The dynamic triaxial test is conducted on the straw-reinforced red clay under different conditions (straw reinforcement rate, confining pressure, and frequency) using British GDS dynamic triaxial apparatus. Five parameters, which are slope k of the long axis of the hysteretic curve, the ratio alpha of the long axis to the short axis of the hysteretic curve, the distance d between the two centers of the adjacent hysteretic curve, the area S of hysteretic curve, and the residual strain epsilon p, are used to quantitatively study the morphological characteristics of hysteretic curves. The results show that S, d, alpha, and epsilon p of the hysteretic curve of straw-reinforced red clay increase prominently with the increase of dynamic load, and decrease with increasing confining pressure and frequency. k decreases logarithmically with the increase of dynamic load, and increases with rising frequency and confining pressure. Under the same dynamic stress amplitude and five different reinforcement conditions, the specimen is characterized by the highest dynamic strength, optimal deformation resistance, and excellent reinforcement performance when the reinforcement rate is 0.2%. Compared with the red clay unreinforced with straw, the straw-reinforced red clay has an increased stiffness and elastic modulus, and decreased viscosity, energy dissipation capacity, microscopic damage, and residual plastic strain of soil. To sum up, the dynamic properties of straw-reinforced red clay are significantly improved compared with that of red clay unreinforced with straw.

Traffic cyclic loading is the key factor that leads to the deterioration of the long-term service behavior of subgrade. A series of cyclic triaxial tests was carried out by the large-scale dynamic and static triaxial apparatus (LSDSTA) to study the dynamic behaviors of coal gangue subgrade filler (CGSF) under multi-step cyclic loading using the morphological characteristics of hysteretic curves (MCHC). MCHC was quantitatively characterized by four parameters, i.e., the unclosed degree (epsilon phl ), inclination of long axis degree (k hl ), area (S hl ) and fullness degree (alpha hl ). With the increase of dynamic strain, epsilon phl increases exponentially. k hl of the coal gangue sample first decreases and then shows an increasing trend with the increasing dynamic strain. The values of S hl are close to each other, and the energy dissipation in the sample is small. However, with the increase of dynamic strain, the specimen failure degree is increased, S hl increases exponentially, and the damping ratio increases. With the increase of dynamic strain, alpha hl increases approximately linearly. Confining pressure has a certain effect on the four parameters. There parameters can be recommended and used for quantitative analysis the dynamic behaviors of subgrade filler under traffic 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 (omega), 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 (epsilon(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 epsilon(p) increase nonlinearly, while k decreases logarithmically. For the same vibration level, d and epsilon(p) decrease with the increases of P, k(c) and f, and first decrease and then increase with the increase of omega.k increases with the increases of P, k(c) and f, and increases first and then decreases with the increase of omega.S is positively correlated with P, increasing first and then decreasing with the increase of omega, 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 omega.