Abstract
The mechanical properties and envelope curve predictions of polyurethane-improved calcareous sand are significantly influenced by the magnitude and direction of principal stress. This study conducted a series of directional shearing tests with varying polyurethane contents (c = 2.5%, 5%, and 7.5%), stress Lode angles (θσ = -19.1°, 0°, 19.1°, and 30°), and major principal stress angles (α = 0°, 30°, 45°, 60°, and 90°) to investigate the strength and non-coaxial characteristics of calcareous sand improved by polyurethane foam adhesive (PFA). Key findings revealed that failure strength varied significantly with the major principal stress axis direction, initially decreasing to a minimum at α = 45° before increasing, with a 30% decrease and 25% increase observed at c = 5%. Non-coaxial characteristics between strain increment and stress directions became more pronounced, with angles varying up to 15°. Increasing polyurethane content from 2.5% to 7.5% enhanced sample strength by 20% at θσ = -19.1° and α = 60°. A generalized linear strength theory in the π-plane accurately described strength envelope variations, while a modified Lade criterion, incorporating polymer content, effectively predicted multiaxial strength characteristics with less than 10% deviation from experimental results. These contributions provide quantitative insights into failure strength and non-coaxial behavior, introduce a robust strength prediction framework, and enhance multiaxial strength prediction accuracy, advancing the understanding of polyurethane-improved calcareous sand for engineering applications.