Significant ageing effects for axially loaded piles in sand and clay verified by new field load tests

Abstract

Abstract The paper describes key results from a series of new load tests to verify the impact of time or ageing on the axial bearing capacity of driven tubular steel piles. Load tests in two different sand deposits (a silty fine loose sand deposit and a medium dense, medium sand deposit), and four different clay deposits (low and medium plastic normally consolidated clay, over consolidated glacial clay, and a very highly plastic and over consolidated clay) were undertaken. Six tubular test piles were driven at each test site, with diameter 400 to 500 mm and length around 20 m. The piles were loaded to failure typically 1, 3, 6, 12 and 24 months after pile driving. At both sand sites, the shaft friction approximately doubled from the 1 month to the 12 months tests, but tended to level off after that. The ageing effect for piles in clay vary with the clay type, and comes in addition to the normal set-up due to dissipation of excess pore pressures. After 2 years, the gain in capacity range from a factor of about 1.1 to 2.0 compared to the capacity after full re-consolidation. The tests in the low-plastic low-OCR clay deposit showed the largest gain, and the tests in the highly plastic high OCR clay deposit the smallest. The study also revealed that repeated load testing on the same pile to failure can give both lower and higher capacity than first time testing, and therefore, can lead to misleading interpretation of ageing effects. One pile at each test site, was subjected to sustained loading at 60 % of the assumed failure load before being loaded to failure after 2 years. The impact of such sustained loading was generally to increase ageing effects for piles in clay, and reduce ageing effects for piles in sand. The overall results confirms that ageing effects are a significant positive factor to account for in future pile design practice.