Owner: Confidential Client
Drilled Shaft Designer: Foundation Test Group
Constructed along the east bank of the Schuylkill River in the heart of Philadelphia, the Riverwalk Tower project is a mixed-use multi-story structure consisting of three podium levels emanating from street grade and parking platforms, providing a panoramic view of the city skyline. The site geology, located in a historic coal yard, consisted of a top urban fill layer, underlain by variable alluvial clays, and a transitioning disintegrated rock layer into bedrock at depths of 20 to 50 ft below grade. The original foundation design provided by the owner’s geotechnical report consisted of a drilled shaft option with dimensions of varying telescoping diameters up to 8-ft, more than 60-ft in depth, and rock sockets up to 20-ft long. Due to the weak expected nature of the disintegrated rock stratum in comparison to bedrock, original design parameters estimated allowable end bearing capacities in the range of 45-100 ksf and side shear capacities in the range of 5-10 ksf.
A load test program was developed with two test shaft locations; one terminated in the weakest of rock conditions and the other terminated into competent bedrock. For analysis of both end bearing and side shear resistance values of the materials, a sacrificial bi-directional hydraulic jack and load cell was installed within a steel reinforcing frame lowered into the shaft prior to concrete placement to a depth of six inches from the bottom. The jack was able to simultaneously apply a maximum test load of up to 4,750 kips in both upward and downward directions.
Results of the load testing indicated that allowable design capacities between the weak disintegrated rock and competent bedrock ranged from 867-990 ksf end bearing and 28.5-42.7 ksf side resistance per shaft. Values of up to 10 times original design capacity! The redesign enabled on-site tooling to be minimized to two auger diameter options of 36 and 60 inches with less reinforcing steel and total drilled depths not to exceed 50-ft, rock sockets not to exceed 10-ft. Direct overall project savings brought a value of $1.5 million along with three months shorter schedule duration.