On the Cyclic Resistance of Dense North Sea Sands-Paper

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The paper evaluates the cyclic strength of dense sands at the Oseberg and Gullfaks fields in the North Sea, where shallow skirted foundations are commonly used. The authors analyze 36 cyclic, anisotropically consolidated undrained triaxial tests on reconstituted sand specimens, generally prepared at in-situ relative densities above 80%. Tests simulated foundation-related stress conditions and wave loading, with failure defined by accumulated cyclic or average strain reaching a specified threshold.<br /><br />The sands were broadly similar in composition and grading, though fines content varied. The authors developed a simple model to estimate cyclic resistance from the initial stress state, expected number of loading cycles to failure, and fines content. They grouped the test data by fines content and calibrated the model parameters for each group. The fitted relationships indicate that both key model parameters decrease as the number of cycles to failure increases, while the model’s strength intercept increases with fines content. Relative density and fines plasticity were not included as variables because the tested sands were uniformly very dense and had low-plasticity fines.<br /><br />Model predictions generally agree with published results for clean Dogger Bank sand, although that sand showed higher monotonic compressive strength and, in some cases, greater cyclic resistance at low initial shear stress.<br /><br />The model is intended specifically for cyclic-strain accumulation under stress histories and loading conditions representative of offshore gravity-base foundations. Its applicability to substantially different stress states, drainage conditions, or loading scenarios is uncertain. Because the model is based on a limited dataset and some assumptions are only indicative, the authors advise caution in applying it directly to design.

Keywords

cyclic strength

dense sands

North Sea

Oseberg

Gullfaks

triaxial tests

cyclic resistance

fines content

offshore foundations

strain accumulation