December 3, 2013, update.
Mill Creek has made progress in selecting a consulting firm to design the fix for 35th Avenue’s flooding problem at Penny Creek.
On December 3, 2013, the City announced that they have selected the finalist in the Request for Qualifications process that began in September. The announcement said, “The engineering firm of KPFF in Seattle was selected as the finalist, and negotiations are underway for the contract scope of work and fee, which will be taken to City Council for approval in January 2014.”
The Mill Creek City Council unanimously approved the Request for Qualifications process on September 3, 2013, as the first step in reconstructing 35th Avenue SE.
The negotiations with KPFF will result in a scope of work for the complete design of the new roadway, which can then be used as a bid document for the construction phase of the project. The initial estimate for the roadway design was $250,000 to $400,000.
State Senator Steve Hobbs has included funding for Mill Creek’s 35th Avenue SE reconstruction project as a line item in the comprehensive transportation plan now being negotiated in a special legislative session. 35th Avenue SE is a major north-south transportation corridor on which many Boeing commuters depend.
The City of Mill Creek has been forced to consider a multi-million dollar project to reconstruct 35th Avenue SE to alleviate flooding.
Significant flooding on 35th Avenue SE between 144th Street SE and 141st Street SE periodically forces Mill Creek to close the road and divert traffic through neighborhoods. This winter’s continuous heavy rains caused the water level to rise two or three feet above the roadway at this location. The road was closed for several days.
Mill Creek’s Public Works Department has been monitoring the roadway for settling since 2009. Shannon & Wilson, the City’s on-call geotechnical consultant, was hired early in 2013 to perform a study of the settlement data, Snohomish County records, and field observations. The goal of the study was to provide options for rebuilding the roadway to alleviate the flooding.
Tom Gathmann, Mill Creek Director of Public Works, presented the geotechnical study to the Mill Creek City Council on March 26, 2013.
According to Gathmann, the flooding is caused by a combination of factors, “One of the biggest areas of settlement is over the Penny Creek crossing between 144th Street SE and 141st Street SE, which also has the deepest deposit of peat material as a base. The water level in the surrounding wetlands has also increased due to several factors such as upstream development and beaver activity.”
The geotechnical report states, “In the area of flooding, the roadway appears to have settled about 2 feet in the past 10 to 12 years since the reconstruction.” Snohomish County performed the “reconstruction” during a 2002-2003 road-widening project.
Until 2005, when the City of Mill Creek annexed 553 acres in the northeast portion of the City, Snohomish County had sole maintenance responsibility for 35th Avenue SE.
In 2002-2003 Snohomish County widened the road to three lanes between Seattle Hill Road and 132nd Street.
According to the geotechnical report the newly widened roadway was designed to utilize lightweight fill material consisting of wood debris and wood chips (hog fuel) on top of the large peat deposit near Thomas Lake to minimize settling.
But at the road’s low point the construction contractor used heavier gravel fill because the groundwater was too high, “When the Contractor reached the low point in the road (about Station 14+82 to 16+80), hog fuel was not placed because the ground water was too high. Gravel borrow was placed in this area and it is in this area the settlement has been the greatest.”
At the time of the 2005 annexation Mill Creek and Snohomish County negotiated an Interlocal Agreement to specifically define 35th Avenue SE maintenance issues and thresholds with proportional cost sharing.
Snohomish County repaved 35th Avenue SE under the Interlocal Agreement in 2006. At this time the road had not settled enough for proportional cost sharing to continue, so Mill Creek took over all maintenance responsibilities.
Road rebuilding options
Shannon & Wilson recommends rebuilding 35th Avenue SE between 144th Street SE and 141st Street SE to raise the road profile by three feet. They analyzed three construction alternatives designed to minimize future settlement.
At the March 26th City Council meeting Gathmann summarized the construction alternatives as follows:
- Pin Piles - Effectively construct a "bridge" by driving many small piles through the peat to solid ground with a concrete slab on top. Most expensive option but least likely to have future long-term settlement, also minimizes dewatering issues during construction. Total cost: $4.0 million.
- GeoFoam - Dig out heavier existing fill and replace with lightweight structural foam material, topped by a concrete slab. Less expensive than pin piles, but would require excavation below the water surface and a significant dewatering system, which could drive up construction costs. Will still have future differential settlement on roadway. Total cost: $3.5 million.
- Cellular Concrete - A less expensive lightweight fill option made from a special type of concrete, with similar dewatering and future settlement issues to GeoFoam. Total cost: $3.1 million.
Gathmann stated, "This reconstruction project is not programmed in the 2013-2014 budget. Of the remaining $369,000 balance in the 35th Avenue Repair Fund, only $55,000 is currently budgeted. The budgeted amount was for this preliminary geotechnical study and continued survey monitoring."
"We have known for years that a long term solution would be needed, and that it would be very expensive. We just did not know how much 'very expensive' really was. Given the tight budget situation we have been in the past few years, we are doing this on a step-by-step basis, and were intending to include it in the update of our Capital Facilities Plan."
Gathmann’s recommendations to the City Council were as follows:
- Pursue the pin pile option to elevate the roadway because this option involves less construction risk and has the least amount of future roadway settlement.
- Continue efforts to secure Washington State funding for the design phase of this project.
- Once the project is "shovel ready," pursue opportunities for grants or other funding sources for actual construction.
- If Council directs staff to prioritize work on this project, the portion of 35th Avenue SE between 141st and 144th Streets will be deleted from the 2013 overlay project saving some expense.
By consensus the City Council directed Gathmann to pursue the pin pile option for rebuilding 35th Avenue SE per his recommendations, and to delete the 35th Avenue SE overlay between 141st and 144th Streets SE planned for this year.