Tag Archives: bridge scour

Bridges and Structures

Riprap grout protects bridge abutments

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Bridges over Minnesota waterways need to be protected from currents by a field of interlocking angular rocks called riprap. Without these rocks along the abutment, moving water could wear away the soil that supports a bridge’s foundation. The faster the water, the larger the riprap must be to provide adequate protection.

While some parts of Minnesota have quarries rich with angular rock, other parts don’t – particularly the northwest and western regions. Bridge projects in those areas sometimes resort to the expensive practice of trucking in stones. Other times field stones are used, but they are less effective and must be replaced more often.

There soon could be a better option thanks to research coordinated by the Minnesota Department of Transportation and funded by the Minnesota Local Road Research Board.

At a few test sites around the state, researchers have used a grout mixture to cement smaller, rounded rocks together at a bridge abutment. Once applied to the rocks, the mixture forms what is called “matrix riprap.” The concept is in use in Europe for many bridge piers, but MnDOT was more interested in learning how it could be used on bridge abutments.

Matrix riprap is currently in use in Minnesota at the following bridges:

  • Highway 23 over the Rum River in Milaca
  • Highway 8 over Lake Lindstrom channel in Lindstrom
  • Prairie Road over Coon Creek in Andover
A MnDOT crew applies grout to rounded rocks at a bridge abutment in Milaca in May 2012. The grout cements the rocks together to form matrix riprap, which has shown to be significantly stronger than conventional riprap.
A MnDOT crew applies grout to rounded rocks at a bridge abutment in Milaca in May 2012. The grout cements the rocks together to form matrix riprap, which has shown to be significantly stronger than conventional riprap.

In May 2012, matrix riprap was placed at the Milaca bridge, which sits alongside a high school. Researchers hoped the use of matrix riprap would prevent vandals from removing the riprap rock and throwing it into the river. According to Nicki Bartelt, a MnDOT assistant waterway engineer, the matrix riprap has proven to be extremely strong and effective.

“Not only is matrix riprap significantly stronger than regular riprap, but it helps prevent vandalism as well,” Bartelt said. “The Milaca installation has been in place for three years now. It looks pretty good and it’s weathering well.”

In the lab, matrix riprap held up extremely well on mechanical pull tests and hydraulic flume tests. In fact, researchers were unable to determine the matrix riprap fail point on many tests, even after applying 10 times the shear stress that regular riprap can withstand. Matrix riprap was tested with both angular and round rock with no change in performance.

A new matrix riprap installation recently went in on the Highway 95 bridge over the Rum River in Cambridge. Later this summer, plans call for an installation on the Highway 60 bridge over the north fork of the Zumbro River in Mazeppa.

“The Highway 60 bridge is being replaced, and the river there has extremely high velocities, so we’re using the matrix riprap instead of regular riprap just because of the size of rocks that would be needed,” Bartelt said.

At least two more installations are planned for 2016. In the future, researchers plan to determine the fail point for matrix riprap. They also hope to study potential environmental effects the grout may have underwater.

MnDOT has also worked with local governments that have tried matrix riprap for themselves. One municipality is trying it as a heavy duty erosion control measure. The concept is catching on outside Minnesota as well.

“We have gotten a lot of inquiries from other states, and we have lent out the spec a lot,” Bartelt said. “Iowa, New Hampshire, Maine, Indiana, Wisconsin and Illinois are among the states to express interest. We have talked to a lot of people about it, so they tend to use our research.”

Read the research

Bridges and Structures

Continuous Scour Monitoring Improves Bridge Safety

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A leading cause of bridge failure is bridge scour, which occurs when rapidly moving water erodes riverbed soil around abutments or piers.

Monitoring bridge scour with traditional inspection methods can be dangerous and difficult, so MnDOT has been working with researchers from the University of Minnesota’s St. Anthony Falls Laboratory to develop a continuous monitoring system to test certain bridges more safely and efficiently.

