Water is being drawn out of the state’s aquifers faster than it is being replenished, so public agencies like MnDOT are increasingly interested in figuring out how to reduce water usage.
A two-year research project underway at MnDOT is investigating how the agency can re-use wastewater at its safety rest areas and truck-washing stations. In addition to preserving groundwater, MnDOT hopes to reduce utility and septic system costs.
MnDOT owns and operates over 1,000 buildings, including 68 safety rest areas, 137 truck stations, 18 regional/headquarters maintenance sites and 15 weigh stations and truck scales.
These facilities either discharge their wastewater to a subsurface sewage treatment system or a wastewater treatment plant.
Researchers from the University Of Minnesota’s Onsite Sewage Treatment Program have been hired to investigate the potential avenues for wastewater re-use at MnDOT. They will consider when re-use makes sense from a regulatory, environmental, economic and management perspective; recommend the most appropriate applications for reuse and identify any challenges with implementation.
Potential benefits include:
Preserve ground and drinking water for potable drinking.
Reduced life-cycle costs in areas where low-producing wells could meet drinking water needs while reused wastewater could be used for toilet flushing and equipment wash-down.
In areas with municipal water, lower water utility costs.
Increased longevity of septic systems due to decreased loads.
As the state, counties, or cities construct new facilities or upgrade existing ones, this research will provide insight into what options are readily available to reduce water consumption and improve water efficiency. If these types of reuse systems are demonstrated by MnDOT, then they could lead to usage by other properties across Minnesota.
The city of Shoreview, Minnesota was on the right track when it took the unusual step of paving a residential neighborhood with pervious concrete to help control stormwater and pollutant runoff into a nearby lake, according to a recently released seven-year performance study.
Typically used for parking lots and sidewalks, porous paving material allows stormwater to filter through the pavement and an aggregate base into the soil rather than run off the pavement and drain into storm sewers.
Shoreview bucked convention by using pervious concrete in a traffic application — low-volume, low-speed roads in the Woodbridge neighborhood near Lake Owasso. The city thought pervious pavement could help meet community sustainability goals and federal clean water regulations by reducing pollutants in waterways and groundwater while keeping water safely off driving surfaces.
Traditionally, pervious concrete hadn’t been used for roadways because engineers didn’t consider it strong enough for traffic (this and other projects have now demonstrated its application for low-volume roads like neighborhood streets). The impact on ride quality, tire-pavement noise and filtration was also not well understood, particularly in cold climates with freeze-thaw cycles like those in Minnesota.
Pervious concrete also presented a maintenance challenge: Organic debris, sand and other grit can clog the pavement’s pores. Periodic vacuuming is required to maintain the intended flow of water through the pavement. Concerned about how best to maintain the pavement and interested in tire-pavement noise levels and filtering performance, Shoreview, MnDOT and the Local Road Research Board monitored the Woodbridge roadways for seven years.
Installation and Evaluation
Shoreview replaced 9,000-square -feet of asphalt roads with 7 inches of pervious concrete over 18 inches of coarse aggregate base; near the lake, highly drainable sand served as the base. About twice each year for five years, researchers tested sound absorption, water infiltration and ride quality one day after the pavement had been vacuumed. In 2015, they repeated these tests without vacuuming the day before.
The pervious pavement performed well in filtering stormwater. By 2012, at least 1.3 acre-feet of water had filtered through the pavement and ground, and by 2015, nearly 2 acre-feet of water had filtered through the surface—all of which would otherwise have run directly into Lake Owasso.
Water infiltration and sound absorption rates were higher than traditional concrete, although rates declined over time because organic material continued to clog pavement pores despite vacuuming twice a year.
Initial construction of the pervious concrete streets and stormwater filtration system was slightly more costly than construction of comparable asphalt pavement with culverts. Life-cycle costs, including projections of maintenance costs over 15 years, however, showed somewhat lower costs for pervious pavement. While the pervious concrete pavement may require diamond grinding after 10 years, monthly vacuuming could make this unnecessary. Asphalt pavement would typically require a mill-and-overlay at year 15, and culverts would require periodic cleaning.
