The N7 Limerick Tunnel river crossing reaches the halfway milestone this week, as precast concrete tunnel sections are tested for water tightness.

Engineers face an anxious week as construction workers pump water into a 600m-long and up to 100m-wide casting basin on the north side of River Shannon before checking each of the five, 100m-long sections.

These will then be floated into a 12m-deep Shannon channel between Bunlicky and Coonagh.

The immersed units will have cut and cover sections at either end to create the new 900m-long tunnel, part of a wider €400M road scheme that is being built by the Direct Route (Limerick) consortium for the Irish National Roads Authority.

Tunnel immersion will take place in the coming months, with the whole project due for completion by the fourth quarter of 2010.