Metro Vancouver’s new Port Mann Water Main, a joint venture of McNally/AECON, is designed to more than double the capacity of the existing system to accommodate future growth. The 7-ft (2.1-m) diameter steel water main itself lies within an 11.5-ft (3.5-m) diameter, 3,280-ft (1.0- km) long tunnel mined between deep vertical shafts constructed on the north and south banks of the Fraser River. Mining was well underway and the invert of the Earth Pressure Balance tunnel boring machine was approximately 160 ft (48 m) below river mud line and 650 ft (200 m) from the north bank exit shaft when an unanticipated mechanical failure occurred in the cutterhead, halting mining operations. Initial approaches to allow access for repair included dewatering of the soil formation to provide a watertight zone around the cutter head and depressurization of the cutterhead itself. However, these options were ultimately ruled out due to environmental issues. With time now very much of the essence, McNally/ AECON reached out to Moretrench, which developed a liquid nitrogen ground freezing solution that would not only allow safe access for inspection and repair but could also be implemented quickly. The ground freezing program was designed, installed and operated by Moretrench subsidiary Moretrench Canadian Corporation (MCC).
Ground Freezing Design
The intent of the ground freezing program was to isolate the cutterhead from surrounding earth and hydrostatic pressures, thus allowing safe access to the chamber immediately behind the cutterhead for the repairs. Liquid nitrogen was chosen as the cooling agent to meet two critical considerations. Liquid nitrogen boils at -196°C (-320°F). Because of the extremely low temperatures achieved, freezing with liquid nitrogen is rapid and high strengths of frozen soils can be achieved. Closure of the freeze is typically achieved in days rather than the weeks that would be needed with brine as the cooling agent. The low temperatures would also readily counteract thermal heat transfer to the surrounding soils from the EPBM.
The ground freezing program was focused around the creation of a block of frozen soil to encapsulate the cutterhead, providing a watertight cut-off and alleviating the hydrostatic pressure. The experience- based design developed by MCC consisted of 11 freeze pipes installed to 10 ft (3 m) below the TBM invert and spaced at 3 ft (1 m) apart in plan to avoid any possibility of drilling deviation at depth resulting in damage to previously installed pipes. Copper pipes rather than the typical carbon steel were used in front of the TBM face so that they could be easily mined through once the TBM was mobile again. Monitoring pipes instrumented with resistance temperature devices (RTDs) were incorporated into the design to measure ground temperatures. Thermal finite element modeling (FEM) was used to determine the time required for freeze build-up, verify closure projections, and confirm that the required thickness of the freeze could be achieved in the timeframe. The final design consisted of a discrete block of frozen soil 29.5 ft (9 m) in height, 23 ft (7 m) in width and 6.5 ft (2 m) thick.
The Port Mann project represents the first time a freeze has ever been attempted in the middle of a river. The ground freezing work had to be to be accomplished from a pile-supported platform installed through water directly above the stalled TBM. This presented the crews with a number of logistics challenges:
- All equipment and materials, including the drill and liquid nitrogen storage tanks, had to be crane-lifted on to a barge, transported to the working platform, and off-loaded by service crane.
- Lack of a consistent local supply, together with transportation cost considerations, meant that liquid nitrogen had to be trucked from Seattle, WA, which was closer than the nearest Canadian source.
- The liquid nitrogen supply needed to be re lled day and night during freeze build-up, and daily during the maintenance freeze period, requiring frequent barging of supply tankers to the work zone.
Installation & Operation
Sonic drilling was selected to install the freeze pipes. This
method utilizes an outer casing with an inner core barrel that is extended to extract the soil and bring it to the surface while the outer casing is pushed forward to maintain the open hole. Since sonic drilling does not require uids to assist the return of drill cuttings to the surface, no liquid spoils are generated that could be released into the river environment, an important consideration.
Drilling and freeze pipe installation was a round-the-clock, with a number of quality control measures instituted to ensure a smooth and problem-free operation. An 8-inch (200-mm) diameter surface casing was installed through the working platform and set to 5 ft (1.5 m) below the mud line and surveyed with inclinometers. The 6-inch (150- mm) sonic drill outer casing and 4-inch (100-mm) core barrel were advanced incrementally to design depths as soil was extracted. Surveys were made at the mid-point and immediately above the TBM to verify clearance but also to ensure that the pipes were close enough to the TBM face to achieve a proper bond. The freeze pipes themselves were instrumented with thermocouples placed every few feet to verify that the liquid nitrogen was maintained at a level sufficient to fulfil the design objective yet not extend the freeze outside of the target zone, and to control nitrogen flow to individual pipes.
The liquid nitrogen was delivered to drop pipes within the individual freeze pipes via an on-deck distribution manifold. Gas generated as the liquid nitrogen boiled traveled up the annulus between the freeze pipe and drop pipe and was vented to the atmosphere. After 12 days of freezing, the temperature data indicated that the freeze was sufficiently formed to allow safe entry into the cutterhead chamber to begin repair work. The freeze was discontinued at the completion of the repairs and tunneling operations were able to resume immediately thereafter.