Spatial variability of in situ weathered soil

The founding depths of pile foundations depend primarily on the loading conditions of the superstructure and the strength of subsoil. The depth of Grade III weathered rock, the top of completely decomposed granite (CDG), and standard penetration test N-value of 200 (SPT-200) are often used as indicators for decision-making in arriving at the preliminary founding depth of piles in Hong Kong. The work reported in this paper focuses on evaluation of the spatial variability characteristics of the above founding depth indicators at a construction site, using statistical models based on random field theory. Spatial variability characteristics are evaluated using the measured data in terms of scale of fluctuation. Geostatistics is used to obtain additional data at unsampled locations for mapping profiles of founding depth indicators over the site, using the measured data from borehole records and site investigation results. It is observed from the results that, if faults are not present, the depth of the Grade III surface exhibits the largest scale of fluctuation among the three indicators, and the variability of the ground is observed to increase with the weathering grade. The effect of the size of the sampling domain on the auto-correlation characteristics of the founding depth indicators is also studied. The results demonstrate that the scale of fluctuation increases with increase in the size of the sampling domain.