Additional benefits of the pervious pavement system that were not included in cost calculations—but were clearly significant—included complying with the federal Clean Water Act, recharging groundwater and avoiding direct pollution of Lake Owasso. Shoreview’s investment in pervious concrete has paid off economically and environmentally.
For additional information about this line of research, see these resources:
Like many snow- and ice-control professionals, Carver County Public Works maintenance operations staff are searching for new options to reduce the amount of chloride that reaches our waters from road salt operations. Using food production byproducts such as pickle brine are among the alternatives maintenance staff have been exploring.
Carver County regularly uses salt brine as part of their winter maintenance operations, which has become a widely accepted practice for controlling snow and ice. In the right situation, salt brine can be a more effective alternative to traditional road salt. An opportunity to obtain a free supply of sodium-rich pickle juice from a nearby canning facility seemed like a natural candidate worthy of consideration as a source of brine for county anti-icing and de-icing operations. In addition, recycling the pickle brine could reduce the amount of the waste byproduct.
The Carver County Public Works Department began testing samples of the pickle juice in 2016 with some encouraging results. But further testing showed the brine from the pickle cannery had variable salinity and pH levels that could damage maintenance equipment. Given the variables involved, staff determined it would be difficult to manually control the manufacture of the brine into a usable liquid. VariTech Industries recommended purchase of the Brine Boss, an automated brine blending system to manufacture the 23.3 percent brine solution needed for effective ice control operations. In addition, staff found adding potassium hydroxide to pickle brine neutralizes the pH level.
After extensive testing and analysis, VariTech engineers and Carver County staff concluded that pickle brine acquired from the cannery had to be exactly the same (salinity, vinegar content, and sugar content) for each and every batch or the system sensors would fail. But it turned out that the pickle brine supplier could not provide chemically consistent batches, and the VariTech system was unable to produce a consistent blend of 23.3 percent brine solution using pickle brine. As a result, Carver County staff determined they were unable to continue using pickle brine for snow and ice control.
Nevertheless, this project benefits other agencies considering the use of food production byproducts. The Carver County project demonstrates that there can be an alternative anti-icing product. As technology continues to advance, Carver County may revisit the use of pickle brine as a viable snow- and ice-control option.
For additional information about the project, check out these resources:
Researchers have found that peat has high potential to replace commercial compost in MnDOT’s standard bioslope and bioswale design for roadside ditches, and that taconite tailings performed comparably to the sand currently specified in MnDOT designs, with the additional benefit of removing phosphates.
Finding alternatives to commercial compost and sand for use in bioswales will help MnDOT meet regulatory requirements for stormwater runoff, while reducing the costs and environmental effects of transporting and storing these materials.
“The results of this project will very much facilitate the development of green infrastructure by reducing its cost to MnDOT and Minnesota local agencies, helping them to do more with less,” said Dwayne Stenlund, Erosion Control Specialist, MnDOT Erosion Control and Stormwater Management.
What Was Our Goal?
The objective of this project was to evaluate peat and muck excavated from construction activities, taconite tailings from area mining operations, and other stormwater quality filter media for use in bioswales and bioslopes along Minnesota highways. Laboratory and field tests of these products would examine their capacity to absorb water, retain pollutants and support plant growth to determine if they are beneficial and practicable in these designs.
What Did We Do?
Researchers began by conducting a comprehensive literature review on the use of bioslopes and bioswales as stormwater treatment best management practices. Then they collected peat and muck near a highway construction project, as well as locally sourced sand, compost, taconite tailings and commercial peat.
These materials, as well as various combinations of materials, were used in laboratory experiments to determine how well they:
Absorbed water, using a falling head test to measure saturated hydraulic conductivity, which indicates the rate at which water infiltrates a material.
Retained pollutants, using leaching experiments to quantify how well they removed copper, lead, zinc, nitrate and phosphate.
Sustained plant growth, using bioassays and greenhouse studies.
