Clarifying the geotechnical configuration of the lake in the Buttes-Chaumont Park by compiling historical data, geophysical measurements and geotechnical boreholes[Préciser la configuration géotechnique du lac du parc des Buttes-Chaumont en compilant données historiques, mesures géophysiques et sondages géotechniques]

The Buttes-Chaumont Park in Paris is currently affected by significant signs of aging and ground instability, which have resulted in the closure of several areas to the public. As part of a large-scale redevelopment project, the City of Paris sought to gain a clearer understanding of the site's configuration, particularly in the lake area, which is a key feature of the park. To support this effort, BRGM expanded and updated a historical and technical synthesis of the site's structures and subsurface conditions, initially undertaken in 2022. Historical archives document several collapses that occurred during the original construction of the lake between 1864 and 1867, as well as shortly thereafter. These events are attributed to a complex and sensitive geotechnical context. The lake rests on thick layers of clayey and gypsum-rich fill materials, reaching up to 12 meters in thickness in some locations. These fills were used to backfill former open-pit quarries, beneath which older underground quarries are also present. Originally, the lake was not waterproofed, allowing uncontrolled water inflows that weakened the underlying materials and increased instability. After numerous minor and largely ineffective repair attempts, two major reconstruction campaigns were carried out, first in the early twentieth century and later in the 1950s–1960s. During these works, the lake bottom was transformed into a concrete slab supported by approximately 350 piles. This structural solution significantly improved stability, and no major disorders were reported for several decades. However, in 2021, new signs of deterioration were observed, including instabilities along the lake's edges and cracking in the slab. Geophysical surveys conducted in January 2024, including microgravity and ground-penetrating radar investigations, largely confirmed the configuration of the historical structures while identifying several anomalies potentially associated with unstable zones. Subsequent geotechnical boreholes did not reveal large voids beneath the slab but did detect small cavities and weak materials, likely corresponding to altered fill. Together, these studies contributed to assessing ground movement risks and provided essential data for designing future engineering works.