Mechanical properties and failure modes of stratified backfill under triaxial cyclic loading and unloading

Multiple filling of gobs will lead to a layered structure of the backfill. To explore the influence of layering structure on the mechanical properties and failure modes of backfill, different backfill specimens were prepared with a cement/sand ratio of 1:4, a slurry concentration of 75%, and backfilling times of 1, 2, 3 and 4, separately. Triaxial cyclic loading and unloading experiments were carried out. The results show that with an increase in backfilling time, the peak strength of backfill decreases as a polynomial function and the peak strain increases as an exponential function. The cyclic load enhances the linear characteristic of backfill deformation. The loading and unloading deformation moduli have a linear negative correlation with the backfilling time. The unloading deformation modulus is always slightly higher than the loading deformation modulus. The failure modes of stratified backfill are mainly characterized by conjugate shear failure at the upper layer and tensile failure across the layer plane, and there is usually no damage in the lower layer away from the loading area.