Experimental Evaluation of Shear Strength of Soil–Concrete Interface in Carbonate Sands from Northeastern Brazil
This study evaluates the shear strength and geomechanical behavior of the soil–concrete interface in carbonate sands from the coast of Ceará, with particular relevance to offshore wind turbine foundations. Three sands with different calcium carbonate (CaCO3) contents, namely, 10.6%, 22.0% and 30.0%, were tested under applied normal stresses of 50, 100 and 200 kPa. Conventional direct shear tests were carried out to determine soil–soil shear behavior, whereas controlled-interface tests were performed using cementitious specimens with smooth and rough surfaces. The soil–soil tests indicated effective internal friction angles (φ′) between 35° and 38°, with no cohesion. No direct correlation was observed between shear strength and CaCO3 content. Instead, the results indicate that particle size distribution, particularly the proportion of finer fractions, exerted the main control on mechanical behavior. Within the three materials tested, no monotonic trend between CaCO3 content and shear strength was identified, a finding that should be confirmed with a larger sample set. At the soil–concrete interface, shear stress mobilization depended on surface roughness, with the rough surface mobilizing higher shear stresses than the smooth surface. The ratio between the interface friction angle and the soil effective internal friction angle (δ/φ′) ranged from 0.963 to 0.994 for rough surfaces and from 0.859 to 0.951 for smooth surfaces. These findings show the need for site-specific characterization of carbonate sands and for explicit consideration of interface conditions in offshore foundation design, thereby reducing unnecessary structural oversizing.