Rebuilding SR 530 - Construction, winter weather and the Knoll

What is the loose soil I’m seeing on the knoll?

  • What people are seeing shifting is minor sloughing of fresh material added to the surface of the knoll for planting. This is an intermediate situation before plant roots take hold. It does not indicate any problems with the stability of the entire knoll.
  • Contractor crews added about a foot of compost and amended soil as the top layer of soil to prepare for landscaping.

How crews prepare the knoll surface for landscaping
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  • Starting the week of Nov. 17, 2014, crews plan to begin planting some 6,500 native trees and shrubs, including cedars and other native plants, on the knoll. This work is expected to last about six weeks. Contractor crews originally planned to begin planting during the week of Nov. 10, but were forced to postpone this work because of freezing ground temperatures.
  • Surface slough is a superficial and minor situation that crews often fight when amending and planting freshly cut slopes.
  • Minor surface sloughing may continue in the coming months, even after planting is complete. The “firming” up of the surface will be a yearlong process with routine maintenance. The landscaper for this project is under contract for three additional years of plant establishment.
  • The shifting of this top layer of soil is not indicative of a threat to the integrity of the entire hillside. There is no threat to the road or the memorial site.
What did contractor crews change at the knoll during this project?
  • The knoll area was significantly steeper before the project started. Over the summer, crews dug out a significant portion of the slope to make it less steep. They also added new drainage features to prevent water from collecting inside the hillside and reducing its stability.
  • Over eight weeks this summer, crews removed 75,000 yards of material from the existing knoll.
  • This work was necessary in part because the highway needed to shift 7 to 10 feet farther south than its previous location.
  • Prior to construction, the knoll slope was inclined at between 0.75H:1V and 1.5H:1V. The slope was flattened to 2H:1V. A 2:1 slope means the elevation of the hillside drops one foot for every two horizontal feet.
    • Stated differently, the knoll was once as steep as 50 degrees or greater at certain locations alongside the road. This slope is now less than 27 degrees in all locations.

Sample cross Section A - SR 530 Knoll
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Sample cross Section B - SR 530 Knoll
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  • Crews also installed 25 horizontal slope drains that extend 150 feet into the knoll.
    • The purpose of these drains is to prevent water from building up on the clay layers of the knoll and getting trapped there, threatening the global stability of the hillside.
  • The new drains ensure that rain and groundwater are able to drain out of the hillside.

Knoll area overhead view
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  • Aside from the knoll’s increased stability, an added benefit of the knoll being less steep is that it provides better sight distance for drivers than before.
  • The final slope is a significant improvement to the pre-construction condition.
Why didn’t contractor crews build a retaining wall at the knoll area?
  • A 2:1 slope is considered by engineers to be equally stable as a retaining wall.
  • A retaining wall would add maintenance concerns as well as reduced sight distance.
  • Even with a retaining wall, the freshly cut slope above the retaining wall would encounter the same planting and surface slough challenges.
What type of analysis did the project team conduct for the knoll area?
  • Atkinson Construction hired Geoengineers, Inc., an earth science and technology firm headquartered in Seattle, Washington, to complete the slope stability evaluation, analysis and design for the knoll or "Knob Hill" area.
  • Prior to construction, the team completed six geotechnical borings to evaluate existing soil and geologic conditions for the design.
  • They also installed vibrating wire piezometers to measure and evaluate groundwater conditions.
  • Our slope stability analyses were completed using limit equilibrium methods using the commercially available computer program SLOPE/W distributed by Geo-Slope International. The analyses were completed for both circular and block failure surfaces. The methodology inherently incorporates infinite slope stability surfaces in the analysis.
What was the factor of safety for the slope?
  • The minimum factor of safety calculated for the slope with the horizontal drains is 1.5, which is well above the required safety factor of 1.25.
  • WSDOT's Geotechnical Design Manual and national American Association State Highways Transportation Officials design standards require the 1.25 factor of safety.
How is WSDOT protecting other slopes in the project area?
  • Crews installed large rocks along the new north slope of the roadway to promote good drainage and prevent soil from washing away.
  • Crews will plant a total of more than 30,000 trees, shrubs and other small plants throughout the project area in November and December. When these plants take root, they will also help prevent erosion in other areas.
How did this project protect SR 530 from potential flooding?
  • Before we started designing and building the new roadway, we knew that the North Fork of the Stillaguamish River had changed course as a result of the landslide.
  • Our SR 530 reconstruction plan accounted for potential flooding in the area.
  • To protect against potential flooding, our contractor crews built the new road higher than it was before by four to six feet on the east side and 15 to 20 feet on the west side
Did WSDOT consult with locals when designing the SR 530 reconstruction project?
  • Members of the project design and construction team walked the site with the Department of Fish and Wildlife and local representatives from both the Tulalip and Stillaguamish tribes to gather information about existing conditions. They walked each stream, including those by the knoll, for thousands of feet and used information collected to inform their drainage and culvert designs.
  • The project team has been in close contact with the property owners immediately adjacent to the knoll throughout the length of the project.
  • WSDOT hosted a number of family and community meetings in April and May to hear concerns and priorities regarding the future of SR 530. At public community meetings in Arlington, Oso and Darrington in mid-June, prior to starting work at the knoll area, Atkinson Construction representatives explained the project, showed visuals of the design concepts and answered questions from audience members. They also hosted three more meetings at the same locations in September, offering the chance for follow-up questions and comments from the community.