Morphological characteristics of hysteretic curves of soil-rock mixture under stepped axial cyclic loading

The strata in urban backfill areas mostly exist in the form of loose soil-rock mixture, with high structural porosity, low strength, and poor engineering performance. They are sensitive to dynamic loads such as tunnel construction disturbance and subway train operation. The hysteretic curve can reflect the deformation, stiffness and energy dissipation of soil under dynamic load. It is of great significance to study the hysteretic curve of soil-rock mixture for the construction and operation safety of subway in backfill area. Using KTLDYN servo-controlled dynamic triaxial test system, the cyclic load test on soil-rock mixture samples in backfill area was carried out by means of cyclic loading. The effects of stone content (P), water content (ω), consolidation stress ratio (k_c) and loading frequency (f) on the morphological characteristics (including adjacent center spacing (d), long axis slope (k), enclosing area (S) and degree of non-closure (ε_p)) and backbone curves of hysteretic curves are investigated. The results show that the typical hysteretic curves of soil-rock mixture are in long fusiform shape on the whole, with pointed lobes at both ends. With the increase in vibration level, d, S and ε_p increase nonlinearly, while k decreases logarithmically. For the same vibration level, d and ε_p decrease with the increases of P, k_c and f, and first decrease and then increase with the increase of ω. k increases with the increases of P, k_c and f, and increases first and then decreases with the increase of ω. S is positively correlated with P, increasing first and then decreasing with the increase of ω, and decreasing with the increases of k_c and f. The dynamic stress and slope of backbone curve increase with the increases of P, k_c and f when the dynamic stress variation is the same, and they first increase and then decrease with the increase of ω.