When it comes to construction projects, you hardly hear about things coming in under budget, but according to reports, the winning bid for the Bellevue light-rail tunnel is 23 percent under Sound Transit’s estimated cost.

The Seattle Times reports Colorado-based Guy Atkinson Construction submitted the winning $121.4 million bid to Sound Transit, which is expected to give final approval Nov. 19 (plus a 10 percent contingency fund).

Before you start thinking of “Bertha the sequel,” the Bellevue tunnel will be done by something called “sequential excavation mining”. The Sequential Excavation Method means the tunnel segments are mined sequentially with supports then the walls are sprayed with fast drying concrete.

The downside to this type of tunnel construction? It could be relatively slow. Experts say the method is found useful in areas where existing structures, such as sewer or subway, could not be relocated.

Construction of the light-rail tunnel in downtown Bellevue, which is all part of the East Link light-rail project, could begin as early as next month.