Measured end resistance of CFA and drilled displacement piles in San Francisco Area alluvial clay

Continuous Flight Auger (CFA) and drilled displacement (DD) piles in the San Francisco (California, USA) area are typically designed using a combination of side- and end resistances. For moderately-sized buildings, these piles are typically 50 to 100 ft (about 15 to 30 m) in length and often bear in Pleistocene epoch alluvium consisting primarily of clay with interbedded sand seams. It can be unconservative to rely upon the higher consistency sand seams because their depth, thickness, and consistency can vary dramatically over short distances. A more robust design approach assigns an end resistance based on the strength of the clay. The fully mobilised end resistance from fifteen (15) high quality axial compression loading tests performed on cast-in-place piles are compared to the average net cone resistance for one diameter below the pile tip. The comparison suggests that direct estimation of the end resistance using local load testing data will result in higher end resistances than will the conventional bearing factor of 9 times the estimated undrained shear strength derived from the cone penetration test.