Axial bearing capacity of piles in carbonate sands – Case study: Hormoz, Gheshm, and Hengam Islands

The soil at a particular site may sometimes be inadequate to support engineered structures like dams, pavements, rail tracks, foundations, and so on. Carbonate sands, also known as calcareous sediments, have been classified as ‘problematic soils’ in many offshore and ocean engineering applications. This study investigates the axial bearing capacity of piles embedded in carbonate sands, focusing on the unique mechanical properties of these soils, such as high compressibility, particle breakage, and low shear strength under high stress. Using a newly-developed tall oedometer apparatus, the base bearing capacity of piles in four types of sands (Firoozkooh Quartz Sand No. 161 and Hormoz, Gheshm, and Hengam Carbonate Sands) was examined under dry conditions. The experimental values, obtained from the end-of-cell pressure gauge and logarithmic spiral failure surface based on soil internal friction angles, were compared with computational values derived from cavity expansion theory. Discrepancies of 18% for Hormoz soil at 80% compaction and 14% at 30% compaction were observed, demonstrating the oedometer device's effectiveness in measuring pile base resistance. The study also explores the relationship between particle breakage and the internal friction angle, employing Hardin's method to quantify breakage indices. Results indicate that the bearing capacity of carbonate sands is notably lower than that of quartz sands, with reductions of up to 36% in dense soils and 24% in loose soils due to particle crushing. The research highlights the significant particle breakage occurring in the shear zone near the pile base, particularly within the 0-5 cm and 5-10 cm layers, with reduced breakage observed at greater depths. The findings highlight the importance of understanding particle breakage and shear zone characteristics for designing stable pile foundations in carbonate soils, improving the safety and efficiency of geotechnical practices in challenging conditions.