Abstract: Boundary effect has a significant influence on the permeability coefficient measurement of coarse grained soil in laboratory seepage tests. Based on the composition characteristics of the pore Vb between soil particles and the seepage equipment wall, a parameter called boundary pore ratio of the soil sample eb was defined according to the ratio of the volume of boundary pore Vb to the particle volume Vs which concerns the constitution of boundary pore. Furthermore, based on the uniform circle packing principle, the boundary particle accumulation patterns were established, and a plane geometric calculation model of boundary pore ratio considering the wall treatment layer thickness was proposed. The expression of eb was finally derived through space and particle gradation correction. Based on the principle of the equality of eb and sample pore ratio e, a criterion called ‘equivalent pore ratio method’ for confirming the optimal thickness of the wall treatment layer hopt was presented. At last, the relationship between the seepage equipment diameter D and the boundary effect was discussed. The researches indicate that the redundant pore caused by the boundary effect would raise the seepage velocity near the seepage boundary. There are three typical plane accumulation patterns of the boundary particles, including two particles pattern, three particles with acute packing angle pattern and three particles with obtuse packing angle pattern, of which the corresponding eb increases accordingly. Furthermore, hopt is comprehensively determined by sample gradation, compression degree, particle density, boundary particle accumulation pattern and D. The estimate value of hopt agrees well with the test result. The influence of the boundary effect on the seepage test result becomes less significant as D exceeds eight times of the maximum particle size of soil sample.

Key words: seepage test, boundary effect, boundary pore ratio, particle accumulation pattern, optimal treatment thickness