Investigation On Stability Mechanism And Control Techniques Of Immediate Roof Above Backfill Area In Gob-side Entry Retaining

Immediate roof above backfill area(immediate, and even top-coal) plays a key part in transferring the main roof load downward and transferring the roadside backfill action upon the main roof upward in gob-side entry retaining. However, stability of immediate roof above backfill area was poor under mining abutment stress and cyclic loading and unloading from the working face shield support, which was important for gob-side entry retaining with fully mechanized top coal caving, or soft roof or compound roof. Multiple methods including in-mine monitoring, laboratory test, theoretical analysis, numerical simulation and field practice were used in this paper focused on deformation, load transmitting, bearing mechanism, and stability control techniques of the immediate roof above roadside backfill area in gob-side entry retaining. The key findings in this paper can be summarized as:(1) Multiple methods including laboratory test, theoretical analysis and numerical simulation were used to research the strength reduction and dilatancy law of immediate roof above backfill area in gob-side entry retaining. Based on the evaluation of the immediate roof mechanical medium above roadside backfill area and roof abutment stress distribution in gob-side entry retaining, the immediate roof rock specimen stress path for triaxial unloading test was designed. Triaxial unloading confining pressure test with multistage axial force and multi-yielding was used to determine the damage rock specimen mechanical parameters during different stages. Unloading mechanical strain softening model was established based on post-peak rock dilatancy for the cyclic loading immediate roof above backfill area in gob-side entry retaining, which was carried out in the FLAC3 D.(2) Elastic mechanics and damage mechanics with variation method were used to calculate horizontal stress and vertical stress expressions of immediate roof above roadside backfill area in gob-side entry retaining during different periods. Stress state and stress distribution characteristics of immediate roof above roadside backfill area in gob-side entry retaining were discussed during different periods including working face shield support period, period without roadside backfill body, roadside backfill body resistance increasing period and period with constant roadside backfill body resistance. Immediate roof tensile stress region and horizontal dislocation region were also calculated. With the increase of rotation angle, the tensile stress region decreased and horizontal dislocation region increased.(3) Deformation characteristics of immediate roof above backfill area in gob-side entry retaining with respect to influence factors including roadside backfill body width, immediate roof lithology and thickness of immediate roof and coal seam thickness ratio by using numerical calculation. Transmitting load mechanical model of roadside support system and bearing mechanical model were established based on unbalanced load characteristics of immediate roof and unbalanced bearing characteristics of two ribs. Thus, load transmitting capacity expression, unbalanced load coefficient expression of immediate roof and unbalanced bearing coefficient expression of two ribs were derived. Meahwhile, load transmitting capacity law and its influence factors, unbalanced load coefficient of immediate roof law and its influence factors, and unbalanced bearing coefficient of two ribs law and its influence factors were gived out.(4) Theoretical analysis and numerical calculation were used to research rockbolt inhibition for immediate roof sliding dilatancy deformation above roadside backfill area, abscission deformation control mechanism between immediate roof and main roof above roadside backfill area in gob-side entry retaining. Control techniques of immediate roof above roadside backfill area in gob-side entry retaining was developed. The dynamic regional reinforcement stability control technology included rockbolt and cable support for solid coal rib ahead of working face mining activity; rockbolt support with high prestressed, strength and elongation reinforcement immediate roof for dilatancy deformation of immediate roof above roadside backfill area, cable support for abscission between immediate roof and main roof, hydraulic support shifting with high resistance during the period with working face hydraulic support; high resistance hydraulic prop supplement support immediate roof support strength, determining an appropriate roadside backfill width and applying backfill hydraulic support outside of backfill area during the period without roadside backfill body; applying roadside backfill material with rapidly increasing resistance to construct backfill body during the period with roadside backfill body resistance increasing and the period with constant roadside backfill body resistance.Gob-side entry retaining was performed in 3107 auxiliary ventilation roadway of Xinyuan Coal Mine, whose width and depth are 5.2m and 500 m respectively by using the control techniques of immediate roof above roadside backfill area in gob-side entry retaining. Field pratice suggested that those control techniques achieved the stability of the immediate roof above roadside backfill area in gob-side entry retaining.