All posts by micaelaresh

Using Debonded Strands to Reduce End Stress in Bridge Beams

A new MnDOT-funded research study has found that most agencies in states with weather similar to Minnesota’s use debonded strands in prestressed concrete bridge beams. MnDOT may begin piloting debonding as an alternative to draping, which manufacturers claim is time-consuming, challenging to worker safety and expensive.

What Was the Need?

Bridge designers often prestress concrete beams with steel strands to improve the performance of the beams. The strands precompress the beams so that when external loads like vehicle traffic are applied, the concrete is less likely to crack under loading.

When the beams are fabricated, the strands are stretched from one end of the concrete form to the other, and then concrete is poured and hardens around the stretched cable. Once the concrete is cured, the cables are released from the precasting bed. When the cables shorten, they shorten or squeeze the concrete they are bonded to in the beam, precompressing it.

Because concrete is effective in compression and poor in tension (it cracks), precompressing the concrete leads to beams that may not crack in service conditions. It also leads to less deflection of the beams under loading. Both outcomes improve the strength, serviceability and durability of the system.

“We were assessing the current state of the practice of debonding strands in prestressed concrete, learning what other agencies have done and how much success they’ve had,” said Catherine French, CSE distinguished professor, University of Minnesota Department of Civil, Environmental and Geo-Engineering.

Prestressing causes high stress at beam ends, which is conventionally mitigated with some combination of two common design approaches. Cables can be debonded at the ends, typically by using a sleeve of a limited length that prevents concrete from directly bonding with the strands where covered. Draping strands can also help with end stresses by reducing the eccentricity of the strands. While regular strands run parallel to the length of the beam, draped strands are pulled in a somewhat V-shape, from the top of the beam at each end to the bottom of the beam in the middle.

MnDOT currently uses draping to relieve the end stresses, but it does not allow debonding due to its potential to provide a path for chlorides to enter the concrete along the debonding tubes, which could lead to corrosion. Manufacturers would like to rely less on draping, which requires more time, cost and care to safely fabricate. Local agencies are interested in debonding because draping requires thicker and more expensive concrete beams. National standards offer the choice of draping, debonding or a combination of both.

What Was Our Goal?

Researchers investigated the state of the practice for using debonded strands in prestressed concrete beams. MnDOT and the Local Road Research Board (LRRB) needed recommendations for using debonded strands to position the agency to adopt current and imminent national debonding standards for prestressed beams and to use debonding as an alternative to draping, where appropriate. 

What Did We Do?

Researchers studied current MnDOT prestressing specifications, prestressing and debonding guidelines established by the American Association of State Highway and Transportation Officials (AASHTO), and research on debonding and draping. They also surveyed 11 agencies in 10 states with climates similar to Minnesota’s about their use of debonding and its performance in terms of reducing beam end stresses and resisting corrosion. They followed up with some respondents to gather more detail on respondents’ practices and experience.

Debonding sleeves protect prestressing strands from bonding with concrete, reducing stress and cracking at beam ends. 

Plastic sheathing is wrapped around a portion of bonded prestressing strands.
In addition to conducting the survey, the research team met with two fabricators who produce MnDOT prestressed concrete beams to review prestressing, debonding and draping procedures, and visited the plants to observe the process.

What Did We Learn?

Debonding appears to reduce cracking at beam ends. Currently, AASHTO allows debonding of up to 25 percent of prestressing strands in concrete beams, though this may soon be revised to allow a higher debonding limit. AASHTO’s T-10 Technical Committee proposes allowing up to 45 percent debonding, while NCHRP Research Report 849 recommends allowing up to 60 percent of strands be debonded.

“Researchers did not find that there was any excessive corrosion with debonded strands. The team is recommending we start at debonding 40 percent of prestressed strands,” said Brian Homan, State Aid bridge plans engineer, MnDOT Bridge Office.

Ten of the 11 responding agencies use debonding, typically in coordination with sealing beam ends with silicone or similar material to protect sleeved cables from water and salt intrusion. Five of the 10 reported debonding as their primary method for reducing end stresses, and three indicated draping as their favored approach. Six limit debonding to 25 percent of strands, though others allow a higher percentage, including Michigan DOT, which allows up to 40 percent of the strands to be debonded. Respondents reported few problems with debonded strands.

Researchers recommend that MnDOT begin debonding up to 40 percent of its strands to refine the practice before it considers adopting the 60 percent standard. Two split sheath tubes, one over the other, should be applied to the strands to achieve debonding in the end regions.

Concrete ends should be sealed as MnDOT currently requires, and silicone sealant should be applied to exposed strand ends. The research team recommended a sequence for releasing prestressing cable to minimize cracking at the beam ends—bonded before debonded and shorter debonded lengths before longer lengths.

What’s Next?

Debonding strands costs less than draping and is favored by local agencies; the practice will reduce prestressed beam fabrication costs for MnDOT and the LRRB. Draping procedures present safety concerns that will be relieved by a reduction in draping, and debonding is expected to reduce end cracking.

