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Minnesota Steeps:
Stabilizing a Hillside Secures Scenic Value

By Donald E. Obernolte, RLA

Several tons of Lake Superior gabbro await compaction to form an approach to the project bicycle and pedestrian bridge. The stone was “harvested” and reused on site, minimizing the need for heavy equipment and expensive fuel.
Photos courtesy of Donald E. Obernolte
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A mile-long segment of Minnesota’s Gitchi-Gami State Trail is a special place where land, sky and water meet the people. Adding a trail and other visitor amenities brought erosion-control challenges, but successfully preserved the historic highway section near Duluth.

A simple, 4,000-foot-bike trail turned out to be one of my most challenging projects in decades, one of the best times in my life. The team dealt with rockslides, bench undermining, and dramatic erosion activity.

Close to 60 percent of the original roadbed had already fallen down the cliff. Stabilizing this loss and providing the traveling public access to this remarkable open space was the primary design goal. Looking back, I view the piece as a legacy type of project.

Most of the project area is seen in this aerial view taken in the fall of 2005. The view looks southwest along Highway 61 towards the new Silver Creek Cliff tunnel, which opened in 1994. The main features completed here in 2004 include the overlook parking lot and the adjacent section of the bicycle and pedestrian Gitchi-Gami Trail. The trail parallels the highway, passes over its own bridge (white section) and follows the old road bed around the cliff bottom and out of this view.

Project Specifics

Silver Creek Cliff Overlook is a 4,000 ft. traverse of a 600-ft. vertical cliff face offering continuous vistas of Lake Superior’s north shore to the traveling public. The project is a mitigation measure for the impacts of recent tunnel construction. This area is one of the premier tourist locations in Minnesota. Additionally, the entire site had to meet accessibility standards.

On one side of the trail, Silver Creek Cliff looms 200-plus feet above the narrow trail shelf. On the other side, you have a continuous 4,000-foot vista with high vantage point panoramas more than 200 feet above the largest fresh water lake in the world. Our guiding principle was simple enough: “stay out of the lake and don’t climb the cliff”—although that is where simple ended.

By the 1940s, the Silver Creek Cliff gate was just a memory. Landscape architect Don Obernolte and staff replaced it in 2004 with this faithful reproduction, located about 2,000 ft. from the tunnel entrance.

Highway 61 around Silver Creek Cliff opened in 1924. The Gitchi-Gami State Trail, (a trail that will eventually continue from Two Harbors to Grand Marais) runs parallel to the highway, which is both an All American Road and a Scenic Byway. We knew we wanted to incorporate historical highway design elements into the trail project. For example, the use of boulder guardrail, dome-topped guardrail, the historic overlook plaque, interpretive signage and restoring the gateway all hearken back to that earlier era.

Technically, the project is considered mitigation for the tunnel project, but it was cut out of the tunnel job as “taking too long and being too complicated” for doing with the tunnel.

An old postcard shows the same section of highway in the 1920s when it was unpaved. This section of roadway north of Duluth has long been renowned for its scenery and was then marked by this whitewahsed-pole gate.

The survey itself was difficult in that no single beacon, or control point, could be used to survey the whole site. Also, there is a segment (about 250 feet) where no data can be collected. Couple that with unusual magnetism in the geology, and a sharply-tipped county coordinate plane and we’re off to work under “field engineering circumstances.”

Horizontal control was also difficult, in that a wedge of property up to 27-ft. wide had to be removed from what we thought was right-of-way. These were issues from early in the plan process and well on into construction.

Nevertheless, we held onto our accessible gradient and kept earthwork change orders to a minimum.

Plotting Erosion Zones

GEOPAK Site soil engineering software was instrumental in overcoming many of these challenges. Because of the techniques that we used for gathering data, we also did some detail mapping of rockslide history. We located the break lines, where the bench in-slope erosion cascades down the lower portion of the cliff to the beach. The path and drainage alignments were coursed through the space between these outer limits—of recent rockslides and the line where the bench erosion drops to the lake.

