Construction on the Port Mann Main Water Supply Tunnel (PMMWST) project began back in the spring of 2011 with the construction of the South Shaft, and this summer, a significant milestone was achieved on June 26, when "Squirrel", an EPBM designed by Caterpillar completed its final advance of the 1,000m tunnel drive. This article provides an update of the tunnel construction and highlights some of the unique challenges that had to be addressed during construction.
PROJECT OVERVIEW
The project site is located approximately 30km southeast of downtown Vancouver, British Columbia. The deep shafts and under-river tunnel will house a new water main that will replace the existing crossing constructed in the mid-1970s. The pipeline, owned and operated by the Greater Vancouver Water District (Metro Vancouver), is a critical link for the supply of high quality drinking water, connecting the Coquitlam reservoir to municipalities south of the Fraser River. A project plan is shown in Figure 1.
PROJECT TEAM
The project is being constructed by the McNally International – Aecon Constructors Joint Venture (MAJV). Construction management is being provided by Hatch Mott MacDonald. The design for permanent structures as well as engineering services during construction are provided by the Fraser River Tunnel Group (FRTG), a team comprised of Ausenco, McMillen Jacobs Associates, and Golder Associates. McMillen Jacobs Associates was responsible for the tunnel design and initial lining design for the two shafts.
DESIGN REQUIREMENTS
The selected project configuration consists of a 3.5m diameter bored and segmentally lined tunnel mined between two 65m deep vertical shafts. The tunnel is located approximately 30m below the bottom of the river. Two new valve chambers will be constructed integral to the shafts to control water flows through the tunnel.
The inside diameters of the initial lining of the North and South shafts are 8 and 13m in excavated diameter, respectively. They were constructed between 2011 and 2013 using slurry walls for initial ground support. The final linings of the shafts are 5m and 11m in inside diameter, respectively, and are heavily reinforced, permanent concrete linings.
The initial tunnel lining consists of six segments (5 + 1 key) forming a bolted and gasketed ring, each 1m long and 250mm thick. The steel fibre-reinforced segments were manufactured locally by Armtec and have a compressive strength of 50 MPa, and provides the temporary support for the installation of a 2.1m diameter, 25mm thick steel water main pipe. The tunnel is located below the depth of riverbed scour and is designed to remain functional following a major earthquake.
During design, it was determined that there was a risk of lateral spreading, which could cause permanent ground deformation along both riverbanks towards the river, resulting in structural demands on the steel final lining. An extensive geotechnical drilling program combined with seismic analyses and structural modelling were undertaken to complete the design. The results indicated that the tunnel and the steel lining could potentially see significant compressive and tensile strains.
Special welding and steel material requirements were specified to accommodate these strains.
Tunnelling Conditions
Tunnelling was completed on a slight downward vertical alignment, with the 3.5m diameter EPBM working its way from the South Shaft in Surrey toward the North Shaft in the City of Coquitlam on the other side of the Fraser River. The TBM was designed to handle high groundwater pressures, with 6 bar pressure anticipated.
The tunnel alignment intersects two primary geologic units, as shown in the tunnel profile Figure 2.
Tunnel Soil Group 1 (TSG1), a till-like unit, comprises variable dense glacial material, consisting of silty sand, sand and gravel, and silty clay with some cobbles and boulders.
Tunnel Soil Group 2 (TSG2) consists of silty clay, with limited granular materials. Approximately 70 per cent of the tunneling was in TSG2 (silty clay), with the remaining 30 per cent in TSG1 (till-like soil).
TBM launch
Several TBM launch challenges had to be overcome because of the tunnel depth as well as limited workspace at the base of the South Shaft. Worker access to the base of the shaft was typically via a scaffolding stair tower (at 55m depth, which is 24 flights of scaffolding), complemented by a crane and man-basket. At only 11m in diameter, the launch shaft made for a very constrained work site.
Typically in shaft launches, a starter tunnel is excavated to provide extra room for equipment. However, because of the seismic requirements of the project as well as the high groundwater level, a starter tunnel could not be constructed. Therefore, the TBM had to be assembled one gantry car at a time as the machine advanced and more space became available. This assembly process was lengthy as each time a new section of the TBM back-up was lowered down to tunnel elevation, new temporary configurations had to be implemented for the utility connections and various operations, such as muck handling. Both shafts utilised "ground replacement" to facilitate launching and receiving of the TBM. A rectangular block of concrete panels at the break-out and break-in zones was constructed using a hydromill and backfilled with a low strength concrete.
To ensure adequate protection from high groundwater pressure, a steel launch can, from which to launch the TBM, was installed with three inflatable Bullflex seals as well as a rubber gasket. A steel jacking frame was set up at the base of the shaft and used to advance the TBM until enough segmental lining had been installed to resist the full thrust of the machine.
Key Requirements
In order to balance the external soil and groundwater pressures, the TBM had to be operated at high chamber pressures. TBM excavation chamber pressures up to 5.5 bar occurred during tunnel excavation where the full hydrostatic pressure was realized. For a portion of the drive under the Fraser River, the TBM was operated typically in the 3 to 4 bar range because of slower groundwater recharge.
Annulus grouting (between the segmental lining and the ground) was initially conducted via the tail shield. Because of issues with grout line blockages, this was soon aborted and annulus grouting was completed by injection through the precast segments for the majority of the tunnel drive.
For each advance, ball valves were installed, approximately at the springline on either side of the tunnel, and drilled out. A two-part cementitious grout (2 MPa design strength) was injected at pressure. Hydrophilic O-rings were installed into the drilled out grout ports at a later date. These were highly effective in reducing any leakage occurring from the grout ports and greatly reduced tunnel water inflow. The TBM cutterhead was equipped with rippers, gauge disc cutters, scrapers and a fishplate (or nose cone). The cutterhead was designed such that disc cutters could be installed to replace the rippers on two of the four spokes.
The rippers were generally effective in breaking up the ground so that soil and rock fragments could pass through the grizzly bars into the excavation chamber. The rippers saw variable wear, with some displaying very significant wear and others minimal wear.
Some of the cutting tools were inspected and replaced during the tunnel drive, particularly during tunnelling through the granular TSG1 material.
Current status
With tunnel excavation complete in late June, MAJV has now begun installation of the final tunnel lining. 9.1m-long sections of 2.1m diameter, 25-mm thick welded steel pipe are now being placed. Each stick of pipe is installed with a rail-mounted pipe carrier and then circumferentially butt-welded. The tunnel annulus will be backfilled with a low strength cellular concrete mix installed via grout ports in the steel pipes. Because of the tunnel grade, several bulkheads will be used to divide the tunnel into backfill reaches.
Completion of final lining and backfilling work is anticipated for later this year, with Project Completion scheduled for late 2016.