MnDOT is developing two pilot projects in which up to 45 percent debonding may be used. Future research may be warranted to identify the best percentage of strands that should be debonded and evaluate debonding sleeve materials, designs and performance.

This post pertains to Report 2019-30, “Debonded Strands in Prestressed Concrete Bridge Girders,” published July 2019. For more information, visit the MnDOT project page.

Collaboration with Minnesota Zoo aims to conserve wildlife

Turtles and other wildlife are at risk along Minnesota roadways.

MnDOT is collaborating with the Minnesota Zoo on a new research project installing small animal exclusion fencing. The fencing is intended to redirect turtles (and other small animals) to culverts and bridges where they can cross the road safely.

Blanding’s and wood turtles are listed as threatened species by the Minnesota Department of Natural Resources.

A proactive approach to wildlife conservation will hopefully reduce small animal-vehicle collisions and prevent these species from becoming listed under the federal Endangered Species Act.

Reducing small animal collisions also improves driver safety. Hitting (or trying to avoid) a turtle on the road can cause significant damage and injuries to motorcyclists and bicyclists. It can also be unsafe for drivers to attempt to pull over and assist small animals across the road—especially in high-traffic areas.

Exclusion fences were installed along four Minnesota roadways this past year (Waconia, Highway 5, Scandia, Highways 97 and 7, Eagan, Highway 2) and will be evaluated over the next year.

The research project started in Sept. 2017 and is estimated to be completed in June 2021. The end goal is to develop a standard set of designs and recommendations for future installation along other Minnesota highways. Christopher Smith, MnDOT’s wildlife ecologist is the technical liaison leading this project.

Visit the MnDOT Office of Research & Innovation for project updates.

Low-Temperature Cracking Test Produces Repeatable, Reliable Results

Researchers ran a sophisticated low-temperature asphalt cracking performance test at multiple labs to study the test, its variability and repeatability, and its additional promise in studying reflective cracking susceptibility of overlays. Results put MnDOT closer to implementing test specifications for low-temperature cracking test for pavement mixes.

What Was the Need?

In very cold temperatures, asphalt pavement shrinks and fractures as it pulls from its various restraint points. Low-temperature or thermal cracking is the most widespread distress found in asphalt pavements in cold climates like Minnesota’s.

Pavement designers select an asphalt binder performance grade (PG) based on expected seven-day average maximum and minimum temperatures that the asphalt pavement is expected to experience. A PG 58-28 binder, for example, is supposed to maintain good performance at maximum temperatures of 58 degrees Celsius (136 degrees Fahrenheit) and minimum temperatures of -28 degrees Celsius (-18 degrees Fahrenheit).

PG binder ratings and tests do not characterize asphalt mixtures precisely because PG does not account for aggregate types and gradations, recycled material in the mix, and plant and field aging of asphalt mixtures. Asphalt mixtures must be tested at relevant temperatures.

“The disc-shaped compact tension fracture energy provides a good basis as a reflective cracking performance predictor, as well as for thermal cracking,” said Eshan Dave, associate professor, University of New Hampshire Department of Civil and Environmental Engineering.

For MnDOT to fully implement performance-based specifications for asphalt mixtures, it must better understand mechanistic tests of the susceptibility of asphalt mixtures to certain kinds of stresses. The disc-shaped compact tension (DCT) test has emerged from a decade of study as the best tool for measuring fracture susceptibility of asphalt mixtures at low temperatures and other conditions. Recommendations from a 2015 pilot DCT low-temperature fracture study included repeatability testing trials for DCT testing of low-temperature fracture energy.

What Was Our Goal?

The goal of this research was to evaluate the DCT test to support its use in performance-based specifications for examining fracture properties within asphalt mixtures at cold temperatures. Researchers would develop a fracture energy database for Minnesota asphalt mixtures, refine the test as necessary, characterize the variability and reproducibility of test results by testing mixtures at multiple labs, recommend test specifications or parameters, and evaluate the potential for DCT testing of the susceptibility of asphalt overlay mixtures to reflective cracking.   

What Did We Do?

Researchers identified pavement and overlay projects with asphalt mixtures that could be sampled for DCT testing. Then they compiled MnDOT data on asphalt pavement construction projects, mixture designs and DCT test results, developing a single database for this information to be updated continuously and managed by MnDOT’s Office of Materials and Road Research.   

Using this database, researchers selected 16 asphalt projects representative of pavement and overlay mixtures, and gathered samples of mixtures from each of these projects for distribution to four testing laboratories. The team analyzed test results for all samples at all labs to assess repeatability.

A disc-shaped asphalt pavement mixture sample in the test setup. The disc was instrumented with a temperature sensor prior to testing.
For the DCT test, a disc-shaped asphalt sample was instrumented with a temperature sensor and prepared for pulling tension to simulate its cracking susceptibility while contracting at low temperatures.

The research team then collected field performance data on 15 selected asphalt overlay projects also in the database and ran over 100 finite element models of the mixtures to examine how their fracture energy test results may predict reflective cracking performance in the field.

What Did We Learn?