Finally, researchers conducted pilot field tests on three plots containing a 50/50 percent peat and sand mixture, and another three plots with a 50/50 percent compost and sand mixture. Between April and August of 2017, they monitored water infiltration, discharge water quality and vegetation establishment for these sites.
What Did We Learn?
“Ultimately, a combination of peat and taconite tailings will compare favorably with current MnDOT specifications for bioslope and bioswale design,” said Kurt Johnson, Research Fellow, University of Minnesota Duluth Natural Resources Research Institute.
Researchers found that peat has a strong potential for replacing commercial compost in MnDOT’s standard bioslope and bioswale designs, and that taconite tailings also performed comparably to the sand currently specified in these designs. However, muck has little potential to replace commercial compost or peat due to its low permeability, poor infiltration and filtration properties, and lack of support for plant growth.
Results for the three properties of interest follow:
Infiltration rate: While muck had an unacceptably low hydraulic conductivity, peat performed at least as well as compost, and taconite tailings as well as sand. Pilot tests showed that a 50/50 mix of peat and taconite tailings had a similar water storage capacity to a 50/50 mix of compost and sand.
Pollution retention: Muck absorbed only 50 percent of metals; salvaged peat, commercial peat and compost performed well, absorbing more than 80 percent. However, only taconite tailings showed the potential to remove phosphate. None of the materials removed nitrate.
Plant growth: Mixtures of compost or peat with sand or taconite tailings all performed well in providing a viable substrate for plant growth. Mixes containing compost performed the best in plant growth trials. Muck was difficult to mix with any other material, and its value for plant growth was minimal. Greenhouse study results showed no difference between sand and taconite tailings in their effect on plant growth response.
In a second phase of this project, “Development and Regionalization of In Situ Bioslopes and Bioswales,” MnDOT will conduct further laboratory tests on alternative materials for bioslopes and bioswales, and expand field tests to several sites in Minnesota that have been constructed using these materials. Researchers also recommend the development of specifications and detail drawings for the use of these materials.
Low-cost, low-maintenance mussel spat rope can help small fish species navigate through culverts by reducing current velocity and providing protected areas for fish to shelter and rest. Recent research in New Zealand demonstrated the effectiveness of mussel spat rope—rope with long, dense fibers used in mussel aquaculture— to assist small species fish passing through steep, perched or high-velocity culverts. The successful results from this research led MnDOT to investigate mussel spat rope as a method to facilitate fish passage in Minnesota’s culverts.
“Minnesota is a headwater state, and we have a responsibility to keep our fish population healthy. Mussel spat rope will be one more effective tool in the toolbox of methods we have to assist fish passage through culverts,” said Petra DeWall, Bridge Waterway Engineer, MnDOT Bridge Office
What Was Our Goal?
The objective of this project was to determine whether mussel spat rope was an appropriate and effective tool in helping small fish species pass through Minnesota culverts.
What Did We Do?
Investigators conducted a literature review to evaluate previous studies. Then researchers from St. Anthony Falls Laboratory conducted experiments in the laboratory and in the field to investigate the use of mussel spat rope as a fish passage aid.
Hydrodynamic performance. Hydrodynamic performance tests were performed in a 20-inch-wide by 30-foot-long flume fed by water diverted from the Mississippi River into the laboratory. Researchers measured velocity, depth and water surface slope, and sediment accumulation around arrays of ropes. They installed single- and multi-rope configurations and examined many variations of flume flow and depth, recording the rope’s effects on water velocity and turbulence.
In a second experiment, researchers released fine sand into the flume containing two- and four-rope configurations to investigate the rope’s effect on sediment transport. Because the ropes slowed local water velocity, deposits were observed on, between and under the ropes in two different depth tests after one and two hours of sediment feed.
Rope durability, performance and use by fish. Researchers installed mussel spat rope in two Minnesota box culverts: one in the northeast serving a fast current trout stream and one in the southwest serving a slow current prairie stream in critical fish habitat. Double strands of mussel spat rope were installed near a wall in each culvert and examined many times for approximately two years. Each observation included photographic and video recordings of the installations.
