Fasano, Gianluca (2020) Experimental and numerical investigation of the effectiveness of some innovative techniques to mitigate liquefaction risk. [Tesi di dottorato]

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Many urban areas in the world have been strongly damaged by soil liquefaction during strong earthquakes. This phenomenon is caused by a progressive increment of the excess pore pressure that leads to a significant drop of the shear stiffness and strength, resulting into large settlements and a reduction of the safety margins in terms of bearing capacity of existing structures. As a consequence, when liquefaction occurs damages are more critical than those induced by inertial seismic actions. Large economic losses induced by this phenomenon encouraged in the last decades the scientific community to spend strong efforts in the study of some techniques to reduce liquefaction risk. Conventional mitigation techniques have been used to reduce settlements and bearing capacity loss, but they require massive and expensive retrofitting interventions. So, the development of some innovative mitigation techniques characterised by lower cost and by the possibility to be used in the urban area is becoming more and more a necessity. The project called LIQUEFACT (H2020-DRS-2015), financed by the European Committee, moved in this direction. Dynamic geotechnical centrifuge experiments were performed in this project to evaluate the effectiveness of two innovative techniques: horizontal drains (HD), which are innovative mostly for their installation procedure, that allows to place them directly under existing structures, and induced partial saturation (IPS), which is innovative for the process, never used in the past to tackle liquefaction risk. Horizontal drains were studied to define their capability to dissipate the excess pore pressures that rise during the earthquake, while the induced partial saturation was adopted to reduce the bulk stiffness of the equivalent pore fluid, reducing the attitude to increase the excess pore pressure induced by distorsional deformations of the soil. Each centrifuge model, realised with a loose liquefiable sand (Ticino sand), was equipped in order to measure accelerations, pore pressures and displacements. Moreover, a simplified SDOF structure was used in some tests to evaluate the interaction between the liquefiable soil and the structure, with and without the adoption of one of the mitigation techniques. The reliability of numerical modelling of the mitigation techniques was checked by the comparison between numerical and experimental results. The presence of horizontal drains was modelled by tubes with a finite permeability, estimated on the bases of experimental tests, with an inside hydrostatic fluid pressure. Some centrifuge tests with free-field configuration were reproduced by numerical analyses, using PLAXIS 2D, to check the possibility to calibrate two constitutive models used to study soil liquefaction (UBC3D-PLM and PM4SAND) with the aim of catching the experimental results in terms of pore pressure increments and settlements of the ground surface. The induced partial saturation was implemented by a reduction of the fluid bulk stiffness with a function of degree saturation. The results indicate that the numerical analyses underestimate the settlements of the ground surface in free-field conditions but capture reasonably well liquefaction-induced building settlement in the centrifuge experiments. Some parametrical analyses of the mitigation techniques were also performed, changing the HD geometrical layouts and soil mechanical properties. Moreover, an evaluation of an optimised horizontal extension of the mitigation techniques was performed for a reference building. A simplified design method for the horizontal drains was finally proposed, for the case of three horizontal rows of drains disposed with a staggered arrangement. A simplified design method for the induced partial saturation based on the analytical procedure was proposed as well.

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