SR 99 Update: TBM Repairs and Settlement Reports

SR 99 Update: TBM Repairs and Settlement Reports

An aerial view of Bertha, the SR 99 tunneling machine, as Seattle Tunnel Partners makes repairs in Seattle.

The Washington State Department of Transportation (WSDOT) on May 18 posted on its website updates about the SR 99 TBM repair and reports related to ground settlement in the area of vicinity of tunneling activities.

Related to the TBM, Seattle Tunnel Partners has confirmed that disassembly of the SR 99 tunneling machine is complete, and assessment of the damage is ongoing. STP will not provide a revised schedule for resuming mining until they fully understand the scope of repairs, WSDOT reported.

STP has indicated that crews will replace the main bearing and outer seals of the machine as expected. STP also decided to replace the inner seals to make them more compatible with the new outer seals and easier to access should the need arise. The new inner seals were designed and manufactured in Japan and are scheduled to arrive in late May.

Damage to the machine was more extensive in some areas than anticipated and some minor damage occurred during disassembly. For example, the outer seals and the steel retainers that hold them in place were destroyed. There was also damage to the cutter drive motor pinions and the main bearing bull gear.

As owner of the tunneling machine, STP and Hitachi are responsible for all aspects of the repair effort, WSDOT said.

Settlement Issues

After months of study, experts agree: there are no simple answers regarding what caused the ground near the SR 99 tunnel access pit to settle approximately an inch last November.

Settlement near the pit and in the surrounding neighborhood was caused by a combination of historic and ongoing natural ground movement in the region, dewatering related to tunneling machine repair work and dewatering related to other construction in the area, according to two reports released by WSDOT.

One report, conducted by geotechnical firm Shannon & Wilson Inc. and commissioned by WSDOT, concluded that dewatering related to tunneling machine repairs was the primary cause of the settlement. A second report , conducted by Brierley Associates and commissioned by tunnel contractor Seattle Tunnel Partners, concluded that natural settlement and other dewatering activities are the primary reasons for the settlement, and tunnel-related dewatering only contributed in areas immediately surrounding the pit. Both reports relied on the same data points.

The issue began late last year, when the tunnel project’s monitoring system detected settlement in the vicinity of the 120-ft deep pit STP built to access and repair the tunneling machine. In response to the settlement, WSDOT and STP increased the frequency of monitoring – which includes hundreds of instruments near the access pit – and assessed the viaduct and nearby buildings.

Both reports demonstrate that settlement related to dewatering has since stabilized; they also agree the ground movement was minor and caused no structural damage. The Shannon & Wilson report, utilizing in part information from satellites, also identifies settlement in areas surrounding the project – in some cases, outside the ongoing monitoring area. Though satellite imagery is helpful to indicate trends, ground monitoring is the most reliable gauge of ground activity, which is why WSDOT’s monitoring program relies mostly on ground sensors throughout the project area.

WSDOT and STP are continually evaluating ground conditions and taking proactive steps when needed to prevent further project-related settlement. That includes reviewing existing procedures for reducing dewatering should it become necessary. WSDOT has asked STP and city officials to work with project staff to further analyze the data and conclusions in the two reports to find consensus. The agency is also increasing monitoring in some areas and expanding the overall monitoring program in keeping with the analysis provided in the reports.

Alaskan Way Viaduct Replacement Program Administrator Todd Trepanier made the following statement regarding the reports:

“We all agree that public safety and protecting infrastructure are our top priorities. This is an incredibly complex issue, but all of us – the state, our contractor, the city – have a shared interest in reaching consensus and acting in the interest of public safety. Having reliable information is essential to any decision-making process. These studies will help inform future decisions about construction as we work to replace the viaduct.”

Report links

WSDOT response to settlement reports (pdf 34 kb)

Shannon & Wilson report (pdf 26 mb)

Brierley Associates report (pdf 7.4 mb)

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