Small fish species’ interaction with the rope. Laboratory investigations of fish behavior with the rope were conducted in a 5-foot-wide by 32-foot-long flume with a raised section representing a box culvert. Two Plexiglas windows allowed viewing. Researchers installed two sets of double-strand ropes along a wall, similar to those in the field sites. Four video cameras tracked motion, recorded overhead views of the flume and captured fish behaviors at the midpoint and ends. Researchers used three species of small fish common to Minnesota: fathead minnow, white sucker and johnny darter. Five fish were released into the test area at a time and allowed to swim for an hour. Their progress and behavior were filmed and analyzed.
What Did We Learn?
Key observations from these investigations follow:
Mussel spat rope created small corridors (about 6 inches) of reduced velocity and turbulence along its length, which was sufficient to aid the passage of small fish. Sediment collected in, between and beneath the ropes. The presence of culvert floor sedimentation may assist fish passage.
The rope displayed wear over two years in the field, raising a concern about plastic microparticle release into streams. Sediment covered some ropes over time, suggesting a need for maintenance in some culverts. Only a few fish were observed at the field installations.
In the laboratory flume, test fish swam near and between doubled rope lengths, apparently taking advantage of the reduced current near and beneath the ropes. While there was variation among species, most fish that swam upstream through the simulated box culvert ended their passage on the rope side, evidence that the rope provided cover and refuge from the current.
Mussel spat rope will be a low-cost, low-maintenance tool to help small fish pass through culverts. The final report for this study includes guidance for installing the rope. The low-cost method will also be included in an upcoming guide for designing culverts that allow aquatic organism passage.
The accumulation of chloride in our waters has become a widespread concern. In a recent study sponsored by MnDOT and the Local Road Research Board, researchers measured the transport and accumulation of chloride from road deicers in a metro-area watershed. The findings revealed a greater infiltration of chlorides into soil and subsurface waters than previously assumed.
“The results of this research provide us with knowledge we did not
have before,” says William Herb,a research associate with the University of Minnesota’s St. Anthony Falls Laboratory and the study’s principal investigator. “It will help investigators and policymakers explore ways to capture chlorides and mitigate their damaging environmental effects.”
Road salt (sodium chloride) is used in most states that experience snow and ice, with growing impact. For example, chloride levels in some lakes and streams in the Minneapolis–Saint Paul metro area exceed state and federal water quality
standards, and a recent study showed that levels in more than one-quarter of shallow groundwater wells in the metro were above drinking-water taste standards.
“This is a real concern because even in low concentrations, chloride can be lethal to sensitive plants and some aquatic species, and many of our lakes, wetlands, and streams show acute or chronic levels of chloride,” Herb says.
To learn how chlorides from road salt deicers are transported in urban watersheds, researchers installed field instruments at eight sites in a Roseville watershed. They monitored water and chloride levels nearly continuously over three winter seasons; this included runoff directly from sources (roads and parking lots), transport in ditches and sewer networks, and retention in and release from detention ponds and wetlands. Computer modeling was used to generalize results.
Overall, the team observed substantial chloride retention via infiltration to soils and groundwater. For example, monitoring the runoff from a vegetated highway ditch showed that more than 95 percent of the chloride applied to the highway infiltrated
from the ditch into the soil, and less than 5 percent was exported from the site in surface runoff. “Interestingly, substantial chloride export from the ditch was observed in November rainfall runoff prior to application of any new road salt for the upcoming winter, suggesting long-term storage in soils and groundwater in and near the ditch,” Herb says.
Researchers also found that winter rain-on-snow events and the first major
prolonged thaw each season moved surface chlorides most effectively into the watershed.
The research team then used the data and modeling to examine potential strategies for reducing or mitigating the spread of chloride, including capturing low flows, seasonal runoff capture, and capture based on salinity.
Wayne Sandberg, deputy director of the Washington County Department of Public Works, chaired the study’s technical advisory panel. “Based on this research, we now know that deicer chemicals are staying in the soil and moving in the watersheds, and this should change how we manage ice and snow control,” he says. “The next questions are what can we do with that knowledge and what changes can we make.”
