Plan for Sprigg Street sinkholes moves forward

Typically, when city officials say there's been some activity in the South Sprigg Street sinkhole area, it's a bad thing. But recent discussions about the sinkholes have been a little more hopeful.

That's because the city received federal funding to perform an investigative study, design and construction to repair the sinkholes and hopefully open South Sprigg Street, which has been closed since last year.

In July, the Cape Girardeau City Council approved an ordinance authorizing the city manager to enter into an emergency relief program agreement with the Missouri Highways and Transportation Commission for the study and remediation of the South Sprigg Street sinkholes. The agreement allows the city to perform an investigative study, design and construction for the restoration and mitigation of damage caused by the sinkholes to the bridge and roadway.

The program funding is an 80/20 grant through federal highway funds administered through the Missouri Department of Transportation. The federal share will not exceed $1.2 million. The city also received a $100,000 Delta Regional Authority Grant to put toward its 20 percent match on the project. Remaining funds would come from stormwater sales tax revenue.

The city awarded the bid to Horner & Shifrin in September. The cost of the investigative work is not to exceed $462,339, per the agreement, and qualifies for 80 percent reimbursement under the Missouri Highways and Transportation Commission's emergency relief program.

The South Sprigg Street area has battled sinkhole problems since 2007. One large sinkhole collapsed South Sprigg Street and another formed near the South Sprigg Street bridge over Cape LaCroix Creek, causing concerns about its structural security.

Temporary fixes have been attempted by the city. Cape Girardeau public works director Tim Gramling said the sinkholes have been filled with gravel and concrete and the road repaved on at least four occasions.

But Gramling said the city is moving beyond the temporary solutions because at least one sinkhole is growing and causing even more worry about the bridge.

"[The sinkhole problem] became a little more serious and that bridge is a big investment and very important to the crossing there for the street," he said. "So we wanted to make sure before we just fill everything in and pave over it and move on that the structural integrity of that bridge is there and it's safe and going to be safe for a long time to come."

Horner & Shifrin worked with architecture and engineering firm Stantec, which spent three weeks completing a geotechnical survey to better understand the sinkholes and the surrounding subsurface. Geotechnical engineer Kurt Schaefer said the firm's goal was to characterize the subsurface.

"We're trying to identify the sinkholes there and look for others in the area to develop a pattern if there is one," he said. "We're also trying to determine ways to make the bridge safer, either through redesign or realignment."

Gramling estimated fewer than 5 sinkholes are in the area, but said determining an exact number is difficult. What might appear to be multiple sinkholes on the surface could actually be part of one underground system.

Schaefer said when it came to actually studying the sinkholes, options were limited. Ground penetrating radar and electrical resistivity imaging wasn't an option because water, gas and cable lines running in the area would interfere.

Drones also were not an option. Schaefer said they work best for studying wide, cavernous sinkholes.

"The sinkholes we're dealing with at the Sprigg Street bridge are more narrow and deep, so there's not really room for maneuvering a drone," he said.

The equipment used by the seven-person field team primarily consisted of drills. Schaefer said they used drill rigs to dig down to the rock or actually drill into the rock to search for any voids underneath its surface. The team drilled in a grid pattern to search for any disturbances.

"We're looking for irregularity in the rock's surface to try and develop a pattern for the sinkholes and we're using that to help the bridge designers come up with better span lengths or prudent foundation systems," he said.

Excavators also were used to expose the rock adjacent to existing sinkholes, where Schaefer said it might not be safe to use the drill.

Information gathered by SparTec during the study will be shared with Horner & Shifrin to complete the bridge design. The engineering consultant will then prepare a report and recommendations for the city. Gramling said the report is expected to be complete in February.

"We hope to execute some design documents to start the design portion of the project in March," he said. "We're hoping to advertise for whatever construction we're going to do on the project in June and be under contract by September."

The federal funding the city received for the project requires it to award a contract by September. It also requires the funding be used for sinkholes directly related to the South Sprigg Street bridge and roadway.

While the sinkholes in the South Sprigg Street area have garnered the most attention -- the closed street hampers accessibility to the city's transfer station -- it's not the only area affected. Gramling said past estimates place the number of sinkholes in and around the city above 40.

srinehart@semissourian.com

388-3641

Pertinent address:

2524 S. Sprigg St, Cape Girardeau, Mo.

---

Sinkhole development

• As rain falls, it absorbs carbon dioxide, making the water slightly acidic. The water migrates downward through tiny cracks, slowly dissolving carbonate bedrock and creating bedrock voids.
• Over time, the underground void grows. Soil and rock from above fall into the cavity and are washed away.
• The void continues to grow and slope upward, toward the surface.
• Eventually, the structural integrity of the surface material is breached and a sinkhole forms.

Source: Missouri Department of Natural Resources