In-Situ Pore Presure Measurements at Rio Blanco

The nuclear test Rio Blanco offered a unique opportunity to observe in-situ pore pressure increases caused by seismic motion. In-situ pressure and ground motion measurements were made at five stations which ranged in distance from 2 km to 22 km from the test. Observed peak component accelerations ranged from 0.3 to 18 times the earth's gravitational acceleration. The observed sustained pore pressure increases were well under the values associated with soil liquefaction and no surface evidence of liquefaction was found. The excess pore pressures decayed in a minute or less. Soil samples were taken at the levels at which piezometers were installed. These samples were subjected to cyclic triaxial tests. The pore pressure increases observed on these tests were correlated with field observations considering the observed ground motion history. Field pressure increases were generally 10% or less of the values predicted from laboratory tests. Possible reasons for this discrepancy and the implications of rapid pressure decay are examined.