Effect of groundwater-table rise on seismic liquefaction hazard of loessial soils: An SPT-based assessment at a residential site in Tashkent, Uzbekistan

Abstract

Rising groundwater in urbanised territories of Uzbekistan changes the seismic behaviour of loessial soils from collapse-dominated to liquefaction-dominated, yet liquefaction hazard is usually assessed only for the groundwater position observed during the site investigation. This paper quantifies the effect of the forecast maximum groundwater level on the computed liquefaction hazard at a residential site in Tashkent rated at 9 points by the seismic microzonation map. Sixty-eight standard penetration tests in three boreholes to 35 m were processed by the NCEER simplified procedure, and the hazard was integrated into the liquefaction potential index for the observed (4.3–4.5 m) and the forecast maximum (2.8–3.0 m) groundwater positions at a peak ground acceleration of 0.4 g and a design magnitude of 7.0. At the observed level the index is 29–34 (very high category); raising the water table by 1.5 m increases it to 43–52, a mean growth of 43 %, and extends the liquefiable zone upward by 1.5 m in every borehole. Liquefaction assessment based only on the survey-time groundwater level can substantially understate seismic hazard, and the forecast maximum position should be adopted as the design scenario for loessial soils.