Micro-scale characterization of carbonate sands with nanoindentation

Many offshore energy production and recovery structures such as wind turbines, offshore tidal turbines, and oil/gas platforms are built on carbonate sands, which predominantly consist of the skeletal remains of marine organisms. Particles of carbonate sands have highly porous grain structures with intra-granular voids. Such porous grain structure results in a much more compressible soil grain than terrigenous (i.e., quartz-based) sands, which have solid grain structure. As a result, calcareous sands exhibit engineering and mechanical characteristics that are different than that of quartzitic sands. While there is a significant amount of experience on the performance of foundation systems on terrigenous sediments, expanding offshore infrastructure construction on carbonate sands brings considerable challenges. This study focuses on micro-scale characterization of carbonate sands as a first step to overcome this challenge. For this purpose, nanoindentation tests were performed on ten different carbonate sand samples. Mechanical properties of carbonate sand grains were determined using nanoindentation techniques. A series of peak loads are applied by the nanoindenter on the grain surface and load-displacement curves are developed to estimate Young's modulus and hardness values of the carbonate sand grains. Modulus and hardness of the carbonate sand grains were considerably lower than those reported for quartzitic sand grains. Test results cumulatively show that modulus and hardness values of carbonate sands are highly dependent on indentation depth. Considerable softening with increasing indentation depth was observed; both the modulus and hardness values of carbonate sand grains show a significant decrease at larger indentation depths. Furthermore, high inter-sample and inter-grain variation of mechanical properties was observed. This study on the micro-scale characterization of carbonate sands provides insight on their distinctive macro-scale behavior.