The Alaskan Way Viaduct that the Seattle SR99 tunnel will eventually replace is reopened four days ahead of schedule following a successful undercrossing by TBM Bertha.

Structural engineers from WSDOT completed a thorough inspection of the viaduct on Sunday (8 May), and made the decision to reopen the viaduct to commuter traffic in time for Monday’s rush hour.

Originally the contractor team of Dragados and Tutor Perini (Seattle Tunnel Partners) had intended to drive the 17.48m diameter mega-TBM a distance of 385ft (171m) out of the concrete-encased Safe Haven 3 – and fully clear of the viaduct – before making a recommendation to reopen it to traffic.

In the event, however, that decision was taken early, at 342ft (104m, 52 rings), following completion of a series of critical rings lying just 15ft (4.5m) below the viaduct’s foundation piles. This is the closest that Bertha comes to any foundational structure along the 2.7km alignment, and completion of the concrete rings at this location also coincides with transition of the machine out of the most complex soils and into more competent ground.

By Wednesday (11 May) STP had mined the full reach of 385ft (171m), which takes the machine clear of the fourth and final viaduct column and into a location that is better suited for a rest stop.

According to Dave Sowers, WSDOT Deputy Director of Highway 99 and a geotechnical engineer, height measurements taken during the drive under the viaduct had proved that Bertha’s earth pressure balance systems were fully functional. Recorded movement of the viaduct during the undercrossing – within a tolerance range of 1.5–3mm – was a positive sign ahead of the machine preparing to break out under downtown, he said.

Bertha is now shut down for a crew rest stop following 12 days of continuous 24-hour operation on 12-hour shifts.

After the break the usual working pattern of 2 x 10hr shifts per day, followed by a four hour daily shutdown for inspection and maintenance, will resume.

Acting Transportation Secretary Roger Millar said: “The end of this closure marks a new beginning for the SR99 tunnel project. Much work remains, but we are encouraged by the contractor’s performance during this phase of the project.”

STP is currently anticipating breakthrough by December this year (2016).

Progress in Seattle on the SR99 replacement tunnel is completing the critical viaduct underpass before heading on under the city to complete the new double-deck highway route.

Mechancial excavation by the 17.48m diameter Hitachi-Zosen EPBM resumed on April 29 (2016) following a 47-day stop for scheduled maintenance within the protective concrete-encased underground structure known as Safe Haven 3 (SH3). By Wednesday 4 May, excavation was more than a third of the way through the critical 385ft-long (117m) reach under the viaduct.

During the maintenance stop at SH3, STP, the joint venture contractor team of Dragados and Tutor Perini, completed nearly 100 shifts under hyperbaric conditions Along with other maintenance tasks, 11 of the more than 700 cutting tools on the mega-TBM cutterhead were replaced.

SH3, which runs parallel to the viaduct and to within a distance of 40ft (12m) of its foundation piles, was constructed prior to machine launch in July 2013 as added protection of the elevated roadway against ground movement. Project owner WSDOT closed the viaduct to all traffic for passage of the TBM for a period that is expected to last two weeks. The scheduled closure is an added safety precaution to enable real-time monitoring of ground movement and to ensure that immediate repairs to the viaduct, which has already been externally reinforced, can take place if necessary.

At its closest point the tunnel crown passes to within just 15ft (4.5m) of the lowest extent of the pile foundations that support the viaduct. Currently the EPBM is just 80ft (24m) below the surface – less than one-and-a-half times the shield diameter – but from here the machine will continue gaining depth as it mines on a 4% downward gradient until reaching its lowest elevation of 215ft (66m) below ground level between Madison Street and Spring Street. Bertha will then climb at a 1.6% gradient to Lenora Street; then on a 3.6% upward gradient to Wall Street; and finally at a 4% gradient to the end of the tunnel at Thomas Street.

Since leaving SH3, STP has completed a further 163ft (50m, 25 rings) of tunneling, for a total advance since machine launch in July 2013 of 1,700ft (518m). This week WSDOT released video footage of progress to date, shot from a video camera installed on a remote-operated drone that was flown into the tunnel.

Chris Dixon, Project Manager for STP, said: “We are looking at averaging four rings a day but there are going to be days when we will only complete two rings, some when we will do four, and some where we will get up to six or seven. What we are really aiming for is an average of four rings per day to complete mining through this 385ft section in the two-week viaduct closure period. For the four days we have been mining, we have achieved progress of one, three, seven and six rings respectively.

“We expect the pace to pick up a bit after we pass under the viaduct but this is dependent on the soils that we encounter. The deeper we go the more competent the ground is expected to be, which should enhance our tunneling rate.”

Once STP completes the short drive under the viaduct Bertha will head under downtown Seattle where there will be no opportunity to carry out repairs from the surface should there be another mechanical breakdown. The latest published schedule anticipates breakthrough at the end of the 2,700m drive in February next year (2017).