DETERMINATION OF AXIAL PILE CAPACITY OF PRESTRESSED CONCRETE CYLINDER PILES

This study focused on determining unit skin and tip resistance of large diameter open-ended steel and concrete cylinder piles in different soil types using insitu Standard Penetration Test (SPT) data. A total of 35 piles with pile diameters ranging from 36 in. to 84 in. (most common 54 in.) were collected from Florida, California, Virginia, Maryland, etc. Each had a static load test with a few (9) instrumented with strain gauges along their length. Twenty-one of the static load tests reached Davisson's capacity used to assess failure. Using the database of piles, which reached failure, equations of unit skin friction and end bearing vs. SPT N values were developed. For those piles that didn't have strain gauges to differentiate skin from tip resistance, DeBeer's method of plotting load vs. displacement on log-log plot was used to differentiate skin from tip resistance. In determining end bearing, the use of pile cylinder area or total cross-sectional area was studied. It was found from dynamic analysis of large open-ended piles that the unit skin friction inside the pile did not generally overcome the inertia force acting on the soil plug (i.e., pile was unplugged during driving). However, FEM studies on static or slow rates of loading (i.e., no inertia forces) suggested that many cylinder piles acted plugged during static loading. Load and Resistance Factor Design (LRFD) resistance factor analysis suggested that the pile's total end area gave better prediction than the pile's cylinder area (% of Davisson capacity available for design = 0.65 vs. 0.60). A range of LRFD resistance factors versus reliability values were determined for design.