The DCT testing database included over 6,000 test results as of the end of 2018 and can be updated with new test results as they become available to optimize the use of DCT testing in performance-based specifications.

Typically, DCT testing requires holding a specimen at low temperature during testing for up to 12 hours; investigators refined the method to reduce the holding time to approximately 2.5 hours, in part by instrumenting specimens for interior temperature monitoring. Researchers also determined that reheating plant-produced loose asphalt mixtures to compact samples for DCT testing produced lower fracture energy results.

Researchers identified a fracture energy limit of 90 joules per square meter over which low-temperature fracture energy test results reproduce poorly and found that testing of 12 replicated specimens lowers variability and minimum and maximum fracture energy range values. Increasing binder content and the high- and low-temperature range of PG binders improves fracture resistance in mixtures, as does lowering recycled asphalt content.

“We need to continue to evaluate DCT for mix design. The researchers made good recommendations for overlay projects to combine thickness and fracture energy,” said Shongtao Dai, research operations engineer, MnDOT Office of Materials and Road Research.

Overlay fracture resistance, determined by combining overlay thickness and mixture fracture energy, correlates with improved reflective cracking performance in asphalt overlays. A fracture-resistance value of 50 joules per square meter found in this study may be used to minimize reflective cracking potential in overlays.

What’s Next?

MnDOT is closer to implementing DCT test specifications for asphalt pavement. Turnaround time in DCT testing limits its use in quality assurance and control of asphalt mixtures during production, and MnDOT is researching alternative tests for this purpose. The DCT database may be valuable in calculating and evaluating other performance parameters for their ability in predicting field cracking performance.

Use of the DCT for overlay performance testing and specifications proved promising. Validation and field implementation, as well as further analysis of fracture energy, mixture composition and overlay thickness relationships to reflective cracking performance all warrant more research. 

This post pertains to Report 2019-24, “Disc Shaped Compact Tension (DCT) Specifications Development for Asphalt Pavement,” published June 2019. For more information, visit the MnDOT project page.

Investigating Wastewater Reuse at Rest Areas and Truck Stations

Researchers have provided MnDOT with a comprehensive and practical evaluation of what the agency would need to do to develop wastewater reuse systems for its truck stations and rest areas. Two sites will install the research project’s recommended systems soon. 


Despite Minnesota’s abundance of water resources, 75 percent of the state’s water comes from aquifers at increasingly unsustainable rates. The recycling of wastewater for other uses would allow potable groundwater aquifers to be used more conservatively. Gov. Tim Walz and former Gov. Mark Dayton issued executive orders directing state agencies to make efforts to save energy and water. Since every MnDOT building uses water, the potential for substantial water reuse exists.

MnDOT sought to learn the potential for on-site wastewater reuse as well as the barriers to implementing, operating and maintaining water reuse systems. Developing a system of water recycling at MnDOT facilities could create sustainable water resources for toilet flushing and vehicle washing.

“The great success of this project arises from its close examination and evaluation of all aspects of industrial wastewater reuse in Minnesota, from regulation to the practical choice of technology MnDOT will be able to sustain into the future,” said Sara Heger, research engineer, Water Resources Center, University of Minnesota.

To that end, MnDOT initiated this research project to evaluate when reuse would make sense from a regulatory, environmental, economic and management perspective at more than 50 rest areas and 137 truck-washing stations and storage facilities.  

What Was Our Goal?

The objective of this project was to investigate and evaluate the potential for wastewater reuse within MnDOT. As part of the project, researchers would identify regulatory challenges, properties of wastewater streams, contaminants that would be difficult to remove or manage, potential treatment technologies and the MnDOT processes that would be suitable candidates for wastewater reuse. In addition, researchers intended to examine the possibility of recapturing salt-laden wastewater for further use in winter road maintenance.

What Did We Do?

The project’s work progressed in three phases:

  • Review of current wastewater reuse policies and regulations in Minnesota, wastewater reuse programs in other states, as well as international guidelines for water reuse. Researchers reviewed case studies about successful wastewater reuse systems implemented in Minnesota and examined potential regulatory barriers to wastewater reuse implementation.
  • Sampling and data collection from 11 MnDOT truck-washing facilities to determine common contaminants. Samples were taken year-round from an equal number of facilities with holding tanks and from those served by city sewer systems. Researcher collected 37 winter samples. In addition, a nonwinter sample was collected from each site to compare the characteristics of winter versus nonwinter samples.
A MnDOT maintenance worker and a University of Minnesota employee take wastewater samples from a waste trap at a MnDOT truck-washing facility.
At truck facilities connected to city sewers, researchers took wastewater samples from flammable waste traps while trucks were washed.
  • Evaluation of existing wastewater treatment technologies to identify those meeting MnDOT’s needs. Researchers investigated methods that could most effectively remove the identified contaminants from the wastewater while allowing chlorides to remain for brine production. Researchers also determined cost estimates over an assumed 25-year life span.

What Did We Learn?

