Describing Soils: Calibration Tool for Teaching Soil Rupture Resistance

Rupture resistance is a measure of the strength of a soil to withstand an applied stress or resist deforma- tion. In soil survey, during routine soil descriptions, rupture resistance is described for each horizon or layer in the soil profile. The lower portion of the rupture resistance classes are assigned based on rupture between thumb and forefinger. The tactile sense of the pressure can vary considerably between individuals. A calibration tool was developed, to help the students and soil scientists calibrate their thumb and forefinger for the correct amount of pressure. Instructions on how to assemble four pressure calibration tool assemblies measuring 8, 20, 40, and 80 N of applied pressure are presented. The calibration tools are demonstrated on how the combinations of spring/knobs are put together to get different rupture pres- sures. Manufactured or substitute fragments for natural fragments can be used in the instruction of rupture resistance in the classroom. In soil survey, this tool has been effective in calibrating the finger-force range for rupture resistance class placement for the last two decades.