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.

In order to study the deformation and non-coaxial characteristics of fiber cement modified iron tailing sand (FCIT) under different stress paths. The evolution law of the dynamic stress ratio (eta) on the cumulative deformation and non-coaxial angle of FCIT under different tension-compression amplitude ratios (alpha) was explored through hollow torsional shear tests. The deformation behavior of FCIT is analyzed by using the stability theory. The research results show that: 1) The increase of dynamic stress level will deepen the cumulative deformation of FCIT, while the increase of alpha will make the deformation mode of FCIT develop into bulging failure-shear failuretensile failure. 2) According to the development trend of cumulative strain and cumulative strain rate of FCIT, it can be divided into three stages: plastic stability, plastic creep and incremental collapse, and the cumulative strain rate development prediction model is established, and the prediction error distribution is within +/- 20%. 3) The stress path determines the non-coaxiality of FCIT to a certain extent, and the non-coaxial angle fluctuates and decreases with the increase of the principal stress direction angle. When alpha=0.125, the maximum and minimum values of the non-coaxial angle of FCIT are 1.5 rad and 0.4 rad respectively. 4) Cement hydration reaction generates calcium alumina and hydrated calcium silicate (C-S-H) to make FCIT structure more compact, fibermatrix interfacial connections more dense, and play a role in filling the pores.