Evolution of critical state of calcareous sand during particle breakage

Based on the particle breakage characteristics and development laws of calcareous sand from physical triaxial test results, a discrete element numerical model is established for conducting numerical triaxial tests. First, the specimens with different initial gradings are generated by pre-crushing a uniformly graded calcareous sand under different pressures, and the numerical tests without breakage during the following triaxial shear process are carried out to determine the relationship between the critical state and the fixed grading. The results show that the fixed grading has a fixed critical state line, and the critical state lines of different gradings are basically parallel, but their position decreases gradually with the broader grading (i.e., the increasing breakage) in the e-p compression plane. Afterwards, the numerical tests of crushable particles during loading process are conducted on the specimens with the same uniform grading so as to reveal the evolution mechanism of the critical state during real triaxial tests on crushable soils. The results show that in the three-dimensional e-p-B_r space, the points of the critical state from the crushable tests fall on the surface of the breakage critical state determined by the fixed critical state lines from the non-crushable tests, indicating that the critical state depends only on the final grading regardless of the intermediate process to achieve the final grading. In real physical tests, the particle breakage extent of the points at the measured critical state line is different, and thus the critical state line exhibits complicated nonlinear form. Under triaxial compression conditions, the particle breakage increases with the increasing mean effective stress, leading to the rotation of the measured critical state line.