Issaquah staff recommends ground-anchor wall and keep center turn lane for Black Nugget Road

The Mobility and Infrastructure Committee on Feb. 11 reviewed conceptual design alternatives for the Black Nugget Road retaining-wall replacement and the roadway immediately adjacent to it and the administration recommended a ground-anchor wall (Option 3) and retaining the center turn lane (roadway Option 1).

Jim Jacoby, Senior Transportation Engineer in Public Works, presented three wall concepts and three roadway cross-section options. The wall alternatives were: (1) a new soldier-pile wall constructed in front of the existing soldier-pile wall, (2) a new soldier-pile wall constructed behind the existing wall (requiring a large berm and likely full road closure for staging), and (3) a ground-anchor/shotcrete-type wall installed in front of the existing wall with smaller surface impact. Jacoby said the project team expects all three approaches to provide long-term structural life and estimated useful life at 50-plus years.

Jacoby presented construction and traffic impacts and rough 2027-inflated cost estimates (including 20% contingency): Option 1 about $7.8 million, Option 2 about $9.7 million, and Option 3 about $6.2 million. The city’s Capital Improvement Program currently lists roughly $5.5 million for construction, financed with debt that is not yet secured.

For roadway cross-sections staff presented three choices: keep the center turn lane and retain two 12-foot travel lanes with narrowed lane widths (preferred by staff); remove the center turn lane and add bike lanes; or remove the center turn lane and provide bike lane downhill with a planter strip on the downhill side. Jacoby said retaining the center-turn lane best preserves safety on the steep, winding approaches to the wall and helps maintain traffic level of service through the commercial corridor between Fred Meyer and Home Depot.

On the roadway decision, Jacoby said: “The Administration is recommending that we proceed with wall alternative number 3 and roadway option number 1, the ground anchor wall and retaining center turn lane.” Committee members supported the recommendation, citing safety concerns for drivers entering driveways and businesses in the commercial area, and the community outreach that indicated preference for retaining the turn lane. Deputy Council President Barbara DeMichele described an experience entering the roadway in dark, rainy conditions and said the center turn lane was “an absolute safety feature of that road.”

Committee members asked about construction staging and the extent of traffic disruption. Jacoby said Option 2 (building the new wall behind the existing wall) would require the most extensive staging, likely a full or extended road closure because of the need to bring construction equipment up the slope; Option 1 and Option 3 could allow portions of the road to remain open during work. The administration noted that constructing a new wall behind the existing one (Option 2) would create more right-of-way but would add substantially higher cost and construction impacts; staff also said gaining more than the two feet afforded by Option 2 would require additional right-of-way acquisitions and thus materially more cost.

Jacoby said aesthetic treatments for the wall face will be developed with future design phases; options include stamped or textured concrete, murals or public art, or other architecturally treated finishes.

Next steps: the administration seeks committee concurrence to proceed with the preferred design alternatives, advance to 30% design by summer 2025, continue community outreach during design, and pursue funding and final design for construction in 2026–2027. Committee members expressed support for the administration’s recommended wall and roadway options.