PEER has just published Report No. 2021/03: "Effective Stress Analysis of Liquefaction Sites and Evaluation of Sediment Ejecta Potential." It was authored by Daniel Hutabarat and Jonathan D. Bray, Department of Civil and Environmental Engineering, University of California, Berkeley.
Sediment ejecta mechanism contributes significantly to the severity of liquefaction-induced ground failure (e.g., excessive land subsidence). Estimating the amount of ejected sediment is a key step to assess the severity of ground failure; however, procedures to quantify it are currently lacking. Sediment ejecta is a post-shaking phenomenon resulting from the migration and redistribution of excess-pore-water-pressure (ue) generated during earthquake shaking. The dissipation process of residual ue can trigger high-gradient upward seepage, which can exploit cracks in the upper non-liquefiable crust layer. Once cracks in the crust layer are fully formed and there is sufficient artesian water pressure, the seepage flow can produce artesian flow above the ground surface while ejecting the fluidized sediment to the ground surface. As more sediment is transported to the ground surface, additional ground subsidence is produced.
The characteristics of liquefiable sites that did and did not produce sediment ejecta manifestation after the 2010–2011 Canterbury earthquake sequence in Christchurch, New Zealand, remain unclear. The severity of liquefaction-induced ejecta manifestation for the 2010–2011 Canterbury earthquakes was overestimated or underestimated using liquefaction-induced ground failure indices, such as the Liquefaction Potential Index (LPI) or Liquefaction Severity Number (LSN), at several sites in Christchurch. By capturing the sediment ejecta mechanism, it is possible to have a reliable estimate of ground failure severity and prevent costly or unconservative ground improvement designs in mitigating liquefaction hazards. This research proposes a new way to quantify the quantity of sediment ejecta and hence the severity of post-shaking liquefaction consequences due to sediment ejecta for level ground.