GEOLOGIC MAPPING FOR HIGHWAYS IN ILLINOIS: PROVIDING INFORMATION FOR TRANSPORTATION PLANNING AND CONSTRUCTION

Three-dimensional geologic mapping was conducted, materials testing was performed, and supporting digital databases were compiled for an area surrounding a 24-mile segment of State Route 29 in central Illinois. The highway parallels the west side of the Illinois River for 15 miles at the base of a 200-foot bluff, composed entirely of glacial sediments or glacial sediments over shale. For 9 miles the highway traverses river terraces and a floodplain. Three-dimensional geologic mapping was conducted using a combination of existing records and new fieldwork. Existing records included logs of 693 water wells and highway borings. Data from new fieldwork included describing and analysing 12 borings (1760 ft. of drilling), 251 outcrops, 40 hand-augered holes, and 9 miles of seismic reflection profiling. Resulting maps included isopachous and structure maps of the glacial and post-glacial sediments between land surface and bedrock, depth to bedrock, and bedrock topography. Multiple cross sections and fence and block diagrams were also produced to best portray the three-dimensional geology.

The geologic maps and sections were used immediately by highway contractors to identify geologic conditions that were most/less favorable to construction. Particularly, seismic data were used to identify old mines beneath the highway corridor that could potentially result in subsidence. Detailed lithologic descriptions of geologic units were used by consultants to help define the character of units and to anticipate unexpected changes in the cobble and boulder content of materials. The 3-D mapping can help reveal sediment characteristics and geometric relationships that may be critical to understanding hydrological conditions, slope stability, erosion and sedimentation, and evaluating how local geology affects habitat and sensitive ecosystems. Mapping also facilitates assessment of aggregate and groundwater resources that will be needed for rest areas and future development along the upgraded highway.