Full-scale Pile Load Tests using Osterberg Cells in Meta-sedimentary Rock

Abstract

Meta-sedimentary (MS) rocks are the dominant geological formation in the northern district of Hong Kong, an area earmarked for extensive urban development in the upcoming decades. This geological context presents challenges for foundation design, as existing practices typically employ conservative design parameters for piles founded in these formations. To investigate opportunities for optimisation, a series of full-scale pile load tests utilising Osterberg Cells (O-cells) combining with the loading kentledge have recently been conducted. The O-cell method integrates a sacrificial hydraulic jack at the base of the test pile enabling direct transfer of applied load to the rock for accurate measurement of both rock socket friction and end-bearing resistance. Unlike traditional pile load tests by loading kentledge, this eliminates the uncertainty of load shedding to the ground above the rock socket. Furthermore, the experience of using fibre optic technology for real-time strain measurements during the tests will be discussed. This novel approach is rigorously evaluated against the traditional vibrating wire strain gauge with a view to assessing the operational accuracy and overall efficiency of both techniques. The first batch of test piles has been carried out in Yuen Long South and Long Bin. This paper presents details of the test set-up, the loading sequence, the load-movement behavior of the test piles and the mobilised end-bearing capacity in the MS rocks. Rock Mass Rating (RMR) of the MS rocks at the founding levels have been determined, based on information obtained from ground investigation or pre-drill boreholes. The evaluation method of RMR and its applicability will be discussed. The findings of this study contribute valuable insights for future instrumented pile load tests in Hong Kong.