Canyon Springs

Canyon Springs is located six miles east of Snoqualmie. The spring is located in a deep canyon on the north hillside bank of the North Fork of the Snoqualmie River, approximately 60 feet above the riverbed, and is located on 20 acres owned by the City. The adjacent forestland is owned by THR LLC. Access to the springs is limited along a trail that parallels the transmission main from the springs. The water line and collectors were constructed around 1950 at the base of the hillside bank, a steep and geologically unstable slope. The spring collectors and water line were upgraded in 1985 and have been functioning normally since that time.

November 2017 Landslide and Emergency Response Project

In mid-November 2017, a landslide occurred at Canyon Springs that could cause loss of the pipeline if slope stabilization efforts are not completed soon and potentially impact the supply of drinking water to residents and businesses. Furthermore, long-term solutions are needed to preserve the Canyon Springs facility as a reliable source of drinking water as slides and slope failures over the current pipe alignment are inevitable in the present condition. In the event of a failure to the Canyon Springs line the North Wellfield and South Wellfield will be used to fulfill Snoqualmie’s drinking water requirements. However, water conservation measures may become necessary during higher demand periods such as summer.

Following discovery of the landslide in mid-November 2017, public works staff were initially concerned, but uncertain about the level of threat to the water supply pipe system that conveys drinking water from the spring heads to the city. Engineering staff were alerted in early December 2017 and quickly engaged Aspect Consulting (Aspect) to get an immediate assessment on the stability of the slope. Aspect currently provides services to the city and are known by engineering staff for geotechnical capabilities in the assessment of landslide and slope stability issues and related geotechnical engineering skills.

Simultaneously, staff began research on the existing water system and site geologic conditions to prepare an initial assessment presentation for executive staff and the City Council. Executive staff determined sufficient cause to declare an emergency and seek authorization by council to allocate funding for initial engineering assessments on a non-competitive basis (Resolution No. 1433). Council authorized the allocation of funds (AB#18-019) on January 8, 2018.

Following field explorations, geologic research activities and meeting with contractors, Aspect submitted their initial assessment report in a memorandum, which concludes that the recent slope failure presents an immediate high-risk concern for function of the water supply system. Slow raveling of the over-steepened embankment will continue if left unchecked and likely result in failure of the water line, and that a program to address the stability of the slope should be implemented immediately.

Additionally, the memorandum provides a list of feasible alternatives for slope repair and a brief impact assessment of each option. Aspect recommends the application of Anchored Mesh Support of the Upper Slope as the preferred alternative:

“In our preliminary assessment, the anchored mesh alternative is the preferred option for repair of the November 2017 slope failure area as it requires the least excavation and the system can be installed with small equipment. We anticipate that it will effectively stop retreat of the area of upper slope treatment, and buttress the pipeline and path for many decades at modest cost. An expected design life would be on the order of 50 years.”

Aspect will require additional scope and budget to acquire survey, conduct targeted field exploration and data collection activities, perform geotechnical analyses and design, prepare plans and specifications, provide support through bidding, and provide construction management and inspection of the work. Aspect submitted a scope and budget proposal.

Additional funds will be required for the construction effort. A preliminary cost estimate assumes a conservative area of treatment, materials selection, access and equipment mobilization issues, and possible site Civil improvements for work staging and security at approximately $1 million to complete the construction work.

  1. Todd Saxberg

    Utilities Operations Manager
    Phone: 425-888-8011

  2. Jeff Hamlin

    Project Engineer
    Phone: 425-831-4919

Timeline


Mid-November 2017:
Landslide; potential impact to the water line and collectors

January 8, 2018:                    
Emergency resolution authorizing selection of engineers to quickly stabilize the slope near the water line approved by City Council

February 2018 – May 2018:
Field and data collection, design analyses, permitting, property/access coordination, preparation of plans and specifications.

June 2018:
Invitation to Bid

July 2018 (estimated):
Contractor selection and approval

August 2018 – October 2018 (estimated):
Construction