Tunnelling under the Fraser River at 6 bar Design and Construction of the Port Mann Main Water Supply Tunnel

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1 Tunnelling under the Fraser River at 6 bar Design and Construction of the Port Mann Main Water Supply Tunnel Stephanie Fekete McMillen Jacobs Associates Steve Skelhorn McNally Construction Inc. Tim Langmaid Hatch Mott MacDonald Allen Mitchell Metro Vancouver Abstract. The Port Mann Main Water Supply Tunnel project provides a critical water main crossing of the Fraser River for the owner, Greater Vancouver Water District (Metro Vancouver). The project consists of two 60-metre-deep slurry panel shafts and a 1-kilometre-long, 3.5-metre excavated diameter tunnel located near Vancouver, British Columbia, Canada. The initial tunnel lining is a precast steel fiber reinforced segmental lining. The final lining is a 2.1-metre-diameter welded steel pipe. This paper describes the challenges encountered during tunnel construction and the solutions implemented. Key challenges include earth pressure balance tunnelling at up to 6 bar pressure the highest to date in Canada a water-crossing without surface access for the majority of the tunnel drive, ground freezing from a river platform for a critical TBM intervention, and boring through cobbles and boulders in dense glacial till. INTRODUCTION Metro Vancouver supplies drinking water to 18 municipalities, one electoral district and one treaty first nation, serving a population of more than 2.4 million residents. Water is collected in three mountainous watersheds to the north of the Greater Vancouver area, in southwestern British Columbia, Canada. To supply the growing demands of Metro Vancouver s customers, treated water must be conveyed across two significant water bodies: the Burrard Inlet and the Fraser River. The existing Port Mann water main is one of several key links between the watersheds north of Vancouver and the municipalities south of the Fraser River. The Port Mann Main Water Supply Tunnel (PMMWST) is the first of a series of upgrades, to increase capacity, enhance scour protection, and improve seismic resilience. The new tunnel is 3.5 metres (11.5 ft) in diameter and 1 kilometre (0.6 mi) long. When completed, it will contain a new 2.1 metre (7 ft) diameter steel water main that will help ensure the continued, reliable delivery of drinking water to these municipalities, and will more than double the capacity of the existing 1.2 metre diameter main. Construction of the PMMWST began in the spring of 2011, with ground being broken for the construction of the south shaft. A significant milestone was achieved on June 26, 2015, when Squirrel, 1

2 an earth pressure balance tunnel boring machine (EPB TBM), completed the final advance of the tunnel drive. This paper focuses on the tunnel portion of the work and highlights some of the unique challenges that had to be addressed during design and construction. Project Team The PMMWST project is being constructed by the McNally Construction Inc. Aecon Constructors Joint Venture (MAJV). Construction management is being provided by Hatch Mott MacDonald. The design for the permanent structures as well as engineering services during construction are provided by the Fraser River Tunnel Group (FRTG), a team comprising Ausenco, McMillen Jacobs Associates, and Golder Associates. Project Overview The tunnel was driven between shafts sunk on either bank of the Fraser River, see Figure 1. The launch shaft on the south bank in Surrey is 11 metres internal diameter; the reception shaft in Coquitlam has a 5 metre internal diameter. The two 60 metres deep shafts were constructed using unreinforced slurry diaphragm walls for temporary support. A tremie slab plug was placed before the permanent, cast-inplace, reinforced concrete liners were installed. At both the North and South sites, a valve chamber was constructed integral to the shafts to control water flows through the tunnel pipe. The tunnel is 1 kilometre long with a 3.5 m excavated diameter. The tunnel is located approximately 30 m below the bottom of the Fraser River. The initial tunnel support (precast concrete segments) provided ground support during mining and steel pipe installation. The final lining consists of a 2.1 m diameter steel pipe, which will be the final conduit for drinking water. The tunnel is located below the depth of riverbed scour and is designed to remain functional following a major earthquake. The annulus around the steel pipe is backfilled with cellular (foamed) concrete. Figure 1. Project location. Left: Geographic location; Right: Tunnel alignment 2

