Measuring method for membrane penetration capacity of coarse-grained soil in triaxial tests

The membrane penetration effects displayed in the coarse-grained soil tests will distort the measurement of volume deformation of samples, thereby affecting the measuring accuracy of strength and deformation index. To this end, by using the same test apparatus, multiple series of isotropic consolidation triaxial tests on the samples of coarse-grained soil with different diameters are implemented. The changing rules of membrane penetration capacity and the related impact factors are analyzed, and an empirical formula is proposed to calculate the membrane penetration capacity of coarse-grained soil. It is shown that the membrane penetration capacity increases with the increase of the confining pressure, and the relationship roughly follows a power function. Under the same confining pressure, with the increase of the sample diameter, the capacity of membrane penetration decreases. The influences of the confining pressure on the membrane penetration capacity are significantly smaller than those of sample size which gradually decreases as the sample diameter increases. Therefore, the strength and deformation tests on coarse-grained soil should be carried out by preparing larger-diameter samples, so that the influences of the membrane penetration effects can be greatly reduced. Due to the great changes in the nature and gradation of coarse-grained parent rock, the existing formulas for the membrane penetration capacity will significantly underestimate the membrane penetration effects of rock-fill samples. It is recommended to use the proposed method to estimate the membrane penetration capacity of coarse-grained soil.