Permeability characteristics of broken sandstone and its stability analysis under step loading

When the permeability characteristics of coal and rock mass and pore pressure gradient meet the seepage in- stability conditions,water inrush and gas outburst disaster will easily occur. In the slow deformation process of broken rock over a long time,its permeability characteristics will change with its porosity variation,and the different ratios of particle sizes of broken rock will affect its permeability characteristics. According to the Talbol grading theory,and by the application of DDL600 electronic universal testing machine and the permeameter test equipment,five level constant axial load pressures are set. The four different osmotic pressures corresponding to each level constant load pressure are tested to explore the permeability characteristics of broken sandstones with different particle size ratios. The test results show that:① When the axial load increases from the first level to the fifth level,the rate of seepage velocity Dv gener- ally shows a tendency of decrease with the increase of effective stress of the broken sandstones in the cylinder;② Within the later period of constant load deformation,the permeability shows a decreasing tendency generally with the porosity decrease of broken sandstones,and the absolute value of non-Darcy flow β factor increases;③ There exists a difference between the permeability of broken sandstones with different Talbol power exponents and the non-Darcy flow β factor sizes. When n = 0. 5,the permeability of broken sandstones is minimum,while the non-Darcy flow β factor is maximum. When n = 1. 0,the permeability of broken sandstones is maximum,while non-Darcy flow β factor is small overall. The following conclusions can be reached from the above research results:when the particle size of broken sandstones is very small or when the breakage of refinement is very serious,the negative value of the non-Darcy flow β factor appears easily,the seepage instability of broken sandstones occurs.