Research On The Pre-splitting Blasting Weakening Mechanism Of The Hard Roof Under The Deep Environment

In the process of the actual mining, many coal mine working face roofs are made of hard limestone, sandstone and other hardness greater layers. Under the condition of hard roof coal, coal mining will induce a series of problems, such as large area roof hanging and more likely impact, so for hard roof usually need to adopt compulsory caving stope measures to control the caving of roof. Because of its strong adaptability for coal geology and effectiveness of alleviate widespread pressure, Pre-splitting blasting forced caving method is more widely used. The deep rock mass will have high in-situ stress as the increase of mining depth, as a result,weakening of deep rock blasting mechanism is different from shallow. So this paper studies hard roof mechanism of pre-splitting blasting weakening under the deep environment through theoretical analysis and numerical simulation method. The main research work is as follows:(1)This paper summarizes the physical properties and fracture properties of the roof. According to the characteristics of hard roof of stope, through simple derivation of hard roof breaking theory, the hard roof first distance of first weighting and periodic weighting interval estimation method are obtained and the related breakage location is calculated.(2)Broken damage mechanism of rock blasting is discussed and the blasting stress wave attenuation characteristics is analysed in this paper. It is concluded that rock mass occurs compressive deformation firstly, then shear deformation. It can be learned that the stress wave can make the low frequency through and resists the high frequency.In addition broken scope of rock blasting of the rock mass is get under the condition of cylindrical charge.(3)The mechanism of dynamic response and fracture patterns of rock mass under single hole blasting load is simulated and analyzed using ANSYS/LS-DYNA explicit dynamic finite element software. How the change of elastic modulus and rock buried deep influence the development regularities of rock fracture and howdynamic response and double hole blasting in rock mass blasting delay influence the rock blasting dynamic crushing are studied. Analysis shows that with the increase of buried depth, the fracture zone after blasting will reduce, the fractures after blasting area and the damage degree of fractured rock mass will increase along with the increase of the elastic modulus, and blasting hole formed between transfixion crack zone will decrease, and the damage degree of fractured rock mass will reduce, along with the increase of the detonating delay and the blasting stress wave superimposition.(4) In the engineering background of na Lin two ore 3-1 coal seam mining face,pre-splitting blasting forced caving measures is taken for the hard roof is simulated using state of discrete element. Results show that because of the existence of pre-split line after the presplit blasting caving compulsory measures, stope roof caving interval decrease obviously, the first step to break down from a drop of nearly 50%, cycle to crush falls step distance between a drop from 25% to 25%, roof peak stress concentration decreases in advance about 5.1 Mpa, and stope roof weakening effect is obvious, the goal of controlling the stope roof breakage caving step distance artificially is achieved.