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The main objective of the thesis is the analysis of the liquefaction potential of a typical pyroclastic soil, under both cyclic laboratory tests and seismic actions expected during strong-motion earthquakes. The particular choice of volcanic soils was addressed by the peculiar interest for such soils in the Campania region, widely covered by this kind of materials, which affect significantly the stability of the territory versus both seismic and hydrologic extreme events.

The assessment of the seismic safety of underground structures, either tunnels or large station boxes, should not be overlooked especially in densely populated areas, even with low to moderate seismicity. For underground structures, an important issue is the estimation of the seismic actions acting on the structure; only few experimental evidences are available for multi-level propped walls. For tunnels, it is generally assumed that their seismic behavior in soft ground is governed by the surrounding soil, while the inertial load contribution of the underground structure itself is negligible. In both cases, recent numerical studies proved that advanced dynamic analyses can provide satisfactory interpretation of non-linear soil-structure interaction during earthquakes. In this paper, a real case study, represented by a large open multi-propped excavation and a circular segmented tunnel in a densely urbanized area of the city center in Napoli, has been used to investigate some of the mentioned aspects. Accurate geotechnical characterization and choice of the reference input motions lead to a first estimate of the free-field ground motion, which was subsequently used for pseudo-static decoupled analyses. For the complexity of both excavation geometry and staged construction, a full dynamic analysis was considered neither affordable nor reliable for the multi-propped station box; thus two conventional pseudo-static analyses, applying either a displacement-based or a force-based approach, were carried out. In the case of the tunnel, the seismic increments of internal forces in the lining could be calculated through both a simplified pseudo-static analysis and a full dynamic analysis, showing a satisfying agreement. Overall, the results of the study demonstrated that the seismic increments of internal forces in the diaphragm walls of the station and in the segmented lining of the tunnel were quite significant. The case study encourages improving the reliability of simplified methods based on the more advanced dynamic approaches.

This paper shows the results of a multidisciplinary research aimed at defning the seismic microzonation of the district Bagnoli–Fuorigrotta in the Western part of Naples metropolitan area, which is subjected to a signifcant urban requalifcation project. The area is located in a complex geological setting within an active volcanic feld and seismic hazard is characterized by both tectonic and volcanic seismicity, which frequently afected the urban setting. Following the recommendations of National and International Guidelines, the microzonation of the area was carried out adopting a multi-level approach. The maps obtained by applying Grade I and II approaches were the starting points for the numerical prediction of the site response carried out within the Grade III approach. The results of linear and non-linear dynamic analyses were synthesized in terms of amplifcation factors of peak ground acceleration at surface, PGA, and Housner Intensity, this latter computed through two diferent period ranges, i.e. 0.1–0.5 s and 0.7–2 s. The above factors were fnally mapped adopting a rational approach to extend the local values of the surface amplifcation to broader areas. The results of the dynamic analyses highlighted that the seismic response of the whole area is highly afected by soil non-linearity and that the amplifcation of the ground motion mainly depends on the depth of the seismic bedrock, rather than on the local variability of shallow soil layering.

A rational evaluation of the seismic risk of urban areas cannot neglect the variability of the ground motion due to site amplification and the induced effects, such as soil liquefaction and slope stability. In this framework, the microzonation maps certainly represent the most adequate tool to account for this element in the seismic risk and for planning mitigation strategies. This study shows the preliminary results of a multidisciplinary research, which deals with the seismic microzonation of the Western area of Napoli. The selected case study is a challenging choice, as the seismic hazard is affected by both tectonic and volcanic seismicity, which in historical time differently affected the urban setting. The latter, in turn, results highly heterogeneous, as consisting of a mixing of both masonry and concrete structures. The adopted approach followed the recommendations of both the National and International Guidelines, with reference to the multi-levels methods. The collection, homogenization and synthesis of a significant number of existing data permitted the development of reliable geological and geotechnical subsoil models, leading to Grade I and II seismic microzonation maps, characterized by different expected amplification of ground motion and instability due to the liquefaction. Such maps represent a key for a quantitative assessment of seismic performance of buildings and infrastructures, in view of the expected urban requalification of the area.

The paper analyzes the cyclic liquefaction behaviour of a pyroclastic silty sand typical of Neapolitan area through an extensive laboratory investigation. Cyclic undrained stress-controlled triaxial tests on natural and reconstituted samples were addressed to study the influence of state variables, such as relative density and effective confining stress, as well as of constitutive factors, i.e. fabric, particle breakage and non-plastic fine content. The results highlight that liquefaction resistance of the natural soil is significantly enhanced by fabric with respect to that of reconstituted samples tested at the same state conditions; on the other hand, the cyclic strength of the reconstituted soil appears poorly affected by the relative density and confining stress. Furthermore, the non-plastic fine ash was observed to increase the cyclic resistance of the pumice sand with respect to that of the same coarse matrix added with plastic clay. Finally, no appreciable evidences of grain crushing were detected on all tested soils containing pumice sand.