With Paris’ faubourgs falling into crevices caused by the collapse of centuries-old abandoned mines, ensuring the safety of the city’s outer reaches was an enormous but urgent task. In 1777 the exploration and reparation of some of its main thoroughfares had already begun, but the extent to which parts of Paris and its outskirts was fragile wouldn’t be fully discovered until almost the end of the 19th century. In the meantime, the Inspection Générale des Carrières, just created that year under its head Architect Charles-Axel Guillaumot, had at once to invent and execute a means of finding and repairing all abandoned mines to prevent any further collapse.
Three of the earliest consolidations were executed well before Guillaumot’s time. The first known was under the foundations of the former Chateau Vauvert, a country house built by Robert II (“the pious” – 996-1031) for his unlawfully wedded wife (his cousin, for which he was temporarily excommunicated). Standing near the corner of today’s rue Auguste-Comte and the avenue de l’Observatoire, the Chartreux order monks would level the last of its ruins from 1259 to build a new monastery. In the rough caves below, they erected solid walls of masonry and a stairway and transformed the finished whole into a brewery and distillery. Some of these grottoes still remain today, but the stair and wells to the surface are gone.
Also near the rue Saint-Jacques to the south of Paris was a terrain occupied by the Feuillantine nunnery from 1623. In their first year there, in awaiting the construction of a new residence to replace the ancient house they had then as a temporary habitation, they happened across an access to what looked to be a very a very ancient mine below it. The caves were crumbling and seemed ready to collapse, and centred in them was a fontis that had practically reached the foundations of their home above. They called in workers to make repairs there at the same time as they worked on the new monastery above; on Easter 1627 the whole collapsed, but as it was a holiday the workers (numbering 500 in texts) were absent. Several other incidents of a similar nature occurred before the caves and Coventry were complete, and it seems that, because of the extra stone they extracted to build their Coventry above, they left the caves in an even worse state than they found them.
Not 200 metres to the south from there in 1645, Anne d'Autriche's Val de Grace church and Coventry was already built to its first floor before its architect, François Mansard, had discovered the state of the ground under its foundations. After giving the enormous caverns he found there a thorough examination, he called a halt to all further construction until the ground below his building’s’ foundations could be solidly reinforced. In a map that a plan that echoed the plan of the Coventry plan above, but twenty metres below, he built to the roof of the caverns immense masonry arches that would later form the walls for an enormous cellar that would later serve a cellar and refuge for the Coventry. The nuns could access the underground by a long stairway, where there was a dormitory, latrines, and even a source - this water was also accessible by a well from the Queen's apartment in the Coventry above.
A more curious case concerning underground cavities was that of those discovered under the Paris Observatory built in 1672. Its site over long-abandoned mines was put to use by the scientists of the time - a spiral staircase without a central column led between the caverns and the ground floor of the observatory. This lack of column left a shaft of a diameter of about ten centimetres through the centre of the staircase, above which holes were pierced through all of the floors until the top of the observatory. From the dome that topped the observatory tower, an object could be dropped through the holes in the floor and through the stair to the bottom of the mines, an overall distance of about 60 metres. The shaft was also intended for a precise observation of the path of stars that passed over its zenith, and for the installation of a large-scale water thermometer.
All of the above grottoes became a novelty in later years, especially those under the Observatory.
Rebuilding Tunnels, Closing Caves
Strengthening the walls and ceiling supports of a degraded mine was no easy task. The Administration des Carrières began this work the day Guillaumot was sworn in; known mines under roadways that presented signs of imminent collapse were the highest on their list. Examining the state of the ground under all existing roadways came a close second, and this included finding yet unknown cavities. Remember that Paris in 1777 was still less than half the size it is today. The southern boulevards "du Midi", lain during Louis XIV (1643-1715)’s time would serve as a base for the Fermiers-Généraux tax wall built from 1784. So, in a few short years, the Administration's priority of assuring the safety of Paris streets grew considerably. The total undermined ground within Paris’ Left Bank’s limits then was estimated at 340 hectares.
A map of Paris' underground seems complicated in its modern version, but it is simple compared to its pre-consolidation state. Imagine, in as many as twenty mines at a time, workers burrowing caverns through the earth in any direction until the end of a mineral bank or until they had taken enough to fill their order. If the mine was still exploitable, they sometimes covered its entryway to preserve it for future use, but if it was exhausted quite often just abandoned a mine without a second thought. To add even more opacity to this mine-tracking scenario, mines were often forgotten, as, since the literate were few in those times, the death of a mine’s owner was enough to erase any record of its existence. Repeat the above scenario over several hundred years, and one has a better understanding of what sort of task the Administration des Carrières had before it.