Policies and regulation. Minnesota’s current state and federal regulatory framework addressing wastewater reuse is dispersed over multiple agencies, and rules can be contradictory. Currently at least four agencies are involved in wastewater regulation. Wastewater reuse systems require a variance and in some cases additional permits from local agencies. Researchers reported that the regulatory framework for water reuse in Minnesota needs to be simplified and streamlined to create a more effective permitting process.

Researchers noted that California has been a pioneer in water reuse since 1929. Florida is a national leader, recycling more than 727 million gallons per day, while Arizona developed effective water reuse regulations in 1972 and has been a regulatory model for Texas, New Mexico and Montana. Investigators suggested that Minnesota could adopt water use standards modeled on Arizona’s as the state clarifies its regulatory framework.

In addition, researchers reviewed four case studies in which wastewater reuse systems were successfully implemented.

Wastewater sample data. Researchers tested samples for volatile organic compounds, metals, fecal coliform, mercury, suspended solids, biochemical oxygen demand (BOD) and chlorides. They found excessive levels of only one chemical, ethylene dichloride, which is commonly used as a solvent; MnDOT could reduce its use of it. Across all samples, the critical contaminants to be removed for water reuse were organics (BODs) and suspended solids. Excessive chlorides were not considered a contaminant since chloride-rich water could be reused as brine in winter road maintenance operations.

“This innovative study’s findings will result in the implementation of wastewater reuse technology at two MnDOT sites. The project promises to be a continuing success,” said Neile Reider, administrative engineer, MnDOT Office of Maintenance.

Recommended technology. Researchers recommended that MnDOT use either a recirculating sand filter or a membrane bioreactor to treat wastewater. An economic evaluation comparing long-term costs of the two technologies indicated that the membrane bioreactor is the most economical system for MnDOT.

What’s Next?

A pilot implementation is planned for the Granite Falls, Minnesota truck station, a facility that is already plumbed to separate and collect washdown water. Evaluation of the system’s treatment effectiveness and maintenance requirements will inform the broader scale implementation of wastewater reuse systems for MnDOT.

This post pertains to Report 2019-22, “Investigating Wastewater Reuse at MnDOT Truck Stations,” published May 2019. Visit the MnDOT project page for more information.

Smartphone App Alerts Drivers Exceeding Speed Limits on Curves

Researchers have developed a proof-of-concept curve speed warning system for use with mobile phones, a technology they hope car manufacturers might adopt for in-vehicle systems. The proof-of-concept system uses data from local road agencies on curve locations, speed limits and signage with geofencing to trigger cloud-based data alerts to road users driving faster than recommended speeds for curves.

What Was the Need?

Over one-quarter of fatal highway crashes occur at horizontal curves. In Minnesota, these areas are a contributing secondary factor in 49 percent of fatal crashes. Each year, accidents on two-lane, two-way highway curves injure over 4,000 people and result in 70 deaths, almost one-fifth of annual roadway fatalities in the state.

Research has shown that dynamic message signs with speed detection components work well in warning drivers to reduce speeds, but these systems require power supplies and cost approximately $14,000 per site. Minnesota’s Otter Tail County alone has over 400 reduced speed curve sites. The Local Road Research Board (LRRB) and MnDOT have been funding research that examines alternative approaches to speed warning systems for drivers approaching curves.

“This smartphone application stitches together existing technologies for GPS, GIS and mapping to provide an inexpensive, cloud-based warning system for drivers,” said Richard West, public works director, Otter Tail County.

In a 2015 study, researchers developed an in-vehicle, vibration-based warning system tested in a driving simulator that relies on data from highway sensors and other sources. Research in 2018 focused on the use of GPS signals to calculate and recalculate a vehicle’s trajectory on roadways to issue warnings. A new phase of this study is refining the approach to draw on vehicle-to-vehicle data.   

What Was Our Goal?

The goal of this research was to develop a dynamic curve speed warning system that would employ cloud-based data sharing. The system would not require significant infrastructure investment and would be applicable to all reduced speed curves in the MnDOT highway system.

What Did We Do?

Following a literature search, researchers focused on developing a proof-of-concept smartphone app that would warn drivers of upcoming curves and speed reduction requirements. They also created a database for county road agency managers to input curve locations within their jurisdictions, speed limits and sign facing direction for use with the smartphone app.

Researchers layered the database into their geographic roadway inventory tool, which draws on GPS and mapping data, and combined data from the sources into a cloud-based curve database. Then they developed a geofence system that triggers alerts as the tracking device crosses virtual geographic boundaries.

A smartphone app uses GPS and GIS to trigger a warning via the cloud to smartphone users traveling above the curve speed limit as they pass through a geofence, or virtual geographic boundary, before the curve.

An illustration of the system overview that includes a curved road with geofencing overlays marking two warning areas on the curve, a cloud-based warning database and a smartphone screen showing a “Reduce speed” message.
A smartphone app uses GPS and GIS to trigger a warning via the cloud to smartphone users traveling above the curve speed limit as they pass through a geofence, or virtual geographic boundary, before the curve.

