By Victoria CavaliereSEATTLE (Reuters) - Contractors have created an alternative plan to extract the world's largest tunnel-boring machine stalled deep underneath downtown Seattle if the faltering equipment cannot move itself to a rescue pit, transportation officials said on Tuesday.The machine, known as Bertha, stopped working in December, 2013, after digging just 10 percent of a planned tunnel to replace an ageing waterfront highway, stalling a $3.1 billion roadway overhaul project until the 2,000-ton drill could be rescued and repaired.The snag, and other unexpected construction issues, has left the project two years behind schedule and sparked concern about massive cost overruns.In recent months, crews have dug a pit to reach Bertha's location 120 feet underground, and the Dutch company Mammoet, which specializes in heavy lifting and transport, has finished a massive, specialized crane designed to lift the machine.The rescue plan also involves Bertha tunneling through about 20 feet of concrete to reach the rescue pit's opening, the Washington State Department of Transportation said.If the broken machine cannot get through the concrete, contractors Seattle Tunnel Partners (STP) will have to chip through the walls, potentially further delaying the project, transportation officials said."The length of time it takes Bertha to reach the pit will depend largely on her ability to mine through and digest the concrete," the Washington State Department of Transportation said in a statement."If she's unable to mine through the wall, STP will create an opening from within the pit to give her an unobstructed path forward," it said.While the machine overheated and had problems with its main bearing in 2013, officials are hoping it has enough strength to grind through the final feet to be rescued.The cost of the so-called Plan B rescue was not immediately known.Seattle Tunnel Partners did not immediately respond to a request for comment.The original highway overhaul was expected to cost $2 billion and be completed by 2015. The new timeline for replacing the busy Alaskan Way Viaduct, damaged in a 2001 earthquake and nearing the end of its lifespan, is 2017.Last month, lawmakers in Olympia proposed a bill to kill the tunnel project, saying it was a money-losing venture and that the existing highway could be repaired.(Reporting by Victoria Cavaliere in Seattle; Editing by Eric M. Johnson and Cynthia Osterman)Join the conversation about this story Click here to read full news..