Guidance on applied pressure heads for quantifying cohesive soil erodibility with a jet erosion test (jet)

The Jet Erosion Test (JET) is one of the few instruments available for measuring cohesive soil erodibility in situ, but uncertainty remains regarding an appropriate initial applied pressure head for the test. Users typically iterate on an initial applied pressure head setting when testing soil. This iteration is necessary to ensure a reasonable erosion rate and the total amount of scour while imposing applied shear stresses that match the expected application range when using JET-derived erodibility parameters. This research used a numerical analysis of simulated JETs to determine both minimum and maximum applied pressure heads, ensuring a logistically appropriate estimation of soil erodibility parameters. First, the minimum head was set to generate at least 25 mm of scour, established based on data from previous in situ JETs. Second, the maximum applied pressure head was set to ensure that no excessively large initial applied shear stress impacted the estimation of erodibility parameters from a linear regression on erosion rates. Analyses were conducted for four selected soil erodibility classes: highly erodible, more erodible, erodible, and moderately resistant soils. Curves showing the ideal applied pressure ranges were generated for initial time intervals of scour depth measurements of 60, 90, 120, 180, 240, and 360 s and dimensionless initial nozzle heights of 1.00, 1.25, and 1.50. The appropriate range in the applied pressure head depended not only on the soil erodibility classes but also on the initial time interval for scour depth measurements, total test duration, and dimensionless initial nozzle height above the soil surface. Users should ensure that a minimum applied pres-sure head is exceeded for resistant soils. Maximum applied pressure heads should be considered for erodible, more erodible, and highly erodible soils, dependent on the initial time interval for scour depth measurements and dimensionless initial nozzle heights. Wider ranges of acceptable applied pressure heads were observed with smaller initial time intervals. The procedure presented in this research can be readily adapted by JET users to reflect specific testing conditions (e.g., different data collection intervals and test durations) for ensuring the a priori use of effective pressure head settings.