TESTING AND DESIGN OF A PILED FOUNDATION PROJECT A CASE HISTORY

Design for a large refinery expansion was undertaken at a site reclaimed from a lake about 40 years ago. The natural soils consist of sand deposited on normally consolidated, compressible post glacial lacustrine clay followed by silty clay till on limestone bedrock found at about 25 m to 30 m depth below existing grade. The site will be raised an additional 1.5 m, which will cause long-term settlement. Some of the new units are 30 m to 70 m in height and will be supported on piles—several thousand in all. In anticipating negative skin friction to develop, the initial design called for subtracting the drag load from the allowable load determined from the pile capacity. Initial design also expected the piles to be constructed to bedrock. However, a review of the design made clear that a drag load is a problem for the axial structural strength of a pile and should not be subtracted from an allowable load based on bearing capacity. Moreover, analysis of the results of full-scale static and dynamic loading tests demonstrated that it was not necessary to reach bedrock, but the piles would develop adequate capacity in the clay till and they would not experience excessive down-drag due to the settling clay. The final, revised design resulted in a saving of close to 25 million dollars and considerable construction time.. The piles selected for the foundations were 457 mm (18inch) diameter bored piles installed to about 1.5 m into the glacial till. The paper presents site conditions, tests results, and the design principles employed.