Soil

Slope Whisperers

Taming hazards with slope stabilization

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DUKE BITSKO, HATCH
Protecting Water In Massachusetts
Cambridge, MA, maintains a series of three terminal reservoirs for drinking water. Fresh Pond has been one of the most prominent reservoirs in the city’s fleet of water sources and a favorite with the community, not just as a source of water, but also because of its role as a place of passive recreation. Surrounded by a golf course and playing fields, the grounds of the Fresh Pond Reservation are also canine friendly. It’s one of the few places where dog owners are permitted to let their pets cut loose and run wild and free off leash. The area is a reservation, rather than a city-owned park, and residents have been going there seeking refuge from the densely populated urban scene for generations.

Duke Bitsko describes Fresh Pond and nearby Walden Pond as kettle ponds. Kettle ponds represent a geological feature unique to New England. “They are like divots formed under the weight of the deepest areas of glacial ice” during the last ice age, he explains. When the remnants of the glaciers retreated, these depressions filled in with groundwater rather than surface flow. Because of their relative isolation from surface water, today, under natural conditions, kettle ponds offer some of the highest water quality in the region.

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Credit: DUKE BITSKO, HATCH
A diverse native plant community was added to the slope.

As far back as the turn of the 19th century, however, the area around Fresh Pond had begun to experience degradation from near-constant human activity, ranging from farming to golfing. Various efforts to restore the area dating back to that era bore little progress, and concern arose about water quality; however, the pond was resilient and remained one of the Cambridge Water Department’s most reliable and best-quality terminal sources.

In 1999, the city of Cambridge developed a comprehensive master plan to protect long-term water quality from its reservoirs and preserve the local ecosystem. A big part of that plan was to repair and restore an area along the banks of Fresh Pond known as Glacken Slope.

Wear and tear on the authorized footpaths in the woods and on the slope had contributed to erosion. Degradation also occurred in wooded areas, where informal trails cut by free-ranging retrievers, hounds, lapdogs, and their enthusiastic owners foreshadowed a future for Fresh Pond mired in excessive sediment. Adding to the potential woes of the sub-watershed, the slope had been almost entirely overrun with invasive plants, says Bitsko, the landscape architect and director of interdisciplinary design with Bioengineering Group (now Hatch), who has served as a long-term consultant. By the 1990s, the slope was populated by up to 80% invasive vegetation, including a large compliment of allelopathic black locust trees, dense stands of the pernicious Norway maple, and garlic mustard and celandine groundcovers.

The weak soil binding capacity of the invasive plants and the continuous foot traffic had resulted in gullies several feet deep. Slope failure was evident in multiple locations. David Kaplan, Cambridge Water Department’s watershed manager assigned to supervise Glacken Slope restoration efforts, says that on much of the hillside, “there was no soil left to speak of.”

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The slope received erosion control blankets and compost filter socks.

A Satisfying Plan
Nonetheless, the reservation remained an attraction with area residents hungering for a taste of the wild in the congested Boston region. Therefore, the city adopted a plan to satisfy the major roles the reservation played in the life of the community. It planned to protect and enhance water quality in Fresh Pond and its open spaces, and to restore the ecosystem to a more natural and diverse plant community. The method chosen was to start at the top of the slope, on the grounds of the city-owned athletic fields and golf course clubhouse, and work downhill in five phases, addressing erosion through a comprehensive and holistic program of structural modifications, protective installations, and adaptive management practices.

“It is a long slope and a wide area, so we did it in phases because of funding—starting at the top and then working downslope,” says Bitsko. The philosophy was to address the worst issues and root causes first.

For the slope stabilization project to be successful in the long run, the city reasoned, the first practical step would be determining and eliminating the cause of erosion on the hillside. That meant diverting stormwater runoff draining from the city-owned golf course clubhouse and athletic fields away from the face of the adjacent Glacken Slope.

The project, following Bioengineering Group’s design, implemented a level spreader and vegetated swale at the top of the slope to calm, gather, and infiltrate runoff from the athletic fields and clubhouse. Six years later, the project culminated with the installation of a 12-foot-wide porous loop road and granite cobble drainage swale located at the bottom of the slope to mitigate historic flooding patterns. In between, the team performed a range of tasks and installations to control erosion, divert water, block sediment, and remedy damage and neglect, all following the comprehensive blueprint.

