Theme Lecture – Soil liquefaction earthquake response at high overburden pressures: new findings
The current State of Practice (SoP) of liquefaction triggering evaluation of saturated sand and other cohesionless soils uses mainly charts based on field penetration tests such as SPT (Standard Penetration Test), and CPT (Cone Penetration Test). For engineering projects involving values of s’v0 on the order of 1 atm or lower, the values from the chart may be used directly. The situation is very different in projects involving earth dams, tailings dams and other impoundments, where the weight of the embankment causes the soil foundation layer to be under a s’v0 much greater than 1 atm, sometimes even higher than 8 atm. The current SoP uses a coefficient Kσ < 1.0 to adjust the sets of SPT & CPT based curves. The Presentation discuss recent research finding that is shedding new light on soil liquefaction earthquake response at high overburden pressure. Centrifuge test results at s’v0 = 1 & 6 atm on loose and dense Ottawa sand, indicate that under the field conditions modeled in the centrifuge, the values of Kσ were above 1.0 and at least 50% higher than the Kσ < 1.0 used in current practice based on undrained cyclic testing. The results were also confirmed by numerical simulations. These findings suggest that the current SoP of using undrained small sample testing to extrapolate behavior from 1 atm to much higher overburden pressure seem to underestimate significantly the liquefaction soil resistance as σ’v0 increases. Such underestimation could lead to unnecessary remediations, with the cost of remediation due to liquefaction hazard under individual dams sometimes reaching more than hundred million dollars.