SOUTHFIELD, MI  NTH Consultants, Ltd. was retained as the prime consultant by the Oakland County Water Resources Commissioner's Office to conduct an investigation into the cause of a sinkhole that developed in Southfield, Michigan. NTH's analysis lead to a repair effort to stabilize the undermined foundations of the culvert and floor of the box sections that carry the drain beneath the service drive of Northwestern Highway (M-10) in Southfield, Michigan.

NTH's unique contribution to the project was the inspection team's discovery that the natural soil bottom within the portion of the culvert that extends beneath the MDOT ROW was eroded due to flow conditions within the drain. NTH was able to quickly identify that the discharge from these pipes projected away from the culvert walls in a jet that struck the channel bottom within the culvert. Because the pipes are connected to the roadway surface drainage system and convey flow from relatively nearby catch basins and other stormwater management structures, during short, intense precipitation events the maximum discharge from the pipes likely would occur before maximum flow is reached. Accordingly, the water depth within the culvert could remain relatively shallow and not provide much "cushion/dissipation" for the jets from the two pipes, leading to erosion of the channel bottom.

Through NTH's quick response, planning and action, the team was able to:

Stabilize the undermined features
Fill remaining void space beneath and surrounding the three-sided culver and box drain
Create a floor to prevent future undermining at the downstream end of the box drain
Allow for traffic to be maintained on both the Northwestern Highway mainline and the service drive without interruption
Avoid the use of access shafts, which speeded construction and reduced impact on the operations of nearby businesses

This project demonstrates a practical approach to addressing scouring and loss of support beneath culvert footings of similar construction. Because a similar design approach was used at other locations along Northwestern Highway and elsewhere, it is possible that this type of repair may be required elsewhere along the highway. In addition, it appears that since similar design was common for other MDOT projects built in the same time frame, routine inspection of such culverts is necessary, and similar repairs may be required on many other roadways throughout the state.