Tortuosity is an important parameter to study the permeability of soil

which reflects the degree of twists and turns of fluid flowing through porous media. Existing studies have shown that the tortuosity is not only related to porosity

but also closely related to particle arrangement of soil. Based on the new geometric tortuosity model of soil

a modified calculation model of tortuosity of soil unite is then proposed with emphasis on the coupling effect of porosity and particle arrangement on the equivalent flow length. The results showed that the proposed model agreed well with the experimental results in the literature

and the effects of particle arrangement and porosity on the tortuosity were further discussed. Furthermore

the minimum sample size needed for calculating the overall tortuosity of soil was determined under certain confidence and error limits based on the sampling reliability theory and this method has been proven to be reasonable and effective for representing the whole soil sample. Finally

the theoretical solution of soil tortuosity based on the proposed method was compared with the numerical simulation solution of soil tortuosity based on COMSOL Multiphysics

and the satisfactory agreement proved the reasonability and validity of the proposed method. The study provides an alternative approach for the calculation of soil tortuosity.