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Eliminating erosion runoff at a Canadian quarry’s stockpile slope

August 1st, 2007 / By: / Environmental, Erosion Control, Erosion Control Materials, Feature

Introduction

With ownership of some 900 quarries around the world to sustain its building materials operations, the Lafarge Group claims a commitment to achieving sustainable development hand in hand with environmental compliance. However, attempts to stabilize a large stockpile at its Woodstock, Ontario, quarry were challenging that goal. Lafarge conquered the challenge with help from a specialized growth medium and geosynthetic erosion control tubes.

The situation

The Woodstock quarry is an open pit operation that supplies limestone for Lafarge’s cement production. In 2001, Lafarge officials realized that erosion control measures were not working on an accumulation of mined “overburden” in one of the quarry’s stockpiles. Overburden is a mixture of clay, sand, and rock material that has no topsoil, making it extremely difficult to revegetate and susceptible to erosion. Unless an effective control method was implemented, runoff from erosion of the stockpile could eventually affect water quality in nearby streams.

The problem

Acceptable cover had been established on the stockpile’s western face that drained into the quarry. However, the eastern face was not able to sustain similar vegetative cover. This facing had a slope gradient of approximately 2.5H:1V, is about 300ft from top to bottom, and totals about 10 acres. It was apparent that corrective action was needed in this area when rain and snowmelt began creating sizeable rills and movement of sediment down the unstable mixture of materials, pooling sediment at the foot of the slope.

The first attempts to stem this erosion and create better growing conditions were fairly conventional. Lafarge contracted with a company to have topsoil hauled up the slope, graded in, and then hydroseeded.

“We tried this (conventional method) twice, with little success,” said Michael Bart, environmental engineer for the Lafarge Woodstock operation. “The combination of rain, snow, freezing, and thawing would wash out the topsoil we’d graded in and create large gullies. The only evidence of growth was in a few areas where the soil hadn’t eroded away.

“It was not only discouraging, it was costly,” Bart continued. “As a result, we were unwilling to make further investments until someone could demonstrate an effective technique on this site.”

The solution

In the spring of 2003, John Reynolds of Mulch-It Inc., located in Putnam, Ontario, and Dwight Johnson, regional manager for Profile Products, met with officials at the Woodstock operation. Aware that Lafarge needed proof before the company would make further investments in its erosion control plan, Reynolds and Johnson proposed applying a growth-inducing medium to a 30-ft-wide test strip down the problematic east-facing slope of the stockpile.

Bart learned from Reynolds and Johnson that this medium material featured patented technology that combined refined wood fibers; crimped, interlocking, man-made fibers; and performance-enhancing additives to provide erosion protection and seed germination on slopes. It also featured claims of more than 99% effectiveness in controlling soil loss.

This medium required no cure time, providing immediate protection against erosion upon application. It created an intimate bond with the soil surface to form a continuous, porous, and erosion resistant blanket that allowed for rapid germination and accelerated plant growth. The product was also promoted to absorb and hold 15 times its weight in water, delivering more moisture to the seedbed for faster vegetation establishment.

Lafarge officials accepted the proposal, and work on the test site began that October. The area was graded with a bulldozer to smooth out the numerous rills. Next, the area was track-packed by a dozer with the cleat marks running horizontally across the slope to act as a basic erosion retardant. Finally, a mixture of the growth medium, seed, and fertilizer, which included a slow-release nitrogen, was hydraulically applied at the rate of 3,500lbs/acre.

“We knew the history of problems with the slope, so we tailored a seed mix containing grasses and legumes that we had observed would grow in that environment,” said contractor Ken Wray. “This combination was designed to help establish a quick cover while providing deep rooting and nitrogen fixation. We knew we were at the extreme edge of the growing season, but our experience [with this combination] was that it works well in situations where dormant seeding is needed.”

“This combination,” as Wray called it, was quickly tested—heavy rains fell the day after the installation was completed.

“We observed very little erosion,” Bart said. “But we wanted to see how the test area would hold up over a winter of snow, freezing, and thawing.”

In the spring, Bart said the strip stood out like a “beacon of green.” Lafarge officials continued to monitor the area and, by that fall, decided to rehabilitate the entire eastern slope of the stockpile—an area of about 10 acres.

“Compared to their previous method, they were pleased with how the [test area] performed,” Reynolds said. “However, over nearly 2 years, there had been some minor rilling in the test area. To ensure a permanent solution, it was agreed that we would incorporate tubes as a slope interruption device in conjunction.”

The team agreed to use the geosynthetic erosion control tubes, based on the product’s construction—an engineered composite of thermally processed wood fibers, man-made fibers, and performance-enhancing polymers encased in heavy-duty, knitted tubes. They had been proven through independent testing to be highly effective stormwater treatment devices, designed to effectively trap, filter, and treat sediment-laden runoff.

The entire slope (with the exception of the original test strip) was then graded and track-packed. Next, 4 rows of 6-in.-diameter tubes were placed along the face of the 2.5H:1V slope. Each row was spaced about 75ft apart. Finally, the previously evaluated mixture of growth medium, seed, and fertilizer was hydraulically applied at the rate of 3,500lbs/acre, as per the test section. The project, beginning from initial grading to placement of the tubes, was completed in 2 weeks.

Ken Wray, the contractor, specifically pointed to the ease of application forthe growth medium/seed/fertilizer mixture, as well as the geosynthetic tubes: “They certainly served their purpose in slowing down the flow of water over a long slope.”

The results

The winter of 2005-06 was, according to Michael Bart of Lafarge: “tough to the ‘nth’ degree.” The winter brought numerous rain, snow, and freeze/thaw events, and temperatures were as low as 0°F in November. Circumstances didn’t improve much in the spring of 2006, which ushered in cold and wet conditions. “Because of the unfavorable weather, we didn’t see much germination and growth until late May,” Bart said. “But then it seemed thatthe vegetation literally exploded out of the ground.”

During the spring and summer of 2006, Bart said he made frequent trips to inspect and photograph the project. “It was apparent that we had achieved a good, stable cover,” he said. “We were able to find only one minor rill in the entire 10-plus acres. The rill was only about 10in. wide x 3in. deep, which we believe was caused by an unforeseen flow pattern. However, by the end of August, the rill was fully grown in and virtually indiscernible.”

The future

The combination of the right products and correct methodologies used at the Woodstock quarry is becoming well-established as a “best management practice” (BMP) for controlling erosion and improving water quality in slope management, according to Stephen Zwilling, market development manager for Profile Products.

A good indication of this BMP establishment is the fact that Lafarge, and the companies involved in the east-slope project, are consulting on further applications in progressive restoration of other stockpiles at the Woodstock quarry.

And Lafarge affirmed its environmental commitment by stabilizing its erosion-prone slope and returning the stockpile to a natural appearance, per the expectations of Ontario’s Ministry of Natural Resources. As a result, Lafarge had a secure, established slope and, even more importantly, a blueprint for handling similar challenges in the future.

Sarah Willnerd, a writer for Swanson Russell Associates of Lincoln, Neb., and Ron Bygness, editor of Geosynthetics magazine, contributed to this article.

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