## 1 Introduction  
As a typical mining layout form that saves coal resources, improves the resource recovery rate, and simplifies mining and excavation replacement, gob (after the coal is mined, the original working face position is called the gob)-side entry (GSE) has been used extensively in Chinese coal mining roadways [1, 2]. The development of the surrounding rock joints and fissures of the GSE is affected by factors such as multiple mining at the working face, advanced support pressure and other factors, and is often accompanied by engineering disasters, such as the collapse of slabs of coal pillars on the side of the goat [3, 4]. The small coal pillar is an important part of the surrounding rock of GSE, and its stability affects the roadway stability directly. Extensive research has been conducted on coal damage and destruction and related mechanical properties. To reproduce the force process of coal pillars in underground engineering, cyclic loading is usually used to test the long-term mechanical properties of coal. [PERSON] et al. [5] analyzed the mechanical characteristics of fracture damage during the uniaxial cyclic loading and unloading of rocks and studied the propagation conditions and fracture criteria of microcracks through experimental results. [PERSON] et al. [6] conducted uniaxial cyclic loading and unloading tests on sandstone and proposed that the macroscopic fatigue failure mode of rock under uniaxial cyclic loading and unloadingconditions depends on the fracture propagation form of internal microcracks. The impact of mechanical performance is more obvious. [PERSON] et al. [7] carried out uniaxial loading and unloading tests on coal samples, proposed that the plastic properties of each cycle are the dissipation energy of new cracks, and fitted the elastic energy index correction formula. [PERSON] et al. [8] proposed that the main factors that affect the rock fatigue life are the upper limit stress and amplitude of the cyclic load. [PERSON] et al. [9] studied the deformation and strength response of sandstone and granodiorite under multi-stage uniaxial cyclic compression loads. It is believed that the different deformation and failure modes of these two rocks under cyclic loading are mainly due to the difference in their microstructures. [PERSON] et al. [10] conducted a permeability test on the sawtooth fractured rock subjected to periodic axial loads. The results show that the increase in axial displacement roughly contributed to the permeability reduction, and excessive amplitude of cyclic load posed a limited boost to the permeability enhancement. However, few people discuss the applicability of the cyclic load loading method. The difference in mine development layout yields a certain time interval between multiple mining disturbances, so the coal pillar is not in an ideal continuous cycle loading and unloading state. Different from general sedimentary rock, multi-level fissures in coal develop visibly [11; 12]. The long-term continuous force expands the internal fissures of the coal fully, and its mechanical properties and pore characteristics are changed [13], which affects the bearing capacity strength of the coal pillar. Therefore, the influence of the continuous load-holding time interval in discontinuous cyclic loading on coal damage was the focus of this work. Nuclear magnetic resonance (NMR) is a new type of non-destructive testing technology that utilizes quantum magnetism at the atomic scale [14]. It has been used extensively in tests and research in the fields of crack identification, pore distribution, rock meso-structure damage, and rock physical and mechanical characteristics under different conditions, and has become an important rock physical test analysis method [15; 16]. By analyzing changes in the number of pores with different diameters in coal-rock masses after microwave treatment by NMR, [PERSON] et al. [17] obtained the evolution of the pore diameter in coal-rock masses after microwave-induced fracturing. [PERSON] et al.