The tunnel construction market is one of constant evolution – and sometimes revolution. Dating back to ancient civilizations through today’s modern marvels, designers and constructors have kept pace with the needs of society by applying new technology to an ancient craft. The 19th century saw the advent of subaqueous tunnels (Thames Tunnel) and drill-and-blast tunnels (Hoosac Tunnel), that changed the way we travel, while the 20th century ushered in a whole new era of underground construction with the introduction of Tunnel Boring Machines.
Today, we are seeing TBMs that are dealing with a broader range of ground conditions, with larger diameters and under greater pressures. These machines are opening doors for projects that may not have been feasible in the past, improving the quality of our infrastructure and contributing to a sustainable urban environment.
To get a sense of the impact of technology in today’s tunnel construction market and where it is headed, we talked with HNTB’s Mike Wongkaew and Tony Bauer. Specifically, the topics of large-diameter TBM tunneling, hyperloop and tunnel lining design were discussed.
Anthony Bauer, PE, is HNTB Corp.’s national tunnel practice operations manager-West. He is based in Los Angeles and supports tunneling projects throughout the West Coast and nationally. Bauer’s high-profile, complex underground project expertise includes working with clients such as Virgin Hyperloop One, Valley Transportation Authority, California High-Speed Rail, London Underground, Sound Transit (Seattle), Washington Metro and others.
Mike Wongkaew, Ph.D., PE, SE, PMP, is HNTB Corp.’s national tunnel practice lead-Northwest and associate vice president. He is based in Bellevue, Washington, and oversees all tunneling work on the Sound Transit West Seattle and Ballard Link Extension Project, as well as projects across the nation. Previously, he served as chief tunnel engineer for research and development of an innovative underground transportation system.
TBM: There has been a trend toward large bore tunnels internationally and we are now seeing it in the United States – Seattle, San Jose, Miami, Hampton Roads. Why are owners becoming more receptive to the idea of using large-diameter TBMs?
Wongkaew: With increasing numbers of successful projects and the availability of experienced suppliers, contractors and consultants internationally and in the United States, owners begin to feel more comfortable with large bore and consider it among the feasible alternatives that could address their infrastructure needs. Large bore also provides several unique advantages over twin bores.
TBM: What are the challenges associated with building and using large-bore TBMs?
Wongkaew: From the designer’s perspective, portal site constraints and logistics are key physical challenges for building large bore tunnels. Access to the site for TBM and material delivery, power requirements, muck processing (especially for slurry TBM) and hauling requirements are important factors that must be considered in portal area planning. Additional challenges include exponential increase in cutterhead torque requirements, increased cutting tool wear and replacement, large variation of pressure across the tunnel face and large perimeter of the annular gap between the excavated diameter and the lining extrados. The last two challenges must be well managed to mitigate the risk of settlement impacts.
Challenges for using large bore TBM include increased tunnel profile depth at the portal to address the buoyancy effect and under existing infrastructures to manage settlement impact risk, and increased minimum alignment radius to accommodate TBM articulation and lining installation. An additional challenge, although this could be viewed as opportunity, is optimization of the space utilization within the large bore as void spaces may require additional ventilation and fire and life safety consideration.
TBM: What technological improvements have been made to increase the practicality of large TBMs
Wongkaew: Improvements are continuing to be made to address some of technical challenges discussed above. For example, TBM manufacturers have developed methods for replacing cutting tools under atmospheric pressure, improving mixing of the muck within the chamber, addressing abrupt or large change in the face pressure across the height of the chamber, backfilling and lubricating the shield gap, and for rapidly filling the annular gap between the liner and the excavated ground. These improvements, among many others, mitigate several key risks of large TBM tunneling and demonstrate how TBM’s advanced controls add to the machine’s ability to control the ground settlement impacts on large bore projects.
TBM: What are the advantages of large bores vs. twin bores?
Wongkaew: There are several advantages of large bores. For example, WSDOT and the City of Seattle saw the benefits of reducing the SR 99 tunnel project footprint as compared to two smaller bores and reducing the surface impacts and disruption as compared to surface and elevated alternatives. For mass transit projects, VTA in San Jose and another transit agency in the Pacific Northwest began exploring a large bore option because of its ability to accommodate two tracks and station platform(s) inside the bore. This would reduce the need for cut-and-cover station construction including impacts on streets, traffic and utilities which can be very disruptive to communities and businesses situated within dense urban areas. The continuous space within the large bore also increases flexibility in locating stations and helps identifying station locations and track crossovers readily along the tunnel alignment, which enhances user experience and operational reliability.
TBM: How does a single large bore compare to twin bore when it comes to cost of construction? Cost of maintenance?
Wongkaew: The cost of tunnel construction is higher for large bore compared to twin bores as the TBM cost, tunnel liner and muck handling and disposal costs are greater; however, for many projects tunneling is not the only major cost items. For underground mass transit projects in urban centers, as an example, the increased cost of large bore tunneling would be offset by the reduction in station and crossover construction, right of way acquisition, and surface disruption mitigation costs, adding to an overall project schedule reliability. It is difficult to provide a generalized conclusion as each project is unique, but for several projects that HNTB have been involved with, the large bore can be cost competitive to twin bore when all cost aspect of the project is thoroughly analyzed.
We do not yet have enough case histories of life cycle costs, but similar tradeoffs could be foreseen. Large bores would have more and larger elements to be maintained; however, inspection and maintenance access in large bores would be easier and less impeded by space constrains and generally feasible during revenue service hours for large parts of the tunnel structure.
TBM: What are some of the safety considerations of large bore tunnels?
