Large-scale pile tests in Mercia mudstone: Data analysis and evaluation of current design methods

Full-scale load tests were carried out on six instrumented large diameter bored, cast in-situ piles formed in Mercia mudstone, as part of the design of a new Viaduct in Cardiff, UK. In this paper, the results from six test piles and extensive data from 218 ground investigation boreholes are systematically processed in order to study the load transfer and resistance mechanisms in Mercia mudstone. Data from strain gauges embedded in each pile are first analysed to calibrate the load-deformation relationship of each pile as-built, taking into account (i) the non-linearity of concrete and (ii) the effect of partial steel encasement on pile stiffness at various levels. The shaft and base capacity of the piles are each predicted using 10 calculation methods belonging to the four basic categories: (i) Undrained analysis, (ii) Drained analysis, (iii) Mixed approach and (iv) Empirical correlation. It is found that the shaft capacity prediction methods are moderately consistent. The standard deviations of the ratio Q _sp/Q _sm of predicted to observed shaft capacity lies in the range 0.06–0.24. However, 8 of these methods are over-conservative, giving Q _sp/Q _sm values in the range 0.29–0.67. The remaining two methods yield Q _sp/Q _sm = 1.01 and 1.49. In contrast, the prediction methods for base capacity are found to be much less consistent. The ratio Q _bp/Q _bm of predicted to measured base capacity falls in the interval 0.52–1.93, with corresponding standard deviations of 0.16–0.82. This revised version was published online in July 2006 with corrections to the Cover Date.