Known as the Voûte du Malpas (Vault of Malpas) or the Grotte du Malpas (Cave of Malpas), the Malpas tunnel was dug under the Malpas mountain pass. The mountain to be drilled into was 13 "toises" high (25 meters) and 367 "toises" wide (715 meters). On each side of the future tunnel, Riquet had the mountain knocked down over a length of 140 "toises" (272 meters) and a depth of 45 to 50 feet (14 to 16 meters). These measurements, as well as those of the tunnel, are provided by Matthieu de Mourgues, in the book “ Report of the second solemn navigation of the Canal Royal…, 1683 »
Remember that in the seventeenth and eighteenth centuries, length measurements were expressed in "toises" and feet. In theory, a "toise" is equivalent to 1 metre 949 and a foot is 33 centimetres.
For the first time in history, a navigable canal is made to pass under a "mountain", hence the amazed astonishment of contemporaries all the more accentuated as it is the one and only tunnel of the Canal du Midi. . Its construction took place between the fall of 1679 and the year 1680.
Riquet, arrived in front of the mountain of Malpas, at the foot of the Oppidum of Ensérune, finds himself in great difficulty. It is formed from a sandy tuff (called tap in the XNUMXth and XNUMXth centuries). A very hard ground hid a mountain of very crumbly sandstone, prone to landslides. Colbert, one of Louis XIV's main ministers, was Comptroller General of Finance, Secretary of State for the King's Household and Secretary of State for the Navy, was alerted by a letter in which Riquet's contemporaries accused him of 'have the head of his channel in a mountain of sand. Colbert stops work, the project is threatened. Riquet will lose his bet because his choice to prefer to pierce this "mountain of Malpas" rather than follow the advice of the Chevalier de Clerville, architect of Louis XIV, who proposed to cross the Aude. Colbert announces the visit of the royal commissioners to decide on the future of the canal route. But Riquet is tenacious: he wants his canal to pass through his native town, Béziers. We must therefore break through.
Surrounded by his engineers, he sought advice from Nissan's master builder "Pascal", who was in charge of the maintenance at the drainage tunnel at Montady swamp, and decided to continue the work in secret, despite the risk of collapse. A test tunnel was dug out. When the emissaries sent by Colbert and intendant Aguesseau arrived, he managed to persuade them to let him continue the work. This was the last major project carried out by the brilliant Pierre-Paul Riquet. He died a few months after the completion.
The length of 85 "toises" (165 m), the width of 5 "toises" (9 m) and the height of 4 "toises" (5 m) were impressive for that time. A 8 feet wide (3 m) platform runs along the entire length of the tunnel. It was used as a towpath for the boats. Riquet had built the same opposite which was removed very early so that larger boats could pass through, especially for the transport of wine barrels on barges.
In the era of Riquet, the tunnel was not arched. This was done later, in around 1700. However, near Capestang, 48 to 50 m still don't have a protective arch today, and the rock of the mountain is visible. Some holes, the deepest of which measures 60 cm, have appeared due to humidity and erosion over three centuries. This tufa rock is therefore not as soft a rock as was initially believed.
Where the stone archway begins, there is an opening to allow for passage between the arch and the mountain (2 m gap) so that maintenance of the structure can be carried out. According to legend, in the nineteenth century, a pious hermit lived there and to feed himself, he simply dropped a basket down on a rope, when the boats passed by.
Speaking of the Malpas tunnel, Jacques Morand in “Le Canal du Midi and Pierre-Paul Riquet”, in 1997 says it: “Far ahead of his time, Riquet has just created a very modern work that was unique at the time. It was not until the XNUMXth century and the Railways to see breakthroughs of this kind made ”.