## 1 Introduction  
In 2016, the coal-fired power generation accounted for 37% of the world's electricity generation capacity, and the share is predicted to be above 30% over the next decade [1]. Therefore, coal production is playing a very important role in the global economy. However, long-term ground stability is becoming a major concern to the coal mining industry [2]. Underground coal mines commonly operate at a depth of hundreds of metres, and some very deep mines even reach and exceed one thousand metres [3]. Deep mining is accompanied by high overburden stresses, complex geologic conditions, and, in some cases, tectonic stresses [4]. In addition, typical coal-measure sedimentary rocks are relatively weak, especially the coal. When experiencing high stress conditions, the time-dependent deformation of coal over time is significant and can become a very important factor in triggering the violent failure of underground openings. Creep is the name applied for the time-dependent deformation of solids resulting from constant stress [5]. The discovery of the creep of rock and rock-like materials can be tracked back to 1907; [PERSON] [6]observed a slow deformation of concrete under a constant environmental condition. Since then, many studies have been devoted to improving the understanding of concrete creep. Successes have been achieved both in the physical mechanism and the mathematical model. Physical mechanisms proposed for concrete creep can be summarized as plastic flow [7; 8], nonlinear cracking and reformation [9; 10; 11], and solidification theory [12; 13]. The mathematical models are established mainly for two purposes: facilitation of structural analysis and close agreement with experimental data. Based on the effective modulus method [14; 15], the rate-of-creep model [16], rate-of-flow model [17], and Arutyunian-Maslov model [18] were proposed to simplify the structural analysis. The B3 model [19] and Double power law [20] were formulated with a view to improve the data fitting and predication. In term of progresses in rock creep, the pioneering work was done by [PERSON] [21] who first developed two types of apparatus to investigate the creep behaviour of shale and limestone. Since then, extensive investigations have been conducted on the time-dependent behaviour of many different rocks by using experimental, theoretical, and in situ approaches [22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33]. When being expressed as a function of time, it develops in three stages: primary (transient) creep, secondary (steady) creep, and tertiary (accelerating) creep (as illustrated in Figure 1). Prior to the primary stage of creep, instantaneous elastic strain is generated as the first response of the rock to the applied stress. Several researchers suggested considering the instantaneous strain as a part of creep process [34; 35]. As time passes, the strain increases without any change of the applied stress. At the primary stage, the strain rate gradually decreases, and the creep strain is governed by the elastic deformation mechanism [36]. During the secondary stage, the strain rate stabilises at a nearly constant value, the behaviour of which is determined by the visco-plastic properties of rock [34; 37]. If the secondary stage is allowed to continue, then the strain rate begins to increase (tertiary stage). The tertiary stage accounts for only a small portion of rock lifetime. However, it is associated with the structural failure caused by the process of micro-fracturing, and therefore it is crucial in predicating service life of rock and rock mass. Compared to the fruitful investigations of creep in rock salt, shale, and sandstone, very few results on creep in coal have been reported. [PERSON] [38] investigated the time-dependent behaviour of stress on bituminous coal and anthracite by performing compression and bending tests. The results showed that an extremely slow strain rate may induce the non-explosive breakage of bituminous coal but cannot prevent the explosive breakage of anthracite.