On 27 August 2022, a collapse occurred in the Schieburg tunnel in Luxembourg (built in the 19th century) during repairs to the vault, following a particularly dry period. Around 400 m3 of debris flowed out in a matter of seconds, completely obstructing the tunnel and blocking the only railway line serving the north of Luxembourg towards Belgium. The rock slump occurred 60 m from the southern entrance, at a point where the overburden above the tunnel is 31 m, and under forest cover.
The geological and geotechnical studies carried out by our office included reconnaissance drilling with parameter recording to a depth of 80 m, geophysical prospecting using electrical and seismic tomography, and a structural study of all outcrops in the vicinity of the tunnel.
These investigations revealed a very complex geological and hydrogeological situation, with the collapse zone located at the intersection of a system of conjugate faults not recorded on the geological maps. Significant water infiltration was observed during the works, which complicated the stability of the excavation face and was a major cause of the weathering of the rock. The results obtained made it possible to estimate the volume and shape of the deconsolidated zone above the rock collapse and to propose safety measures to reinforce the deconsolidated zone above the tunnel, safely remove the pile of scree and repair the damaged vault. These safety measures were carried out using injections of the deconsolidated zone around the vault and anchors from the tunnel and the surface. The final support system consisted of metal T-beams and steel plates. Geotechnical monitoring of the securing works was carried out from start to finish, enabling the tunnel to be reopened less than one year after the collapse, despite very difficult access conditions both for exploratory drilling and for the works.