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Trends in Tunnel Linings

Figure 1: Three lane road tunnels in twin and single bore configurations

Figure 1: Three lane road tunnels in twin and single bore configurations

Perhaps the most visible trend in tunnel linings at the moment is the size of the linings we’re being asked to construct. The size just seems to keep increasing. While there has been a well-publicized delay of the country’s largest TBM in Seattle, Bertha is now progressing well. Other projects have been successfully completed at similar diameters.

The pressures that push up TBM sizes are not abating, either. Probably the next step up in size will come from a double-deck, three-lane road tunnel. These have a narrower footprint than their twin bore equivalents, as shown in Figure 1, making them suitable for congested urban environments. The double-deck solution could be cheaper, too, with a smaller overall excavated area and the elimination of cross passages.

Economics are also changing the way we view underground space. Tunneling has been seen as expensive, but TBM tunneling has now got to the point where it can be cheaper (per cubic yard of excavated space) than cut-and-cover, particularly deeper excavations in soft ground. This means that we will more regularly seek to increase the size of the tunnel to make room for infrastructure that we might historically have put in cut-and-cover structures or even above ground. This concept was used to great effect on Barcelona Line 9, where the footprint of the deep shafts was limited by placing platforms, plant rooms and crossovers in a larger bore tunnel.

While size grabs the headlines, other trends have also been apparent. One is how modern lining designs trade labor (particularly in the tunnel) for material costs. For example, the cheapest way of connecting segments together is with steel bolts, but we often use dowels because the time savings during erection mean less labor and lower overall costs. Similar commercial drivers exist behind other advances, such as steel fibers to replace conventional reinforcement, cast-in gaskets which eliminate the gluing of conventional gaskets, or the provision of cast-in ferrules in the lining to fix tunnel services to the lining that save money on drilling and fixing bolts.

RELATED: Innovations in Precast Concrete Segmental Linings

A third trend is for improvements in the durability of linings, and this is particularly notable in sewer tunnels. It is well known that hydrogen sulfide and other contaminants can attack the concrete, leading to corrosion. To protect against this and provide the 100-year design life that is routinely specified for such assets today, it has become common to install a secondary permanent protection to the lining, such as a GRP pipe or full secondary lining with an HDPE inner face for protection. While such solutions are undoubtedly robust, the additional cost and time required to install such secondary protection is substantial. To avoid this, integrally protected linings — segmental linings with cast-in HDPE protection — have gained traction. Meanwhile in less aggressive environments like combined sewer overflows durability can be assured by careful attention to mix design. As clients increasingly ask designers to give proper consideration to the life-cycle costs of the assets we design, this is likely an area where we will see greater attention in the future.

Figure 2: Hexagonal lining

Figure 2: Hexagonal lining

Technology in Segmental Linings

Precast concrete segmental tunnel linings have been around for more than 100 years and have been used in conjunction with pressurized face TBMs for at least half that time. As TBMs extend their reach to deeper tunnels, more difficult ground conditions and ever bigger diameters, segmental linings have followed without fuss, quietly incorporating the changes required to adapt to the new conditions. The changes have helped make TBMs more reliable, more productive, safer and more cost effective than ever before.

The main drivers of change on the tunnel lining are primarily in erection speed, cost of the linings themselves and quality. As per the previous discussion on labor costs, erection speed and quality both have cost implications: less time tunneling means less time paying for crews, equipment, etc.; while better quality means fewer repairs.
The individual changes in linings might be considered small, but taken as a whole they add up to big changes in time, cost and quality, ensuring that segmental linings remain one of the great workhorses of the tunneling industry.

Recent Advances

From the old style rectangular segments that took longer to place and were easier to damage, we now routinely use trapezoidal systems that build more quickly and more accurately. When combined with modern dowels on the circumferential joints, the ring builds are more accurate than ever before and completed quickly — helping TBM progress.

Meanwhile, hexagonal lining systems don’t even need the TBM to stop for a ring build. The TBM can place half the segments while thrusting off the other half, and then thrusting off the erected segments while placing the remaining segments of the ring, as shown in Figure 2. Hexagonal linings have been used in some long, straight tunnels where water ingress is not a major concern. However, gaps between segments tend to open over successive rings and that makes the segments difficult to seal. If this problem can be overcome then the use of hexagonal linings could become more commonplace.

Another area where technology has advanced significantly is gasket design. The latest products are highly engineered and can maintain resistance to water pressure under the full range of gaps and offsets that might be expected with contemporary lining erection. This, combined with the improvement in erection accuracy itself, leads to much drier environments. I was recently on a walk-though on a tunnel project in Washington, D.C., where the dust kicked up by a group of us waking in the invert was enough to reduce visibility to less than a 100 ft and we had to wear dust masks. Fifteen years ago, the risk would have been slipping in the water flowing in the invert.

Future of Lining Design

While we have a plethora of circumferential dowel products in the marketplace, we’re still using bolts to connect segments across the radial joint, and they have to be inserted during the ring build process. At least two suppliers are currently working on connectors for the radial joint that will connect on segment placement in exactly the same way as dowels. This will not only save time and money, but also make the erection process safer by removing operatives from the build area.

However, probably the biggest change that the industry could see is a change in the concrete itself: moving from ordinary Portland cement (OPC) to geopolymer. Concrete generates a phenomenal amount of CO2. More than 5% of the worlds emissions are from the production of concrete and 80-90% of this comes from the production of OPC. Reducing these emissions would make a big dent on the industry’s carbon footprint.

Geopolymers, on the other hand, use industrial byproducts such as fly ash and slag along with an alkali activator to produce strength and strength gain rates comparable to conventional OPC, but while eliminating over 75% of the embodied carbon in installed segments. Geopolymer cements also require almost no curing, which would save on the cost of steam curing often employed with OPC segments. Fly ash and slag can be more expensive than OPC, particularly in economies where there are fewer coal fired power stations and less steel production. There are also no major design standards that cover this material. This has hampered the development of this field to date.

However, geopolymers appear to show improved corrosion resistance over OPC, and new geopolymer cements are under development that may perform even better. This may have serious implications for concrete linings for sewers. Indeed, I was recently contacted by a client who was looking at the possibility of using the material for a sewer project. Perhaps it will be the desire for cost effective durability that will finally push this material into the tunnel industry.

Table 1: Notable recent developments in segmental tunnel linings.

Table 1: Notable recent developments in segmental tunnel linings.

Case History – Riachuelo Outfall Project, Buenos Aires

Being an outfall tunnel, the internal environment is not as aggressive as a sanitary sewer, and this means that a conventional concrete with a robust mix will provide the required 100-year design life. The real challenge is the sustained internal pressures for which the lining must be designed. Segmental linings are extremely structurally efficient in compression, but the internal pressures on the project push the lining into tension. This means that you need to find a way to hold the joints closed.

One solution that has been used a couple of times now is post tensioning. A cable is inserted in a duct that runs the entire circumference of the lining. Once tensioned, it creates compression in the concrete, so that the concrete itself never goes into tension, even under the highest internal pressure. However, providing assurance of the 100-year design life provides a challenge: how do you ensure that the steel cannot corrode? One solution is to provide double or even triple corrosion protection. However, this adds cost to an already expensive installation process. For cost and assurance reasons we decided not to proceed with a post tensioned solution, and selected a solution with bolts between segments. While this also presents challenges given the high loads, the installation is expected to be more manageable.

Anthony Harding is the Global Lead Technologist for Tunnels at CH2M.

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