# Review on Engineering Application Status of Gob-Side Entry Retaining Technology in China
## 2 The Current Status of GSER Technology Development in China  
After over 70 years of development, a relatively comprehensive theoretical and technical framework has been established for the application of GSER technology in China's mining areas. The development process and research status of GSER technology are roughly as follows: In the 1950s, GSER technology was proposed in China and applied to thin coal seams (below 1.5 m) with simple engineering geological conditions. The construction method and roadway support were both in a simple passive form, with poor GSER effectiveness and high labor intensity. With the accumulation of engineering experience and further exploration, GSER technology has been applied to medium coal seams with a thickness of 1.5-2.5 m since the 1960s. The dense pillars, blocks, and gangue walls were gradually used to construct the entry-side structures. The I-beam support was added to the roadway support, and the implementation effectiveness of GSER technology was improved. Until the 1970s, the above stage can be referred to as the initial and exploratory phase of China's GSER technology [7]. In the 1980s, Chinese researchers began to explore the mechanism of ground pressure appearance in the entry retaining, proposed the relationship between the roof subsidence of the retained entry and the mining thickness [8], and recognized that the \"triangular hanging plate\" structure at the end of the longwall working face could affect the stability of the surrounding rock of the retained entry [9]. Since then, mechanized entry-filling technology has been employed with quick-setting cement and hard gypsum as materials, with the development of metal yieldable support in the roadway. These advancements have significantly enhanced the implementation effectiveness of GSER technology under simplified engineering conditions. In the 1990s, mechanized coal mining technology for comprehensive working faces was vigorously boosted in China. The increase in mining height and the accelerated advancing speed in the working face have significantly augmented the difficulty of maintaining the surrounding rock of the retained entry. Although  
Figure 1: The schematic diagram of the concrete mechanized filling process of the GSER. a set of high-water material filling was used in GSER technology, the low strength, slow resistance increase, susceptibility to weathering, and high cost of such materials were inadequate for accommodating the ground pressure in the retained entry. Moreover, the intricate nature and slow construction speed of GSER technology limited its further promotion and application in engineering. During this period, researchers proposed calculation methods for the support resistance and strength of entry-side structures under specific engineering conditions [10] and proposed a supporting principle of \"controlling roof and yielding pressure\" in the retained entry support [11]. At the same time, the position and time of the triangular hanging roof of the retained entry roof were predicted, providing an important basis for determining the timing of retained entry reinforcement support [12]. The above stage can be referred to as the sustained development stage of China's GSER technology. After 2000, China's GSER technology entered a stage of rapid development and widespread application. The control theory, support technology, equipment machinery, and filling materials for retained entry surrounding rock have achieved significant advancements. The fundamental movement characteristics of retained entry rock layers were clarified, a mechanical model of the filling body and roof was established, and a preliminary calculation method for the resistance of retained entry support and the subsidence of the roof was formed [13]. It is also clearly pointed out that the relevant parameters of the filling body have a significant impact on the subsidence of the retained entry roof [14], and a calculation method for the compression amount of the filling body was proposed [15].