As a mixture of soil and rock medium

coarse-grained soil has the inherent unique nature of inhomogeneity

discontinuity and difficulty of sampling

so as to bring about great difficulty in study. Coarse-grained soil is a typical porous medium

and its permeability is closely related to the content of gravel. Using home-built constant head permeameter

we mensured the permeability coefficient of coarse-grained soil of different gravel contents

and found that there exists index relationship between them. Changing gravel content rationally is a feasible method to control the permeability of coarse-grained soil in engineering. An empirical formula for calculating the composite permeability coefficient of soil and rock mixture is put forward based on power averaging method; and the formula is verified by the experimental results; and it provides a concise and useful computational tool for the theoretical calculation of coarse-grained soil permeability coefficient.