Construction on 10th Street was supposed to be a straightfoward mill and overlay project. It wasn't.
Washington County began a construction project on 10th Street in July that was simply supposed to repave the road from the I-694 bridge to about 300 feet east of Inwood Avenue.
Once the project wrapped up they had never encountered before. Now officials are preparing to fix it.
County Engineer Wayne Sandberg went before the Washington County Board of Commissioners on Tuesday regarding the project's unforeseen problems in July.
"This was originally designed as a simple mill and overlay," Sandberg said. "It's very straightforward and something we do regularly around the county."
County officials found that the roadway was uneven when the project was completed. The county hired a geotechnical engineering consultant to help investigate the problem and also consulted with the Minnesota Department of Transportation.
Through different testing methods they found water present about four feet below the road as well as a sand sub-grade. The road was originally constructed in a "swampy area" and sand was used to replace the "muck" from the swamp.
Though the road has had structural problems since it was built in 1994, Sandberg said other factors contributed to the road's strange result.
In the paving process rollers are used to compact the road. During the compaction, water was drawn up through the sand from the compactor's vibrations, which caused the road to become a "Jello-like substance." The soil compacted further once the water drained back down, which caused additional settlement.
Current practices call for faster and better compaction, which extends eight to 10 feet below the surface, most probably causing the shifting.
Sandberg said 10th Street will not get any worse. During the repair project, the surface will be ground flat, and the roadway will be paved using a different compaction method with less vibrations. A consultant will be onsite during the repaving, to monitor the process.
State aid funds will cover the entire cost of fixing the road, which is $175,000. The pavement repair will begin in mid-September.
"Everyone believes this is a unique situation," Sandberg said. "It was the combination of the wet area, vibrations and the water sucked into the sand."
Further study will be done to see if the same soil conditions could occur in the future when a highway is repaved.