Abstract: Particle loss of karst collapse column fillings is one of the major causes of water inrush accidents in mines.In order to investigate the relationship between initial grading,pore structure and particle size distribution of karst collapse column fillings during particle loss,the steady state percolation method was used to obtain the particle evolution and particle size distribution of karst collapse column fillings under graded loading conditions using a variable mass crushed rock percolation test system,and then to analyze the pore structure and water flow mutation during particle loss.The results show that ① A large number of microscopic pores,cracks and particle splicing structures exist in the karst collapse column filler samples,and the particle loss depends largely on the properties of the specimen itself,with external load F and permeation pressure P only playing a facilitating role;② The filler particles before and after the loss meet the fractal conditions,with fractal characteristics,and the proportion of fine particles in the entire permeation loss process is as high as 34.4%.The fractal dimension D decreases with the increasing value of grading n,and the specimens with grading values of n=0.3 and n=0.9 in the lost particles are easier to destroy,resulting in a structural instability;③ There are two abruptly decreasing porosity segments in the whole permeation course,the values of which appear in the 63 s and 484 s of the permeation course respectively,and the adjustment of the void structure is related to the particle size grading,and the reciprocal action between the water flow and the specimens will be more effective in the inlet and outlet.After infiltration,there are holes,collapses and cavities in the filling body;④ Particle loss under the specimen bursting process can be divided into three stages,namely initial seepage,catastrophic destabilization and pipe water flow.The relationship between flow rate and time at each stage is generally consistent,and the bursting process will generally last 5-10 s.During infiltration,the col lapse mass m and Talbol power exponent n can be modelled with available exponential function.The mass of collapse increases with the value of Talbol power exponent n,and the significant water spattering,floating water level difference,and head drop occur during the collapse of the fill structure.