Tom Shire

Fabric and effective stress distribution in internally unstable soils

Internal instability is a form of internal erosion in broadly graded cohesionless soils in which fine particles can be eroded at lower hydraulic gradients than predicted by classical theory for piping or heave. A key mechanism enabling internal instability is the formation of a stress-transmitting matrix dominated by the coarse particles, which leaves the finer particles under lower effective stress. In this study, discrete element modeling is used to analyze the fabric and effective stress distribution within idealized gap-graded samples with varying potential for internal stability. The reduction in stress within the finer fraction of the materials is directly quantified from grain-scale data. The particle-size distribution, percentage finer fraction, and relative density are found to influence the stress distribution. In particular, effective stress transfer within a critical finer fraction between 24 and 35% is shown to be highly sensitive to relative density.

Closure to “Fabric and Effective Stress Distribution in Internally Unstable Soils” by T. Shire, C. O’Sullivan, K. J. Hanley, and R. J. Fannin

The authors have performed numerical tests to determine the distribution of fabric and effective stresses in internally unstable soils. Different grain size distribution curves as well as different densities were investigated. The authors have used the methods of Kezdi (1979) and Kenney and Lau (1985, 1986) to assess the internal stability. The relative density has a significant effect on the internal stability (Ahlinhan et al. 2012; ICOLD 2013). Accordingly, it is important to use a method takes into consideration the effect of density to assess the internal stability. Neither Kezdi’s method nor Kenney and Lau’s method takes the density in consideration. A suitable method is the one suggested by Dallo et al. (2013), which takes the effect of soil density in consideration. For instance, the soil (Gap med 18) is classified as border-line according to Kezdi’s method, whereas it is classified as unstable according to Kenney and Lau’s method, regardless of its relative density. According to Dallo et al. (2013), soil (Gap med 18) is unstable, transition (or border-line), and stable for the loose, medium, and dense relative densities, respectively. Also the internal stability prediction accuracy of Dallo et al. (2013)’s method is better than those of the Kezdi or Kenney and Lau methods (Dallo et al. 2013). The discusser used the method of Dallo et al. (2013) to assess the internal stability of the gap-graded soils tested by the authors (Table 1). It can be seen that the soils (Gap wide XX) are unstable, the soils (Gap narrow XX) are stable, whereas the internal stability of the soils (Gap med XX) depends on the relative density of the soil. Another discussion point is related to the authors’ adoption of the findings of Skempton and Brogan (1994) that the critical finer fraction (S ) falls between narrow limits of finer fractions by mass, Ffine 1⁄4 24–29% for dense and loose samples respectively. Also, the finer fraction (Smax) at which the finer particles completely separate the coarse particles from one another is given as Ffine 1⁄4 35%. The discusser believes that the critical finer fraction can be computed more accurately according to Indraratna et al. (2011), Eq. (1), or Dallo and Wang (2012), Eq. (2), as Smax 1⁄4 1 − nl 1 − nc nc ð1Þ