GEOTECHNICAL SUBSURFACE INVESTIGATIONS FOR THE INTERPLANT CONNECTION PROJECT IN INDIANAPOLIS, INDIANA

A three-phased subsurface investigation was performed for the Belmont/Southport Interplant Connection project which will connect the Belmont Advanced Wastewater Treatment plant to the Southport AWT plant in Indianapolis by a 12-foot inside diameter tunnel with approximately length of 34,000 feet.

Because of complexity of the glacial soils and in order to better evaluate the subsurface soil conditions along the alignment, a total of 91 conventional hollow stem auger borings, 14 sonic borings, 7 large diameter bucket auger borings, and 9 test pits were performed. A geophysical refraction exploration was performed to delineate the bedrock surface between borings where relatively shallow bedrock could be encountered during tunneling.

The results of the subsurface investigations and geologic review indicate that the subsurface stratigraphy, from the ground surface downwards, consists of the following strata. Near surface fill materials overlie post glacial aeolian, overbank, alluvium, and lacustrine deposits. The post-glacial materials overlie glacial deposits consisting mainly of outwash sand and gravel generally extending to bedrock. Within the outwash, there are discontinuous interstratified or underlying layers of lower permeability glacial till. The outwash sand and gravel is the thickest and predominant stratum within the tunnel envelope.

In conjunction with the subsurface soil borings, 26 slug tests and 2 pumping tests were performed to evaluate field hydraulic properties of the outwash and till deposits. A series of laboratory tests were performed to characterize the subsurface soils, including grain-size analyses, unconfined compressive strength tests, undrained shear strength tests, consolidation tests, moisture contents, and Atterberg limit tests. A series of rock tests were performed on large cobbles and the New Albany Shale cores, including uniaxial compressive strength tests, point load tests, Brazilian tensile strength tests, Cerchar abrasivity tests, punch penetration tests, slake durability tests, pyritic sulfur contents, petrographic analysis and liner cutter tests.

The results of the field and laboratory investigations were complied in a Geotechnical Data Report (GDR) and the results will be used to baseline the subsurface conditions along the Interplant Connection alignment in a Geotechnical Baseline Report (GBR).