Bitsko says the project partners, including the City of Cambridge Water Department and the Fresh Pond Master Plan Advisory Board, not only sought to keep the reservation accessible during the restoration project but also wanted to garner attention for the improvements as they were being made, to maximize public support. According to Bitsko, work tasks were scheduled strategically so that welcome, visible improvements would be noticeable to visitors, the better to gain and maintain public support for a long-term project that might, periodically, disrupt their dog walking, exercise, and recreation rituals with trail closures, construction fences, and detours.

To address eroded hillsides, technicians stacked 12-inch-diameter coir fascines, or logs, to fill the gullies that had formed and backfilled the voids with soil to retard sediment migration. Bioengineering Group’s plan called for smaller coir fascines, 8 inches in diameter, to be staked in place at vertical intervals on the slope to act as slope breaks every 4 to 6 feet across any slope contours steeper than 2.5:1. “The point was to slow the water,” says Kaplan. Every invasive Norway maple was removed, along with 75% of the black locusts.

To bring vitality back to the hillside and encourage native growth after removal of invasive vegetation, restoration workers amended the depleted soil with compost, compacted it with hand rollers, and anchored the system with BioNet biodegradable erosion control blankets from North American Green. Although care is required when using natural fiber on a restoration to make sure it is not compromised as work proceeds, Bitsko says he often prefers using biodegradable blankets rather than plastic because of the synthetic material’s potential to interfere with wildlife habitat. “I have witnessed birds getting their feet caught. I once had a bird caught in plastic and it was chewing its leg off to get free. I had to use my keys to cut it loose,” he says.

“These are fairly steep slopes, exceeding 1:1,” says Kaplan, and at times the crews would need to use a rope and pulley system to haul soil, materials, and tools upslope. The steepness of the slopes sometimes compelled workers to improvise, in some cases setting back rather than advancing progress on the site. For example, says Bitsko, to get across the slope to perform required tasks, “sometimes the planting contractors would walk on the coir fascine slope breaks, they’d come loose, and we’d have to stake them in again. That was something we didn’t anticipate.”

Happier Trails
To repopulate vegetation, contractors applied a cover crop seed mix to hold the soil until more permanent native plants could get a foothold. In some areas, says Bitsko, the cover crop persisted “longer than we expected, and it may have inhibited long-term herbaceous growth.” However, in spite of the delayed growth in some spots, he says the restoration plantings are healthy and well-established. “We chose a native plant palette that would work on a north-facing sandy slope. The plants we put in have survived, and the root mat is now holding the soil in place.” He believes the project was successful. “It took something that was limited in biodiversity and now has made it very diverse.”

Informal footpaths that had been subjected to excessive wear have been fenced off, with plant establishment fencing holding hikers and dogs at bay. Visitors to the reservation have been encouraged to stay on authorized trails. Bitsko says the staff, noting the importance of the fence to sustaining habitat recovery, often refer to it as “long-term temporary plant establishment fencing.”

“We’ve had your regular maintenance issues with the porous asphalt path relating to keeping the footpath clean of leaves and litter,” says Kaplan. But, he says, aside from occasional wear from utility vehicles, there have been few issues in caring for the rehabilitated loop road or slope. “So far, so good,” reports Kaplan two years out from the completed installation.

In aggregate, he says, water quality at Fresh Pond is the best of all of Cambridge’s three terminal source water reservoirs. He believes the slope stabilization and restoration project will help the site maintain its reputation for providing clean fun, healthy environs, and fresh water for the residents of Cambridge.

Credit: GREENVISTA LANDSCAPE AND EROSION CONTROL
Greenvista placed Geoweb on the slope above the retaining wall.

A Bridge in California
Plans to widen the Route 49 approach to the South Fork American River bridge in preparation for constructing a new bridge required cutting into a slope adjacent to the current roadbed and installing a retaining wall along the 200-foot extent of the cut. Above the wall, the slope loomed another 45 feet at a 1:1.5 gradient. “You could barely walk up it,” says Tim Jones of Greenvista Landscape and Erosion Control. His firm performed the installation of Presto Products’ Geoweb to stabilize the slope along its 200-foot run.

The California Department of Transportation (Caltrans) chose Geoweb for the task. “It comes in 8- by 10- or 12-foot pieces that, when folded to ship, are only three inches tall,” says Jones. “It accordions together and becomes like a web. It comes with clips to connect the panels together, and rebar stakes it to the ground. There’s an ‘ear’ that goes on it to lock it down on the soil.”

After contractors completed work cutting back the slope grading and excavating to exclude jagged edges and boulder-sized stone from the slope, Jones’s crew got to work laying the Geoweb. Jones reveals that it was his first time using the product; however, he says, a representative from Reed & Graham, the local Presto Products distributor, “came out on the first day and showed us how. In about 10 minutes, my guys knew everything they needed to know and were ready to start the installation.”

