ENGINEERING PROPERTIES OF NUCLEAR CRATERS. REPORT 5. RESIDUAL SHEAR STRENGTH OF WEAK SHALES.

The report is concerned with the laboratory determination of the residual shear strength of weak shales such as might be encountered in construction within the Canal Zone. This laboratory measurement is proposed mainly as in index property to estimate the potential for shales or deeply weathered rocks to lose substantial strength with time. In addition to the residual strength parameter, five other index properties are outlined to form a comprehensive means for expediently evaluating potential loss of strength. By definition, the residual shear strength is a constant for any given soil regardless of stress history, and is the lowest 'ultimate' strength. In practice, however, the laboratory measurements of residual strength can vary simply because of failure to reach this lowest strength. The main effort in this present research is to develop the equipment and testing procedures to reliably measure residual strength. It was found that the repeated direct shear test, with about 30 repetitions of loading, on mechanically disintegrated and resedimented samples resulted in an adequate index property test. The results of comprehensive testing of two Canal Zone shales (from the Cucaracha and the Culebra formations), Pierre shale from the Oahe damsite in South Dakota and a kaolinite clay are contained in the report. (Author)
Sponsored by Army Engineer Nuclear Cratering Group, Livermore, Calif., See also AD-626 899.