Microstructural properties of unsaturated Q₃ loess and their influence mechanisms on strength properties

Structure is the important factor affecting the mechanical deformation characteristics of unsaturated undisturbed loess, but there are few reports on the structural differences between undisturbed and remolded loesses from the perspective of combining micro and macro-scopic views. Taking the unsaturated undisturbed Q₃ loess and its remolded soil under the same physical and mechanical conditions as the research objects, the unsaturated triaxial shear tests with the control suction and net confining pressure as constants are carried out, and the influence mechanism of the structural differences between the two on the strength characteristics is revealed by micro and meso-scopic scannings. The tests show that the cohesions of the undisturbed and remolded Q₃ loess increase linearly with the increase of matric suction, and the cohesion of the undisturbed soil is greater than that of the remolded soil. The angle of internal friction is basically constant and less affected by the matric suction, and the difference between the two is not significant. The undisturbed loess particles have obvious edges and corners, which are in bracket contact and have good cementation, and the structure of the undisturbed loess is strong. The remolded loess particles are close to round and in inlaid contact, and the cementation between them is destroyed. The soil skeleton function of the remolded loess is not obvious and the structure is weak, the formed aggregate structure is easy to compress, and the connection between aggregates and particles is not stable, thus forming the undisturbed soil with better resistance to the external shear failure. Under the same physical conditions, the pore sizes of undisturbed and remolded loesses less than 50 μm account for 22.7% and 16.3% of the total volume of the samples, respectively. The pore sizes between 50 μm and 200 μm are 71% and 83.7%, respectively, and those larger than 200 μm are 6.3% and 0%, respectively. The open structure composing of the bracket macro-void does not provide deformation space for the undisturbed soil, but the good cementation and structural properties ensure that the undisturbed loess has high strength characteristics.