Sammamish council reviews Sahali Way corridor study, weighs two high-cost alternatives and slope mitigation

The Sammamish City Council heard technical findings and cost estimates Feb. 11 for the Sahali Way corridor study, a master-planning effort that covers the roadway from Northeast Eighth Street south to State Route 202 and will be carried to public engagement this spring.

The city’s project team and consultants described three geotechnical zones with past landslides and seven distinct slide scars along the corridor, recommended monitoring and targeted instrumentation, and asked the council whether to advance alternative 7 or alternative 8 as one of the public-facing options. “This is what we're asking council for. Do you like alternative 7 or 8?” said Jed Ireland, senior project engineer and the project manager for the Department of Public Works.

Why it matters: the corridor plan will set a long-term framework for walking, biking and traffic safety and guide later design and construction. It also intersects urgent stability issues: consultants said some slope areas are close to the existing fog line and will probably require mitigation within a 10–25 year horizon even if the city takes no immediate capital action.

Geotechnical findings and monitoring

Donald Huling of HWA Geosciences, the project geotechnical lead, divided the corridor into three geotechnical zones. He said the northern portion includes slides that were mitigated in the past using horizontal drains and that several slide scars date to recent decades. For deeper, previously active slides north of NE 30th, HWA’s analysis found static factors of safety near acceptable levels but predicted measurable movement under a design seismic event. “If those drains stop functioning, then water's gonna build up and you'll start to see movement again under regular static loading conditions,” Huling said.

Huling said the team is installing slope inclinometers at priority locations to provide early warning and described structural options — tangent or secant pile walls — for the steepest sites. He recommended a “do no harm” approach in areas with previous mitigations: avoid solutions that add conventional fill or extra permanent load unless offset by lightweight fill or other techniques.

Cost and funding

City and consultant staff provided a high-level planning estimate for slope reinforcement in the three most critical zones south of NE 30th: roughly $15 million to $20 million. Brent Powell, PERTIT project manager, told council the estimate is preliminary and that no large dedicated funding source has been identified. He noted the state and federal funding programs the team has reviewed, including FEMA, are typically focused on sudden, life‑safety failures and may not be competitive for the slower, progressive slope problems described here.

Alternatives and policy trade-offs

Project consultants narrowed many initial concepts to a short list of public-facing options. The primary difference between alternative 7 and alternative 8 is the posted speed north of NE 205th: alternative 7 would retain a 45 mph limit with barrier-separated shared-use paths in the north segment; alternative 8 would reduce the posted speed to 35 mph and rely on roundabouts and intersection treatments to produce the slower, calmer conditions. Janessa Donato, Pertit engineer, said both alternatives meet the city’s multimodal level-of-traffic-stress targets but drive different costs and travel-time effects.

The corridor planning-level cost estimate for the full north segment (NE 30th to NE 8th) was roughly $60.8 million for alternative 7 versus $82.7 million for alternative 8 (both figures exclude the separate slope-mitigation line). Council members signaled interest in a three-option public outreach package: (1) a low-cost spot-treatment alternative (2), (2) a mid-range alternative (3B) with continuous sidewalks and shoulders, and (3) either alternative 7 or 8 as the higher-cost “transformational” option. Staff said they will take one of those high-end alternatives to public outreach in April and return with public feedback and a detailed risk analysis in June.

Council direction and next steps

Staff will: present three public-facing alternatives at an April community open house; deliver an enhanced risk matrix and updated cost/phasing analysis to council in June; and aim for corridor plan adoption this fall (tentatively October), which would authorize about 30% design work. The consultants emphasized phasing and coordination with utilities, noting, for example, work already coordinated with a local water district’s replacement program to avoid repeat relocations.

What the study does not decide

Council was not asked to adopt a preferred alternative at the Feb. 11 study session. The slope‑reinforcement cost estimate is preliminary; funding sources remain unconfirmed; and the timing of any mitigation (before, during, or after corridor upgrades) will be evaluated with the risk analysis and public input.

Ending

Staff and consultants will present alternative materials and maps at public outreach planned for April, collect feedback, refine costs and phasing, and return to council in June with the risk analysis and a recommended path for a preferred alternative and funding/phasing options.