The curve speed warning system was tested on roadways in Otter Tail and Pope counties. After county agencies input curve location and speed limit data into the system, researchers tested the system by running the app while driving a number of highways selected for a high density of reduced speed curves. They adjusted the system based on these field tests to accommodate GPS signal speed, travel speed and cloud data transfer bandwidth.  

Researchers then evaluated roadside dynamic speed warning system safety impacts to determine the potential safety and cost benefits from the cloud-based warning system.

What Did We Learn?

The curve speed warning system worked in proof of concept. GPS and cloud data can be drawn on fast enough to provide warnings in time for drivers to respond. Researchers refined geofencing parameters to only pull data for curves within 30 miles of the vehicle to keep data volumes to manageable levels within standard parameters for mobile phone data packages.

“Study results show that this system works accurately. If data from county and state roads were input, the application could be made available to everybody,” said Bradley Wentz, program director, Advanced Traffic Analysis Center, Upper Great Plains Transportation Institute, North Dakota State University.

The core of the system is the curve database, which requires accurate data input by county road agencies. Testing resulted in one performance error, which was traced to incorrect data for the facing direction of a warning sign.

Image of the secondary warning on a smartphone screen showing the message “Slow Down,” the recommended speed of 45 mph and the driver’s current speed of 51 mph in a red circle.
The smartphone app sends a second warning with this message and an audible signal to a driver’s phone.

When vehicles are traveling faster than the speed limit for an upcoming curve, the smartphone app issues a silent, on-screen warning of the approaching curve and speed limit. If the vehicle does not slow its speed sufficiently, the app flashes and issues another warning with an audible signal. 

The smartphone app sends a second warning with this message and an audible signal to a driver’s phone.

Safety implications may match crash and speed reductions identified in research on the safety benefits of dynamic sign warning systems. Researchers believe the cost to maintain the software and warning database roughly matches the cost to maintain a traditional dynamic speed warning sign system. But using a single cloud-based system for the entire roadway inventory offers a dramatic cost savings over installing expensive warning sign systems at every curve.

What’s Next?

Researchers have prepared presentations for local audiences and presented findings at the 2018 National Rural ITS Conference. County road agencies can easily update the database, and the system can accommodate not just reduced speed curve locations, but any reduced speed needs, such as seasonal bumps and cracks in pavement, work zones, special events and controlled intersections.

This post pertains to Report 2019-19, “Cloud-Based Dynamic Warning System,” published June 2019. For more information, visit the research project page.

Using Regional Materials to Manage Stormwater Runoff

Researchers determined that natural soil amended with locally sourced materials performed well in bioslopes and bioswales. This practice will allow MnDOT to avoid hauling in costly commercial materials for stormwater management installations.

What Was the Need?

Following the requirements of the Minnesota Pollution Control Agency, MnDOT designs and constructs roadways with means to contain and manage the first inch of stormwater runoff—the “first flush”—flowing from impervious pavements. The agency often accomplishes this with low-impact development (LID) practices such as bioslopes and bioswales (shallow ditches) along roadways that mimic the original landscape. Constructed of tested soil mixtures and vegetation, bioslopes and bioswales effectively absorb runoff and filter sediment, heavy metals, chemicals and other pollutants that wash from the roads, preventing entry into the watershed.

In previous MnDOT highway construction projects, soils that are unsuitable for supporting pavements, such as peat and muck from wetlands, are dug out and hauled away as waste. Commercial compost mixtures are transported to the site for bioslope and bioswale construction. These hauling operations are expensive. To reduce costs, MnDOT has been investigating the use of natural materials close to construction sites and previously discarded as waste as possible filtration media in LID stormwater management.

A cross section drawing of an engineered bioslope design shows the edge of the pavement, a filter strip of grass at the top of the slope and the section of filter material with an underdrain surrounded by small rock buried beneath it. Blue arrows show the flow of water.
This cross section of a common engineered bioslope shows the position of a vegetation filter strip, a section of biofiltration material and an underdrain pipe beneath it. The blue arrows indicate the flow of water.

Phase I of this project examined the infiltration capabilities of compost mixtures and naturally abundant peat and muck in northeastern Minnesota as alternatives to commercial soil and sand in bioslopes and bioswales. Researchers conducted laboratory tests and constructed pilot plots, monitoring their performance. The current project continued the lab and field investigations, and developed a new peat-based biofilter based on research findings.

What Was Our Goal?

The primary objective of this project was to apply the results of the first phase of study to the design and construction of bioswales and bioslopes using local alternative media. An essential corollary objective was to determine whether lab tests and procedures could accurately predict and monitor the performance of these media in the field. Further, researchers sought to determine the effects of aging on alternative media performance.

What Did We Do?

The multidisciplinary research team addressed all aspects of alternative filter media suitability, from water retention and infiltration capacity, to contaminant-filtering effectiveness, to the ability to grow and support vegetation. First, researchers conducted a comprehensive literature review of bioslope and bioswale designs and methods of monitoring filtration media properties on-site. They examined best management practices, including state and federal regulations. Next, they collected treatment media from across Minnesota, including salvaged peat and muck, and commercial peat from an approved source. Researchers evaluated the media and determined potential filtration mixtures based on the results of the previous project and MnDOT standards.

