Shaft Construction in Glacial Overburden and Karstic Limestone on the Lower Olentangy Tunnel Project - NAT2024

The 5180 m EPBM drive on the Lower Olentangy Tunnel (LOT) is predominantly within glacial overburden. To connect LOT with the existing OARS deep tunnel in limestone bedrock, the upper 15 m inside diameter Vine Street retrieval shaft telescopes down to 6.7 m in diameter and extends into the karstic Columbus Limestone to a depth of 51 m, where a 104 m SEM tunnel will connect to OARS shaft 6. Pre-excavation grouting was re􀁔uired prior to lower shaft excavation to reduce potentially high groundwater in􀃀ows. This paper discusses the secant pile installation, grouting program, shaft excavation, and hydrogeologic observations.