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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."
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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 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.
H&D, a division of Reith-Riley
Construction Co., Inc., of Bay Shore, Michigan installed
the walls. The five-man H&D crew consisted of an
operator for the backhoe, an operator for the skidsteer,
two laborers and a foreman. ‘’The biggest challenge for
this project was accessibility. Unfortunately, we were
on the railroad grade and the entire grade was only 13
ft. wide before it dropped off into the lake. We had to
stage everything quite a ways back and bring the blocks
in one pallet at a time,’’ said H&D foreman Mark Wagner.
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The crew had to install
extensive drainage media behind the wall to accommodate
the groundwater flow and ensure the structural integrity
of the wall. |
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.
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Wagner explained that his crew
brought the blocks down one pallet at a time and placed
them using a backhoe and the Rosetta lifting device. Due
to the massive size of each block, installation
progressed quickly.
To prepare the base, the foundation
soil was compacted and the crew installed a four-inch
perforated sock drain pipe at the back of the excavated
trench. The drain was daylighted below and away from the
wall. The crew then installed a stone leveling pad.
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.
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.
Because the bike path is situated
20-30 ft. above the beach, lake access required a
staircase. The city chose to use Rosetta’s line of
‘’irregular’’ steps because the steps have a natural
stone shape and look, but have the height consistency of
a manufactured product. The consistent rise of each step
allows for fast installation and also safe access for
pedestrians. |
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.
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