Drilling and Grouting of Water Bearing Rock Formations to Facilitate Tunnel Excavation: Fid-Bau Portal

Drilling and Grouting of Water Bearing Rock Formations to Facilitate Tunnel Excavation

Hill, Jeffrey / Boonstra, Greg

The Baumgartner Stormwater Tunnel project in St. Louis, MO, consisted of boring a 6,096 meter-long (20,000-foot-long), 3.6-meter-diameter (12-foot-diameter) tunnel to store excess water during rainfall events, and to connect an existing wastewater treatment plant (WWTP) with a proposed wastewater treatment plant for additional treatment capacity. An open graded layer of rock 45.7 meters (150 feet) below grade and covering 1,066.8 lineal meters (3500 lineal feet) of the tunnel alignment, with direct connection to the Meramec River, was discovered during tunneling. The layer was unknown until probe holes drilled from the face of the tunnel encountered water containing unsafe levels of hydrogen sulfide. In order to complete the tunnel excavation, this formation needed to be sealed with neat cement grout. The unsafe levels of hydrogen sulfide required all grouting of the formation to be completed from the surface. The water bearing rock formation was drilled and grouted to a depth of 51.8 meters (170 feet) along the 1,066.8-meter-long (3500-foot-long) segment of the proposed combined sewer tunnel. A total of 425 grout points, drilled at 4.9 meter (16-foot) centers along both sides of the alignment, and nearly 700 tons of Portland cement were necessary to complete the project. Challenges included drill hole alignment and injecting enough grout to seal the formation without affecting the formation or wells outside of the tunnel alignment. Additionally, access conditions were difficult as the owner had not planned on surface access for completion of the work. This paper describes the construction methods, quality control and verification, including water testing and visual observation at the completion of the tunneling, used to help to ensure precise drilling and grouting of the tunnel alignment.