3 PROJECT GEOLOGY The tunnel alignment intersects two primary geologic units, as shown in Figure 2, a simplified geologic profile. The geology is typical of the Fraser Valley, with overconsolidated glacial units at depth and overlying normally consolidated fluvial and marine deposits. Tunnel Soil Group 1 (TSG1), a till-like unit, consists of dense glacial material, including silty sand, sand and gravel, and silty clay with some cobbles and boulders. This unit is quite variable with zones of predominantly fined grained materials as well as sand and gravel zones. In general, this unit has low permeability, although this varies in the more granular subunits. TSG2 consists of silty clay, with limited granular materials. TSG2 is a more uniform unit and proved to be an excellent tunnelling medium. Seventy percent (70%) of the tunnelling was in TSG2 (silty clay), with the remaining 30% in TSG1 (till-like soil). The interface between TSG1 and TSG2 was encountered at both the north and south sides of the river. This interface was known to have the potential for nested cobbles and boulders. A Geotechnical Baseline Report was developed during project design, incorporated into the contract documents, and was a valuable tool throughout construction. TUNNEL DESIGN Figure 2. Simplified geologic cross section for the 1 km long Port Mann Tunnel Initial Lining The initial tunnel lining is a universal gasketed ring consisting of six segments (5 + 1 key), each 1 m long and 250 mm thick. The steel fibre-reinforced segments were designed for MAJV by Aecom. To facilitate the installation of a tunnel air lock in the event of a hyperbaric intervention, the segment design incorporated a deep caulking groove around each circumferential joint. The segment moulds were fabricated by Herrenknecht in Germany, before being shipped to Vancouver, where Armtec fabricated the segments a short distance from site. They are designed to have a compressive strength of 50 MPa, and provide temporary support for the installation of the steel water main pipe. Final Lining The final tunnel lining consists of 9.1 metre long sections of welded steel pipe, 2.1 m in diameter and 25 mm wall thickness. Each stick of pipe was installed with a rail-mounted pipe carrier and then circumferentially butt-welded. Once all of the pipe was installed, the tunnel annulus was backfilled with a low strength cellular concrete mix installed via grout ports in the steel pipes. Because of the tunnel grade, several bulkheads were used to divide the tunnel into backfill reaches. 3

4 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 significant 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 large compressive and tensile strains. Special welding and steel material requirements were specified to accommodate these strains. TUNNEL CONSTRUCTION: OVERVIEW Tunnelling was completed on a slight downward vertical alignment, with the 3.5 m diameter Caterpillar EPB TBM working its way from the South Shaft toward the North Shaft on the other side of the Fraser River. The TBM was designed to handle high groundwater pressures, with a 6 bar pressure anticipated. Tunnelling began in February 2014 and was completed in June TBM Launch Both shafts utilized ground replacement to facilitate TBM launch and reception. A rectangular block of concrete panels at break-out and break-in zones was constructed using a hydromill and backfilled with 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 (or wiper seal) (Figure 3). 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. Figure 3. TBM launch: approach to manage high groundwater pressures. Left- South Shaft configuration for launch. Right- Launch can lowered by crane for installation. TBM Design Because of the risks associated with a cutterhead entry under the anticipated conditions (6 bar), the primary machine design requirement was to mitigate or eliminate the need for a compressed air intervention during the 1 kilometre drive. The following measures were incorporated into the machine s design: 4

5 1. A combination of rippers, scrapers, and discs were included in the cutterhead design to maximize the possibility of performing the drive without having to change tools. A total of 36 rippers were installed on the head and 4 disc cutters were added to the periphery. Figure 4 shows the ripper design; Figure 5 shows the TBM cutterhead configuration. Adapter boxes allowed for the interchange of rippers with disc cutters along two spokes; however, this alternative configuration was not used. 2. Tool wear indicators were also installed and monitored from within the machine. All tools were back-loading to enable tool changes from within the plenum if necessary. 3. Robust grizzly bars were welded in place to prevent oversized material from entering the head and jamming the screw. 4. Flood gates were not to be used; this was deliberate to minimize the potential for mechanical related failures. 5. Hard wearing plates were welded to the face of the machine. 6. A TBM mounted airlock was eliminated to maximize clearance in the TBM for a larger screw conveyor. A tunnel airlock was to be utilized if a compressed air intervention was needed. Figure 4. Ripper teeth with state-of-the-art wear protection Figure 5. Squirrel cutterhead configuration 5