We were faithful to the original highway alignment, though much (60 percent) of the original roadbed has eroded away. We did not cross the erosion break line to the lake and we oversized our swales to catch rockslides from above.

Smooth-bore, chrome-plated pipe was used for all culvert drains. This drainage entrance is surrounded by rip-rap, biolog and compost that has been impregnated with seed mix.

There is no instability going from trail to lake slope and most stone gets trapped in the swales. We have had two boulders make it onto the path in two years. Despite this instability, a (helmeted) climbing group does some regular scaling of loose and weathered rock in this area.

Poor soils composed of rock, a little clay and virtually no topsoil were some of the challenging conditions. Given the high level of topographic relief (600 vertical ft. in 225 horizontal ft.) and the sensitivities of the regulatory-defined “most protected waters” Lake Superior resource, both temporary and permanent erosion control measures were carefully crafted.

More than 7,000 linear feet of compost-filled “biolog” were created on site by using a truck-mounted blower to fill nylon socks. Here two sections of sock are being joined to create an uninterrupted section of erosion-control.

Erosion Control: Bag of Tricks

The project used innovative erosion controls to keep the project in highest regulatory compliance while working within the watershed of Lake Superior–the most protected waters in Minnesota. Compost logs, compost topsoil, mulched stockpiles, blanketed foundation excavations, special seed mixes, plunge pools and catchments are among many devices, practices and ideas used to stabilize natural forces.

Close to 60 percent of Highway 61’s original roadbed had eroded 200 ft. down into Lake Superior by 1994, when the new tunnel bypassed this shaky section. This angle gives a sense of the cliff’s verticality, and shows a portion of erosion control straw at the lower left.

We had great teamwork, good communications and timely resolution of choices, with satisfying results observed on site. This was critical, because there was not much time for irresolvable differences between diverse agencies and functional groups. We had private and not-for-profit interests closely involved.

“Original guardrail items including boulders and dome-top guardrail posts; both were installed to recapture the historic aura of the old touring experience of the North Shore.”—author

The erosion-control solution set included the use of compost logs at the perimeter of the site, drainage inlet and outlet structures, and to make plunge pools along flow lines of the drainage structures. Control of soil stockpiles was achieved with temporary seeding and daily straw cover, temporary containment basins around poured concrete locations, and the placement of compost in the absence of topsoil.

The team built a 264-ft. bicycle and pedestrian bridge that parallels the main highway at Silver Creek Cliff. The truss segments are constructed of Cor-Ten steel, as seen here. The light-colored surface towards the bridge’s center is fire-resistant ipe wood.

These were among the erosion control measures successfully implemented.

Compost was also used to “grout” rip-rap and gabion baskets. Rockslides were contained and then harvested to be used to enhance engineered slope stability and to more than double parking capacity and further refine stormwater treatment. This “rock slide harvesting” also eliminated the need for Trunk Highway Maintenance Operations to haul rock material in and out of the area.

Cliff faces were also stabilized, making them available for aesthetics and rock climbing. The product created a well-vegetated soil surface with excellent infiltration. The result is one of the best “seed-takes” ever achieved along the North Shore (the seed mix is native and custom-mixed).

Our work preserved the original Highway 61 corridor roadbed that is listed on the National Register of Historic Places. In environmental assessment, the “no-build” option was considered to be the highest impact to the site—the original road grade was rapidly sloughing off. The work both re-established and stabilized the tourist corridor.

A photo taken during construction shows preparations for pouring one of the bridge’s concrete footings. The structure has three irregular spans that are attached to piers sunk and anchored on to bedrock. The tunnel entrance is visible at upper left.

The project also recreated a historic gateway, and reinstalled the historic overlook plaque–both dating from the early 20th century. The gateway had gone through several iterations in the last century, but had been absent since the 1940s. At the project onset, the original plaque installation was undermined and the plaque itself had to be retrieved from the lake. The plaque reinstallation is in the same approximate location with the base and structure anchored and pinned to the bedrock.

