Repeated Measurements of In Situ Soil Stiffness with Permanently Embedded Geophones

There are times when one would like to perform repeated measurements of a particular characteristic of a given material over an extended period of time. In soils, one possible way to accomplish such a task in situ is to employ permanently embedded geophones (velocity transducers) and measure stress wave velocities. The geophones perform as sources and receivers which are used nondestructively to measure the propagation velocities of compression and shear waves at small strains. The nondestructive nature of the stress wave measurements and the permanent placement of the geophones in designed array configurations allows repeated measurements to be made, either continuously or at selected times, over the life of the geotechnical system. The wave measurements allow in situ stiffnesses in terms of shear and constrained moduli to be calculated at the measurement time and along the measurement directions. Examples of these measurements in a calibration chamber and a pavement subgrade are presented.