The world’s largest tunnel-boring machine is now scheduled to get back to work on November 23, nearly two years after it overheated and stalled while digging beneath Seattle.
The Seattle Times reported that the construction team handling the massive undertaking of drilling the 1.8-mile Highway 99 tunnel released the new timetable and a new completion date of March 2018—a full three years past the original projected completion before Big Bertha broke down.
Unfortunately for Seattle Tunnel Partners, the crews discovered back in May that the damage to the machine was worse than what was expected. Originally, only the seven rubber seals that keep the main bearing free of water and sand were were expected to be damaged. But once the crews started taking apart the tunneler, it was discovered that the steel casing around the seals broke apart, sending fragments into the drive gears.
Since the discovery of the extent of the damage, Hitachi-Zosen, the machine’s manufacturer, resurfaced much of the front end to allow the circular parts to move smoothly and replaced the full set of 24 pinion gears and the outer seals of the main bearing.
The company also lengthened the mixing arms and equipped them with paddles, attached more steel blades to the cutting surface and ordered a replacement for the machine’s cracked center pipe. The full repairs are expected to be completed later this month.
But even after the front of the tunneler is back underground tentatively in August, much work has to be done before digging can resume. In September, workers will begin to connect the front end to the rest of Big Bertha. The construction team will then spend October and much of November open-air testing the tunneler before it gets to work November 23.