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 (theta sigma = -19.1 degrees, 0 degrees, 19.1 degrees, and 30 degrees), and major principal stress angles (alpha = 0 degrees, 30 degrees, 45 degrees, 60 degrees, and 90 degrees) 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 alpha = 45 degrees 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 degrees. Increasing polyurethane content from 2.5% to 7.5% enhanced sample strength by 20% at theta sigma = -19.1 degrees and alpha = 60 degrees. A generalized linear strength theory in the pi-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.