Evidence of Long Term Ageing Effects on Axial Capacity of Piles in Soft Clay

Pile load testing has been carried out on two piles installed in a soft normally consolidated clay deposit found in the Oromieh salt water lake in Iran. The load tests were carried out in connection with the design of pile foundations for a new bridge across the lake. One test pile was a new pile installed for the purpose. It was a 66 m long steel pipe pile with diameter 305 mm, and which was load tested 5 months after installation. The other pile was an old pipe pile that had been installed in the lake 15 years earlier. It was 30 m long pile and had a diameter of 356 mm. The paper documents the ground conditions and the pile load test results, and compares the results against design methods and other published evidence of long-term ageing or time effects on the axial pile capacity in clay deposits. Whereas the new pile show a skin friction that is pretty much in line with current design methods (back-calculated α=0.74), the 15 year old pile gave a normalized skin friction that was as much as 2.3 times larger. Most of this more than twofold gain in normalized skin friction, as compared to the new pile, is assumed attributed to time or ageing effects.