The role of a bentonitic layer on slope stability in bedded limestone: the case study of the December 2004 Ca' Madonna Quarry rock slide (Umbria-Marche Apennines, Central Italy)

Abstract

This work describes and analyzes the unexpected rockslide that affected, on December 2004, the quarry face of the Ca' Madonna extraction site (Municipality of Urbania, Province of Pesaro and Urbino, Italy), where stratified calcareous lithologies are quarried. This movement occurred in correspondence with an unsurveyed clayey intra-bed layer approximately 10 cm in thickness that consists principally of smectite group clayey minerals. These are very active and have a high capacity to absorb water until reaching a swelling pressure of up to 1200 kPa. Mining activities reduced lithostatic load and facilitated rainwater infiltration down to this bentonitic layer, which led to fully softened conditions (Skempton, 1970) with a resulting severe reduction in the shear strength parameters. Under such conditions, the slope stability back-analysis, performed considering a planar rock slope failure with tension cracks and with different water level heights, shows that a small water level was sufficient to exceed the limit equilibrium. Stratigraphically, this layer can be correlated with the Lower Campanian Bentonitic Layer already reported in other Central Italy sites (Mattias et al. 1988;Bernoulli et al. 2004). The potentially great spatial extent of this layer and the possibility that the conditions analyzed here may occur in other similar settings makes it important to identify its presence earlier to prevent analogous situations.

The Historic Centre of Urbino, UNESCO World Heritage (Marche Region, Italy): an Urban-Geological Itinerary Across the Building and Ornamental Stones

“…is generally constituted by regular beds of pinkish and reddish limestones (Fig. 14e), ranging from 10-15 cm to about 30 cm in thickness, and interbedded few millimetres thick reddish marls (Centamore et al 1975(Centamore et al , 1979Arthur and Fischer 1977;Wezel 1979;Tonelli et al 2019). In the lower and the upper part of the formation, limestones are characterised by beds and nodules of red chert, while the middle-upper portion is marked by the occurrence of marly-rich lithofacies.…”