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Translated Jeff Matthews

A demotion tunnel is generally built at the same time as a bastion. It is a defensive structure and meant to partially demolish the bastion, itself, if necessary, in order to turn it into a new defensive structure displaced to the rear.

This typology is quite particular and rare. The tunnel is spacious and follows internally the two external faces of the bastion. The tunnel is equipped with small spaces dug into the ceiling and into the shafts that branch off toward the outside, all for the placement of explosive charges. If the bastion is partially destroyed by artillery fire or explosives, the mines in the tunnels are detonated in order to collapse the two faces of the bastion into the trench. This creates a new salient (a defensive corner facing the enemy) and a new trench (the tunnel, itself, now devoid of the ceiling and uncovered) with an integral counterscarp wall (external pier) and scarp wall (internal pier). The tunnel has the further advantage of serving as a bomb-proof shelter and as a passage within the gallery from one flank of the bastion to the other.

An exceptional example of this kind of tunnel, probably the only one of its kind in Italy, may be seen within the San Carlo bastion of the Cittadella di Alessandria; in 2007 it was studied and mapped by speleologists of the S.C.A.M. Association.

The terms demolition gallery or demolition tunnels indicate those works that often complete a permanent blockade structure, serving to interdict passage either partially or totally. In the first years of the 1900s there were also demolition tunnels that served to temporarily interrupt passage on roads and rail within tunnels (for further information, see: Breda Maria Antonietta, Fumagalli Sara, Padovan Gianluca, “Opere di mina: la galleria di demolizione di Brienno (Como)”, in Breda Maria Antonietta, ed., Luoghi e architetture della Grande Guerra in Europa. I sistemi difensivi dalle teorizzazioni di Karl von Clausewitz alla realtà della Prima Guerra Mondiale, Hypogean Archaeology Research and Documentation of Underground Structures, British Archaeological Reports International Series 2438, Oxford 2012, pp. 81-132).

references:

Padovan Gianluca (ed. ), Archaeology of the Subsoil. Lectures and studies of artificial cavities. British Archaeological Reports, International Series 1416, Oxford 2005.

Basilico Roberto et alii, Italian Cadastre of Artificial Cavities. Part 1. (Including introductory comments and a classification), B.A.R. International Series 1599, Oxford 2007.

Gianluca Padovan (Associazione S.C.A.M. – Federazione Nazionale Cavità Artificiali)

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Demolition tunnel in the Cittadella di Alessandria (photo, G. Padovan)

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