To investigate the mechanical properties and energy evolution laws of rocks under true triaxial unloading conditions, a study was conducted using a true triaxial rock testing system on three different types of rocks: coal, sandy mudstone, and siltstone. The study examined the mechanical behavior, failure patterns, and fractal dimensions of these rocks under true triaxial unloading conditions. The tests revealed significant variations in stress–strain curves and failure patterns among the different rock types. Observation indicated that rocks with lower peak strength exhibited higher fractal dimensions and increased fragmentation upon failure. Subsequently, based on the experimental data of siltstone, the impact of the unloading rate and particle size distribution on the energy evolution under true triaxial single-sided unloading paths was further investigated using the three-dimensional particle flow software PFC3D 6.0, revealing the micro-mechanisms of rock energy evolution. The study revealed that when the initial stress unloading level was low, the total energy and strain energy at the peak strength exhibited a strong linear relationship with the unloading rate. Before the stress peak, the dissipative energy was mainly composed of frictional energy. After the stress peak, the dissipative energy consisted of frictional energy, damping energy, and kinetic energy. The heterogeneity of rock significantly affected the distribution of dissipative energy, with an increase in rock heterogeneity leading to a decrease in frictional energy and an increase in kinetic energy.

When measuring the compressive strength of rock, size and strain rate are the two main influencing factors. To study the rock strength size effect, rock specimens with length-to-diameter ratios of 0.5, 0.6, 0.7, 0.8, 0.9 and 1 were subjected to static loading tests using the RMT rock mechanics test system and dynamic loading with the split Hopkinson pressure bar, respectively. Based on the Weibull size-effect formula, the experimental results were compared with the improved formula obtained. The results show that rock strength is influenced by size and strain rate. Both the dynamic increase factor and rock strength are proportional to strain rate. The different failure modes of rock with size variation and strain rate variation are described according to the failure process of the specimens. The same length-to-diameter ratio specimens produced more fragments with a strain rate increase. Under the same strain rate of impact, the larger the rock specimen, the finer the broken fragments. Considering the factor of strain rate in the Weibull size-effect formula, the calculated result is accurate. The improved size-effect formula could be used to better elaborate the potential mechanisms of dynamic rock strength. In the unified theoretical formula containing static and dynamic loads, the relationship of rock strength, size and strain rate is well described.

In order to study the failure and fractal characteristics of unloaded rocks, with the help of the true triaxial unloading rock test system and the acoustic emission (AE) monitoring system, rock failure tests were conducted under varying intermediate principal stress and the mechanical response features of the rocks were analyzed. An investigation was conducted into the rocks’ AE patterns and multifractal features. The results showed that the rocks’ AE macroscopic and microscopic main failure modes differed slightly under unloading. As the intermediate principal stress ${\sigma }_2$ increased, the fractal dimension of the cracks in the rocks first increased and then decreased. The distribution of rock failure was initially concentrated, then dispersed, and concentrated again at the end. As the ${\sigma }_2$ increased, the number of failure events within a specified area in the rock samples under unloading, as represented by the ring-down count, first increased and then decreased. Meanwhile, the fractal dimension $\Delta \alpha$ first decreased and then increased. These results characterized the process whereby the failure distribution pattern of the rocks changed from being concentrated to dispersed and back to concentrated again.

In underground engineering, the excavation and unloading processes of rocks are extremely complex. In-depth understanding of the fracture patterns of rocks and the characteristics of acoustic emission (AE) during this process is crucial for ensuring the safe and stable operation of underground engineering. In this study, a combination of the true triaxial disturbance unloading rock testing system and the acoustic emission system was adopted to conduct true triaxial single-sided unloading tests on several common rock types, including coal, siltstone, fine sandstone, and granite. At the same time, in order to analyze the experimental data more thoroughly, the Gaussian mixture model (GMM) clustering algorithm was introduced to perform clustering analysis on the characteristic parameters of acoustic emission, so as to explore the damage and fracture patterns of the unloading rocks. The findings indicate that as the peak strength of the rock decreases, the fractal dimension of the ultimately broken fragments becomes larger, and the cracks within the failure zone develop more comprehensively. The higher the peak strength of the rock after single-sided unloading is, the greater its axial average elastic modulus will be. The acoustic emission ringing count and the b-value curve indicate that compared with siltstone and fine sandstone, granite and coal are more prone to rock burst after unloading. In the RA-AF signals of coal and granite based on the GMM, the proportion of shear signals is as high as 66.05% and 69.21% respectively, which makes it easy to form shear cracks. While the proportions of tensile cracks in siltstone and fine sandstone are 41.43% and 56.41% respectively. Under the action of axial stress, they are prone to longitudinal splitting and form tensile cracks approximately parallel to the direction of the maximum principal stress. The research findings of this study are of great guiding significance for a deep understanding of the characteristics of different rocks during the excavation and unloading processes in underground engineering.

To investigate the reasonable width of a coal pillar in the downward mining section of close-distance coal seams, the stress state of any point below the residual coal pillar in the overlying goaf and the width of a small coal pillar were studied by theoretical calculation, numerical simulation, similar simulation and field monitoring. The findings indicate that the width range of the small coal pillar is 7.92~11.42 m. The 4-1 coal seam is in the stress reduction zone when it is more than 16.6 m horizontally from the border of the residual coal pillar above it. In addition, the peak stress is situated inside the elastic zone of the coal pillar and is lower than the coal pillar’s bearing limit when a small coal pillar of 8 m is maintained. With the help of distributed optical fiber monitoring to model the coal pillars’ stress distribution, it is found that 8 m simulated coal pillars have a certain bearing capacity. The practical findings demonstrate that the 8 m small coal pillar that was left on the site satisfies the demand, and the convergence of the roadway’s floor and roof, and its two sides fall within the controllable range. The findings of the study offer a reference for the location of a return air roadway and the width of section coal pillars in the downward mining of close-distance coal seams.