October has flown by, and so has a big project at Black Diamond Open Space (BDOS)!
King County Parks replaced a bridge that crosses Ravensdale Creek in the western portion of BDOS, a park that is located two miles north of Black Diamond and six miles south of Maple Valley.
The design of the new bridge provides elevated views of the creek and improves habitat by providing more clearance for large, woody debris to move downstream. It also can be used by multiple user groups, including equestrians, mountain bikers, and hikers. An important feature connecting all the 17 miles of trails across BDOS as more and more people enjoy this amazing place!
So how do you replace a bridge deep within an extensive backcountry trail system? With the help of a Black Hawk helicopter and a great team of people!
The helicopter used to deliver the materials to the bridge site is a Sikorsky UH-60 with a maximum external load of 8,000 lbs. It is commonly called a Black Hawk.
The bridge replacement required four “fly days” throughout the month of October 2022 with the helicopter which closed the BDOS parking lot and the west side of the park. This was to ensure park users remained safe during delivery of heavy materials, including bridge parts, to the bridge site.
At the start of each fly day, the crew held a safety meeting and talked about the materials being delivered by the helicopter along with the work on the ground. The crew consisted of 20+ people from King County Parks, GLY, and NW Helicopters. In addition to the safety meetings, King County Sheriff Deputies helped with traffic control and provide a safety perimeter around the landing area for the helicopter.
To construct the bridge, crews assembled the steel girders in the parking lot before hooking them up to the helicopter. The girders are held together by nuts that are specifically engineered to support bridges. In addition, crews used this Turn of the Nut tool, which tightens each nut to exactly the right spot for structural integrity.
Then everything needed to be delivered to the bridge site! To construct the bridge in the heart of the trail, the bridge alone was delivered to the project site in 10 pieces. There were two concrete footings, three girders, and five bundles of decking and railings. Many additional flights brought in equipment (including two excavators!) and materials, particularly gravel to construct the approach on each side of the new bridge.
First the gravel was laid out for the approach for the bridge, followed by the installation of the two pre-cast concrete footings. Each footing weighs about two tons.
Then the crew installed the girders, railings and decking.
Pack it in, pack it out! In true trail stewardship fashion, anything not staying as part of the bridge was then carried back out by the crew and helicopter.
The new bridge (66.5 feet long) is 14.5 feet longer than the replaced bridge (52 feet long). We hope when you go out to BDOS, you’ll walk/ride across it and appreciate the engineering and people who made this bridge possible.