The East Side Combined Sewer Outflow (CSO) Tunnel project consisted of the construction of approximately 29,000 linear feet of tunnel, seven shafts and multiple pipe-line connections.
The project
An underground connection between two shafts, the newly constructed Port Center Shaft and the existing Confluent Shaft, required ground modification to be completed successfully. Keller's scope of work for the Eastside CSO project involved providing a ground freezing system to assist in mining the tunnel between the existing confluent shaft and the new port center shaft. Soil stabilization by ground freezing was used as the necessary ground modification to complete this task.
Ground freezing design
Keller's design incorporated a grid of vertical freeze pipes installed from the ground surface to a depth of 140 feet. The vertically welded steel freeze pipes were installed using mud rotary drilling techniques. The upper portion of the freeze pipes was insulated with a plastic casing to minimize the amount of frozen ground above the area required for ground improvement. To ensure the verticality of the freeze pipes, the drill rig was equipped with a heavy stabilizer. This system typically yields a borehole verticality of 1 percent or less.
System installation and operation
Each freeze pipe was connected to supply and return headers via a custom freeze head. The supply and return headers were piped directly into the refrigeration unit, which circulated the coolant, calcium chloride brine, and was delivered to the freeze pipes, extracting heat from the ground. The soil strength in the freeze zone varied with the distance from an active freeze pipe and was typically between 800 to 1,500 psi.
The monitoring program will consist of four temperature monitors and two piezometers. Temperature monitor pipes fitted with thermocouples at select depths to record actual ground temperatures were installed within and adjacent to the freeze area. During the formation and maintenance of the frozen ground, the thermocouples were monitored daily to monitor the ground freezing progress. Piezometers were installed adjacent to the freeze area and were used to monitor the groundwater regime at the work location.
Upon completion of the work, the ground freezing system was turned off, and the surface components were removed from the site. The freeze pipes were abandoned by Tremie grouting in place.