6 Notwithstanding the above, the TBM was designed to allow safe hyperbaric entry should it be necessary. Compressed air dive crews worked with the TBM manufacturer through the design phase to ensure alignment with all components. The TBM gantry was designed to be easily removable to facilitate installation of an in-tunnel airlock as close to the face as practical. 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. In the first 800 m, tool wear was minimal with the tools showing only 10% wear. The following 200 m saw increased wear, with some rippers displaying very significant wear and others minimal wear. Based on the results of the inspections, the cutterhead tools were partially replaced on two occasions and fully replaced on a third. Tunnel Progress and Statistics Tunnelling began February 24, 2014, and was completed June 26, Progress for the first 100 m was slow because of the combination of difficult ground conditions, progressive installation of the TBM equipment at the base of the South Shaft, and some mechanical issues. This segment took 4 months to complete. There was a strong contrast between the TBM progress under the most favorable and least favorable tunnelling conditions. The interface of TSG1 and TSG2, with its concentration of cobbles and boulders, made for the most challenging. Advance rates in TSG1 (till-like soil) were typically slow, with an average of 5 m per day (or 2 to 3 m per 12-hour shift). When the TBM encountered greater resistance (typically cobbles or boulders), advance rates further reduced to 0 to 5 mm/min, with several 1-metre shoves taking 3 hours each to complete. Tunnelling in full-face TSG2 (silty clay), on the other hand, was excellent and resulted in high advance rates. A typical day in TSG2 saw 18 metres of tunnelling completed (9 m per 12 hour shift) with advance rates of 100 mm/min on average and up to 200 mm/min. Tunnel Breakthrough Although highly anticipated, the TBM breakthrough was less visually dramatic than is typical. In order to balance the significant chamber pressure, the North Shaft was backfilled to above the tunnelling horizon and filled with water. This allowed the machine to enter the shaft with minimal change in external pressures. The tunnel eye was heavily reinforced with rebar, which meant that missing the 70 mm alignment tolerance could have had catastrophic consequences. The as-built tunnel alignment was verified continually using the tacs guidance system and was also confirmed with a gyro-theodolite survey on three occasions by the MAJV and another two times by the Owner s third-party consultant. The backfill was removed and the shaft dewatered to reveal that Squirrel had in fact reached its destination. The cutterhead and forward shell were removed by crane via the North Shaft (with just enough room) while the sacrificial skin around the stationary shield was grouted in place. The remainder of the TBM was disassembled and removed via the South Shaft. Figure 6 shows a schematic of the TBM entering the backfilled North Shaft and then beginning disassembly. 6

7 Figure 6. TBM Breakthrough and disassembly PROJECT-SPECIFIC CHALLENGES AND SOLUTIONS Shaft 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, complemented by a crane and man-basket. At only 11 m 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 backup was lowered down to tunnel elevation, new temporary configurations had to be implemented for the utility connections and various tunnel operations, such as muck handling. The TBM umbilicals consisted of electrical and hydraulic connections, which allowed the launch of the TBM while retaining some of the systems, such as the grouting and bentonite systems on the surface. Additionally, the launch through the headwall into the 5.5 bar ground presented its own challenge. The TBM was designed with a double screw conveyor to allow the pressures to be reduced before discharging; however, on launch there was insufficient clearance for the double screw. To allow a full pressure launch, a Putzmeister pump was utilized. This was bolted to the outlet of the first half of the screw conveyor and acted as a pressure reducing valve. The system was successful, and the launch under pressure was achieved with no ground loss. The system did have some issues in that clearance was reduced for installing the segments, and two TBM operators were required for the launch one on the TBM and one on the pump (see Figure 7). The pump also reduced the effective advance of the TBM. Large cobbles could not pass through the pump and were collected in a screen box at the screw discharge. With the number of cobbles encountered, this resulted in the screen requiring cleaning out every 200 mm or so. 7

8 Figure 7. Initial TBM configuration. No TBM backup installed. First part of screw conveyor in place with Putzmeister pump for use during launch. Small-diameter EPB TBM Tunnelling with a small-diameter EPB TBM at high pressure certainly has its challenges. The 3.5 m excavated diameter TBM is significantly smaller than what is used for transportation tunnels (typically 5 to 7 m diameter). Additionally on the PMMWST project, clearances were further reduced by the thicker than normal segments (250 mm) and the large screw size. From the launch, it became obvious that working in such a confined space makes every task more difficult (Figure 8), be it lack of access or difficult access, limitations on number of construction activities that could occur simultaneously (often allowing only one at a time), number of workers, or equipment size. Other limitations of smaller TBMs, such as reduced thrust and torque capabilities and reduced ability to handle larger cobbles and rock fragments, make tunnelling in glacial soils even more challenging than typical. 55cm Figure 8. View inside the TBM gantry a tight working space. High External Pressures To balance the external soil and groundwater pressures, the TBM had to be operated at high chamber pressures. TBM excavation chamber pressures up to 6 bar occurred during tunnel excavation where the 8

