[1] In the design and engineering of embankment dams, the critical shear stress provided by this test indicates the maximum shear stress that a fluid (such as water) can apply to a soil before a concentrated leak forms and erosion begins.
The numerical measure of soil erodibility can be used to predict how quickly this erosion will progress, and it can be found as an input in various computer simulations for dam failure.
where ρ is the density of the liquid, g is the gravitational acceleration, Δh is the difference in hydraulic head across the sample, L is the length of the sample, and Φt is the diameter of the hole at time t. While the diameter of the hole is not directly measured throughout the test, it can be estimated using the measured flow rate as well as an estimated friction factor.
[1] From the change in diameter of the hole over time, the rate of erosion can thus be plotted against applied hydraulic shear stress and fit to the following equation:[1][4]
The difference in hydraulic head used to calculate the shear stress also does not factor in the energy dissipated due to flow recirculation and expansion losses downstream of the test specimen.
Furthermore, estimating the diameter of the hole throughout the test using an assumed friction factor has been reported as problematic.
[5][6] The modified hole erosion test (HET-P) seeks to rectify these issues with the addition of a pitot-static tube.