North Shore Drive Roadway Repair

Project Description

This project intends to repair and restore this portion of North Shore Drive to the original paved width and stabilize the adjacent lakeside slopes to prevent continued erosion and slope failures. This section of North Shore Drive is directly adjacent to Lake Cavanaugh. The portion of the road has been experiencing slope stability issues over many years, but more recently, in 2019, significant failures became evident.

The roadway and adjacent slopes are failing and have been compromised in several locations between Milepost 2.050 and Milepost 2.145; an approximate length of 500-feet.

There are several factors that are attributing to the slope and subgrade failures:

Lack of drainage facilities. This has encouraged seepage and erosion of the roadway subgrade from the hillside;

Wave action from increased range of lake level fluctuations have undermined the roadway subgrade from the lake side.

Original road construction: Constructed in the early 1900’s as part of a logging operation. Materials used for the roadbed, combined with the compaction and construction methods used at the time were intended to facilitate logging operations for a short duration. The road was not intended to serve as a permanent access for today’s passenger and service vehicles.
Maintenance activities and repair efforts have been effective in extending the life of this portion of roadway. Unfortunately, the severity and size of these failures have exceeded the scope and ability of our Maintenance Department to rectify and repair.
Portions of the roadway have been barricaded off and reduced to one travel lane with two-way operation.

Current Work

Public Works staff has contracted with a consultant to perform a geotechnical assessment of the roadway. This assessment will inform Skagit County as to the best strategy to repair the road.

Why is North Shore Drive eroding?

North Shore drive is caused by the following reasons, and not for lack of maintenance. No amount of maintenance could fix the root cause of the problem.

2018 Geotech Investigation (Report)

Causes:

Poorly compacted embankment fill used in the original construction. (Original construction utilized cut/blast and fill placed on sloping bedrock surfaces. Road fill materials were not keyed or benched into bedrock. Fill used was of low-density materials which would not have meet today’s specifications – we know that because of boring holes performed by Geotech)
Undercutting/overhang (fluctuated water levels, increase use of speed boat, or storm events have caused moderate to severe erosion at the base of slope directly above the lake, which leads to over steepened and undercut road bank. Tree roots and other vegetation are the primary containment thus far to retain soil, and slow down erosion.
Water seepage to roadbed: lack of catch basins, and ditches during original construction along with seasonal hillside drainage and standing water seeps beneath the road, and saturate subgrade, which exe contributes to continuous erosion.
Surcharge loading: large trees on the downslope, along with repeated road repairs (gravel, and asphalt) to extend the life of the road, add to surcharge loading, which leads to the eventual needs of road replacement.

What can Skagit County do now?

Maintenance activities

Crack-sealing
Drainage maintenance
Rocks placing along lakeside: Though it appears rocks can be physically placed along lakeside, such erosion control techniques cannot be used nor permitted at this site. The area along the shore is high potential habitat for salmonids so placing rock is actually pretty impactful to them.

According to the 2018 Geotech report, potential exits that new construction could initiate either new or historic landslides from above the road.

Reconstruction of the road with retaining walls along the lakeside is the only option to restore vehicular access. Construction costs range from $3 million to $7.5million (2021 dollars).

What funding can be used for the repair?

Grant fund opportunity for this road is limited due to the remoteness and a relatively low AADT (Annual Average Daily Traffic) number.

Residents/lake users can help slow down erosion rate by minimizing wave energy action by reducing boat speed or limiting/restricting the use of power boats.

Forming a Local Improvement District (LID) would help fund the reconstruction of this road.

Project Timeline and History

Fall of 2018, Initial Geotechnical investigations were completed.

June 2019, an Engineering Consultant was hired to provide design solution alternatives. Conceptual design alternatives were presented to Skagit County for internal review and permitting agencies for informal consultation. The conceptual design proposed did not garner support internally nor from permitting agencies.

Summer 2019, Skagit County Public Works performed maintenance activities to prevent continued slope failures.

2019 to 2020: Failed areas continued to grow. Skagit County Public Works continued to provide maintenance activities in order to mitigate and slow the continued failure of the slope and roadway edge, while simultaneously working with another consultant to evaluate the expanded failures and develop appropriate design alternatives.

2020 to present: Slope and road conditions continued to quickly degrade. The extent of failures triggered the need for significant revision to the design and it became evident that the extent and scope of repairs had exceeded what was originally programmed. Revised design alternatives were presented internally, and held another informal consultation with WDFW on 6/24/21.

July 2021: Skagit County Public Works has gathered enough information and would like to communicate these findings and options to the community and general public via Public Open House to be held in a nearby location.

August 20th, 2021: Open House at the Lake Cavanaugh Community Hall, 5:30 p.m. to 6:30 p.m.
,
September 14, 2021: Work session with the Skagit County Commissioners, 2:00 p.m. - 3:00 p.m.