Kansas Department of Transportation
Abandoned Underground Mines–Pittsburg, Bypass
Richard Ryan, Regional Geologist, Kansas Department of Transportation
Abandoned mines,beneath the US-69 Bypass at Pittsburg, Kansas, have been a headache for Kansas Department of Transportation Road Maintenance. It seems that shallow depressions in the pavement or in the ditches occur about once a year after either the spring or fall rainy seasons. These mine shafts were excavated over 70 years ago to obtain bituminous coal from the Weir –Pittsburg Coal Seam.
Most of the underground coal mining in the Pittsburgarea occurred between the 1890’s and 1940’s to supply fuel for a local zinc smelter. Zinc ore was shipped by rail from the lead-zinc mining districts of southeast Kansas and northeast Oklahoma. Pittsburg had a ready supply of coal within 20 to 100 feet of the surface. Extraction of the coal was done using the room and pillar mining method and it seems that over 80% of the area is undermined.
As these abandoned shafts age, the openings manifest slowly as shallow depressions in the pavement or as small holes in the ditches and backslopes. These depressions seem to appear more frequently after a period of heavy rainfall. Once the swag forms, surface water drains to these depressions and accelerates the sinking of the pavement.
Discovery of a shallow depression in the southbound lane of US-69, on November 10th 2004,initiated an investigation by KDOT Geologists. We found a 5-foot void at a depth of only 22 feet. This prompted emergency repair of a 680 foot section of roadway. PittsburgCity officials were notified in case an alternate route was needed. Warning signs and barriers were placed near the depressions, emergency funds were appropriated and a contractor was contacted. Before repairs began, two additional areas were added to the mine remediation project after another depression was discovered in the shoulder of the northbound lane and one in the middle of the southbound lane at a section 1.7 miles to the north.
Grouting got underway by November 29th, 2004, within 19 days of the discovery. Over 52% of the grout holes drilled in these two sections of roadway intersected mine openings. Low strength grout was used to fill the voids. High slump or stiff grout was used to create barrier walls at the shoulders of the pavement. Low slump grout was used to fill in the 15-foot staggered holes to infill under the roadway. It took two cold winter months, December and January, to complete the project.
The town of Pittsburg has been growing in size and the existing US-69 Bypass,on the west edge of the city, is inadequate to handle the projected future traffic loads. The result is a proposed new alignment further west of the existing roadway. However, this future highway project will be constructed over some of our most hazardous locations. This project will require the most extensive and costly mine remediation to date. The proposed new 18 mile-bypass will cross approximately 2.5 miles of old strip pits and 14.5 miles of underground abandoned coal mines.