The first step to ensure the safety of a roadway was to locate any abandoned mines under it. For this let us use as an example the first street to get Guillaumot’s attention: the route d’Orléans, or today’s avenue Denfert-Rocherau: workers would dig a tunnel the entire length of the street in question as far as grottoes were known to have existed, and depending on the importance of the street (as in this one), the tunnels would sometimes be a parallel two. Any cavities found along the way would be thoroughly examined and consolidated with masonry where needed. Since the hallway itself was burrowed through the sandstone deposit, its walls through any “solid” areas would be of natural stone. The final touch would be to engrave the name of the street above into the stone along with the date on which the work was completed.
If a fontis was encountered on the way, a whole other procedure had to be followed. As the mine floor under the centre of a fontis was usually a pile of massive stone rubble, instead of digging into it, workers would try to find how high it went. If the erosion was already close to the surface, a hole would be broken and the cavity filled in from above, and if it had not progressed too far the space between the fontis dome and top of the rubble pile, or “cloche”, would be filled from below. Once this was done, workers would dig around the cloche (so as not to further disturb it or their completed work) to continue their hallway to the other side. In some special cases where the fontis was regular in form and relatively shallow, it would be reinforced with masonry, covered in plaster, and the date the consolidation completed painted into the tip of its dome. The “official” catacomb visit of Paris (from place Denfert-Rochereau) has a few examples of this latter type of consolidation. Lastly, if the fontis was enormously high, but the surface still far above the highest reaches of its dome (as in some regions around Montparnasse) the entire cavity would be reinforced with masonry walls topped with a concrete or masonry arches or dome.
The Carrières under a larger Paris
Paris’ streets could be considered safe by 1860, the year it doubled in size. Work was still under way in its former faubourgs, now its outer eight arrondissments: the main arteries into the city had been consolidated, but there remained much of the land around the still-operating stone mines to deal with. One change to the positive in this regard was the ban from 1860 on all further mine activity within Paris; Only Belleville's plaster mine “Carrières du Centre” and “Carrières d’Amerique” mine enterprises managed to avoid this interdiction, but these also would end production in 1865 and 1871 respectively. All plaster mines were dynamited upon their cloture, so there exist no known cavity under either hill, with the exception of the re-sculptured 'grotto' in the parc des Buttes-Chaumont. Nor, contrary to the popular belief, are there grottoes under the Basilica of Sacré-Coeur on Montmartre. Its closest plaster exploitation was about twenty metres to its southeast. All the same, a series of wells were dug under the future basilica's foundations and filled with masonry, much in much the same technique as the reservoirs of Montsouris. Unlike this last construction, Sacré-Coeur's foundation columns served not to support a weight above empty caves, but to prevent the movement of underground clay banks.
One of the first suburban consolidations made even before Paris’ 1960 growth was along its Petite Ceinture railway. Running a ring around Paris from 1852, its path crossed many areas previously mined for clay and plaster. The most difficult areas of consolidation were where it undercut hills; the weakening of gypsum deposits and the sliding of clay banks in the Belleville tunnel excavation caused a few landslides and collapses, and almost carried a church into the Ceinture trench at the tunnel mouth. Yet-unknown cavities discovered under the rue des Pyrenees needed repairing before work could progress, and the tunnel walls and arches had to be doubled to provide enough resistance against any movement of the clay banks. All the same, the work undertaken by the State, as they were responsible for the construction of the Ceinture Rive Droite, would have to answer for two tunnel collapses in later years.
The second portion of the Petite Ceinture, the Chemin de fer de Ceinture Rive Gauche, would receive particular attention in 1865 while it was still in its landscaping stage. It crossed many of Paris’ former stone mines, but over an area where the stone deposit had the particularity of being almost at the surface. First there were two tunnels to build under the hills of Montsouris and Ivry, but once these done, the Ceinture civil engineers, under the surveillance of the Administration des Carrières, dug trenches to the bottom of the stone deposit under where the rails were to lie. Though extending further than any existing mine, these ran the entire length of the sandstone deposits under the 15th, 14th, and 13th arrondissments. Once solidly walled, these trenches were capped with a roof in the form of a masonry arch and covered over. These tunnels were accessible both from the greater underground mine network and from stone “regard” houses placed aboveground at even distances along the railway.
The last consolidation work Paris’ underground would see would be from 1898 for its Metropolitain railway. One of the most gargantuan masonry jobs the Parisian underground has seen was for the reinforcement of an enormous fontis under metro line 12 – it measures over fifteen metres high. Many of Paris’ metro lines run relatively near former mines, and the workers digging them sometimes made some interesting discoveries.
In fact, if it is the interesting tidbits about Paris' underground you are looking for, we will be discussing a few in the next and final chapter on Paris’ mines.