Fifteen Mile Creek Bridge No. 493C Replacement Project

Why King County is replacing this bridge

We are replacing the Fifteen Mile Creek Bridge because it is past its useful life. We have repaired this bridge many times over the years, and now it is time to replace it. The new bridge will be designed to meet current national bridge standards. These standards include weight capacity, lane and shoulder widths, and environmental protection.

Improves safety by:

• Adding standard 8-foot shoulders on each side of the bridge for people walking, biking, and rolling.
• Increasing the height of the bridge railings by 14 inches, enhancing safety in the event of a collision.
• Raising the height of the bridge over the creek, providing more room for water to flow underneath during high-water events.

Environmental benefits:

• The existing wooden bridge support piles are treated with creosote, which can be harmful to fish and the environment. This project removes the creosote support piles from the stream and replaces them with an out-of-water steel and concrete support system.

Maintains reliable routes for the community:

• The Fifteen Mile Creek Bridge replacement project is a long-term investment in route reliability, maintaining the connections between Issaquah, Maple Valley, and Renton. In 2009 and 2010, we also replaced two other bridges on the route connecting SR 900 and Issaquah-Hobart Road SE.

Getting through the construction zone

Work is scheduled to begin in summer 2024 to avoid the rainy season and minimize impacts on school bus service during the school year. We studied the option of building a temporary bridge during construction, but our studies showed that a temporary bridge would double construction time, be more expensive, and increase impacts on the environment, immediate neighbors, and the traveling public.

The existing bridge will be closed for 10-12 weeks during the summer months of construction, and a temporary signed detour route will be in place. The detour will add approximately ten (10) minutes to travel time.

Temporary detour route

The general traffic detour route will only be active during summer 2024 when the bridge is fully closed.

Alternate truck routes

Trucks are not allowed on roads in downtown Issaquah, including SE Newport Way, so they must use alternate routes such as State Route 169, State Route 18, Interstate 90, Cedar Grove Road SE, and SE 128th Street.

Project schedule

King County is in the design process for a replacement bridge, with final design, right-of-way acquisition and environmental permitting expected to be completed in 2023 and construction projected to begin in 2024.

Frequently asked questions

1. What will the new bridge look like?

The new bridge will be 38 feet wide, with two 11-foot-wide travel lanes and two 8-foot-wide shoulders. The span (length) of the new bridge will be 65-feet-long. The longer span will allow the new concrete bridge footings to be located outside of the stream channel. The existing bridge is 27-feet-wide and 38-feet-long.

2. Will the new bridge be in the same location?

The new bridge will be built in the same location as the existing bridge. This project will not change anything about the location of the road or the bridge.

3. Will there be weight restrictions on the new bridge?

No, there will be no weight restrictions. The new bridge will be designed to meet current bridge standards. The bridge will be open to all vehicles, including cars, trucks, and emergency vehicles, as well as to people biking, walking, and rolling.

4. Why are there no weight restrictions on the current bridge?

Road Services installed temporary support beams called "stringers" under this bridge. This temporary repair prevented the need to put a weight restriction on the bridge until a new replacement bridge is in place.

5. Why close the road for bridge construction during the summer only?

We are closing the bridge for construction during the summer for three main reasons. First, there is more daylight, temperatures are warmer, and there is less rain, which allows our contractor to get more work done during the day. Second, this time of year is also known as the fish window, which is the period when it is safe to perform in-water construction work without disrupting salmon migration. Third, closing the road during the summer allows us to keep the road open during the school year, which helps buses and people driving children to school avoid detour delays.

6. What is King County doing to reduce the length of the construction closure and minimize impacts to the public?

We know the full roadway closure is very inconvenient to travelers and neighbors and we are doing everything we can to speed up construction. The project team designed the project with several innovative elements to minimize the time that the bridge needs to be closed. We will build part of the bridge offsite, use specialized concrete that cures more quickly, and coordinate with the construction contractor to complete the work as quickly as possible.

7. Will people be able to walk or bike across Fifteen Mile Creek during construction?

No, people walking, riding a bicycle or other non-motorized vehicle will not be able to cross the creek during the full road closure.

8. How will the creek and surrounding environment be protected during construction?

The contractor will protect the creek during construction by routing it through a temporary bypass pipe. Routing the creek through a bypass pipe will prevent construction-related materials from being released into the water. The contractor will also install barriers to prevent demolition materials from entering the creek and surrounding area.

9. How is the project design reducing the risk of flooding?

While flooding has never over-topped onto the roadway, we are preparing for the possibility of higher water events due to climate change.

The new bridge will raise the bridge deck higher from the stream bed than the existing bridge. Raising the bridge deck creates more space for the water and floating debris to move freely under the bridge, even during high-water events. The higher bridge deck also will reduce the risk of flooding onto the roadway and provide a larger opening for floodwaters to pass through, which will reduce flood levels along the stream.

The City of Issaquah is planning to reinstall their flood monitoring device on the new bridge. The flood sensor is part of a citywide network that helps to indicate where water levels are high and predict flooding during heavy rain or storms.

Photos