The final two tunnel boring machines (TBMs) have broken through into a reception chamber marking the end of 25 years of tunnelling work on the Emscher sewer system in Germany.
The two 10km long tunnels constructed by Porr form part of a 400km drainage system between Dortmund and the Emschermündung wastewater treatment plant.
The two tunnels have an internal diameter of 2.6m making them among the smallest segmental concrete lined tunnels in the world.
As well as tight radii turns, the earth pressure balance TBMs also had to negotiate a 300m long geological fault zone with sandy, groundwater-bearing sub-soils.
Commissioning of the final two tunnels on the system and another 31km of sewers is expected to be completed by the end of 2020.