Long-Term Pressure Effects on Shear Modulus of Soils

Long-term resonant-column tests were performed on seven air-dry soils to determine their small-strain dynamic behavior under confining pressures maintained for durations up to 803 days. Test results indicate that: (1) The shear modulus of air-dry soils at constant confining pressure increases with time; (2) the shear modulus after 2 years can be predicted within 15% from 2-day tests; (3) the time-dependent increase in modulus increases with decreasing particle size; (4) the time-dependent increase in density under a constant pressure accounts for only 0-15% of the total time-dependent modulus increase; (5) continuous low amplitude vibration (shearing strain less than 0.00001) has no measurable effect on the soils tested over and above the time-dependent modulus increase; and (6) a 10-psi step increase in pressure may destroy up to 15% of the time-dependent modulus accumulated with time in clay, but does not influence sand or silt.