On the estimate of seismically induced pore-water pressure increments before liquefaction

In engineering practice, the liquefaction potential of a loose sandy soil is commonly evaluated with a semi-empirical method through a factor of safety in free-field conditions, computed as the ratio between the liquefaction resistance and the seismic demand. Even though this approach has proved effective in estimating liquefaction risk, it falls short in assessing the risk of ultimate limit states (bearing capacity failure, excessive settlements) that may take place during seismic shaking - even prior to liquefaction - due to the ongoing reduction of the effective stresses in the sand. Then, a more comprehensive understanding of the evolution of risk during seismic shaking has to be based on the estimate of the seismically induced pore-pressure increments Δu before liquefaction, for which an analytical tool is still missing in the literature. This paper attempts to fill this gap by proposing a simple procedure to evaluate Δu (in the form of normalised pore-pressure ratios ru) from the liquefaction free-field factor of safety FSliq,ff. Simple ru(FSliq,ff) charts are reported for different fines contents and relative densities. The procedure has been successfully verified with some experimental data available in the literature for soils having different grading and densities.