The Kaiser effect reflects the memory of the loaded rock to the irreversible damage and deformation. The stress level, loading rate and lithology are the main factors affecting the Kaiser effect of the rock. To identify the accurate stress point of the Kaiser effect, the MTS 816 rock mechanics testing system and the DS5-A acoustic emission testing and analysis system were adopted. The uniaxial cyclic loading–unloading and acoustic emission characteristic test of 90 rock specimens from three types of rocks under different stress level and loading rate was carried out. The evolution of acoustic emission under uniaxial compression of the rock corresponds to the compaction stage, elastic stage, yield stage and post-peak stress drop stage of the rock deformation and failure process and is divided into the quiet period, transition period, active period and decay period of the acoustic emission. The larger the hardness of rock is, the earlier the stress point of the Kaiser effect appears. The loading stress level (σ_A) has appreciable influence on the Kaiser effect of the rock. When σ_A ≥ 0.7σ_c, the Kaiser effect disappears. Usually, the dilatancy stress (crack initiation stress) does not exceed 70% of the uniaxial compressive strength (σ_c) of the rock, and the stress point can be the threshold to determine whether the Kaiser effect occurs. The influence of loading rate (l_r) on Felicity rate (FR) is relatively large when l_r < 0.01 mm/s, and FR rapidly grows with increase of the loading rate. When l_r ≥ 0.01 mm/s, the influence of the loading rate on FR is relatively small. The findings facilitate the future application of the Kaiser effect and improvement of the accuracy of the acoustic emission data interpretation.

To study the mechanical properties of argillaceous weakly cemented rock under dynamic loading, a sample reconstituted and graded loading scheme is firstly designed, and then the reorganization rock sample is used as the research object. Using a Hopkinson pressure bar test, the responses of an argillaceous weakly cemented rock mass under different reorganization loads and different impact velocities is studied, and changes in specimen shape after impact are also analyzed. The study found that with increased of the recombination load, the amplitude of the transmitted wave increases. With increasing impact velocity, the rate of the increase in the incident wave amplitude is much larger than that of the transmission wave amplitude. The dynamic stress–strain curve can be divided into a compaction stage, an approximate linear elastic stage, a microcrack growth stage and a strain softening stage. The larger the reorganization load is, the less obvious the compression stage of the stress–strain curve is, and the greater the elastic modulus is. The peak strength and elastic modulus increase with increasing strain rate before 500 s⁻¹. When the reorganization load is increased, the deformation decreases, and its impact resistance increases. With increasing impact velocity, the deformation of the specimen increases.

The Kaiser effect of rock has been extensively studied due to its wide application in in-situ stress measurement and rock damage quantification. The uniaxial cyclic loading and unloading (UCLU) test is commonly employed to examine the rock Kaiser effect. However, how the two critical parameters, including prescribed stress in the first loading cycle (σ_A) and loading strain rate (l_sr), affect the appearance of the Kaiser effect lacks thorough understanding. We systematically performed UCLU tests on 75 sandstone specimens under 25 combinations of σ_A and l_sr. σ_A spans from 0.5σ_c (σ_c is the uniaxial compressive strength) to 0.9σ_c, and l_sr ranges from 10⁻⁵ s⁻¹ to 10⁻³ s⁻¹, respectively. The acoustic emission characteristics of all the rock specimens are continuously monitored over the entire tests. We find that the Kaiser effect is unanimously observed in the stable crack growth stage, corresponding to the stress levels of 0.5σ_c to 0.7σ_c because under a lower stress, the Kaiser effect is easily covered by the acoustic emissions generated by microcrack friction. The loading strain rate also heavily affects the occurrence of the Kaiser effect. When l_sr does not exceed 10⁻⁴ s⁻¹, the Felicity ratio (FR) rises quickly as l_sr ascends, whereas FR increases less pronouncedly once l_sr exceeds 10⁻⁴ s⁻¹. Therefore, a relatively high loading strain rate, i.e., l_sr higher than 10⁻⁴ s⁻¹, is suggested to facilitate the appearance of the Kaiser effect.