Jones notes that the Geoweb wrapped easily around the contours of the slope and the tight curves around power poles that dotted the site while still holding flush to the soil. In addition to being lightweight, compact, and easy to transport, he says, “It’s plastic, but it’s pliable and bendable.”

Aside from the difficulty of walking on the steep slope, the toughest part of the installation was driving the anchors into the rock to secure the system. “Once they cut back the slope, underneath it was decomposed granite; that’s why we had such difficulty driving the anchors. It was a struggle.” However, crew members, using Dewalt 35-pound demolition hammers, drove in the 24-inch #4 rebar on 2- or 3-foot centers, anchoring 9,800 square feet of Geoweb to the slope in just three days. “Everything went smoothly, so there was never any need to call Caltrans out,” says Jones.

The slope was hydroseeded with a Pacific Coast native grass seed mix with 2,000 pounds of wood fiber mulch, 35 pounds of native seed, and 150 pounds of tackifier per acre. “It held fast before germination, and the general contractor said nothing had moved; it did its job,” says Jones.

Although Geoweb had been specified prior to Jones’ involvement with the project, he says, “When the opportunity does come up, I will definitely use it again. It hasn’t gone anywhere, and we’ve got a lot of heavy rain.”

Staving Off a Slide in Pennsylvania
The Pittsburgh area in Pennsylvania has seen a lot of rain lately. Mike Sydlik of Earth Inc. observes that 7 inches of rain fell in February alone. “This on top of snow that had been sitting on the ground” slowly melting into saturated the soils, he says. The combination led to numerous landslides. In Allegheny County, up to 75 areas of movement had affected Pennsylvania Department of Transportation (PennDOT) infrastructure. According to Sydlik, many of these slides were occurring in areas of chronic movement, but some of them were brand-new slides in areas where movement had never been detected before. The landslides led the PennDOT to implement emergency repairs, some of which needed to be initiated in a matter of days to reopen heavily traveled roadways.

While the steep topography and weather conditions have contributed to the slides, says Sydlik, there is more to the picture than just soil saturation. “A lot of these areas that are moving can be considered ancient landslides that predate any involvement by man, such as cutting or filling. These were areas that seemed to be stable, in many cases for decades, and then you just had a little too much precipitation that then activated them.” However, he says, some of the slides were “probably related to highway fills that had improper drainage. Ideally, when you have a good fill you want to have free-draining material at the toe that’s benched into something solid below any potential sliding surface.” He says roads in the area that were built many decades ago “were not always benched into solid ground appropriately.” As a result, the slides can be blamed on the combination of “ancient landslides being reactivated, and roads with embankments and cuts that were built perhaps not to 2018 standards.”

The most devastating of the slides in 2018 took place on State Highway 30, wiping out three lanes of a heavily traveled four-lane road that had averaged 11,000 vehicles per day. Along with the loss of travel lanes, the community lost several residential units that collapsed under the mud and rubble, leaving families without shelter. In the face of the urgency to open the road, the governor declared a state of emergency. PennDOT limited the bidding period to just four days for contractors to submit bids to fix the Route 30 slide to put the roadway back into service. According to Sydlik, PennDOT officials determined that a methodical design-build process was out of the question; to incorporate a design-build or value engineering approach in an emergency situation, they would have had to take considerable time to evaluate a variety of proposed solutions offered by bidders. Instead, he says, with a goal of getting the project done and the road reopened as soon as possible, officials found it expedient to require contractors to adhere to a specified design that could be implemented directly. Repair work is currently underway.

Credit: GEOBRUGG NORTH AMERICA
Rockfall netting along US Highway 12 in Washington

Mounting Pressure
However, Sydlik says that detecting a hazard before a slide takes place may sometimes allow for consideration of options, even in an emergency scenario. Although slides related to precipitation can be hard to predict—but must nevertheless be quickly addressed after they occur—not all slides are related to weather. Sydlik remembers an emergency slope stabilization project that his firm worked on several years ago, precipitated not by rain falling from the sky but by seepage rising up from the ground below.

Undergirded by a concrete crib wall, State Route 88 in Washington County passes over the confluence of an abandoned coal mine site, a major stream known as Ten Mile Creek, and the Monongahela River. Dependent on the same concrete crib wall, rail tracks farther up the slope followed the undulating contour of the hillside in this rocky region of central Pennsylvania. Due to a malfunctioning pump, the mine was seeping drainage and held a full head of water, which, leaking from below, began weakening the foundations of the road.

