## 1 Introduction  
Many underground tunnels and slope rock masses were excavated in the processes of mineral resource exploitation, water resource construction, and hydroelectric and railway engineering. However, rock masses experience different excavation unloading damages in certain areas near the slope and tunnel excavation surfaces. During project operation and maintenance, the mechanical properties of the rock masses in the unloading zones formed by excavation and the common bearing carrier formed by the supporting structures determine the stability of the excavation project, which is a core aspect of the development and utilization of rock masses. Generally, the rock masses in disturbed excavation unloading zones are no longer in a simple compression state, with the phenomenon of stress relief occurring along the direction of the vertical excavation surface [1], which experiences a complex stress environment. The mechanical properties of rock (masses) will be different depending on the stress environment. Scholars have conducted indoor mechanical tests on various types of rocks (masses) under complex stress environments and these can be roughly divided into loading mechanical, unloading mechanical [2], cyclic loading and unloading mechanical [3; 4; 5; 6; 7], and damage reloading mechanical tests [8; 9; 10]. Previous studies [11; 12; 13] conducted indoor loading tests and studied the stress-strain curves, deformation, and failure characteristics of sandstone and conglomerate specimens under various confining pressures. The unloadingmechanical tests were based primarily on conventional indoor triaxial unloading tests, considering the effects of confining pressure, unloading rate, and time on the unloading mechanical properties of rock [14; 15; 16; 17; 18; 19]. [PERSON] et al. [20] conducted triaxial loading and unloading tests on salt rock specimens and analyzed the damage characteristics and energy evolution law. Previous studies [21; 22; 23] also conducted uniaxial graded cyclic loading and unloading tests on metamorphic sandstone to examine the failure mode and the crack initiation and development process. [PERSON] et al. [24] conducted triaxial cyclic loading and unloading tests on limestone specimens under different confining pressures and examined the strength, deformation, and expansion characteristics. [PERSON] [25] conducted true triaxial unloading tests on sandstone under cyclic load damage effects and examined the strength, deformation characteristics, and failure modes of sandstone under cyclic load damage effects. [PERSON] et al. [26] prepared damaged specimens by removing the axial confining pressure and then subjected the damaged specimens to triaxial compression tests to determine the relationship between the peak stress, deformation parameters, and damage degree. Related studies [27; 28; 29] obtained damaged white sandstone and limestone specimens through triaxial loading-confining pressure unloading and then subjected them to uniaxial loading mechanical tests to examine the strength weakening mechanism and macroscopic failure characteristics. [PERSON] et al. [30] obtained specimens with different types of initial damage by conducting loading and unloading tests on silty mudstone specimens and examined the triaxial strength characteristics of the damaged specimens under different confining pressures. In some previous studies, uniaxial compression and splitting tests were conducted on rock specimens that were first damaged before being continuously loaded and compacted [31; 32; 33]. In summary, the mechanical properties of various types of rocks under complex stress environments have been studied; however, some gaps remain in terms of understanding the excavation unloading stress environment and subsequent excavation unloading of damaged rock masses.