Darkness box culverts does not present a complete barrier for southwestern Minnesota fish species, according to a new MnDOT study. The findings will reduce the cost and delay of future box culvert replacement projects.
“This research will allow MnDOT to save both time and money when replacing other box culverts in southwestern Minnesota by eliminating the need for a fish passage study for each one,” said Scott Morgan, Principal Hydraulics Engineer, MnDOT District 7.
In this study, researchers developed several objectives in their efforts to assess the effect of low light levels on fish passage through replacement box culverts. As part of this effort, they wanted to determine typical light levels in the replacement culvert and other box culverts in the region. They also sought to determine if the Topeka shiner and other fish move through culverts in the same numbers they pass through control areas in the same stream, and whether light levels affect frequency of movement. Finally, if a barrier is determined, researchers sought to design or recommend a method for mitigating light in the culvert.
What Did We Do?
In the field, researchers characterized light in long box culverts (at least 8 feet by 8 feet) by collecting many light levels with a light meter at the water surface within the three culverts and at control reaches. They also measured light levels within the water column to characterize the light conditions a fish would experience.
To determine whether Topeka shiners passed through culverts in similar numbers as through control reaches of the same stream, and whether light levels affected their passage, researchers employed a mark-and-recapture process. They caught fish upstream and downstream from the culverts or control reaches, marked them with an identifier indicating where they were caught and released, and then resampled to see where fish moved. They also noted other culvert features that could affect passage, such as water depth and velocity.
To control for confounding variables that could affect fish movement, a laboratory study measured Topeka shiner preference for light or dark channels. Researchers used a 25-foot-long double channel box with water diverted from the Mississippi River. Fish could choose to swim along light or shaded lanes as they preferred in this light manipulation experiment.
What Did We Learn?
Although there has been increasing concern over the potential for culverts to create behavioral barriers for fish and other organisms, this was the first study that quantified these behavioral effects for fish passage. Light levels in large box culverts were not identified as a potential barrier to the fish communities present in southwestern Minnesota. Two out of the three culverts monitored showed reduced fish passage compared to the control reaches; however, fish—including Topeka shiners—were able to pass through all three.
The longest and darkest culvert had the greatest difference in movement between the culvert and the control, but this variation could not be attributed solely to light levels. This finding was supported by experiments at the St. Anthony Falls Laboratory, where fish that could select either a shaded or lighted channel showed no avoidance of the shaded channel regardless of the shading level.
The light measurements in three culverts yielded an extensive data set that can be used to model light levels through culvert barrels. Light levels at the water surface depended on the culvert entrance, dimensions, construction material, orientation and elbows, while light levels in the water column were also affected by turbidity.
The conclusions of this study apply only to Topeka shiners and other small warm water fish species, and to large box culverts like those studied. Additional research is required to investigate possible barriers created by smaller, darker culverts to light-sensitive fish species and the interactions between light and other variables such as velocity.
Kentucky bluegrass, the grass species that MnDOT typically uses for roadsides, was sensitive to salt; many installations could not tolerate winter deicing salts and died. Research on salt-tolerant grasses begun in 2009 resulted in MNST-12, a grass mix of fine fescues (with 20 percent Kentucky bluegrass for sod cutting, transport and installation stability) that is more salt-tolerant. MNST-12 was installed at many roadsides sites but by 2013, many MNST-12 installations were not thriving. Research into the reasons for these failures and the ways to best establish and care for MNST-12 revealed that this salt-tolerant grass mix requires a different planting and irrigation regimen than the standard MnDOT protocols that had been used for decades on Kentucky bluegrass. When installed as seed, MNST-12 should be planted in August or September; when installed as sod, it can be laid between May and November if sufficient irrigation is available. MNST-12 roots slowly and needs a particular irrigation regimen in early stages. Moisture replacement of 60 percent of its evapotranspiration rate is sufficient to promote establishment.
“The [highway] construction critical path for program delivery rarely includes a biological requirement for establishing vegetation. For salt-tolerant fescue grasses, planting dates and irrigation regimens matter,” said Dwayne Stenlund, Erosion Control Specialist, MnDOT Office of Environmental Stewardship.
