We've received a number of questions about the ground settlement measured recently near the SR 99 tunnel access pit. Answers to some of these questions can be found below. We'll continue to update this post as new questions come in.
What’s the latest regarding ground settlement near the SR 99 tunnel access pit?
We installed a state-of-the-art settlement monitoring system as part of the SR 99 Tunnel Project. In late November, Seattle Tunnel Partners surveyors detected over an inch of ground settlement near the pit crews are building to access and repair the SR 99 tunneling machine. We also saw similar settlement on the Alaskan Way Viaduct, with lesser settlement in the surrounding area.
We quickly took action to survey nearby buildings and structures, including the viaduct, to confirm that there was no risk to public safety. No significant settlement has been observed in the area since Dec. 5. We are continuing to monitor this situation closely, but the viaduct remains safe for use and this settlement does not pose any safety risks.
What’s causing the ground to settle?
The cause of the settlement is still being determined. We believe that recent dewatering by our contractor may be a contributing factor, but we do not yet know if there are other factors. We will share additional information about this as it becomes available.
Is the ground still settling?
According to recent survey data, no significant ground settlement has been observed since Dec. 5.
What kind of settlement monitoring are you currently doing?
Crews from WSDOT and Seattle Tunnel Partners are conducting ongoing surveys of the Alaskan Way Viaduct and ground near the SR 99 tunnel access pit. In general, the surveys include:
Twice daily manual measurements at the bottom of both the east and west columns of the viaduct.
Approximately every other day measurements of deep survey points. These are survey points more than 80 feet underground.
Ground surveys of sidewalks and streets from Alaskan Way to Second Avenue and from Yesler Way to South King Street. Some areas are surveyed twice a day; other areas are surveyed once every two to three days.
Surveys of some buildings. Data is collected both manually and automatically and monitored daily.
What are your next steps?
We will continue to conduct frequent surveys and analysis, and provide regular updates to the public. In the meantime, STP crews have resumed excavation at the access pit and construction is moving forward.
Did recent settlement damage a water main in Pioneer Square?
On Dec. 15, Seattle Public Utilities officials announced that a 16-inch water main beneath First Avenue may have been damaged. They suggested this damage was the result of the settlement measured recently near the pit STP is building to access and repair the tunneling machine. A memorandum of agreement we reached with Seattle Public Utilities details acceptable levels of settlement for SPU infrastructure. Based on our current data, settlement has not exceeded acceptable levels. Also, no leakage or damage to the pipe has been verified by SPU. We will continue to work with SPU and other utility owners to protect their infrastructure.
Has settlement caused any damage?
The settlement did not cause any new damage to the Alaskan Way Viaduct. We continue to reach out to owners of approximately 30 buildings where the greatest settlement has occurred. By the end of Wednesday, Dec. 17, we had surveyed 38 buildings. This number includes surveys that were requested by property owners. Survey crews have found some cosmetic damage in a handful of buildings, but no structural damage has been discovered.
What percentage of the tunnel project is complete?
Our design-build contract with Seattle Tunnel Partners includes a number of elements. In addition to boring the tunnel and building the highway within it, STP is responsible for building highway ramps and other connections at the north and south ends of the tunnel. They are also constructing buildings at each tunnel portal to house lighting, ventilation and other systems needed to operate the tunnel. Although tunnel boring is on hold, approximately 70 percent of this work has been completed.
Is the viaduct still safe?
Our bridge experts have confirmed that the viaduct remains safe for day-to-day use. If we had any reason to believe it wasn’t, we wouldn’t hesitate to close it. It’s important to remember, however, that the day-to-day safety of the structure does not change the fact that the viaduct remains vulnerable to earthquakes. That’s why it’s being replaced.
All structures, the viaduct included, are designed to withstand some settlement. In fact, we were expecting the structure to settle some as Seattle Tunnel Partners built the tunnel project. It’s challenging to explain in general terms how any ground movement is acceptable. In the case of the viaduct, no one number represents an acceptable level; limits vary along the length of the viaduct based on ground conditions and the condition of the structure. How the ground settles is also important. A structure that settles uniformly is less likely to be damaged than a structure that settles unevenly.
