Repairing the largest tunnel boring machine ever built poses a immense challenge to the ingenuity and resources of engineering experts in Seattle.

Big Bertha is the largest tunnel boring machine ever built in terms of diameter, measuring 17.5 metres, or almost six storeys in height, from top to bottom.

The machine was designed for the specific purpose of burrowing beneath the waterfront of Seattle as part of the US$3.1 billion Alaskan Way Viaduct Replacement Program. The tunnel excavated by Big Bertha would be the biggest part of the program, enabling the replacement of an obsolete viaduct dating from the 1950s which was found to be unstable following an earthquake in 2001.

The machine was originally scheduled to traverse a distance of over 2.7 kilometres in a pit approximately 35 metres below the surface of the earth. Its journey came to an abrupt halt back in December after covering only the 300 metres due to a malfunction the cause of which is still hotly contested by the parties to the project.

According to the tunnel’s contractor, the boring machine struck a 20-centimetre steel pipe which caused rock and debris to enter its seal and bearing system. State-hired transportation experts, however, assert the pipe had no bearing whatsoever on the mammoth machine’s break down.

The ensuing rescue operation, referred to euphemistically by workers as “the Intervention,” will entail immense effort and cost due to the size and complexity of the tunnelling device, as well as its subterranean location.

Workers first have to build a shaft to reach Big Bertha, using as much as concrete as is contained by a medium-sized office complex. The shaft first requires the creation of a rink of pilings, in order to prevent its collapse once excavation work begins in earnest.

Once the shaft is complete, Big Bertha, which still has limited functionality, will be turned around and driven through the concrete pilings to the middle of the shaft, where it will be suspended on a giant cradle.

The front end of the device, which itself weighs 2,000 tons, will then be hoisted from the tunnel using a rail-mounted crane perched precariously at the brink of the shaft.

After being lowered onto the waterfront, workers will repair the retrieved part under the watchful eye of managers from the Japanese company responsible for its construction, adding around 200 tons of steel to the device as a reinforcing and strengthening measure.

The front piece will then be lowered back into the underground tunnel to be reattached to the rest of the boring machine’s body.

Despite the immense rigours of endured thus far, putting Big Bertha back together comprises the most challenging part of the repair process due to the sheer complexity of the machine and the difficulties involved in achieving a precision reintegration of the amended front piece.

Work on insertion of the piling rings commenced toward the end of the northern hemisphere spring, while the entire repair job expected to conclude in March of 2015, with an estimated price tag of US$125 million.