Small Diameter Driven Pipe Piles in Seattle: Summary of Recent Research and Test Results

Small diameter pipe piles called pin-piles are often used in the Seattle area for underpinning small structures, especially at sites where access is difficult or constrained. Pin-piles are most commonly 60 mm [50 mm nominal] diameter and 89 mm [75 mm nominal] schedule 80 black iron pipe driven with a 41 kg jack hammer or a 68 kg Rhino hammer. In the dense glacial soils often found in the Seattle area, piles are frequently damaged during driving to the specified one minute driving criterion. Furthermore, only very small if any settlement is observed under working loads, and even load tests carried out to twice the allowable load often show settlements of only a very few millimeters. The paper begins with a brief history of the development of pin-piles and the basis for driving criterion and the limitations on allowable loads currently imposed by municipalities in the Seattle area. Then the results of three research projects carried out at the University of Washington in cooperation with local consultants and contractors are summarized. It was found from both static load tests and limited dynamic testing that ultimate load capacity is often 3.5 to 5 times greater than maximum loads allowable without a static load test. Finally, we show the results of thirteen production static load tests on 152 mm diameter pin-piles loaded to 180 kN. The deflections were very small and the results tend to support our contention that the presently allowable load capacities are conservative. Recommendations for future research and local foundation design and construction practice are given.