Qualche cenno sull'argomento:

The shelter tunnel under the hospital San Giovanni di Dio, designed by Gaetano Cima and built between 1844 and the end of the XIX century, was excavated in Tramezzario, a carbonatic formation of the Miocene type (11-7 million years ago) typical of Cagliari. The tunnels were built and used as bomb shelters during the Second World War. The tunnel excavation was carried out with straight exploding charges with a length between 25cm and 1.5 m, using compressed air drilling means and explosives triggered by detonators. Laterally to the straight tunnel, other small transversal ones overlook it, some about 6 meters long, other barely excavated with an approximate depth of 50-80 cm.

The transoms were dug starting from the straight tunnel, proceeding with not very deep starting bursts (about 25-50 cm), getting to a depth of about 1 meter once a certain distance from the straight tunnel had been reached. This method was used to avoid damages to the main tunnel. On the opposite wall to the beams dig some perpendicular holes in the gallery course can be actually noticed. Here iron supports were imbedded into a wooden bulkhead placed as a protection for the main gallery wall in order to avoid damages created by pieces of rock scattered around by the blast of the exploding charges. Skilled miners, probably coming from the Iglesiente mining district, carried out the dig of San Giovanni di Dio hospital underground gallery:  the realization of a gallery below a big building, moreover a hospital, required great attention, accuracy and high professionalism. The excavation was carried out using workers divided into teams of two people: a miner (who operated both as a mine driller and an explosive loader) and a labourer (generic). A group leader (called attendant in the mining slang) coordinated the teams.

The progress of the digging can be estimated in an advancement of about 1 meter every 8 hours’ work. Probably the explosive used was the jelly traditionally used in mining.