Complicating the situation, says Sydlik, the locks and dams implemented on the Monongahela River system in the half century or more since the road was built back in the 1950s or 1960s had raised the water level fronting the wall. Whereas the wall had once been on a slope 12 feet or more above the water, by 2013 it was sitting directly in the water. With the flowing river water lapping at the toe of the existing bulwarks and mine drainage weakening the slope from within, Sydlik says, the clock was ticking on how long the road and the tracks above would remain on stable ground. “They were losing a lot of material out of the crib,” he notes.

According to Sydlik, a slope failure and slide, potentially taking out the road and the rail line, would have required a minimum 30-mile detour just to establish roadway connections; there was no real alternative to replace the rail line if it were lost. “It was pretty hairy; more and more voids developed by the water going through. A lot of the backfill was gone.” PennDOT declared the repair project to shore up Route 88 a “super emergency” and Sydlik’s firm, along with subcontractors, developed a “quick design” to address the potential fault, he says.

That design included a wall of steel piles with soldier beams encased in concrete, held in place by a matrix of rock anchors driven into the bedrock. This new wall was to be put in front of the failing concrete crib wall to protect the road from a potential slide.

As the slide prevention project got underway, all but one lane of Route 88 was taken out of service, and traffic was metered through in one direction at a time with temporary traffic signals. Crews drilled soldier beams “through the water, punching through the soil at the riverbed and into rock to create a bond to resist the loading,” he explains. Between each pair of concrete-encased soldier beams, crews installed two sheet piles. To hold the wall in place, rock anchors were drilled through the face of the wall into bedrock behind the streambank, to create “a bond with the bedrock to resist the hydrostatic pressure” from the mine drainage.

Sydlik says that rock anchors, which consist of high-tensile-strength stranded steel cables, are typically driven at a 25-degree angle, but space constraints on the project made that standard impractical. The rock anchors had to be driven through the wall face at a steep 45 degrees in order to keep the footprint and subsurface profile of the installation within the PennDOT right of way.

“It was heavy-duty erosion protection but it required that, given that you were right up against a major river and an important tributary to it,” he says. In light of all the devastation that has recently occurred because of slope failures throughout Pennsylvania, says Sydlik, “It’s satisfying to work on a project like this one where a stitch in time does save nine.”

Credit: GEOBRUGG NORTH AMERICA
Above Highway 12

Binding Boulders in Washington State
Jerry Wood, construction engineer with the Washington State Department of Transportation (WSDOT), says that on US Highway 12 there is “always” rock falling, and shoring up slopes to hold boulders in place is not an unusual project for his department to undertake. Highway 12 is the southernmost pass that crosses the Cascade Mountains in the state of Washington. Numerous slopes near White Pass are active rockfall areas. They have been mitigated over the decades with many solutions, including numerous draped mesh systems composed of traditional cable nets.

One such antiquated cable net at mile post 152 near White Pass was draped across a rockfall chute and, in 2017, was destroyed by severe rock and snow slides. WSDOT decided to update the system.

Geobrugg worked with WSDOT, assisting the department in creating a robust Rolled Cable Net drape. The Geobrugg drape is designed to safely contain rockfall by deflecting loose rock away from the road and into a ditch at the base of the rockfall chute. The updated drape includes upsized and redundant support rope infrastructure. The support rope and flexible wire rope cable anchors are made of 1 1/8-inch ropes having 130 kip strength. Geobrugg’s Rolled Cable Nets are considered to be 30% stronger than the nets being replaced. They also had a reputation for performing well in another nearby installation. Hi-Tech Rockfall Construction Inc. performed scaling at the site and quickly installed the large net panels.

Wood says there are important considerations regarding timing of work on a rock face. “Typically, we want to wait until dry weather, wait until the snow melts, and wait till the rain stops, especially on these areas where you have these rock slopes. When it rains or when the snows melts is when you have most of the rock falling. That’s critical when you’re talking about worker safety. You don’t want them up there when there’s potential that the rocks may be coming down above them.”

Although tying down boulders is a somewhat ongoing task along Highway 12, Wood says the work can have major implications. “It was a very small job,” but an important one. “I wouldn’t say that anybody was imperiled, necessarily, but it’s always a risk on those mountain passes.”

Wood says the project went well. “These are the kind of things that, if they are unobtrusive, they are the best projects you can have, actually. If you don’t notice it, then they’ve done it right. You don’t want people looking up at it and wondering. When you don’t see a rock hanging up in the netting or the net hanging halfway off, it’s a good thing, and it’s done correctly.”  EC_bug_web

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