Earthwork Factors in Weathered Granites by Geophysics

A method utilizing surface refraction seismic geophysical measurements to estimate earthwork factors involving large rock cuts in weathered granites and associated embankments constructed from these materials has been developed. Estimating overall unit weights of a large volume of in-situ rock mass, including fractures or other discontinuities and very weak material that cannot be adequately sampled by standard coring methods, is a major feature of this earthwork factor method. The method uses Percolation Theory (PT) concepts of modulus porosity relationships to relate rock mass modulus values to rock mass porosity values. Surface refraction seismic measurements are utilized to estimate rock mass low strain modulus values. Recovered rock core unconfined compressive strengths are used to estimate rock particle modulus values and unit weights are used to determine corresponding porosities. The PT modulus-porosity relationship is shown to be consistent with the testing of recovered rock core and with test results of weathered granites reported in the literature. Estimates of unit weights of rock masses to be excavated are made based on the measured seismic velocity profiles through project rock cuts. Estimates of embankment fill unit weights are made using seismic velocities obtained through existing fills and from moisture-density relationship (Proctor test) results of materials sampled on the projects. Earthwork factors are determined from the ratios of these in-situ rock mass unit weights and existing or anticipated embankment fill unit weights.