Gypsum quarry under Paris

"The "room and pillar" mining method (piliers tournes), by excavating the deposits into a cathedral-like vault, reinforced the ceiling with a supportive buttress. Given gypsum’s fragility, due to its water-solubility and mechanically weak nature, street and house collapses were common. A famous accident occurred in 1778 where horses, wagons and people were engulfed. Regulations and edicts followed with the establishment of the position of Inspector of the Quarries by King Louis XVI, who’s responsibility was to map and reinforce the quarries. Note the stratification of the gypsum, marl and sand in the quarry walls, referred to as "masses" in the geological French literature. The average mass of gypsum was about 5 to 20 meters thick. Limestone is buried below the gypsum and is prominent in Paris."

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