The Effect of Subgrade Cavity on Pavement. A Case Study

Abstract. The paper reports the finite element analysis of the effect of subgrade cavities on urban pavements. The study starts after a critical collapse event in 2008 in Cagliari (Italy), where a 110 m3 sinkhole suddenly opened up, swallowing one car on an urban street. The cavity, originated by water network failure, occurred in the urban area built in a zone of abandoned limestone quarries. Pressured water erodes the subsurface and effortlessly flows through rock debris voids. The cavity grows until the weight of the above materials and traffic load trigger a collapse of the pavement. In order to evaluate the effect on the dimensions’ stress field and the cavity’s location below the pavement, a parametric study of the finite elements was carried out with ANSYS® software. In a section of 25 x 10 m, which comprises the road pavement and the adjacent buildings, the FEM mesh includes not linear materials and the presence in the subgrade of a circular cavity with a variable radius (0.30 ÷ 1.80 m), depth (0.50 ÷ 3.00 m) and loads (0 t, 4 t, and 8 t). The F.E. simulations’ results define the pavement’s bearing capacity and the developed plastic yielding of the subgrade for each condition analyzed. No trivial cavity collapse conditions and mechanisms have been discussed in detail. The results show how the effects of the dimension and position of the cavity are not linear. Small and deep cavities collapse immediately with depth increases but with progressively smaller effects on the surface. Larger and superficial cavities are critical, and the vehicular load significantly affects the superficial cavities. Future research may address how these events may affect the road network.