Una grande caverna per l'estrazione di pietra ornamentale: il caso della Cava Madre del Duomo di Milano a Candoglia (VCO)

The geological, geo-mechanical and numerical studies carried out for the static design of the underground exploitation cavern of the Cava Madre of Candoglia (VCO, Italy) are presented and discussed in the paper. This underground cavern was constructed for the mining exploitation of a large sub-vertical vein of pink marble that was necessary for the construction of Milan Cathedral. This marble orebody was gifted to the Milan Cardinal by the Duke Gian Galeazzo Visconti in the 1387 and the exploitation is today active (only in the underground cavern and no more in open pits) since it is necessary to provide the marble (slabs, blocks for the statues, etc.) for the cathedral maintenance. The cavern started from the mountain side and entered sub-horizontally inside the mountain for a length of about 50m. Now the cavern has a width of more than 20m and a variable height that, in the portal area, is of about 40m. Due to a stability problem of the cavern wall that occurred more than ten years ago in the deepest portion of the cavern, it was decided to restart the exploitation works in the portal area where detailed “geo” investigations have discovered a large portion of very good marble. To permit a safe extraction of this new portion of the orebody it was necessary to develop both side investigations and numerical computations. Three dimensional and two dimensional FLAC models were developed to understand the static conditions of the cavern in the present situation and after the development of the forecasted exploitation. Thanks to a very good monitoring of the cavern during the years it was also possible to carry out a detailed back analysis that permitted to validate the quality of the numerical results. Before carrying out the new mining works it was necessary to demolish several reinforced concreted beams that were built up in the seventies and that were not able to fulfil the displacements induced by the new excavations as shown by the numerical model results. The design of the new beams with a larger cross section than the previous ones is discussed in the paper together with the numerical model results after the development of the new excavations.