Rapid and accurate measurement of permeability coefficient for the graded macadam used in heavy-haul railway subgrade bed surface layer is of great significance to guarantee the construction quality and period. First

a large-scale constant-head permeameter suitable for graded macadam was developed based on Darcy’s Law

and a series of penetration tests considering wall effects was carried out using the aforementioned device. These initial tests were conducted to explore the influence degree and properties of pore ratio e

unevenness coefficient Cu

and characteristic particle size parameters on the permeability coefficient K20 of a specimen. Besides

based on the Terzaghi model

an improved calculation model that took into account both pore and gradation characteristic parameters was established with reference to test data

and the accuracy of the model was verified through experiments. The results show that the graded macadam permeability coefficient K20 has a linear relationship with e2 and the square of mean particle size d502

and has a negative exponential relationship with Cu; d50 has the greatest influence weight on K20

followed by Cu

and e has the smallest influence. The proposed calculation model demonstrates a high degree of accuracy and can provide a reference for rapid determination of permeability coefficient for the fillers used in newly heavy haul railway subgrade bed surface layer under construction. Using this calculation model

the permeability stability of operating subgrade beds can be evaluated.