CNA Engineers designed the protection for the MN-300 interceptor tunnel under the Bridal Veil Falls in Minneapolis. Bridal Veil Falls is fed by a naturally occurring stream that flows to the Mississippi River. Over several decades the falls receded upstream due to bluff erosion, exposing the 6ft tall unreinforced concrete lined sanitary sewer tunnel to rockfalls from the overhanging limestone. Due to the proximity to the Mississippi River, a structural failure of the interceptor could have resulted in sewage flow to the river.

Working as the prime consultant, we designed an aesthetically pleasing 25-foot-tall mass concrete structure to protect the interceptor and sandstone bluff. The use of mass concrete pours resulted in a need to monitor concrete temperatures to minimize concrete cracking. Formliners were also incorporated in the design to provide the finished product with a stone look that blended into its surroundings. The waterfall is staining the wall naturally.

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Ford’s dam and hydroelectric plant are located in St. Paul, Minnesota, on the Mississippi River. The 170-ft-long by 60-ft-tall rock bluff had large rock sections falling off the bluff, damaging equipment and endangering the hydro plant.

Our firm surveyed the bluff, mapped the bluff from a structural geology perspective - including on rock type, condition, joints, bedding planes and overhangs - and designed a repair. The repair involved removing the large unstable rock blocks, placing concrete protection over power duct lines and placing concrete retaining walls. Our engineers provided the design, contract documents and construction administration for the project.




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A portion of the Gitchi Gami Trail near Split Rock State Park passes between the Lake Superior cliff and T.H. 61. In a critical section about 200 feet long, the trail is between 8 feet and 50 feet from the crest of the 40-ft high cliff. The work included site reconnaissance, including joint mapping, erosion rate assessment, and cliff geometry documentation. After a bridge was chosen to span the critical section, we addressed the stability of the bridge foundation based on rock wedge stability analysis.

Lock and Dam #1 is located in Minneapolis on the Mississippi River. The cribwall, originally constructed in the 1950’s, was tipping and experiencing a number of structural and geostructural problems. The massive retaining system is over 1400-feet long and stands over 70-feet tall. Above the wall was 5 feet of concrete wall and 15 feet of unsupported limestone bedrock that was failing and causing damage to the existing structure due to rock falls. Adjacent to the wall was a 500-ft long limestone and sandstone bluff, 60-ft high that was failing and causing rock fall damage as well.
The limestone and concrete walls were stabilized by placing rockbolts and a reinforced shotcrete wall. The 50-ft tall bluff was stabilized by shaping the bluff to a stable configuration and placing rockbolts, and mesh and a separation wall.
Our engineers evaluated the retaining structure and performed analysis to design a system to stabilize and prolong the cribwalls useful life. In evaluating the structure, our engineers mapped structural deterioration and anomalies, cored concrete samples and conducted concrete and soil examinations. Finally, we performed a finite element analysis and designed a deep anchor tieback system to stabilize the wall.

Tunnel portal construction for the Silver Creek and Lafayette Bluff tunnels along the North shore of Lake Superior required the creation of new rock cuts up to 220-ft high at the tunnel entrances.
CNA Engineers performed the geotechnical investigation and designed the cuts. We also mapped the existing rock structure and performed kinematic analyses of rock slope stability. Rockfalls were modeled to assess the required depth and width of rockfall ditches.

A vacant lot in Stillwater, Minnesota, was purchased by a private individual to be converted into a children’s park and donated to the city. The lot had unstable rock bluffs up to 60 feet tall with overhangs up to 10 feet that needed to be repaired. We were hired to design a repair for the bluff above the park.
Our firm mapped and surveyed the entire 190-ft-long by 60-ft-tall rock bluff. Then designed a repair that utilized removing unstable rock sections caused by overhangs, adverse bedding planes and joints to reduce rockfalls. The upper bluff was left natural and the lower weaker rock bluff was protected with a concrete wall that was formed and stained to match the overlying natural rock, leaving a natural looking bluff.