What is the situation?
The Monday, Dec. 3, record rainstorm resulted in a collapse in the metal culvert that carries Thunder Hills Creek underneath I-405 in the city of Renton. The collapse blocked much of the 48-inch diameter pipe and created a sinkhole 12 feet from the southbound I-405 shoulder that threatened the stability of the freeway. Crews stabilized the sinkhole, but WSDOT and its contractor had to build a pump and pipeline system to divert the creek temporarily before a big storm created another sinkhole or caused the stream to backup and flood over the freeway. Before the pump and pipe system was completed on Dec. 23, I-405 was vulnerable to closure or collapse. This system will be operational until crews install a new culvert underneath I-405.
What is WSDOT doing to fix this problem?
WSDOT is taking a three-step approach to the culvert replacement:
- We have a pump stationed next to the culvert to divert water from the creek and into a nearby storm drain. The pump is effective with creek flows of up to 10 cubic feet per second (equivalent to one inch of rain in 24 hours).
- Under an emergency contract with Tri-State Construction, crews built a 2,000 foot-long temporary pipeline to divert the creek along the northbound shoulder of I-405, on Talbot Road (SR 515) underneath I-405, and into a wetland.
- Under another contract crews will install a culvert underneath I-405 once the diversion pipeline is in place. Work on that contract will begin in February 2008.
What was the worst case scenario?
If the pumping system was not installed and Puget Sound was hit by another big rain storm, the collapsed culvert would not have been able to handle the excess water. I-405 would have acted as a dam, and water would have pooled under and adjacent to the freeway. As the volume of water increased, it would have had the potential to flood sections of I-405 and erode and possibly collapse the hillside supporting the freeway. To keep drivers safe, WSDOT crews would have been forced to partially or fully close I-405 for an indefinite period of time.
Did this work affect traffic?
During the pipeline installation drivers experienced heavy congestion on the Valley Freeway (SR 167) and Maple Valley Highway (SR 169), SR 181 and I-405 as crews closed a northbound lane on I-405 between 10 a.m. and 7 p.m. The culvert installation, by contrast, will only require minimal lane closures that should not have an effect on traffic.
Why did WSDOT choose to open the lanes for the morning commute?
Our engineers studied data gathered by electronic traffic detection sensors located under the highway. After analyzing this data, they found that the morning commute (between 5 a.m. and 10 a.m.) is more predictable and heavier than the evening commute.
Additionally, many of the alternate routes to I-405 are heavily congested during the morning commute, giving drivers few options. While drivers can still expect heavy congestion during the daytime lane closures, traffic volumes on alternate routes are typically lighter between 10 a.m. and 5 a.m.
Does WSDOT realize that this work came at the height of the holiday shopping season?
We realized this work was an inconvenience to drivers trying to finish the last of their holiday shopping or traveling to visit family. However, I-405 was vulnerable to collapse and indefinite closures if this work was not completed quickly.
Why didn't WSDOT perform this work at night?
Crews were working in steep, unstable terrain that required daylight to work safely.