Gebauer, who was in charge overseeing the design and construction of the project, says the softer clay soil conditions are common to projects he's been involved with in the Fargo area. Fortunate for Shaw and other contractors working on the project, Fargo had a relatively warm and dry spring even with some of flooding that occurred earlier.
"Normally we can expect to get about 10 to 15 years of service life out of a parking lot built on normal soil conditions," Gebauer says. "But this project required some stabilization to prevent shifting and rapid deterioration caused by the frost/thaw cycle we experience in this part of the country."
By going with a soil stabilization process, the owner was also able to reduce the amount of aggregate subbase required to be placed over the treated soil before a final surface covering was placed.
"One-third of the parking lot (used for office parking) was designed with a five inch asphalt surface, while the other two-thirds (used for truck access and parking to the service area of the business) was designed to be covered with recycled crushed concrete and recycled asphalt pavement on the surface for dust control," Gebauer says. "We worked with an engineering firm to come up with the optimal mix design (percentage of cement additive that would provide the optimal structural integrity of the soil once the blended mixer was compacted and cured), and we were able to remove four inches of aggregate base out of the section that was covered with bituminous asphalt and three inches out of the section that was covered with crushed recycled concrete and recycled asphalt."
"After taking samples from the site, we conducted tests to determine the optimum moisture content that would allow us to achieve maximum density," notes Mark Kalahar, operations supervisor for Braun Intertec, the geotechnical engineering firm hired to analyze and design a mix-design solution to stabilize the project's soft soil conditions. "The site is in an area that once was an ancient lake bed, so we know it's susceptible to frost and the 'fat clays' tend to shift around more than other clay materials. The cement additive controls the moisture content (stabilizes the material to the point where it maintains a consistent moisture level) and that's what enables the contractor to achieve an optimal density."
"We know that treating the 15,000 square yards of soil with cement added between $70,000 and $80,000 to the cost of the project, but we also know we saved money by reducing the aggregate you would normally bring in to construct the base," Gebauer says. "We also are pretty confident the stabilization process will add years of service to the parking lot structure."