Influence of main roof thickness-span ratio on the initial cracking induced instability in fully mechanized longwall face

The mechanism of the initial fracture instability of main roof in a fully mechanized longwall face is critical to the selection of hydraulic support and the control of surrounding rock. The fracture process and structural instability mechanism of main roof were analyzed by establishing the roof fracture mechanical model. The results show that the in- ternal maximum principal stress σ1 and the maximum shear stress τmax increase gradually,with the decrease of the thickness-span ratio of main roof from the initial state to the limit state. When the thickness-span ratio is close to 0. 2, both σ1 and τmax increase sharply,but the increase rate of σ1 is greater than that of τmax,and the main roof is thus easy to break at the top of bothinitial fracture appears at the main roof. The n1 of hinge structure of main roof that does not appear sliding instability increases linearly with the increase of the rotation angle θ. The hinge structure is easy to occur sliding instability at the beginning of the initial rotation when the rotation angle is 0° ~ 3°. The critical value of the rotation instability of the hinge structure is n1 equals to 0. 624 on a condition that only the roof thickness-span ratio dominates. When the n1 is less than 0. 624,however,the rotation limit balance angle βj becomes larger than the minimum value β1 allowed by the swing balance angle,and the hinge structure occurs rotation instability. In addition,the method for determining the dy- namic load factor of the longwall face when the main roof structure appears slipping instability was obtained. When the separation layer Δh between immediate roof and main roof is 0,the minimum value of the dynamic load factor Fsd should be applied. When Δh is non-zero,Fsd increases with the increase of Δh,but also can be reduced with the in- crease of the ratio of immediate roof to support stiffness ks,the ratio of immediate roof weight to main roof transfer load m. Finally,the liability of the theoretical analysis was verified by the analytical calculations for the initial weighting of different fully mechanized longwall faces. ends. The limit thickness-span ratio n1 of main roof is normally greater than 0. 2 when the