It concluded that following “rigorous evaluation of the situation…the passage of the tunnel boring machine is responsible for the sinking that has been observed in recent months.”

The Toronto Star reported that the sinkholes formed in the environmentally sensitive Herons Marsh area beneath which the TBM tunnelled. The marsh soon started to dry up following the appearance of the first sinkhole in August 2021, which was followed by other sinkholes, some of which appeared in a nearby forest.

However, numerous sources, such as the MELCC and infrastructure developer CDPQ Infra, did not conclude that the first sinkhole caused the marsh to drain. Indeed, a spokesperson for MELCC attributed the draining to the weak snow cover of the past winter and below-normal rainfall since May. CDPQ Infra explained the subsidence as a result of a “very eventful transition zone between loose soil and rocky horizons”, which presents “difficult geological conditions.“

But Stéphanie Pellerin, professor at the University of Montreal and wetlands specialist said that past dry summers had not caused draining of the marsh. She believes the first sinkhole to be the main cause.

The TBM has now completed its passage beneath the marshy zone and has entered rock near Montréal-Trudeau airport, working around 30m below ground and with 7m of overburden.

Backfilling of the sink holes has now been completed with soil taken mainly from the excavation of Marie-Curie station; more recently CDPQ Infra has gradually been injecting water to replenish the marsh.