“Lab and field investigations showed the salvage and reuse benefits of muck and other organic materials for slope and ditch topdressing to retain the first flush of rain from roadways,” said Dwayne Stenlund, erosion control specialist, MnDOT Office of Erosion Control and Stormwater Management.

In situ and lab materials from nine established and newly constructed biofilter sites were tested. These sites were constructed between 1990 and 2014, and the main soil filtration medium was amended with compost, peat or muck. On-site materials were evaluated for compaction, conductivity and absorption while lab samples were analyzed for metal and other contaminant retention.

Researchers also monitored six pilot test sites from Phase I. Three plots were prepared with native soil mixed 50/50 with compost and three plots with native soil mixed 50/50 with peat. Plots were examined and data were retrieved from instruments previously installed to measure rainfall, soil absorption and ambient temperature.

The team also established and monitored a new peat-based biofilter stormwater system using selected filter materials and designs. The new biofilter was installed throughout 5.7 miles of new road construction at Eagles Nest, which included extensive bioslopes and a bioswale enhanced with an underdrain system to resist silt clogging.

What Did We Learn?

Most sites showed deficiencies in nutrient and organic matter to support plant growth, and were dominated by weedy plants. Additional fertilizer and organic matter along with appropriate seeding could assure a good cover crop of grasses.

“This project’s results allow MnDOT to use in situ soil to build bioslopes and bioswales to retain the first inch of roadway runoff and associated pollutants. Using in situ materials rather than transporting new materials to the site will save taxpayer dollars, said” David Saftner, department head, University of Minnesota Duluth Department of Civil Engineering.

Compost and peat showed comparable effective water absorption and infiltration performance in amended biofilters. Lab tests can conservatively predict field performance. Researchers noted that early trends should be reinforced by further monitoring.

Peat’s performance shows it is a good alternative to compost for removing metals and phosphates in biofilters. It removes metals as well as compost and leaches less phosphate. Capacities to retain pollutants diminish with age for both media.

What’s Next?

Alternative local filtration media show great promise in stormwater management bio-installations. The next phase of this project will gather additional data from all plots to more fully assess the alternative media’s capabilities over time.

This post pertains to Report 2019-31, “Development and Regionalization of In Situ Bioslopes and Bioswales,” published July 2019. Visit the MnDOT project page for more information.

Evaluating Iron-Enhanced Swale Ditch Checks for Phosphorus Removal

Researchers documented performance of an iron-enhanced ditch check filter to remove phosphorus from stormwater over three years. The filter was effective, but its performance decreased over time, and it will require relatively frequent maintenance. Several design changes may be considered.

What Was the Need?

The Minnesota Pollution Control Agency (MPCA) requires that developments adding more than an acre of impervious land must try to include methods to infiltrate the first inch of stormwater runoff. This “first flush” of stormwater from impervious road pavement contains pollutants that could contaminate watersheds.

To meet its MPCA permit requirements, MnDOT might consider constructing strategically designed ditch blocks and swales: wide, shallow ditches with mounds of engineered soil and sand mixtures and vegetation along roadways that manage stormwater flow.

“Our success in removing phosphorus from stormwater runoff using iron particles in filtering ditch checks showed strong proof of concept. Modification of the filters’ orientation in the ditches could result in a device that is both more effective and low maintenance for MnDOT,” said John Gulliver, department head, University of Minnesota Department of Civil, Environmental and Geo-Engineering.

Previous research has shown that MnDOT’s linear swales are effective filters of contaminated highway runoff. That study also conducted laboratory investigations into enhanced ditch checks—low permeable mounds placed in a swale to reduce water velocity. Researchers tested the ability of various media in an experimental ditch check filter to remove more pollutants. A subsequent MPCA study examined the capability of an iron-enhanced sand trench to remove phosphorus.

In September 2014, MnDOT constructed two iron-enhanced ditch checks in a swale along CR15 (TH5) in Washington County to test two design versions. One failed to function effectively; the other was monitored for four months in 2015, showing effective phosphorus and metals retention.

What Was Our Goal?

The goals of this project were to investigate the long-term effectiveness of iron-enhanced ditch checks in retaining pollutants and to develop recommendations for maintenance actions needed to support effective filter performance.

What Did We Do?

Researchers monitored the performance of the CR15 (TH5) iron-enhanced ditch check from 2016 through 2018 while temperatures were above freezing. A tipping-bucket rain gauge was connected to a data logger to record rainfall at the site. Water samples were collected automatically through tubing at four points: the inflow and outflow of the check dam and filter insert. (Two monitoring wells had been built into the filter.) An automated sampling instrument was triggered by flow through the filter insert. The data logger recorded water levels, flow rate, cumulative flow volume and rainfall depth information continuously at five-minute intervals.

Near the end of the study, workers used a boring machine to take core samples of the ditch check and the filter insert.

Three workers in safety vests and hard hats stand near a tall boring machine taking a core sample of the ditch check filter.
Near the end of the study, workers used a boring machine to take core samples of the ditch check and the filter insert.