9 full hydrostatic pressure was realized. For a portion of the drive in the clay-rich TSG2 material, the TBM was operated typically in the 3 to 4 bar range because of slower groundwater recharge. In the TSG1 material, recharge was generally rapid and the TBM had to be operated at the full EPB pressures with a target of 0.3 bar above ambient, typically around 5.6 bar. This was very challenging, with the nature of the ground, cobbles and boulders combined with the high pressures. Large spikes in pressures were common and very rapid drops in pressures were observed when clearing cobbles through the screw conveyor. Ground conditioning was utilized throughout the zone with polymer injection used in TSG1. The high hydrostatic pressures also affected the TBM design. The sealing systems needed to cope with the high pressures and needed to be fail-safe to lock pressures in the event of a power failure. This presented some challenges in the initial part of the drive with potential over pressurization of the seals during mining. Additional seal pressure monitoring systems were installed along with safeguards to trip grease pumping systems should pressure spikes be observed. Tunnelling through Difficult Geologic Interfaces The TBM saw considerably lower advance rates in the more challenging geologic conditions. The interface of TSG1 and TSG2, with its concentration of cobbles and boulders, made for the most challenging. The first time the interface was encountered (near the South Shaft, as the TBM was driving from TSG1 into overlying TSG2 unit), the TBM was able to pass through with minor delays. More significant delays occurred closer to the North Shaft, where the TBM returned from TSG2 back into TSG1. Because of the near flat geologic contact, the length of tunnel intercepting the contact was much greater than at the southern contact. A number of free-air interventions were performed at the TSG1/TSG2 interface, with the majority of these occurring in the last 200 m of tunnelling. The interventions were required for various reasons, including TBM repair, replacing cutterhead tools, breaking up boulders/cobbles ahead of the face, and unclogging the grizzly bars/cutterhead openings. During project planning, it was not assumed that the ground behaviour would be favorable enough (i.e., firm ground with minimal water inflow) to perform free-air interventions, so preparation had been made for a hyperbaric intervention. In the end, no hyperbaric interventions were performed. An intervention occurred at Ring 793 that was an exception to the others. Here, a combination of less favourable geology and machine condition would not allow for a free-air intervention. The project team evaluated the benefits and risks associated with each of the alternatives. In the end, ground improvement by ground freezing from an in-river platform was selected as the best method to undertake the critical repairs needed. The river platform was constructed to conduct the ground freeze, and boreholes were drilled to below tunnel depth. Moretrench, a subcontractor to MAJV, performed the design and implementation of the ground-freezing scheme. A liquid nitrogen system was implemented with barges of liquid nitrogen regularly replenishing the on-platform tanks. Once the ground freeze was established, the intervention was conducted to perform repairs, unclog the screw conveyor, and change the cutterhead tools. This intervention was the most significant delay during tunnelling and lasted 6 months. Grouting Annulus grouting (between the segmental lining and the ground) was initially conducted via the tail shield. Because of issues with grout line blockages, and the risk in cleaning these out at high groundwater pressures, this was soon aborted and annulus grouting was completed by injection 9

10 through the precast segments for the majority of the tunnel drive. For each advance, ball valves were installed, approximately at the spring line on either side of the tunnel, and drilled out. Grout holes were added to every segment, but these were blind holes and required drilling at the TBM once the selected ports were identified. 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. CONCLUSIONS This paper outlines some of the practical engineering solutions developed to tunnel through complex geology at exceptional depths and high hydrostatic pressure. Numerous innovative approaches were taken in customization of TBM and tunnelling systems to suit the aforementioned conditions. The design of the TBM, emergency systems, procedures for high-pressure compressed air interventions, and specialized launch and removal procedures are just some of the details that were analyzed in detail. Lessons Learned The Port Mann tunnel project provides a wealth of knowledge and experience for tunnelling in challenging ground conditions, which will be invaluable for other tunnel crossings of the Fraser River and the Burrard Inlet that are currently in the planning or design stage. The Port Mann experience has provided a number of key learning points: It is worthwhile to invest time and resources in risk management measures to reduce the likelihood of interventions. Despite this effort, the need for interventions can never be fully eliminated and should be allowed for. Launching the TBM from a small shaft without a starter tunnel is challenging and time consuming, but with thoughtful TBM design and configuration, the challenge can be overcome. When planning to tunnel at high pressures, it is prudent to prepare for high pressure compressed air interventions. However, there are alternatives, which should also be considered at the design and planning stages. The possibility of safe, free-air interventions should not be discounted, even with potential pressures of up to 6 bar. The tunnelling fraternity must work to educate owners and designers of the benefits of a larger tunnel during construction tunnels should be sized to enable safe and efficient construction, not just for their intended final purpose. While the project did encounter challenges during tunnelling, the diligence given to these technical aspects certainly paid off and resulted in the successful execution of a tunnel under the Fraser River at the highest EPB pressure seen to date in Canada. Acknowledgments The authors would like to thank Metro Vancouver for its permission to publish this paper and for providing its support. 10