A detailed understanding of the engineering properties for grouted sand is a key concern in foundation engineering projects containing sand layers. In this research, experiments of grouting with various grain sizes of sand specimens using a new type of improved chemical material-urea formaldehyde resin mixed with oxalate curing agent (UF-OA), which has rarely been used as grout in the reinforcement of soft foundations, were conducted on the basis of a self-developed grouting test system. After grouting tests, the effects on the mechanical behaviors of grouted sand specimens were investigated through uniaxial compression tests considering the grain size, the presence or absence of initial water in sand, and the curing time for grouted sand. Experimental results show that with the increase in the grain size and the presence of initial water in the sand specimen, the values of uniaxial compressive strength (UCS) and elastic moduli (E) of the grouted specimens decreased obviously, indicating that the increase of grain size and the presence of initial water have negative impacts on the mechanical behaviors of grouted sand; the peak strains (ε_c) were almost unchanged after 14 days of curing; no brittle failure behavior occurred in the grouted specimens, and desirable ductile failure characteristics were distinct after uniaxial compression. These mechanical behaviors were significantly improved after 14 days of curing. The micro-structural properties obtained by scanning electron microscopy (SEM) of the finer grouted sand indicate preferable filling performance to some extent, thereby validating the macroscopic mechanical behaviors.

This research experimentally analyzed the impacts of various water cement (W/C) ratios of ultrafine cement grout material and normal loads F_N applied to fractures on grout nonlinear flow behavior through a rough plexiglass fractured sample. An effective self-made apparatus was designed and manufactured to conduct the stress-dependent grout flow tests on the plexiglass sample containing rough fractures. At each W/C ratio, the grout pressure P increased from 0 to 0.9 MPa, and the normal loads F_N ranged from 666.3 to 1467.8 N. The results of the experiments indicate that (1) the Forchheimer’s law can be used to express the results of grout nonlinear flow through rough fractures. Moreover, both nonlinear coefficient a and linear coefficient b in Forchheimer’s law decreased with the increase of the W/C ratio, but increased with the increase of the F_N value. (2) For normalized transmissivity, with the increase of Re, the decline of the T/T₀–Re curves means that the grout flow behavior through the fracture mainly went through three stages: the viscosity effect, then the weak inertia effect, and finally the strong inertia effect. The three stages showed that with the increase of Re, the grout flow state changed from linear to nonlinear. Moreover, with the increase of the W/C ratio, the Forchheimer coefficient β decreased. (3) At a given F_N, the critical grout hydraulic gradient J_c decreased, but the critical Reynolds number Re_c increased as the W/C ratio increased; at a given W/C ratio, J_c increased, but Re_c decreased as F_N increased.

The mining of the working face has a pronounced influence on the floor roadway, resulting in the intensification of deformation of the surrounding rock of the roadway and posing a threat to production safety. To clarify the failure mechanism of the surrounding rock of the dynamic pressure roadway and to understand the development pattern of the plastic zone shape, in this paper, by integrating the stress distribution law of the floor during the mining process of the working face, a mechanical model of the surrounding rock of the floor dynamic pressure roadway is established. An analytical solution for the plastic zone shape of the surrounding rock of the dynamic pressure roadway is proposed and verified through a comparison between analytical calculations and numerical simulation (using FLAC^3D 7.0 software). The results indicate the following: (1) when the horizontal distance between the working face and the roadway exceeds 35 m, no obvious changes are observed in the development pattern of the plastic zone of the surrounding rock of the roadway; (2) when the horizontal distance is less than 35 m, the rate of change in the maximum value of f increases from 0.02 MPa/m to 0.205 MPa/m as the vertical distance decreases, and the β value increases from 0.09°/m to 0.79°/m. The maximum development depth increases, the butterfly leaf of the plastic zone deflects towards the working face direction, and the shape gradually transforms from a “circular” shape to a “butterfly” shape. (3) The morphological characteristics of the plastic zone obtained by the analytical algorithm are highly consistent with those obtained by the numerical simulation method, suggesting that the analytical algorithm possesses certain validity. (4) There exist certain errors between the analytical method and the numerical simulation results, and they increase with the increase in the vertical distance (the error range is 19.4%~54.9%), but they can be disregarded within the requirements of engineering accuracy.