Pressure transducers installed inside the monitor wells measured upstream and downstream water levels at the filter insert section. Flow rate through the filter was calculated. Researchers retrieved inflow and outflow samples within 24 hours of the end of a rain event. The University of Minnesota St. Anthony Falls Laboratory tested these samples for phosphorus and metals.

In July 2018 researchers collected three core samples from the ditch check sides and the filter to test for retained phosphorus and signs of diminished performance. 

What Did We Learn?

The iron-enhanced ditch check filter successfully removed phosphorus during the majority of the 40 rain events, reducing the phosphate mass loads between 22 percent and 54 percent. However, the cumulative phosphorus retention in the filter decreased from 42 percent in 2015 to 30 percent in 2016, 25 percent in 2017 and 23 percent in 2018. The core tests confirmed that the bottom 3.9 inches of the filter media filtered most of the inflow volumes of the runoff. This heavy runoff load reduced its sorption capacity over three years while the upper part of the filter was active only infrequently.  

“Enhancing ditch checks with iron filings will aid in the removal of phosphorus in stormwater. However, since the bottom of the filter receives the most flow, more frequent mixing of the iron filings is required than originally anticipated,” said Beth Neuendorf, water resources engineer, MnDOT Metro District.

The ditch check itself showed a somewhat lower phosphorus retention performance than the filter insert, though performance varied some years. Researchers considered that the soil and sod covering may have leached phosphates into the ditch check and filter insert, affecting its overall performance.

Neither the ditch check nor the filter insert were very effective in copper and zinc retention, although the metal concentrations in the inflow and treated runoff were generally lower than in typical highway runoff.

Regarding maintenance, researchers recommended the filter insert medium be mixed up every other year to redistribute the filter media at the bottom. They also recommended that the entire filter insert be replaced after six years.

What’s Next?

Researchers presented possible design changes that could improve performance, such as using peat instead of soil and sod to cover the ditch check. They also suggested installing ditch checks in series and re-engineering the filter to address the findings revealing the heavy runoff load taken by the filter’s bottom 3.9 inches. Reducing the depth of the filter berm could avoid the excessive inundation of the bottom media while the upper portion remains unused. Maintenance frequency could then be reduced.

This post pertains to Report 2019-27, “Iron-Enhanced Swale Ditch Checks for Phosphorus Retention,” published July 2019. For more information, visit the MnDOT project page.

New Project: GIS Tools and Apps – Integration with Asset Management

The Minnesota Local Road Research Board recently funded a project to survey and analyze the use of Geographic Information System (GIS)-based asset management tools for city and county public works departments.


Asset management is critical for local and state government to not only track assets but also plan for maintenance of assets that will provide the greatest return on investment for the agency.  Inventorying of infrastructure assets, such as signs, culverts, roads and bridges, and tracking their location, age and condition allows agencies to predict when maintenance or replacement is necessary and allows them to plan ahead and rather than waiting for asset failure. The use of Geographic Information Systems (GIS) applications, tools and geospatial data in asset management can provide agencies with an accurate inventory of assets, cost-effective mobile tracking, and reporting tools to justify asset expenditures. 

However, GIS use for asset management is varied among local agencies within the state of Minnesota. Tools and usage depends on agency size, urban/rural location, and access to accurate and up-to-date geospatial data, making it difficult for agencies looking to integrate asset management and GIS to understand the best data processes, applications, and tools to use.  This research project will survey and analyze GIS tools and applications for existing and new technology, present local agency case studies, and make recommendations for data best practices, and cost-effective mobile technology use.  

This project, sponsored by the Local Road Research Board, will have several benefits to Minnesotans, including streamlined processes, cost-savings, and increased transparency. Local agencies will use best management practices (BMPs) and case studies to improve operations management systems and use mobile devices to track assets, identify maintenance concerns, and record work orders from the field into the database, thereby increasing efficiency and reducing costs. Leveraging the knowledge and experience of existing users and communicating the results to all the city and county engineers will expedite successful implementation, and increase efficiencies in day-to-day usage. This technology will also increase transparency by helping local agencies manage their data in a streamlined and accessible manner that can be shared.

Project Scope

Researchers from Pro-West & Associates will survey staff from local agencies in Minnesota regarding asset management strategies and share results and case studies, demonstrate mobile technology, and document their findings and recommendations. The focus will be on exploring integration of GIS tools and asset management systems, and helping local agencies implement best practices.

Utilizing GIS for asset management is expected to extend service life and improve maintenance and replacement planning for vital assets, while streamlining processes, reducing costs, and increasing transparency.

Watch for new developments on this project.  Other Minnesota research can be found at

New Project: Use of Innovative Technology to temporarily Deter Bat-Bridge Use Prior to and During Construction

MnDOT has funded a study to evaluate the use of non-lethal ultrasonic acoustic devices to temporarily deter bats from bridges before and during construction projects.


