Tunnel ferroviaire de Vierzy : vieillissement, altération des maçonneries calcaires

Abstract

The Vierzy railway tunnel was excavated in 1859-1860 in cuisan sands below 38 m of essentially lutetian limestones. The accident is due to the collapse of the roof of the tunnel on a 15 m long section and of about 4 m thick Lower lutetian formations. The initial cause of the collapse is a defect, at construction time, in the keystone zone of the masonry of the circular vault, at the joint between two sections of excavation. In 1914 a war destruction with explosives created a collapse and a « cathedral» 50 m long, 17 m high, and a cone of 10,000 m 3 debris at 20 m of the 1972 collapse. The shaking provoked the sinking of Lower lutetian formations on the masonry, at a distance, Middle Lutetian staying as a slab. Fumes of steam trains deposited soots with coal and sulphides (pyrite) particles in voids of the ground and of the mansory up to in cavernous mortar. This permitted a sulfuric acid alteration, increased by bacterial processes, changing carbonates into soluble sulfates (gypsum). It lowered the resistance of the vault and permitted the fall of quarry stones. At the inner surface freezing caused a progressive fall of debris and reduction of thickness of the masonry. Thin facing rolls of bricks or concrete were placed here and there as a protection. The removing of a section of such a roll of bricks, altered, to be substituted with concrete was the final cause. One limit of the removed section coincides with one limit of the collapse. On June 16, 1972, two passenger trains crossed into the tunnel at a small interval and collided with the cone of debris. There were 108 deaths. It is the third most important catastrophe in French railway history.