Key technologies for supporting ultra large section tunnels and the evolution law of mining induced stress

In response to the problem of controlling the deformation of surrounding rock in ultra large cross-section tunnels, taking the mining roadway of the ultra high mining face in Caojiatan Coal Mine as the research background, the distribution law of the roof rock strata in Caojiatan Coal Mine was studied, and the structural characteristics of “high+low layer thick hard roof” were analyzed. The characteristics and formation mechanism of the deformation of surrounding rock in dynamic pressure tunnels were clarified, including asymmetry, zoning, and persistence. The influence of tunnel section size on surrounding rock stability and the distribution characteristics of support prestress field were analyzed using numerical calculations. Reasonable anchor bolt support parameters were determined, and a collaborative control technology was proposed to improve near-field surrounding rock stability with high prestress anchor bolt support and reduce far-field mining pressure dynamic load with hydraulic fracturing of the roof. The distribution law of surrounding rock stress and plastic failure under different coal pillar widths was analyzed, and the reasonable width of the roadway coal pillar for ultra high mining face was determined to be 25 m. A scheme for ultra high section roadway anchor bolt support and hydraulic fracturing pressure relief technology was formulated. The self-developed ultra large section roadway mining pressure monitoring and early warning system platform was used to reveal the evolution law of mining stress in the dynamic pressure roadway of the ultra high mining face, and to obtain the evolution law and spatial distribution characteristics of microseismic energy in the fracturing zone roof. The results showed that there was no significant deformation during the excavation of the ultra large section roadway, and the force on the anchor rod quickly tended to stabilize. The advanced impact range of the working face during the backfilling stage is 135.7 m, and the auxiliary transportation roadway lags behind the working face by 229.6 m. The mining pressure basically tends to stabilize, and the peak value of the advanced support pressure of the working face is within the range of 25.3−31.7 m. The stress on the anchor rod and the coal pillar changes dramatically and quickly stabilizes. The maximum stress on the anchor rod is 142 kN, the maximum stress on the anchor rod is 178 kN, and the maximum stress increment on the coal pillar is 8.6 MPa.Compared to ordinary fully mechanized top coal caving working faces, the period of influence of mining pressure in the reserved roadway is significantly reduced, and hydraulic fracturing of the roof significantly weakens the hard and thick roof, effectively reducing the sustained lateral support pressure of the coal pillar. The microseismic energy of the lateral roof of the coal pillar significantly increases from the leading working face 170 m to the lagging working face 220 m, and the roof activity is most intense at the lagging working face 20−60 m, followed by a rapid decrease, indicating that the high and low strata of the roof collapse in a timely and orderly manner. The microseismic energy is consistent with the evolution law of mining stress, revealing the migration state of the high and low strata of the roof and the formation mechanism of mining pressure. There was no significant deformation observed in the surrounding rock of the ultra large section roadway throughout the entire period, indicating that the use of high prestressed anchor support and roof fracturing pressure relief collaborative control technology in the super large section roadway has achieved good control effect on surrounding rock deformation.