Abstract: CFG pile(cement-fly ash-gravel pile) is an important type of ground improvement, and its role is becoming even more prominent as more accessible soft foundations become increasingly exploited by the surging urban industrial development in China. Yet despite its significance, the load-settlement behaviour is poorly understood on this pile type; particularly in the context of probabilistic assessments. Based on a series of repeating full-scale field load tests for CFG piles (21 samples) and the pile-soil intermediates(PSI) or composite foundation systems (32 samples) under static axial compression loading, the data of the loading-displacement curves were gathered from a field site in Beijing city. A regression curve is facilitated by approximating the load-settlement curve for each load test using a two-parameter, hyperbolic or power law curve-fitting equation. Moreover, when assessing the regression parameter values from entire pile load measurements across the test site, the scatter in load-displacement curves attributed to inherent soil variability can be found. Thus, the uncertainties of the load-displacement curves at the site are represented by a relatively simple bivariate random vector containing the regression parameters as its components. A bivariate Copula-based mixed distribution is further applied to capture the dependence characteristics between these regression parameters. For the CFG pile, the Pearson correlation coefficient between the power-law regression parameters is calculated to be -0.621; and the best-fitting Copula is identified as normal through the Akaike information criterion(AIC). In the case of PSI, the correlation coefficient of the regression parameters is -0.645 and their dependence structures are best described by the normal Copula. Finally, a simple Copula-based simulation model(CBSM) is used to estimate the reliability index at any specific allowable settlement for the serviceability limit state(SLS) design. The effect of the various Copulas on the reliability indexes is clarified. These analyses of the load-displacement behaviour give an insight into the probabilistic design and safety assessment of the CFG pile foundation.

Key words: composite foundation, cement-fly ash-gravel pile, correlation, Copulas, uncertainty, reliability index