In Situ Strength Measurements of Weak Rocks

There is a lack of strength data on weak rocks where cores for conventional testing can only be obtained with difficulty. In recent investigations, strengths are obtained over a 300-ft (91-m) vertical section for calcareous fanglomerates, typical of the Southwestern United States, by plate testing and correlation with seismic velocity measurements. A 240-ton (213-metric ton) ore truck is used to provide the large reaction load necessary to induce failure. Values for the compressive strength are computed from Boussinesq stresses, Mohr circle analysis, and by correlation with P-wave velocity, and range from 451 psi (3.1 MPa) at a depth of 100 ft (61 m) to 863 psi (5.6 MPa) at a depth of 300 ft (122 m). Interpretation of the results of the plate tests provides an interesting example of the different engineering approaches sometimes needed in rock and soil mechanics. Both the plate tests and seismic correlations give similar results and, when used with care, represent simple and reliable methods for strength estimation.