“Basing irrigation approaches on evapotranspiration could reduce water consumption and, ultimately, the cost of establishing areas by sod,” said Eric Watkins, Professor, University of Minnesota Department of Horticultural Science.
What Was the Need?
Minnesota has more than 24,000 acres of green, grassy roadsides, ranging from street terraces to Interstate high-way medians. These roadside environments have many stressors, including heat, drought, insects, weeds, traffic and salt.
MnDOT has traditionally used Kentucky bluegrass for turfgrass, but its poor salt tolerance has resulted in many failed installations. Seed and sod research begun in 2009 produced MNST-12, a salt-tolerant grass mix of mostly fine fescues. By 2013, however, many roadside installations of MNST planted under MnDOT’s standard turf care protocols had failed and the reasons were unclear.
Replacing an acre of failed sod costs up to $25,000. MnDOT needed to learn why the turf failed and find the right methods to establish and care for salt-tolerant grass.
What Was Our Goal?
Researchers sought to assess installations of MNST seed and sod across the state to determine the planting and care practices that resulted in successful establishment or in failure. They also wanted to identify best practices for salt-tolerant turf establishment and care.
What Did We Do?
The study had two phases. In the first phase, researchers identified 16 roadside sites located throughout the state with salt-tolerant turf that had failed or performed poorly. They assessed these sites from July 2013 through 2014, and gathered detailed information about the sites from MnDOT, landscape contractors, sod producers and weather data sites. Information included date and time of installation, sod or seed used, temperature and precipitation at installation, and irrigation and mowing protocols. At each site, researchers also took measurements of ground cover, salinity, temperature, moisture content, surface hardness and depth of soil to top of curb. Soil samples were tested for pH, available phosphorus and organic matter.
Beyond variations in soil moisture, it was unclear whether any other soil aspect promoted the success or failure of site turf. Homeowners at various locations suggested that installation date and supplemental irrigation might have influenced a site’s success.
In the second phase of the study, investigators identified the best management practices for MNST by examining three factors that could influence turf performance:
Use of soil amendments during establishment. Researchers examined the effects on MNST performance of seven types of soil amendments, from slow-release fertilizers to various composts, used in trial plots.
Timing of seed and sod installations. Subsections of large trial plots at St. Paul and Blaine, Minnesota, were seeded or sodded monthly starting May 1 through Nov. 1. The watering regimens followed MnDOT’s 2014 specifications.
Post-installation watering regimens. Researchers planted sod plots of MNST and Kentucky bluegrass in a controlled outdoor area using an automatic sheltering system that protected the test areas when it rained. Irrigation was carefully controlled to test seven watering regimens. Researchers studied turf cover, root growth and shear strength of grasses at the sites.
What Did We Learn?
Soil amendment treatments had little effect on turfgrass performance, whether the plots were seeded or sodded.
MNST planted as seed cannot tolerate the heat and frequent drought conditions of Minnesota’s summers during establishment. Seeding should therefore only occur in August and September. Sod may be laid between May and November, provided there is adequate irrigation.
MNST differs biologically from Kentucky bluegrass and has different watering needs. Root development occurs more slowly and requires a longer period of irrigation during establishment, thriving with moisture replacement between 60 and 100 percent of the evapotranspiration rate. With adequate water, MNST establishes well.
MNST should not be mown until roots are established several inches into the soil profile. Drought-stressed turf should not be mown.
Revisions to MnDOT’s specifications and guidelines are needed. In addition, MnDOT will need to adjust its previous recommendations for watering MNST-12 sod to ensure a successful installation. Further, guidelines for designers and inspectors must be updated. MNST is a different grass community than Kentucky bluegrass–dominated sods: The perception of what “success” looks like must be changed, and this change can be best accomplished through images. New methods of irrigation will need to be devised and implemented. Providing water only as the plant needs it could result in considerable savings in water and labor over time. Additional studies related to best management practices are pending.
