Comparing Peak and Residual Soil Pressures Measured by Pressure Bulbs and Stress-State Transducers
Measurement of soil compaction caused by vehicle loading is difficult and often subject to extreme variability. Two types of soil transducers were compared in an experiment conducted in a Norfolk sandy loam soil in the USDA-ARS National Soil Dynamics Laboratory's (NSDL) soil bin facilities. Stress-state transducers (SSTs), electronic transducers developed at the NSDL for measuring six directional pressures and determining the stress state, were used for this experiment. Rubber bulbs connected by a rubber hose to a dial pressure gauge, which measure hydrostatic pressure, were also used in this experiment. Both transducers were buried at depths of 7.5, 15, or 23 cm and were used to measure soil pressures caused by a 30.5L-32 tire with dynamic loads of 19 or 37 kN. The SSTs were buried by inserting them into an excavated hole, while the rubber bulbs were inserted with a special tool designed to leave the soil surface and surrounding soil undisturbed. Peak values of mean normal stress (calculated from measurements of pressure) from the SSTs and hydrostatic pressure measured with the rubber bulbs were found to be affected by both loading and burial depth. Similar magnitudes and variation were observed for each transducer. Residual pressure, defined as the pressure remaining after loading was removed, was found to be affected by both loading and burial depth when measured with the rubber bulbs. Continued development and testing of the rubber bulb transducers could provide a simple method of determining levels of compaction that could damage soils and thus prevent excessive trafficking.