Liquefaction mechanisms and mitigation techniques

Seismic shaking of loose, saturated granular soils causes a transient pore pressure build-up and a progressive reduction of stiffness and shear strength, that may eventually lead to a state transition from solid to fluid when the effective stresses tend to zero. This final stage is called liquefaction. Even before full liquefaction, however, critical mechanisms on structures and infrastructures may be triggered by the reduction of effective stresses. The effects on the built environment differ from the inertial ones typically observed during earthquakes, being basically related to excessive settlements and tilting, rarely resulting in human losses but often producing extremely relevant repair or dismantling-rebuilding costs. The mitigation of liquefaction risk is therefore a relevant issue, also because experimental evidence shows that this mechanism may repeatedly take place on previously affected areas upon new seismic events, which is somehow counterintuitive because of the densification effect of post seismic consolidation. After an introductory section devoted to a description of liquefaction mechanisms and to the effects induced on the built environment, the paper briefly introduces and comments codes rules for liquefaction risk assessment, to finally end up with some design indication for the most popular or promising ground improvement technologies suited to mitigate liquefaction risk. Because of the large and ever-increasing number of technologies and patents, design hints are herein proposed categorizing ground improvement interventions on the base of the modification induced by treatment, without entering in details on the specific technology adopted to the aim.