OFFICE OF SURFACE MINING RECLAMATION AND ENFORCEMENT
2012 Abandoned mine Land reclamation Award
MID-continent Region Award winner
I-72 Piers 3 Sag Subsidence Emergency Project
Sangamon County, Illinois
For quickly responding to an emerging threat, working through difficult conditions, and successfully stopping ongoing subsidence, the 2012 Mid-Continent Regional Award was presented to the I-72 Piers 3 Sag Subsidence Emergency Project in Sangamon County, Illinois
The twin 800-foot Interstate 72 bridges over the Sangamon River in Illinois.
This project not only responded to a life-threatening emergency situation, but proved for the first time that it is possible to stop mine subsidence while it is happening. The State of Illinois discovered that two heavily traveled interstate bridges were subsiding, or slowly sinking, because of the collapse of two underground mines about 200 feet below the surface. The problem threatened both bridges structural integrity and the lives of thousands of people traveling on the road every day.
Skid marks from cars momentarily becoming airborne due to the sinking of the bridges.
In late November 2010, the Illinois Department of Transportation performed a routine inspection of the bridges on Interstate-72, a major east-west artery through the heart of Illinois, and discovered a problem with the bridges over the Sangamon River - tire skid marks in several unexpected places. Starting in 1974, about 15,000 cars each day crossed these 800 foot spans with few problems.
A map of the old underground coal mines. The red circles show where subsidence (or, sagging) was happening underground.
In December, state AML officials began 24 hour monitoring of the bridges, collected data, and confirmed the suspicion that three ongoing mine subsidences - one at each end of the bridges and one in the middle - were causing land under the bridges to sag. They determined the cause of the subsidence were two underground room and pillar mines last operated in 1931 and 1951, about 200 feet down. In turn, the subsidence caused warping and damage to parts of the bridge pier supports. A state engineer warned that because of their unique design characteristics, the bridges could not handle uneven sagging from the sinking ground, and said failing to act could lead to the loss of one or both bridges, and of human life. The state moved quickly to address those threats.
Workers endured long shifts in bitterly cold conditions, during flood stages on the river, and also faced the possibility of the bridges collapsing due to the ongoing subsidence.
Working in bitter subzero temperatures and heavy snowfall, the state drilled 40 boreholes in mid-January, and began filling the old mines with grout in March 2011. A month later, despite harsh weather, flooding, related delays, the constant fear of further subsidence, and the possibility of the bridges collapsing, the grouting was completed. The state identified the problem, contracted for the repair work, and completed grouting to stop the sinking in slightly over four months.
The emergency project was the first of its kind and provided a key finding: the Sangamon repair indicates that stopping an active mine subsidence incident while it is happening is technically feasible.
Link to Illinois Department of Natural Resources, Abandoned Mine Lands Website at http://dnr.state.il.us/mines/aml/recpgm.htm