What structural improvements have been made to the viaduct since the 2001 Nisqually earthquake?
Crews successfully completed a project to demolish and replace the southern mile of the viaduct in fall 2012. The remaining mile-long waterfront section of viaduct remains open today due to a number of important safety measures we have taken since the 2001 Nisqually earthquake.
Immediately after the quake, we repaired damaged portions of the viaduct to make it safe and functional. Additionally, vehicles weighing more than 105,500 pounds are prohibited and trucks and buses must travel in the right-hand lane only to limit the number of heavy vehicles on the viaduct in one location at one time. In 2008, we strengthened four viaduct foundations between Columbia Street and Yesler Way that had settled as much as 5 ½ inches since the earthquake.
As discussion about how to replace the waterfront section of the viaduct continued, we began work on a system designed to close the viaduct automatically in the event of a moderate to severe earthquake in the greater Seattle area. Completed in 2011, the automated closure system consists of traffic gates at all viaduct access points controlled by an earthquake detection system. If the system detects significant earthquake motion, it will simultaneously lower all nine traffic gates and safely close the viaduct in under two minutes.
How much has the viaduct settled since the 2001 Nisqually earthquake?
Viaduct settlement varies by location. The most significant settlement our crews have measured – 5 ½ inches – occurred between Columbia Street and Yesler Way, where crews have since strengthened some of the structure’s foundations.
Identifying the cause of settlement is challenging and in some cases impossible due to the many factors that can contribute to settlement. For example, it’s not unusual for the ground to settle long after an earthquake. As a result, much of the settlement measured along the viaduct could potentially be traced back to the Nisqually earthquake. Other potential factors include natural ground movement and vibration caused by traffic or nearby construction.
How much settlement would it take to close the viaduct?
If the viaduct were unsafe, we wouldn’t hesitate to close it. WSDOT has closed bridges in the past due to safety concerns, including the Murray Morgan Bridge in Tacoma. Because settlement limits vary along the length of the viaduct, there is no single threshold for determining when settlement would be significant enough to require further mitigation or closure of the structure.
Our contract with STP allows up to two inches of viaduct settlement before mitigation is required. Should it be necessary, a number of techniques could be used to strengthen the viaduct and keep it open to traffic until the new SR 99 corridor is completed. These techniques could include strengthening columns or other areas of the structure to provide additional support. We could also reinforce the viaduct’s foundation as we did in 2008.
Does this new settlement increase the seismic risks facing the viaduct?
A few people on Twitter asked whether this settlement makes the viaduct more vulnerable to earthquake damage. The short answer is no. While any kind of ground movement has the potential to damage structures like the viaduct, ground settlement poses different risks, from a structural standpoint, than earthquakes. The main difference is that earthquakes cause lateral movement, while ground settlement causes structures to sink. These differing movements create different strains on the structure that aren’t related. In other words, settlement, if severe enough, has the potential to weaken a structure, but not necessarily in a way that makes it more likely to be damaged in an earthquake. How the ground settles is also an important factor. A structure that settles uniformly is less likely to be damaged than a structure that settles unevenly. This recently measured settlement was uniform.
Bertha repair effort
Has Seattle Tunnel Partners stopped work?
STP is continuing work on the project, including excavation of the access pit. We temporarily required them to stop excavation while we analyzed ground settlement data. Because no significant settlement has occurred since Dec. 5, and because there is no link between excavation and recent ground settlement, excavation was resumed on Dec. 16. Public safety remains our top priority as our contractor moves forward with their work.
How viable now is the plan to get Bertha restarted?
We’re disappointed with STP’s progress to date, and we can’t guarantee they will meet their schedule milestones. A preliminary review showed their plan to restart Bertha would likely work, but an expert technical team is awaiting more information from the contractor once the machine is brought to the surface to provide us with feedback on the plan.
How much more excavation is needed for the access pit to be completed?
The pit, which will be 120 feet deep when complete, is currently 90 feet deep.
