Study On Seepage Characteristics And Particle Size Distribution Of Mixed Broken Coal Samples

In the mining engineering,the deformation of the pressurized and broken coal body is affected by various factors,such as the groundwater environment,external loads and physical properties of itself.The deformation of pressurized broken coal body is the main reason for inducing water inrush from the roadway and coal and gas outburst disasters.According to the seepage process of fluids such as gas or water in the pressurized broken coal,it can be divided into two parts:the compression process of the broken coal sample and the seepage of the fluid in the broken coal sample.With the increase of coal mining depth of coal resources,the study of compression deformation and seepage characteristics of broken coal in complex engineering environment can provide theoretical guidance for supporting surrounding rocks around roadways and predicting water inrush from roof and floor to ensure the safety of coal resources production.The main research conclusions are as follows:The compaction tests for broken coal with different immersion time or under different cyclic stresses have been conducted to study the compaction process of the broken coal.The axial displacement loading method was used to study the effect of water immersion time on the small displacement compaction of coal samples.The graded coals with different immersion time increased with the increase of displacement of each stage,and the fractal dimension of particle size distribution increased gradually.During the small displacement compaction process,the fractal dimension of particle size distribution decreases with the increase of water immersion time,and the relationship between them can be described by exponential function.For the grading coal samples with different immersion time,the relationship between the axial stress and the depression displacement during the compression process can be divided into a slow increase stage and a rapid increase stage,and the two-stage limit value is 15mm.The fractal dimension of the particle size distribution of the broken coal sample during cyclic loading and unloading is affected by various factors such as axial stress,grading index and the load recycle number,and the fractal dimension of particle size distribution satisfies the exponential relationship with cyclic loading times and the grading index,when the axial stress is 8MPa,the n value is 0.2,0.4,0.6,the stress dominates,while the n value is 0.8,the coal sample void is larger,and the initial internal structure form plays a leading role.The fractal dimension of particle size distribution can reflect the degree of fracture of the whole coal sample,which related to the shape,structure and loading mode of the particle size.A two-dimensional contact model between particles in the process of coal particle compression is established,and it is concluded that the volume and shear modulus of coal sample particles are related to the void ratio and the number of contact points between particles.The seepage characteristics of coal samples with different particle compositions were studied,and the cyclic loading and unloading seepage characteristics were studied when the grading index was 0.8.The flow rate of the mixed coal sample is positively correlated with the particle size and osmotic pressure,but negatively correlated with the axial compression displacement.In the seepage process of mixed broken coal samples with different particle size components,the permeability has the phenomena of stable,fluctuation and sharp decline,which indicates that the particle size components cause the difference of the skeleton formation,while the effective pore passage varies with the support of large particle coal samples to skeleton and the filling change of small particle coal samples to the pore between particles,and the support of 20-25 mm particle coal samples to the whole coal sample is obviously,5~10mm small particles have good filling effect on the voids between larger coal samples,and the Forchheimer equation is satisfied in the process of compaction seepage.The seepage process of single-stage compaction skeleton with fine particles presents obvious non-linear seepage phenomenon,and the physical model of lateral restraint resistance is established.The permeability of the broken coal sample during cyclic loading and unloading has a nonlinear relationship with the fractal dimension of the particle size,which can be fitted by a quadratic polynomial.The increasing process of fractal dimension of coal particle size distribution reflects the increasing process of axial stress and cycle times.