Engineering geological properties of the Phlegrean pozzolan soil and effect of the suction on the stability of cut slopes

Pozzolan is an incoherent to a pseudo-coherent pyroclastic soil that is much diffused in the Phlegrean Fields and has been used since ancient times for cement making. The industrial and economic relevance of this geological material gives rise to quarrying activities distributed in the Phlegrean area, which are carried out by means of pit quarries. These issues stimulated engineering geological analyses, in order to comprehend the conditions of stability of the quarry cut slopes discussed in this paper. Pozzolan soil, due to its prevailing sandy and silty components, among the other typical features of pyroclastic soils, does not have a negligible apparent cohesion in unsaturated conditions. These engineering geological issues motivated the interest in the stability analyses of cut slopes in pozzolan soil, also considering the effect due to the variable water content related to hydrologic regime. This paper regards the results of engineering geological analyses carried out on a sample area that corresponds to a cut slope of a pozzolan pit quarry located in Giuliano in Campania (NA), in the Zaccaria-Settecainati locality where the pozzolan outcrops in this area with its typical features. In order to evaluate the influence of the hydrologic regime on the mechanical characteristics of pyroclastic soils, and therefore on the stability of the artificial slopes, different engineering geological analyses have been applied, integrated with those techniques typical of the unsaturated soil hydraulic characterisation. In this paper, the results of field and laboratory experimental analyses carried out on the pozzolan soil including soil identification and the engineering geological classification, the estimation of mechanical and water retention properties are discussed. The application of the unsaturated shear strength model has allowed the estimation of the apparent cohesion effect. From these results, verified by means of direct shear tests in unsaturated conditions, the role of mechanical properties and the suction effect on the cut slope critical height, and therefore on the slope stability, have been comprehended.