What is the schedule for resuming tunneling?
STP’s initial repair plan showed that mining would resume in March, but we are doubtful that they will meet that milestone. The timeline will become clearer after the tunneling machine has been fully examined by the manufacturer and the contractor submits a full list of necessary repairs.
What are the other options, if the 120-foot pit cannot be dug without destabilizing the viaduct?
There are other ways for STP to access the machine, both through the pit and through the tunnel. At its core, this is an engineering problem, one that can no doubt be solved. The current plan was simply chosen by STP for reasons of cost and expediency. If they change course, deciding the best path forward — and assuming the risk associated with that choice — will be up to them.
What is the state’s financial exposure in this tunnel contract?
The answer to this question goes back to the way this contract is structured. Traditional design-bid-build contracts leave design up to the owner of the project – in this case WSDOT – but design-build contracts leave final design and construction up to the contractor. This gives the contractor greater opportunities for reward if things go well, but it also requires them to take on a greater share of the risk. As a result, the contract is perhaps the most important tool we have on this project. It can’t bore a tunnel or build the highway within it, but it can perform the project’s most vital function: setting the terms for completing the project safely while protecting taxpayers’ interests.
Protecting infrastructure in Pioneer Square
What kind of settlement has WSDOT seen near the access pit and in Pioneer Square?
We installed a state-of-the-art settlement monitoring system as part of the SR 99 Tunnel Project. Recently, Seattle Tunnel Partners surveyors detected over an inch of ground settlement near the pit crews are building to access and repair the SR 99 tunneling machine. We have also seen similar settlement on the Alaskan Way Viaduct; the amount of settlement lessens in the surrounding area.
Have buildings been damaged in Pioneer Square?
We have identified more than 30 buildings that warrant further investigation. We are working with property owners and managers to complete interior visual inspections of these buildings. These inspections are being completed by a historic architect and include a survey of the building to look for impacts to significant architectural elements or other indications of damage. A number of these surveys have already been completed and the project team continues to schedule surveys as needed. So far, our team has seen a small amount of cosmetic cracking in masonry and drywall but no indications of structural damage.
How would settlement affect my building?
You may notice new cracks or changes to existing cracks in your building. You may also notice doors or windows that are sticking or do not open properly. If the building settles more than the ground outside, utility services also could be disrupted. If you notice such changes in your building, please keep a list or take photos, and follow the contact information below.
What should I do if I see new damage in my building?
If you are a resident or tenant, please contact your property manager or building owner. Building owners should contact the Alaskan Way Viaduct Replacement Program (1-888-AWV-LINE or email@example.com) to report any settlement-related concerns. If you have photos, please share them with our staff.
How do I file a claim if I have building damage?
If you feel your building has been damaged as a result of ground settlement, you can file a claim with WSDOT. Please contact 1-888-AWV-LINE or firstname.lastname@example.org
for instructions on filing a claim and to speak with project staff.
What is WSDOT doing to protect buildings that may be affected by settlement?
We implemented a comprehensive program to monitor ground movement during SR 99 tunnel construction. Crews are currently analyzing data from this system and conducting extensive surveys near the work zone. If damage occurs to buildings or infrastructure as a result of tunnel construction, we will be responsible for costs associated with repairs.
Can WSDOT expand the monitoring program to include more buildings?
Our monitoring system was designed to measure ground movement during tunneling. As part of that system, monitors were installed both in the ground and on structures. The most significant settlement has occurred in areas that include both ground and building monitors. However, some recent settlement has occurred in areas that are not actively monitored. As a result, we do not have specific settlement data for some of these buildings.
Structural engineers and surveyors are inspecting buildings and infrastructure in all areas where settlement was detected. Decisions about the scope of the monitoring program will be made after that analysis is completed.
Does WSDOT have a plan if settlement continues?
Protecting people and infrastructure is our top priority. Protocols are in place to address a range of potential issues that could arise as a result of ground settlement. WSDOT and STP are prepared to shut off the dewatering system if it becomes necessary.