City Completes "Crown Jewel" Bike Path Using Big Block Retaining Walls

By Lindsey Manthei, Communication Specialist, Rosetta Hardscapes, LLC

The ‘’Resort Bluffs’’ section of the path has seen heavy use since it opened in June of 2009. Many users think the walls are made of natural stone, not precast concrete. Photos Courtesy of Rosetta Hardscapes, LLC

When the City of Petoskey needed to build a 1.3-mile section of asphalt bike path on the edge of a Lake Michigan bluff, they required a system to maintain the structural stability of the slope above and below the path.

Because the trail is along one of the most scenic areas of the route, designers needed a retaining wall that would not only provide a structural solution, but an aesthetic solution as well.

‘’We considered spanning the area with a bridge structure but found it too expensive,’’ said Lucas Porath, P.E. with Northwest Design Group in Petoskey. ‘’We also considered using natural stone, but again, Rosetta was the more economical option.’’

Rosetta Hardscapes Outcropping Collection retaining walls are more economical primarily due to their dimensional consistency, which translates into saved installation time. Where natural stone often requires sorting and cutting to make each piece fit correctly, these large blocks are manufactured from precast concrete. ‘’We were trying to match the natural landscape so we needed a fairly massive product to match the scale of the project,’’ said Al Hansen, director of the City of Petoskey’s Parks and Recreation department.

The crew had to install extensive drainage media behind the wall to accommodate the groundwater flow and ensure the structural integrity of the wall.

The molds for the block system are taken from heavily weathered natural limestone, which gives the blocks the look of natural stone. The blocks come in different shapes and heights, giving walls a random look but keeping design and installation easy. The blocks are also manufactured with integral color; in this case the company’s ‘’Lakeshore Blend’’ pallet was chosen to complement the rock indigenous to the area. ‘’It kind of blends into the actual material that surrounds it; it is irregular and looks more like natural limestone than a regular concrete retaining wall. We’ve used it in several other applications along the path because we like the look,’’ Hansen said.

The design of the lower section of the project called for an 18 ft. tall wall at the highest point, so reinforcement was necessary. The wall was reinforced using both Paraweb strap and geogrid. The walls above the bike path were gravity walls and utilized a cast-in-place concrete backfill design where stability needed to increase.

An 18-ft. tall, reinforced retaining wall supports the heavily used bike path, and a gravity and concrete backfill-reinforced wall holds back the slope above. Natural-looking pre-cast concrete steps provide access to Lake Michigan.

Since hydrostatic pressure is the number one cause of retaining wall failure, installing drains and backfill as specified by the engineer was a critical step in keeping backfill materials dry and structurally sound.

The entire project, including the 5,200 sq. ft. of retaining wall, was completed in about two months. This newest section of the bike path is considered the ‘’crown jewel’’ of the 23 mile path and has seen heavy use from bikers and pedestrians since it was dedicated on June 20, 2009. ‘’The walls have been very well received,’’ Porath said.