Central Concrete completes concrete construction in 30 hours

Concrete pumping technology recently came to the rescue on a failing dam in southern Minnesota. During a routine inspection of the Rapidan Dam new Mankato, MN, maintenance crews noticed sunlight seeping through the bottom of the hollow concrete structure. Upon further investigation, it was discovered that the dam’s concrete spillway had completely broken away. Water action was quickly eroding the sandstone bedrock on which the structure was built. Barr Engineering Company, Minneapolis, MN, whose consulting services include dam break analysis and emergency action plans, informed Blue Earth County officials that most of the 200-foot spillway was undermined, with scour depths of up to 30 feet. It was soon evident that the 92-year-old, 82-foot-high dam was on the verge of collapse.

Failure of the Rapidan Dam could potentially devastate areas of Blue Earth County. The dam prevents several tons of silt laced with 30-year-old farm chemicals from contaminating the Blue Earth River. Mankato area ground water, nearly 450 acres of wetlands, and a $700,000 electrical generator system were all in jeopardy if contractors failed to rescue the structure.

Only days after the discovery, Barr completed designs and prepared documents for emergency bidding. The Emergency Response Team from the U.S. Army Corps of Engineers, St. Paul, Minn. district, provided funding and took over the implementation of the construction phase of the project. The Corps solved the immediate threat of failure through building a rock berm in front of the dam to allow access and to temporarily close off the scour holes beneath it.

General contracting company Southern Minnesota Construction Co., Inc., Mankato, began construction hours after being awarded the project. Crews immediately began work on a haul route running across the Blue Earth River and 600 feet down stream. Crews also built an access road to allow truck mixers and a truck-mounted boom pump from Central Concrete, Inc., Mankato, to back up next to the structure. Superintendents from Southern Minnesota and Central partnered to devise a concrete mixture and the system to fill the voids under the dam buttresses and stabilize the structure.

To supply the bottom of the dam with concrete, Central crews drilled several holes measuring 8 inches wide, some as deep as 10 feet, into the buttresses of the dam. The pumper brought in one of their two truck mounted boom pumps, a 32 XL, to reach through the holes and pump into the undermine.

“Pumping was the only way to complete the project,” said Sheila Reynolds, Operations Manager with Central. “The only alternative would have been to hand carry five-gallon buckets and devise a system to funnel the concrete through the holes. It certainly wouldn’t have been economical or efficient.”

Because of the urgency of the situation, contractors couldn’t afford to waste any time. Armed with the Schwing pump and 40 operational ready-mix trucks, Central supplied and pumped a total 2450 cubic yards of tremie concrete into the undermine in a non-stop 30-hour pour. At the end of installation, concrete construction and stabilization was achieved.

“It was very fast by most standards,” said Reynolds, “But it had to be. There were so many factors involved and too many repercussions if our equipment didn’t perform. It was the perfect job for a pump, and we had the perfect pump for the job.”

In the two weeks following concrete installation, more than 12,000 tons of rock fill was placed on the downstream side of the dam. The Army Corps and contributing contractors completed the entire repair project on April 20, 2002, only two weeks after contracts had been awarded.

Since the completion of the project, the organizations and companies involved have been recognized for their efforts. As a current member of the Aggregate & Ready Mix Association of Minnesota, Central Concrete was awarded ARM’s first Concrete Pumping Award in the organization’s history. Central Concrete, Inc./Superior Concrete Block Co. services the Mankato, Sleepy Eye, St. Peter, and New Ulm areas with ready-mix supply, pumping capabilities and masonry and landscape services.

On February 21 the Minnesota Society of Professional Engineers presented the U.S. Army Corps of Engineers, St. Paul District, with a Seven Wonders of Engineering award, recognizing the organization for their work on the Rapidan Dam.