MnDOT currently monitors 45 scour-critical bridges — and local Minnesota agencies monitor 360 more — using visual inspections or water data websites during flooding events. Once a predetermined threshold is exceeded, portable scour monitoring equipment is deployed to measure scour depth.  If scour has undermined the foundations of a bridge, inspectors close it for repair.

But portable scour monitoring systems can be difficult and dangerous to deploy from the bridge deck or boat in fast-moving water. It can also be difficult to get inspectors to sites quickly enough in areas subject to flash flooding.

A better alternative for such situations are fixed scour monitoring devices that continuously monitor scour and send data wirelessly to bridge personnel, alerting them when scour reaches a dangerous level.

The Highway 43 Bridge in Winona was affixed with continuous bridge scour monitoring equipment.
The Highway 43 Bridge in Winona was affixed with continuous bridge scour monitoring equipment.
New technology

MnDOT has not historically made use of fixed scour monitoring equipment, but as advances in technology have made these devices more affordable and reliable, the agency  became interested in exploring the use of fixed monitoring equipment at locations where the use of portable equipment is problematic. ( A major concern for fixed scour monitoring is damage from debris and ice.)

Researchers have installed fixed remote monitoring stations on four such bridges.

Stations on the first two bridges (Highway 14 over the Minnesota River in Mankato and Highway 43 at the Mississippi River in Winona, pictured at top) ran successfully for three years, with outages due to primarily to power and communication issues.

Researchers learned valuable lessons from these bridges and have now installed monitoring equipment on two more: The Old Hastings Bridge (Highway 61 over the Mississippi River), on which float-outs were installed; and the Dresbach Bridge (Interstate-90 over the Mississippi River), which had a tilt meter and underwater sonar device installed.

“The less familiar personnel are with technical equipment, the less they tend to use it,” said Andrea Hendrickson, State Hydraulics Engineer, MnDOT Office of Bridges and Structures. “This research project gave us the familiarity and technical information we need to be comfortable using fixed scour monitoring equipment on bridges that warrant it.”

Related Resources

*Editor’s note: This article was adapted from an article in the latest issue of our newsletter, Accelerator. Read it online, or sign up to receive it by mail. 

Bridges and Structures

MnDOT tests new technologies to monitor bridge scour

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A research implementation project could provide MnDOT with a new set of tools to help combat a major source of bridge failure.

The MnDOT Bridge Office is testing several new methods of monitoring bridge scour — erosion that occurs around bridge piers and abutments during high water-flow events like floods. Acting Waterway Engineer Nicole Danielson-Bartelt said the project’s goal is to be able to monitor scour-critical bridges remotely rather than sending maintenance personnel out on the water during difficult or hazardous conditions.

“There are a number of bridges that are pretty difficult to monitor, especially during high water events,” she said. “Typically, you need to get out on a boat and do either sonar readings or drop weights. It’s dangerous work to be out on the water during those types of events unless you have the right training.”

The project will evaluate several different monitoring technologies, including continuous monitoring equipment like tilt meters and active sonar. The sonar systems, which allows continuous stream bed and water surface elevation data to be transmitted to a website for graphical display, could provide benefits that go beyond monitoring individual bridges.

“The ability to collect continuous, long-term data could help engineers understand short term scour-fill and long term aggradation-degradation cycles,” said Solomon Woldeamlak, a Bridge Office hydraulic engineer. He added that the data can be used to calibrate existing methods of estimating scour at bridges.

Other devices being tested include “float-out” devices, which are buried in the sand around the abutment and send out a signal only if washed to the surface by a scour. Danielson-Bartelt said these non-continuous monitoring devices might be appropriate for bridges where installing permanent sonar is not advisable due to the presence of debris that could damage the equipment.

Monitoring equipment has been installed at two locations: the Highway 43 Winona bridge over the Mississippi River and the Highway 14 Mankato bridge over the Minnesota River. A final report on the project is expected in late 2014/early 2015. You can learn more about some of the products that are being tested on the website of ETI Instrument Systems, Inc., which provided the equipment.