No Business Creek Tunnel Project located in Gwinnett County, Georgia is a 12 foot diameter TBM tunnel which extends 16,000 ft through the Piedmont Provence. The general geology consists mostly of metamorphic and igneous rocks. The anticipated rock mass conditions anticipated were hard, intact rock with Q-values greater than 10 for 80% of the tunnel length, blocky and seamy rock with Q-values from 1 to 10 for 15% of the excavated length and 5% was expected to be saprolite having a Q-value of less than 1. The corresponding support types prescribed for these anticipated ground conditions included a Type 1-two bolt pattern for the Q-values above 10, a Type 2-four bolt pattern for the Q-values between 1 and 10, and a Type 3-steel ribs and lagging as well as a 12 inch concrete liner for the Q-values less than 1. The ongoing mapping effort documented faults, shear zones, caverns, biotite seams, saprolite zones and discontinuity combinations that when paired with the inflow of groundwater, created initial support challenges that did not fall easily into the originally prescribed categories and had to be analyzed based on their individual characteristics. This paper describes the ground conditions, the analyses, as well as the initial ground support recommendations and measures for the excavated tunnel.
The No Business Creek (NBC) Tunnel and Lift Station Project located in the southern part of Gwinnett County, Georgia at the lower end of the No Business Creek watershed basin required the construction of 4877 m (16,000 ft) of 3.7 m (12 ft) diameter, TBM-driven, sewage transfer tunnel along with a 10-MGD lift station and odor control facilities. The ultimate purpose of the project is to convey and provide storage for wastewater flows from the Jack's Creek Water Reclamation Facility (WRF) to the No Business Creek Pump Station and to provide equalization storage for the pump station. The original contract required supporting structures including a 12.2 m (40 ft) diameter mining shaft (eventually becoming the lift station), one 4.6 m (15ft) diameter and two 7.6 m (25 ft) diameter drop/access shafts, and one 7.6 m (25 ft) diameter receiving shaft. Original design shaft depths ranged from 18.3 to 68.9 m (60 to 226 ft). The TBM excavation was completed in late October 2009.
The contract documents for the project dictated that a complete record be made of the ground conditions on site during excavation of the pilot drift as well as the TBM excavated tunnel for the purposes of determining initial support measures as well as to provide site specific geotechnical data to aid in the optimization and selection of the final ground support measures. The result of this data collection is a comprehensive record of the rock mass and its engineering properties. This information was not only utilized during excavation, but was used as a resource as final lining decisions were made. It will also be valuable as a future reference for tunnels with similar geologic settings.
