The replacement project focuses on the 2-mile-long elevated double-deck road originally built in the 1950s. Damage from a 6.8 magnitude earthquake in 2001 led to the need to stabilize and repair the structure and restrict heavy vehicle traffic.
Most of the existing bridge deck has since been replaced with new roads, and the viaduct itself will be replaced by the tunnel.
The 1.7-mile-long tunnel is expected to boast an inside diameter of 52 feet (15.8 meters), allowing it to hold the planned double-deck roadway.
Bertha was out of commission since December 2013, when dirt got into a bearing, causing the machine to overheat and damaging the seal system after having only dug 1,000 feet.
The drill was returned to the access pit in August, after repairs were made to the cutter head, but the work to reconstruct the front shield and reconnect wires, hoses, cables and pipes was expected to take a few more months to accomplish.
In October, STP announced that repairs were taking longer than expected, and they pushed the restart date back to Dec. 23, a deadline they met.
STP and Bertha’s manufacturer, Hitachi Zosen, are responsible for the repair effort and schedule. Both parties insurance companies were also reported to be involved in a battle over who is liable for covering the costs of Bertha’s repairs.
The entire $1.35 billion tunneling project is expected to be complete by April 2018.