The Tailgate Test Kit quickly and easily identifies flocculants that reduce turbidity in construction stormwater discharge. The mobile test setup efficiently determines which of the many available products works best for a particular construction site. In this study, 13 product combinations were tested. A short list of five tests was developed, as well as worksheets to aid in calculating the amount of flocculant needed and developing scale-up procedures.
“The Tailgate Test Kit is a cost-effective innovation that will help us determine the flocculant and quantity of product to use in the field and in real time,” said Dwayne Stenlund, Natural Resources Program Coordinator, MnDOT Environmental Stewardship.
“It’s important to add to the body of knowledge in this area,” said Joel Toso,
Senior Water Resources Engineer, Wenck Associates, Inc. “The Tailgate Test Kit is already being used in the field to help both contractors and maintenance workers make decisions.”
What Was the Need?
Stormwater runoff from construction sites often carries sediment from soil erosion, causing the water to become cloudy or turbid. Federal, state and local stormwater regulations prohibit construction sites from discharging water that is too turbid into the environment. Instead, the runoff must be sent to ponds to allow the sediment to settle to the bottom of the pond. The remaining clear effluent may then be discharged from the site.
The chemicals in flocculants speed up the sediment settling process by causing the sediment particles to clump together and fall to the bottom more rapidly. A number of flocculating agents are commercially available. The most effective agent for a specific situation is generally deter-mined by testing various flocculants with water samples in a lab. This selection process usually takes one or two days. Only after the appropriate flocculant is selected can the entire pond be treated.
To speed up this process, MnDOT has developed the Tailgate Test Kit, a series of tests that can be conducted in the field to determine the most effective flocculant, as well as the correct amount, for a specific construction site and soil type. What used to take a day or two to process in the lab now can be accomplished by field crews in an hour or two on the tailgate of a truck, enabling workers to begin treating the ponded turbid water much more quickly.
What Was Our Goal?
The overall goal of this study was to build upon the findings of several recent research projects, including “Flocculation Treatment BMPs for Construction Water Discharges” (2014-25), by developing and improving field methods to reduce total suspended sediment from construction stormwater runoff. A specific aim was to create a method for work crews to test water samples in the field using a mobile test toolkit that contains flocculants identified in previous research. Other goals included determining the most effective amount of the flocculant needed, developing the calculations needed for scale-up once the best product is identified and implementing a test for residual unreacted product.
What Did We Do?
To identify a variety of flocculant product types to evaluate with the Tailgate Test Kit, the research team summarized stormwater best management practices from the literature and from other departments of transportation. Since the effectiveness of product types varies depending upon soil and sediment types and environmental conditions, researchers conducted 13 tests of nine flocculant products (alone and in combination) taken from five distinct product classifications: mineral, polyacrylamide, chitosan, bio-polymer and anionic polyacrylamide. They also tested water samples from eight locations in Minnesota to ensure a cross section of representative samples.
What Did We Learn?
Using the results from these tests, the research team developed a short list of five tests that could be conducted in the field and incorporated in the Tailgate Test Kit. The five tests represent a range of flocculant product classifications and reduce the time required to complete the tests.
The team also prepared worksheets with mixing and dosing guidance to help users identify the most effective amount of product to achieve the target turbidity goal. Finally, the team developed scale-up procedures to aid in using test results to determine full-scale dosing rates on-site and procedures for testing new flocculant products.
The researchers investigated four methods for testing residual flocculant to detect any unreacted product in a sample. A preferred method was not identified during the course of this research but would still be a desirable research outcome.
Next steps for this research effort include field implementation and new product evaluation.
First, investigators recommend developing a training module and field guide for using the Tailgate Test Kit to encourage implementation of the mobile kit throughout the state. If users understand how it works and how to use the test results for scale-up calculations, they will be more likely to use it.
Second, the product list should be kept current by testing additional flocculant products. It may also be beneficial to create a category for flocculants on the MnDOT Approved/Qualified Products List.
Finally, methods to identify residual and unreacted flocculant product need to be developed. If excess flocculant product is used in field tests, the residues will eventually have to be collected and removed for disposal. Minimizing the excess flocculant used at construction sites is desirable.
