Gradation characteristics and load-bearing deformation rule of gangue in underground

Filling the gob directly with gangue generated during underground mining can not only reduce the ground land occupation, but also effectively control the stability of the gob roof. However, under the influence of time effect, the gangue will weaken the support of the roof due to load-bearing deformation, and then lead to surface collapse. To investigate the particle size gradation characteristics of underground gangue and its load-bearing deformation rule, image information extraction technology was used to obtain the particle size gradation of gangue, and gangue samples consistent with the distribution of gangue gradation were allocated according to the principle of equal scaling. The load-bearing deformation law of gangue samples under the same loading time of 24 hours and different constant loading of 5, 10, 15 and 20 MPa was studied by using the load-bearing deformation test apparatus.The particle size distribution of the underground gob pile exhibits a trend where smaller-sized waste rock particles predominate in quantity, decreasing as size increases. In terms of mass, smaller particles contribute less, showing an initial increase and then a decrease as particle size increases. The compression process of the gob sample comprises two stages: loading and constant load. With the increase of the constant load, the proportion of deformation occurring during the loading stage shows an increasing trend. The constant load stage can be further subdivided into rapid deformation and slow deformation phases. For the same duration of constant load, samples subjected to lower constant load settings produce greater strain. During compressive deformation, the proportion of smaller-sized particles continuously increases while larger-sized particles decrease. Waste rock particles in the 10-15 mm size range are identified as the stable particle size. Under a specific constant load, the degree of damage to typical waste rock particles gradually decreases as their layer's distance from the load application surface increases. Similarly, the vertical compression of a gob layer diminishes with increasing distance from the load application surface. Furthermore, under different constant load settings, the vertical pressure borne by the gob sample consistently exhibits a gradual decreasing trend as the distance of the layer from the load application surface increases. The vertical load transfer attenuation effect within the gob sample is verified from three aspects: particle breakage, deformation, and stress distribution.