Study On Weakening Of Engineering Rockmass And Control Principle Of Gob-side Entry In Large Height Mining Condition

With the increase of mining depth and scale, proportion of complex geological conditions grows, such as deep high in-situ stress, intense mining influence, soft and fractured surrounding rock. Excessive ground failure and deformation leads to frequent occurrence of safety accident and function decrease of ventilation and transportation, which further affect the safe and efficient production. In general, the surrounding rockmass of coal mine roadway will undergo the after-peak, whose weaken effect is the key to ground stability analysis and support optimization. Various intensities of fractrues development will occur in surrounding rockmass under the effect of mining-induced stress, which lead to ground failure, weaken or even instability. Thus, the studies of weakening model of soft rockmass and control principle of gob-side entry in large-height mining(GELM) have great theoretical and application significance for improving engineering rockmass mechanical model and numerical simulation methed, ground stability analysis and control.Based on the field problems of sever ground failure and deformation in Zhaogu No.2 coal mine, by employing a variety of research methods including field test, laboratory experiment, analytical analysis, numerical simulation etc. and knowledge of elasto-plastic mechanics, underground mining, numerical finite difference, computer programming etc., the research has been systematically studied, and following reseach results and conclusions are obtained:(1) Taking the GELM in Zhaogu No.2 coal mine as engineering background, the mechical properties before and after peak are obtained from laboratory experiments; characteristics of ground failure and deformation are analyzed. Under the effect of last mined panel and ground condition, the surrounding rockmass of gob-side entry with a 8m wide pillar undergoes severe failure and creep deformation, coal of ribs and floor excessively converge, making it extremely difficult for ground support and maintance.(2) Based on the non-rigid or deformability that soft rib behaved on concentrated stress after roadway excavation, an analytical model of roof beam resting on Winkler foundation is built by considering the two ribs as deformable foundation. The expressions of roof bending moment and deflection are derived, the distribution laws are analyzed according to typical roof structure, thus the roof bending characteristics with support of deformable foundation are revealed. Soft ribs notablely deform in compression under concentrated stress, and roof strata bend and sag along with rib foundation, the maximum bending moment and deflection along roof beam differs unnoticeably. A parameteric study has been carried out to study the relationship between roof bending and roof rigidity, foundation rigidity, foundation thickness, stress intensity and roof span, the effect of foundation rigidity on roadway roof stability has been proposed.(3) According to the after-peak mechanical behavior of rockmass and characteristics of strain-softening model, the machism of Young’s modulus decrease after tension-induced fractures and its significance in numerical simulation is studied. A rockmass weakening model and the relationship between residual Young’s modulus Er and tension-fracturing intensity GSIt is built, expression of weakening parameter A is propsed, the tension-weakening model implemented into FLAC3 D and hereby the simulation technique of engineering rockmass weakening is developed.(4) Taking the rockmass properties derived from laboratory experiment using Hoek-Brown failure criterion as input material properties, parameteric studies between Mohr-Coulomb model, strain softening model and rockmass weakening model are carried out, and the simulation results are compared to field monitoring data. The results show that the tension-weakening effect is notale and the rockmass weakening model provides more rigorous result, and feasibility and validity is verified. The proposed model can provide more rigorous and reasonable simulation in analysis of soft rockmass stability and ground support, especially for the engineering condition that tensile failure is easy taken place in rockmass.(5) Based on the structural characteristics of mining strata, extimation and prediction methods of mining-disturbed scope is analyzed, mechanical behavior duiring caved rock consolidation is studied and back analyzed with double yield model to derive reasonable gob material. Evoluation laws of mining-induced stress and the effect of mining-height are studied with propsed rockmass weakening model. Results show the large mining-height has notable effect of abutment pressure distribution and stress recovery in gob. Larger mining-height leads to larger disturbing scope and more intense mining influence.(6) By employing the rockmass weakening model, the fracture-induced weakening effect of mining stress field is studied, the effect of fracturing intensity of overlying fracture zone on mining-induced stress evolution and its mechanism is revealed. The fracturing intensity has significant influence on overlying strata movement, with the increase of fracturing intensity, abutment pressure ahead of face increases, scope of lateral stress concentration and distance to face decreases, and stress recovery also varies. In numerical simulation of mining stress field, pillar stability, support design, the rockmass weakening model should be employed according to field condition to ensure process and result validity.(7) FLAC3 D numerical models with different pillar width are built, production procedures, including excavation, support and mining, are simulated to study the stress state, failure pattern, deformation evolution and effect of pillar width. A pillar width of 5m is determined after comprehensive comparison. By taking ground support, gob isolation, ventiliation, resource recovery rate etc., the pillar width in GELM condition should be optimized to 5m, which also provides reference for gob-side entry layout in similar geological condition.(8) Based on the effect of foundation rigidity and support theories, a support principle “ground control by enhacing ribs”(GCER) is proposed. The field case study shows ground improvement hereby verifies the effectiveness and feasibility. Three GCER support designs are simulated to study the control effect duiring GELM excavation and mining. The results show that, failure extent and deformation significantly decrease through GCER technique, in addition, deformation and stability of roof and floor also notablely improves; the design with extension bolts have the best support performance and achieves effective gob-side entry support with low cost.(9) According to the pervious studies, a support principle “ground control by enhacing ribs”(GCER) is proposed. This support technique aims to improve the stability of entire roadway surrounding rockmass by enhacing anchoring deep and rigidity of ribs. Verfied by field case study and numerical simulation, GCER is an effective tool for ground control of underground coal mine roadway.