Abstract：The dense coarse grained soil has the content of fine particles generally between 25% and 35% and has excellent mechanical properties. However，the seepage failure like piping or boiling cannot be easily discriminated. After a series of tests on seepage deformation，a parameter called the “equivalent specific surface area ”reflecting the specific surface area(S0) and tortuosity(T) was introduced based on Kozeny-Carman equation and measured permeability coefficient(K). A model of equal diameter capillary with porosity(n) and S'0 was established. A new criterion called the“equivalent permeability & average diameter method”for assessing the type of seepage failure of the transitional coarse grained soil was proposed according to the capillary diameter(D) and characteristic particle sizes of gradation curve(d3). The relationship between T and n of the transitional coarse grained soil was discussed on the assumption of uniform sphere particles. The gradual increase of density will lead the transitional coarse grained soil to have the continuous piping being transformed to the discontinuous piping and finally into boiling. Unlike the current“pore diameter method”and“variable cross-section method”ignoring the tortuosity or considering only the constant tortuosity of uniform soil，the parameter D controlled by is able to reflect the effect of actual tortuosity on permeability. The predicted results according to the proposed method are the same as the experimental data. The calculated tortuosity T1 increases linearly with the decrease of n，reflecting the actual growth of seepage length.
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2017, Vol. 36 