Wongkaew: The larger cross section of large bores means larger volume of air will need to be ventilated during construction, normal operating condition and emergency (this aspect could be controlled by guideway cross-section area control). Heightened attention is also needed in the areas of fall protection, ladders and stairs, scaffolding, hoisting and conveyors, etc., for the construction of sizable interior structures inside the large bores. Large bore also increases the importance of face pressure control, muck volume balancing, shield gap grouting and annular gap grouting for ground movement and settlement risk management.
TBM: How do you see the future of large bore tunneling in the US and internationally? Will it continue to grow?
Wongkaew: We expect the demand for large bores to continue to grow as the technology matures and owners find innovative ways to utilize the large underground space.
TBM: What are some areas that perhaps can be improved on to increase the effectiveness of large TBMs?
Wongkaew: Muck handling and ring erection continue to be two slow steps in the mining cycle that limit the production rate of TBM tunneling.
TBM: What is the current status of hyperloop development in the US and internationally? Are we close to building routes?
Bauer: Hyperloop is an emerging technology that is still in the early development phases; however, it is closer to reality than many believe. There are several projects in various stages of development within the United States – St. Louis to Kansas City and the Colorado Front Range being the most prominent.
Obviously, there is no operating commercial hyperloop system to use as a precedent. Therefore, the challenge for each of these projects will be to demonstrate to government agencies that the technology has achieved a level of technical maturity sufficient to justify the investment of public funds. If they cannot demonstrate this level of development, the hyperloop companies will be compelled to rely on private financing or government grants to build the first systems.
TBM: Tunneling is not necessarily needed for hyperloop. What do you envision as the role of tunneling in hyperloop development? What impact will it have on the tunneling industry?
Bauer: The physics of high-speed travel – whether with a hyperloop, high-speed train, or airplane – require straight routes to maintain acceleration forces which are tolerable to passengers. Passenger hyperloop systems would likely connect densely populated cities and there are not many transportation corridors available to build a new transportation network. The practicalities of designing the infrastructure for hyperloop in an urban setting will necessitate going underground. The tunneling industry should encourage the development of hyperloop systems because it will result in more underground construction all around.
TBM: What special considerations would be needed for tunneling for hyperloop vs. typical water or transportation tunnels?
Bauer: Current construction and design practices can be used for hyperloop tunnels. If the industry can continue to mechanize, standardize and automate the tunnel construction methods with the goal of driving costs lower, this can unlock other projects which have previously been considered uneconomical.
TBM: What advantages does tunneling have for hyperloop development vs. surface or aerial alignments?
Bauer: Using tunneling for hyperloop infrastructure will lead to simpler alignments, faster speeds, shorter travel times, and less disruption to the communities around the project.
TBM: What areas of tunneling are ripe for improvement?
Wongkaew: In the near term, I would like to see more research into fiber reinforcing. Steel fibers have been used in segments for quite some time. However, we could use more research to ensure fibers are consistently and uniformly distributed in the precast segments. Macrosynthetic fibers may help in this regard as we will get more fiber count for the same cost. However, we need more case history and research done to confirm the performance of macrosynthetic fiber reinforced segments for this technology to be widely accepted.
We also would like to see TBM technologies that can deal with manmade steel obstruction such as steel piles and tieback ground anchors. These are growing problem for TBM tunneling in dense urban areas.
TBM: A lot of talk recently has focused on the construction cost of tunneling, as well as production rates. What can we do to make tunneling less expensive? How can we increase production?
Wongkaew: Muck handling and disposal continue to be a major cost factor for TBM tunneling. We applaud the effort by Elon Musk and others to find ways to reuse the muck.
Ring erection is another step in the mining process that takes time and human intervention. We expect to see more innovation in this area including the use of boltless segments that could simplify automation of ring erection.
TBM: What are some of the issues related to liner design? Speed to produce? Ease of installation? Cost? Performance?
Wongkaew: Corner spalling of lining segments remains an issue that has not been fully eradicated. It is challenging to provide proper reinforcing in the corners. The use of steel fiber reinforcement is helping but only when proper distribution of fibers is achieved. We would benefit from more research and best practice guidance on how to ensure proper distribution of fibers in the lining segments.
I also personally would like to see more standardization of tunnel lining segment as the sizes, thickness and details of the segments now vary from project to project often unnecessarily. The bridge industry developed standard precast girder sections quite some time ago and everyone in the ecosystem has benefited from it, including owners, contractors, precasters and designers. Standardization has the potential to reduce the cost and improve the quality of lining segments.
TBM: How have liner designs changed to allow tunneling under high pressure (Lake Mead, Delaware Aqueduct)?
Wongkaew: Most of the challenges are in the TBM tunneling process, e.g. advance grouting to reduce permeability and inflow, maintaining suitable active face support, muck conditioning and hyperbaric intervention. The liner design approach itself has not changed significantly to allow tunneling under high pressure. Suitable gaskets for high pressure application have been available for quite some time. The tunnel liners need to be designed for the concentrated reaction forces from the compressed gaskets and for the relatively high external ground loads. Some projects opted for the use of double gaskets for redundancy.
TBM: How have liner designs changed to allow tunneling in seismic areas?
Wongkaew: We see more utilization of advanced methods of analysis, such as nonlinear time history analysis and three-dimensional analysis, to increase our understanding of the seismic load effects in the liner and how the liner would behave during seismic events. We also see increased use of double gaskets or combination gasket (EPDM and hydrophilic) in seismic areas to provide redundancy.
Traditionally, gaskets have been tested under static condition. For SR 99 tunnel in Seattle, the gasket was also tested under dynamic loading-unloading condition to better understand the potential adverse memory effect of the gasket material and to confirm water tightness under seismic condition. For Istanbul, special rings and expansion joint details were provided at geologic transitions between rock and soft ground in addition to the interfaces with stiffer cut and cover structures.