The mechanical consequences of internal erosion on gap-graded soil

Abstract

The mechanical consequences of internal erosion (suffusion) on a gap-graded soil are investigated.

A new soil sample formation procedure is proposed which results in homogeneous particle size distributions along the length of an eroded sample. Triaxial tests are conducted on homogeneous samples formed using the new procedure as well as heterogeneous samples created by the more commonly used approach. The results show the samples with homogeneous post-erosion particle size distributions exhibit slightly higher peak deviator stresses than those which were heterogeneous. The results highlight the importance of ensuring homogeneity of post-erosion.

Additional triaxial erosion tests on the gap-graded soil having undergone different amounts of internal erosion at varying confining stresses are conducted. The hydraulic gradient, confining stress and initial density have significant influences on the erosion characteristics. The peak deviator stress tends to decrease as the amount of erosion increases. The volumetric strain at large shear strains decreases as the volume of seepage water and the amount of erosion increase. Internal erosion also causes the critical state line to move upwards. The upward movement of the critical state line is lesser than the increase of the void ratio due to internal erosion.

A direct mathematical link between the fractal dimension and void ratio for soils with single or double fractal particle size distributions is developed, incorporating some practical and realistic assumptions. In gap graded soils, different amounts of internal erosion result in the fractal dimension of the finer component to vary. The changing particle size distribution due to internal erosion may be expressed solely in terms of the change in fractal dimension. The change in the void ratio can also be expressed in terms of this changing fractal dimension.

A modified grading state index is proposed using fractals enabling the effects of the shape of the particle size distribution to be considered. The dominant effects of the increase in void ratio and changing grading are incorporated into an extension of the Severn-Trent sand constitutive model. The extended model describes the stress-strain behaviors of soils subjected to internal erosion. Model simulations fit well with the experiment data.