Design guidelines for lattice-type soil improvement with deep mixing for liquefaction mitigation: Coulomb Conference 2024

One of the most dramatic causes of structural damage during earthquakes is the occurrence of liquefaction in loose saturated sand deposits. Several techniques have been developed for mitigation of liquefaction hazard in seismic areas which, for some of them, rely on design methodologies, validated through observations of improved soils during earthquakes. A specific form of soil mixing improvement, the lattice-type technique (DSM lattice), is the focus of this paper. One example is the Oriental Hotel in Kobe, Japan, which experienced the Yogo-ken-Nambu earthquake in 1995. A visual inspection after the earthquake revealed no damage to the piles, although significant lateral sliding of the soil towards the sea caused severe damage to the quay walls. Due to the limited number of existing design guidelines and shortcomings in recent studies, a research programme involving centrifuge testing and advanced numerical analyses was launched by the Fédération Nationale des Travaux Publics (FNTP) and Soletanche–Bachy (SB) to develop design recommendations for this technique. To this end, a different from the state-of-the-art approach is implemented to evaluate the liquefaction potential based on a strain-based methodology: the criterion for liquefaction triggering is formulated in terms of cyclic strain and the average strain induced within the cells of the DSM lattice is computed from parametric dynamic analyses considering a nonlinear constitutive behaviour for the soil. Based on these numerical computations a relationship relating the average strain induced within the cells of the DSM lattice to the geometric parameters of the DSM lattice, soil properties, parameters of the input motion and freefield induced shear strain is developed. This strain is compared to the liquefaction triggering strain to assess the effectiveness of the soil improvement.