Modified Soil Strain Gage

While it appears that the greatest effort regarding in-situ monitoring of soils has been directed as stress measurements, strain monitoring is of equal importance. The reason for the past emphasis on stress measurement is obviously due to inherent difficulties associated with strain measurements in a discrete particulate mass such as soil. Recently, however, a new technique for soil strain measurement has appeared and has resulted in a commercially available system marketed by Bison Instruments, Inc., of Minneapolis, Minn. The procedure utilizes a pair of sensors that are disk-shaped coils placed free floating in the soil in parallel and coaxial alignment. The separation distance between the coils is sensed by their mutual inductance. An inductance bride produces a voltage that varies as a function of strain, since a change in spacing of the coils from the initial value produces a bridge unbalance. An external instrument package contains the necessary driving, amplification, balancing, readout, calibration controls, and a self-contained power supply.