Question: When I look at the coal mine maps I can see that the ENTIRE structure of the I-39 bridge and approximate 2 mile lowland bridging up to it are all undermined... heavily... I would think that some of the finest engineers in the world looked at the geological data prior to approving this route.

The area below according to the maps I am looking at are all room and pillar.

Perhaps I am asking this question of the wrong message board or people. I will say some of what I have read on this web site... I find EXTREMELY interesting!!!!!!!!! Thanks.. John Ryba LaSalle Illinois

i can't answer your specific question about what investigations were done on the section of I-39 that you mentioned, but here in pennsylvania we do extensive investigations to determine a project's suseptability to mine subsidence early in the design phase. most of our old mines are mapped. so i know if mining is an issue to consider during design. if i am concerned about the potential for mine subsidence, i will have core borings extended through the old mine workings. the quality of the rock core helps me to determine if the mine roof is in the process of collapse. the depth to the coal seam plays a major role in me determining the suseptability of an area for subsidence. some rules of thumb that work pretty well here in pa are: less than 100 ft of cover over the mine -- high probabilty; between 100 & 200 ft of cover -- medium probabilty; between 200 & 300 ft of cover -- low probability; and over 300 ft of no potential. there are exceptions to these rules of course but they serve my needs in southwestern pa. we also must consider the cost of treatments versus the potential of subsidence and make decisions using our best judgement

I cannot specifically address I-39 in Illinois. However, many states are now aware of the need to consider abandoned underground mines during the investigation and design of highway projects. Unfortunately it is not always possible to avoid the mines in areas that are heavily mined or where the alignment is constrained by other factors.

Existing abanonded mines are becoming more of a problem as the mines become older and the overlying rock and the supports deteriorate. Years ago the mines were younger and in better condition and did not exhibit as many problems. Consequently, in the past, abandoned mines were not considered as much of a problem and, although they were considered in the design, the measures taken to address them were not as significant as those taken today.

We obtained many good years of service without significant problems from our older highways that were constructed over abandoned mines. We are now learning from our experience and including more extensive consideration of abandoned mines in the design of highway facilities. Each highway project is unique and the investigation and design are tailored to the specific needs of the project.

The Interstate Technical Group on Abandoned Underground Mines was formed to promote the transfer of experience and technology to assure the safe and efficient design and maintenance of transportation facilities over abandoned underground mines.

I would encourage you to further research subsidence reports and research projects out of Southern Illinois University, Carbondale. Dr. Yoginder Chugh was the spearhead of an extensive research program and biannual conference on the subject for many years, and is still associated with the university. --Matt DeMarco, FHWA Denver, CO

Prior to any road or bridge construction or replacement project, the Department investigates the area. Our investigations include borings and land use, maintenance issues, etc. When an area was mined, either open pit or underground, the investigation includes mining history and potential for settlement at open pits or collapse for underground mining. For this project, a report was written by the staff geologist for IDOT's Bureau of Bridges and Structures. According to his report the Danville (No. 7) Coal, Herrin (No. 6) Coal and the Colchester (No. 2) Coal were the primary interests of the coal companies. The report was for the entire alignment from Oglesby to Bloomington. The Mining depths for the higher No. 7 Coal was between 326' and 370' below the ground surface and between 517' and 567' for the No. 2 Coal. An Illinois State Geological Survey (ISGS) publication, The Cooperative Coal Mining Series, Bulletin 17, similarly shows the No. 2 Coal in LaSalle/Peru being between 336' and 440' deep and only 3' 4" thick. A little south near Oglesby the No. 2 Coal layer is 464' and is 3' 6" thick.)

At the subject structure, the reported conclusions were: "Other than concerns regarding the possibility of additional future subsidence over the mined-out areas in the vicinity of Wenona, Rutland and Minonk, there is little, if anything, that at this time, appears to present major geotechnical concern. Remedial treatments may be appropriate to insure adequate stability and settlement aspects of any major fills; such conditions should be identified and resolved during the design stage of any project in the FA-412 corridor. There are no unique geological deposits or features in the corridor which would require protection and/or preservation. Likewise, there are no geologic materials or resources of present economic value that would likely be damaged by development of a freeway in the FA-412 corridor."

The mines were looked at prior to choosing the alignment of I-39, but the coal layer is so deep that the concerns were minimal. As a further note, in Coal Mine District 1, which included portions of Bureau, LaSalle, Kendall, Grundy, Putnam Marshall, Woodford and Livingston Counties (the subject area), not all the mines were room and pillar. Practically all the longwall mines in the state are found in the above mentioned counties.