# Study of the Constraint Pillar Test and the Application in Gob-Side Entry Retaining
## 1 Introduction  
Roadside support is a support method which is set outside the roadway section to be reserved and on the side of a goat. For simple conditions such as thin and medium-thick coal seams, shallow buried depths, and stable roofs, the direct roof of the working face can fall in time after mining, the hinged rock beam structure supporting the basic roof can quickly reach balance, and the support and strengthening support in the roadway can effectively maintain the stability of surrounding rock, so the roadside support may not be set. Except for this, roadway retaining along the gob-side entry generally needs to set up roadside support [1, 2, 3]. Chinese scholars have performed a lot of research on the roadside support of gob-side entry retaining. The roadside support has gradually developed from gangue wall, dense pillar, wood crib, and gangue belt with low initial strength, poor shrinkage, and tightness to various forms of roadside filling support structures, such as high-water filling, paste filling, and concrete filling. The renewal and progress of roadside support has promoted the development of gob-side entry retaining technology [4]. [PERSON] et al. [5] used high-water materials as roadway backfill and reinforced the backfill with anchor bolts to enhance the overall support performance, whichwas tested and applied in S2-6 fully mechanized top-coal caving face of Changcun Coal Mine, and it achieved good results. High-water material has good plasticity. When the load reaches the peak strength, its bearing capacity gradually decreases, but it is not destroyed immediately and still has high residual strength, which is helpful to adapt to the activity law of the basic roof. [PERSON] [6] studied and matched the high-water filling materials and mechanized filling technology for gob-side entry retaining, which promoted the wide application of high-water materials in the filling body beside gob-side entry retaining in thin and medium-thick coal seams. [PERSON] [7] studied a complete set of technology and equipment for filling and supporting flexible formwork concrete beside roadway in gob-side entry retaining. Under the cover of retaining equipment, a flexible formwork diaphragm wall was built beside the roadway to support the roof and floor, and the goat was enclosed. It has strong technical superiority, with a simple supporting structure, convenient construction, high strength, and good tightness. [PERSON], [PERSON], and others studied the supporting structure of concrete-filled steel tubular pillars beside the roadway. A layer of gangue wall was built at the edge of the goat and shotcreted, and steel tubular pillars were set close to the gangue wall to form the supporting body beside the roadway. The pillar was composed of steel tube with a \(\varphi\) 219 mm and 7 mm wall thickness and C40 concrete. The resistance of pillar support can reach 3100 kN, the bearing capacity is larger, and the structural stability is better [8; 9; 10]. [PERSON] et al. studied the retractable steel pipe pillar, which is composed of two seamless steel pipes with a length of 2000 mm nested, and its interior is filled with a certain proportion of sand and gravel granular materials, with a bearing capacity of about 3300 kN. The pillar has certain flexibility [11; 12; 13; 14]. [PERSON] and others conducted research on the material ratio, filling process, and supporting equipment for filling gangue gypsum bodies in goat areas, and they carried out a large number of underground filling experiments. After the gangue is lifted into the well, it enters the crushing system and is ground to a particle size of less than 5 mm to serve as the aggregate for filling paste. The final setting strength of the material is controlled at around 3.5 MPa. Reserve tunnels are used for the next working face while filling the goat on a large scale. This process can consume a large amount of coal gangue and also has a significant effect on reducing surface subsidence, but the filling cost is relatively high.