Another project highlight is the 264-ft. pedestrian bridge that the team installed in complex conditions (that were well stabilized and experienced no slippage incidents during construction). Original guardrail items including boulders and dome-top guardrail posts; both were installed to recapture the historic aura of the old touring experience of the North Shore.

A section of plastic blanket covered by compost protects a slope that falls 200 ft. down to Lake Superior. This section is located at the edge of one of the pedestrian and bicycle trail abutments.

Rest Stop is Visitor Focus

As part of the trail project, a parking and Wayside Rest was developed for motorists near the Silver Creek Cliff Tunnel on Highway 61. The trail segment offers the traveling public the opportunity to get out of their vehicles and take a walk to enjoy a spectacular lake vista. The overlook trail now offers the public its longest continuous vista of Lake Superior on Minnesota’s North Shore.

The scenic overlook with the historic overlook plaque, interpretive signage, gateway and pristine natural beauty draws people to this remarkable place. When the trail is connected to the community of Two Harbors it will offer residents and visitors to the area a premier route to use for transportation, recreation and physical activity.

Another view of the composted-and-blanketed area near one of the bridge footings. The blanket is anchored with local stone. The seed mix used here and elsewhere was custom-created for the site and includes more than 40 woody, grass and wildflower components.

Currently where the Gitchi-Gami State Trail connects to local communities, local residents are using the trail as a walking route because the highway itself has a very high Average Daily Traffic count and the shoulders are narrow.

The 264-ft. trail bridge was developed to provide a safe crossing apart from the highway near the edge of the cliff. The bridge has three irregular spans between carefully-controlled pier and abutment structures.

Precarious temporary conditions during construction required careful temporary erosion-control measures, so open structural excavations were blanketed as if they were permanent restorations. This was done before footings were formed. The superstructure is on a slope, and the truss is Cor-Ten steel with a durable and fire-resistant ipe wood deck. Trail safety fence is used in areas where needed. The fence is a match to the bridge palette of Cor-Ten steel, with close-grained wood stained to match.

A nearby landslide facilitated a time and money-saving change from the original plan with the creation of these gabion baskets that form a portion of the project trail bed. The original plan called for a reinforced soil wall at this point.

Making the Grade

Trail grades were reduced where needed to meet AASHTO, Minnesota Bicycle Transportation Planning and Design Guidelines, and the Americans with Disabilities Act. All features of the site are fully accessible by the most direct route. The trail is wider near interpretive signs and the overlook area to allow all visitors to pause without obstructing other visitor traffic. This trail segment gives cyclists and pedestrians complete separation from intense motor vehicle traffic in the tunnel. The trail shoulders and recovery areas are restored and re-vegetated.

The State of Minnesota, through the Department of Natural Resources and the Pollution Control Agency, has identified waters of concern. This includes water bodies that are either damaged/impaired or “special.” The goal of the Clean Water Act is to maintain swimable, fishable and drinkable waters.

Lake Superior is state-listed as a “special” water body. As a result, all new construction activities that drain into it must utilize best management practices that will preserve the essence of what makes the lake special to all citizens, including Native American groups that view the water as a spiritual resource.

A worker blows compost over a check dam in a stormwater ditch along the side of the highway. The truck at rear carries the same blower that was used to fill the sock-contained “biologs” used extensively on the project.

On-site Erosion Solutions

The Gitchi Gami Trail section at the overlook was designed after many site visits that explored soil, geology, plant conditions, slopes and drainage from the watershed.

The soils are very thin, and infertile. Plant specie density is sparse and of low diversity.

According to the NPDES rules, where soil sits directly on bedrock you do not have to use traditional methods for treating the storm water, but must still incorporate some treatment. The concept of living swales and green treatment space helped achieve this goal. Tight language was used to make sure the contractor knew what was involved in protecting Lake Superior (with the federally-mandated Storm Water Pollution Prevention Plan).

Compost and the ubiquitous seed mix were blown into the trail bed gabion baskets. “It did a nice job of sticking,” Obernolte said. “It hung in nicely and we got a fair amount of growth to cover the baskets.”