Brown bat on bridge

When protected bats roost or form colonies on bridges, bridge repair and replacement projects have to follow regulatory requirements to minimize impacts on species protected by state and federally regulations – including the Endangered Species Act. These regulations protect bat populations that have already declined due to white-nose syndrome, which is estimated to have killed more the 5.7 million bats in eastern North America since 2006. The northern long-eared bat (Myotis septentrionalis) is listed as a threatened species as it is one of the species most impacted by white-nose syndrome. Other species are anticipated to be protected as the disease spreads.

To minimize disturbance to protected bats, which may use bridges both for day roosting habitat and sites for maternity colonies where bats give birth and raise their young, MnDOT is evaluating the feasibility and efficacy of ultrasonic acoustic devices to temporarily deter bats from work areas on bridges. These devices emit a sound, inaudible to humans, that disrupts the bat’s ability to echolocate and therefore discourages bats from approaching. This new technology has been utilized for wind turbines with positive results.

Ultrasonic device to deter bats

Temporarily deterring bats from a work site saves taxpayers money and increases bat safety. Regulations for protected species can limit activity that is potentially harmful, including bridge work during times when bats may use bridges for roosts or maternal colonies. Without the use of deterrents, work may be delayed until bats vacate the bridge, which may not occur until bats retreat to hibernacula (such as caves and mines) for the winter. Having control over when bats are present will provide more predictable timelines to projects and reduce engineering and administrative costs associated with delays and changes to work plans. Without a control measure, projects must adhere to timing restrictions that increase construction costs and may even reduce bridge life expectancy. And if bats are kept away from construction sites, they will not be directly harmed or disturbed by the activity.

Project Scope

This one-year study will investigate the efficiency and feasibility of ultrasonic bat deterrent technology for temporary exclusion of bat species on bridges by monitoring bat presence before the ultrasonic devices are installed, during a trial period, and after devices are removed. This technology will be tested on two bridges in Minnesota. Findings (expected in May 2020) will determine if ultrasonic bat deterrent technology can be utilized to exclude bats from construction and maintenance work zones, thereby reducing costs and ensuring the safety of protected species.

Watch for new developments on this project.  Other Minnesota research can be found at

Study Underway to Harness Renewable Energy from Minnesota’s Highways

Sound barriers and snow fences along highways have the potential to provide clean energy in Minnesota.

A newly funded MnDOT study, Harnessing Solar Energy through Noise Barriers and Structural Snow Fencing, is investigating how to utilize existing noise barriers and snow fences on Minnesota highways to harvest clean, cost-effective energy.

“Snow fences and noise walls are structural barriers with a singular purpose. Snow fences are intended to limit snow from drifting onto our highways and noise walls are intended to reduce noise to a comfortable level for communities living near our highways. Finding a way to integrate solar that maintains their structural integrity could transform the use of these barriers from single purpose to multi-purpose,” says Dan Gullickson, MnDOT’s Blowing Snow Control Shared Services Program Supervisor, who is overseeing the research project.

Solar energy is energy captured from the sun and converted into thermal or electrical energy. It is a clean and abundant renewable energy source and generally requires very little maintenance after installation. Solar energy has a variety of uses, including providing electricity to power street lamps and homes, heating and cooling spaces, and heating water.

“We’ve seen some applications of solar panels on noise walls—primarily in European countries—but the addition of solar panels to snow fences is an entirely new concept,” says Gullickson.

The innovative nature of this project brings many unknowns that MnDOT hopes to answer, such as: Is it possible to engineer these structures without disrupting their functionality? What safety measures need to be taken to ensure the public and MnDOT workers stay safe if they come into contact with the panels? What are the lifecycle costs of installing and maintaining solar? How much energy could they generate and how does that connect with existing power grids?

One estimate shows that a thousand miles of solar panels could power all the street lights along Minnesota highways or 43,333 residential homes. (Assuming each solar panel is 330W and 1,000 panels could generate up to 330kW per mile.)

Harnessing solar energy on Minnesota highways: Solar energy can be used for heating, cooling, lighting, and warming water. 1,000 miles of solar panels on Minnesota highways could power: All of minnesota highway lights o 43,333 residential homes.

“We know Minnesota and North Dakota winters bring a lot of snow—which is disruptive to our travelers and farmers. We hope to create a sustainable solution that aids drivers and farmers, but also harnessing energy which would be able to offset the cost of construction and installation,” says Mijia Yang from North Dakota State University, the lead researcher.

Gullickson and a diverse team of MnDOT experts – from the field of environmental stewardship to traffic engineering – will guide the research and review findings.

The study will include surveys, lab testing or modeling of possible design options and a cost-benefit analysis—planned to be completed by of June 2021.

Currently, the research team is developing surveys to better understand public opinion on solar energy (including energy prices and solar panel infrastructure), power companies’ interest in purchasing solar energy generated through the right-of-way and legal considerations for harvesting solar energy through the rights-of-way.

 “Surveying the public and utility providers may uncover questions that we hadn’t previously anticipated. We hope to address those hurdles throughout the study,” says Gullickson. Follow along for project updates on MnDOT’s Office of Research & Innovation website.