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

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.

Performance of CPT-based methods to assess monopile driveability in North Sea sands

“…Based on static and dynamic load tests, the 492 authors proposed that the end of driving shaft capacity of piles was 70% of the resistance predicted 493 using the ICP design method. This proposal was consistent with field tests reported by Gavin et al 494 (2013) and Karlsrud et al (2014) summarised in Gavin et al (2015). For the UWA-05 approach, the 495 base resistance is further modified herein using the base-settlement model in Fig.…”

Design Procedures for Marine Renewable Energy Foundations

“…Chow et al (1996) reported an increase in the shaft resistance of open-ended steel piles driven into dense sands. Karlsrud et al (2014) verified significant ageing effects in sand and clay by performing pile load tests. The tensile capacity computed using the ICP method is mobilized about ten days after driving.…”

“…This time-dependent increase may be due to a chemical process, particularly corrosion, cementation, or creep, which reduces arching effects. The trend reported by Karlsrud et al (2014) could be used to evaluate the increase in shaft resistance in both clay and sand due to ageing. The trend reported by Jardine et al (2005) shown on Figure 3, could be used to evaluate the increase in shaft resistance in sand due to ageing.…”

A new and openly accessible database of tests on piles driven in sands

“…Simple expressions were developed that captured the above shaft capacity phenomena and also pile end bearing capacity. Subsequent research has considered additional factors, including N the influence of load cycles imposed during installation (White & Lehane, 2004;Jardine et al, 2013a) N time effects (Jardine et al, 2006;Gavin et al, 2013;Karlsrud et al, 2014) N how particle breakage under the tip and surface abrasion affect the stresses and the development of a well-defined interface shear zone (Yang et al, 2010) N the stress regime developed in the surrounding soil mass (Jardine et al, 2009(Jardine et al, , 2013b N the influence of cyclic loading (Tsuha et al, 2012). Yang et al (2014) and Zhang et al (2014) have gone on to relate stress measurements from experimental investigations and numerical analyses by other workers.…”

“…However, only 11 piles (from just three sites) were open-ended, had D > 600 mm and full CPT profiles. Further tests are required to augment this sparse dataset, obtain information from a wider range of international sites and gain further insight into uncertain factors such as the effects of layering on base resistance (Xu, 2006) and the effects of pile age on capacity (Jardine et al, 2006;Gavin et al, 2013;Karlsrud et al, 2014).…”