The treatment measures used on the project include using leaf-and-grass-clipping-origin compost for a sustainable-perimeter control BMP for sediment capture (with compost logs), and compost logs in the ditch swale as a “speed bump” to slow and trap storm water (this feature mimics the channels and plunge pools of natural streams). Additionally, two-inch-thick compost blanket serves as a storm water sponge. Calculations show that the site will treat the first 1.25 inches of storm water, with an additional potential to treat up to a six-inch rain event (a 100-year storm event).

With its 31,820 square miles, Lake Superior is the largest freshwater lake in the world by surface area. (Siberia’s Lake Baikal is larger by volume.) The lake was known as Gichigami (or “big water”) in the Ojibwe language, which accounts for the Gitchi-Gami State Trail’s name. The trail passes to the left of this point, along Highway 61’s old roadbed. In 1994, the new tunnel bypassed this segment, so cyclists and pedestrians now enjoy this view in relative silence.

As of January, there has been no evidence of storm water overwhelming the system. This is gratifying, as it means the system is retaining the rain where it falls, boosting growth of an exceptional native plant community that will provide wildlife habitat and provide the maximum aesthetic experience for hikers and cyclists.

An auto safety barrier marks the sunset along the famous sightseeing drive. Like other site features, these stanchions are similar to ones used along the highway in the early 20th century.

The water that is not utilized by plants and microorganisms will slowly filter through the bedrock fissures, discharging clean and clear into Lake Superior.

Canine and human visitors enjoy the freshly-laid asphalt of the Gitchi-Gami State Trail at Silver Creek Cliff. The group at the top of this view is pausing to read two interpretive signs, including one on local history (“Coastal Travelers”) and on erosion control (“Outsmarting the Cliff”).

Gitchi-Gami State Trail and Highway 61

Minnesota’s Gitchi-Gami State Trail is an 86-mile non-motorized trail now being completed between Two Harbors and Grand Marais along the North Shore of Lake Superior.

Trail segments are being built through a joint effort of the Minnesota Department of Transportation, the Minnesota Department of Natural Resources and the Gitchi-Gami Trail Association. Construction of the 86-mile trail system is divided into several segments.

The Silver Creek Cliff segment is one of five sections that have recently finished or will soon finish construction.

The approximately one-mile Silver Creek Cliff portion parallels the new Silver Creek Tunnel and is constructed on the old scenic Highway 61 road bed. The segment offers 4,000 feet of elevated and scenic views of Lake Superior.

Parking is available at a new visitor area east end of the tunnel.

More information:

Meet Don Olbernolte

Don has been a consulting landscape architect in private practice since 1986 when he graduated from the College of Architecture and Landscape Architecture in the Institute of Technology at the University of Minnesota. He first began working in custom landscape design and project management in his teens working on Valleyfair Entertainment Center in Shakopee Minnesota starting in 1976.

He began drawing extensively in grade school. Don’s first project with the Minnesota Department of Transportation was the major state entry monuments starting in 1997. His comprehensive master planning work in transportation site development includes, entry monuments, safety rest areas, playgrounds, scenic overlooks, trails, headquarters buildings, and operations centers. His comprehensive summary of professional services ranges from market research and site and feasibility analysis to site selection, and on through concept planning, design development, custom detail design, original art, elements, features, systems, human factors, contract documents, construction management, project oversight, field engineering, contract administration, and site operations in all levels of regulatory compliance.

Don likes to work with local culture, history and personalities, enjoys natural and available materials, themed design and diverse aesthetics; he embraces technology. He works throughout Minnesota, and lives with his wife and three children.

Don and family divide their time between their home in Saint Paul, and family farm on Round Lake in north central Minnesota. His volunteer focus is on facilities for the developmentally disabled, and he is also a scoutmaster. His youngest daughter while a kindergartener once introduced him to her student teacher as, “This is my dad; he draws pichers an’ builds stuff.”

“That’s the job description I go with!” Obernolte says.

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December 14, 2019, 8:49 am PDT

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