CYCLIC CONSTANT NORMAL STIFFNESS TESTS ON SAND

Abstract

Pile-supported jacket structures were used for the Block Island Wind Farm project, the first offshore wind farm in the United States (DEEPWATERWIND 2012). Due to the significant length of the piles (60 m in some cases), the frictional resistance along the sides of these piles, termed shaft friction, is the main component of axial capacity. Shaft friction can be degraded due to cyclic loading from wind and waves, and understanding this behavior is critical for safe design. It is hypothesized that degradation of shaft friction can occur due to the contraction of a thin layer of soil, called the shear band, immediately in contact with the pile (DeJong, White & Randolph 2006). In the laboratory, shaft friction is often modeled by performing interface shear tests with soil and the pile material. Interface shear tests, whether monotonic or cyclic, are commonly performed under constant normal load (i.e. direct shear). This boundary condition does not accurately model the shear behavior of piles as they do not account for changes in the normal stress during shearing. LeHane and White (2004) showed that, during loading, contraction of the shear band caused a reduction in the normal stress acting on a pile. In contrast, dilation of the shear band caused an increase in the normal stress. This behavior can be recreated in laboratory interface shear tests by imposing a constant normal stiffness condition (CNS) (e.g. with a spring) on the sample. The primary objective of this thesis was to modify an existing cyclic simple shear device to be able to perform cyclic shear tests under constant normal stiffness (CNS) conditions. A second objective was to perform a series of monotonic and cyclic tests in support of a research project funded by the United States Bureau of Safety and Environmental Enforcement (BSEE) to understand the behavior of piles under cyclic loading. Both monotonic as well as cyclic CNS tests were performed on samples of Monterey Sand at various densities and values of normal stiffness. In the monotonic tests, dilation caused an increase and contraction caused a decrease in the shear strength of the samples compared to constant normal load (CNL) tests. Contraction of the soil along the interface occurred in all the cyclic tests, resulting in a decrease in shear resistance with each cycle of loading. The influence of initial normal stiffness and displacement amplitude was also investigated.