Researchers identified 14 sites representing destabilized roadway slopes in Minnesota. Following site investigations, lab testing and modeling, researchers recommended eight slope stabilization techniques that local engineers can undertake without the help of outside geotechnical engineers. The methods were packaged in a simple, accessible field guide for county engineers.
“When most studies end, further research is needed. This project, however, created a user guide that local engineers can use right away to repair destabilized slopes,” said Blake Nelson, Geotechnologies Engineer, MnDOT Office of Materials and Road Research.
“This guide includes an easy-to-use flowchart that steers local engineers toward an appropriate slope stabilization technique,” said David Saftner, Assistant Professor, University of Minnesota Duluth Department of Civil Engineering.
What Was the Need?
Winter weather and spring storms leave their mark on slopes along highways and at bridges. Erosion and other forces cut gashes and ravines into slopes. Some damage such as failing pavement at shoulders or sloughed off sections of a slope can be obvious to road users. Other, more subtle signs of creeping embankments may only catch the attention of engineers.
Slope failures must be repaired to prevent damage to roadways and embankments. When slope damage is severe, a geotechnical engineering firm must step in at some expense. By the time the first soil sample bore is pulled, county engineering departments may already be facing a bill of $20,000. But when damage is less severe, the county can often stabilize the slope using local materials and simple techniques.
Determining whether slope damage can be completed by local engineers or requires outside help remains a challenge for county road departments that often lack geotechnical expertise.
What Was Our Goal?
The Local Road Research Board (LRRB) funded a research project to determine effective methods for stabilizing damaged roadway slopes. These methods would be incorporated in a guide that local engineers could use to identify the type of slope failure and then select an appropriate repair method.
What Did We Do?
Investigators began by surveying Minnesota county engineering departments to identify sites that needed to be stabilized. Local engineers also provided details about both successful and unsuccessful stabilization methods that have been tried in the past. The re-search team inspected 14 destabilized sites identified in this effort and took soil samples from each site.
Then they conducted a literature review of slope stabilization methods, identifying 12 stabilization techniques. Based on this review, researchers tested the soil samples with direct shear tests to identify shear strength parameters such as effective friction angle and cohesion. They ran soil classification tests to measure plasticity, granularity and gradation, and moisture content. These properties were then used as inputs in slope modeling and parametric studies to examine viable repair techniques for each site.
Investigators summarized their analysis of each case and documented stabilization methods that would meet the needs identified in the case studies. Finally, the research team prepared a slope stabilization guide that local engineers could use in the field to identify the type of slope failure and the appropriate solution.
What Did We Learn?
Five of the destabilized sites featured primarily sandy soil, eight had fine-grained soil, and one was rocky. Slope failure was visible at nine of the sites. Groundwater management figured prominently in most sites and repairs.
The literature search identified approaches for specific types of failures. Managing groundwater and drainage improves shear strength in slide-prone areas; surface covers protect slopes from erosion; vegetation and plant roots stabilize soil; excavation and regrading reduce failure forces; and structural reinforcement features directly support slope materials.
Investigators identified eight slope failure mechanisms that encompassed the full range of destabilization scenarios presented in the case studies. Each method had been identified in survey responses as a technique used successfully at the local level. The site conditions that contributed to the failure were identified along with a repair solution for each failure type.
Using the findings from this project, researchers created a slope stabilization guide for Minnesota local government engineers. This field guide describes common slope failures and conditions that may contribute to each. It includes a simple, three-step flowchart that guides engineers to the appropriate repair technique by determining whether the damage is a creep or rotational failure, whether the soil is cohesive or granular, and if there are groundwater concerns.
Based on engineers’ answers, the flowchart directs them to one or more of the eight slope stabilization techniques, providing photographs and repair methods that have been successful in addressing slope problems along Minnesota roadways.
The Slope Stabilization Guide for Minnesota Local Government Engineers will be sent to each of the 87 county engineering departments. Local engineers can keep the guide on hand when they investigate slope failures along their roadways, and with it quickly identify what work needs to be done to repair the damage.