear capacity of the socket, which is evaluated through empirical expressions that require the uniaxial compressive strength of the intact rock (qu or UCS). And, quite often in practice, a comprehensive test program is not available, and only boring logs are available. This paper examines data from a BRT (Bus Rapid Transit) project in Rio de Janeiro, with 8 bridges, in which some 30 dynamic tests were performed on piles partly embedded in residual soil and partly in rock - a gneiss. These tests produced profiles of mobilized side shear. For the evaluation of the mobilized side shear, a series of laboratory tests were performed on rock samples with different RQDs, taken from borings at the pile sites. A relation between qu (uniaxial compressive strength of the intact rock) and RQD could be established for gneissic rocks of Rio. Values of mobilized side shear are compared to qu derived from the RQD correlation. Finally, an expression for the prediction of mobilized pile shaft capacity is put forward.