Gravity retaining walls in unsaturated pyroclastic materials: Observations and numerical modeling

The safety assessment of an existing structure is a sensitive issue and is certainly a sound benchmark for understanding the mechanical behaviour of the geotechnical structure. In Naples, Italy, most of these structures are gravity retaining walls made of Neapolitan Yellow Tuff, a soft rock abundant in the area and used as primary architectural material for its excellent technical properties. Over time, many of these masonry structures suffered erosion of the tuff bricks as well as moderate reductions in strength. Fortunately, throughout the centuries, they have maintained their retaining functions thanks to the unsaturated nature of the backfill. This paper concerns a parametric study on the stability of these retaining structures under the effect of rainfall or water infiltration. A coupled hydraulic and mechanical analysis is carried out by applying the two-phase flow module of the FLAC2D code [ITASCA, 2007] in order to assess the unsaturated state of backfill and masonry. Particular attention is paid to the analysis of the usually disregarded role of tuff masonry. The analysis highlights the importance of soil suction in the stability of soil-wall system.