Pore Pressure Enhancements Observed on Rio Blanco

Measurements were made of the change of in situ pore pressures and the associated accelerations in the saturated clayey silts (ML) of Fawn Creek and Black Sulphur Valleys at 2.1, 4, 7, and 10 km from surface zero. The early pore pressure changes correlated well with accelerations while the later values correlated better with velocities. The pressure changes ranged from 17.1 to 0.36 psi far the peak pressures and from 1.7 to 0.10 psi for the average pressure increases 10 sec after the detonation. Pretest estimates indicated that liquefaction was considered probable but it was not observed and the pore pressure increases were one-quarter or less than those associated with this phenomenon.