Rehabilitation of tunnels: An owner’s perspective

Since ancient times, storing and conveying water via underground water systems and tunnels has been touted for its strategic advantages. Underground cisterns and tunnels assured protection of essential fresh water supplies from man made contamination and especially from enemies who would gain an advantage by disrupting an adversary’s water supply in a siege. Modernday engineers do not usually specify tunnels for reasons of avoiding deliberate contamination or siege — however, they are just as enamored with the idea of building tunnels for water conveyances.

Such was the case for the San Francisco Public Utilities Commission (SFPUC). The SFPUC provides drinking water to 2.6 million residential, commercial and industrial customers in the San Francisco Bay Area. The SFPUC’s water system includes 257 km (160 miles) of transmission pipelines and tunnels from the SFPUC’s largest reservoir, the Hetch Hetchy Reservoir inside Yosemite National Park, to San Francisco. About 129 km (80 miles) of the system consist of tunnels. A 30-km (19-mile) section of the tunnel network, called the Mountain Tunnel, was thought to be at risk of a catastrophic failure in its concrete-lined section of 18 km (11 miles). For much of the project planning process, a new replacement tunnel was presumed to be required according to the tunnel planners.

Originally constructed between 1917 and 1925, the Mountain Tunnel has been in continuous service since 1925. The 30-km (19-mile) tunnel is located downstream of the Hetch Hetchy Reservoir water source in Yosemite National Park, between the Early Intake and Priest Reservoir (Fig. 1). Approximately 18 km (11 miles) of the tunnel is lined with unreinforced concrete. Though the condition of the tunnel was being monitored through water sampling and periodic inspections, there was growing concern that the tunnel lining was at risk of a partial collapse that could interrupt water delivery for up to nine months. The SFPUC was faced with a decision to construct a new bypass tunnel to replace the lined section of the Mountain Tunnel or to rehabilitate the existing tunnel.

In order to decide between constructing a new tunnel or rehabilitating the existing one, the SFPUC embarked on a detailed alternatives analysis study, developing a set of performance standards for the tunnel, and identifying different alternatives. The study included four alternatives for indepth review (McMillen Jacobs, 2017):

• Rehabilitate the existing tunnel, focusing on repair and contact grouting of the 18 km (11 miles) of concrete lined section.
• Relining with smaller-diameter steel pipe in the 18 km (11 miles) of the concrete-lined section.
• Construct a new bypass tunnel within the tunnel right of way to replace the 18 km (11 miles) of concrete lined section.
• Construct a new bypass tunnel outside the tunnel right of way to replace the 18 km (11 miles) of concrete lined section.

A 60-day shutdown of the tunnel was scheduled in early 2017 to perform a visual inspection, gather detailed information regarding the location and type of defects in the lining and establish the current structural condition of the tunnel (Fig. 2). The inspection indicated the many defects in the existing tunnel lining and the feasibility of repair (McMillen Jacobs, 2017).

The study also performed geotechnical investigations to ascertain site conditions for the new tunnel alignments, a condition assessment of the existing tunnel and a detailed hydraulic analysis to assess the flow capacity of the existing tunnel and evaluate effects of various improvements. The overall assessment was that existing tunnel was found to be not beyond repair and many sections to be in relatively good condition (McMillen Jacobs, 2017). The hydraulic analysis indicated that relining the tunnel would significantly reduce hydraulic capacity (McMillen Jacobs and Black & Veatch, 2017). So, the selection of the preferred project came down to comparing the rehabilitation and the new bypass tunnels against the performance standards and the other considerations of construction, environmental, permitting, cost and schedule described in this article. After thoroughly reviewing all this information and data at hand, the SFPUC ultimately decided the best course of action was to rehabilitate the Mountain Tunnel.

Why rehab?

The purpose of this article is to explain why tunnel rehabilitation can sometimes meet the needs of water agency owners better, or just as well, as new tunnels or bypass tunnel projects. Many owners are resource limited. They have competing program priorities, limited capital resources and limited staff expertise to undertake and properly manage mega size new tunnel projects.

Water agency capital budgets typically include many improvement projects competing for the same capital. The project list can include improvements to treatment facilities, transmission pipelines, pump stations and other facilities besides tunnel projects. Not every project gets immediately funded and some are postponed for later funding in a 10-year plan. Owners want to stretch every dollar to cover as many priority projects as possible.

Water agency owners typically have to sell bonds to fund their capital projects. The sale of bonds is a longterm financial commitment that many water districts are more hesitant to undertake in fiscally conservative times. Smaller water districts, in particular, are not willing to mortgage the future operations of the district to significant long-term debt.

Some districts have to also contend with stakeholder and customer opposition to the raising of water rates to service the debt payback. After years of increased water rates to pay for improvements on other parts of the SFPUC water delivery system, ratepayers would be understandably wary of additional increases to their water bill to pay for a new Mountain Tunnel.

It’s about less money and time

When choosing to upgrade major transmission facilities, most water districts would look seriously at cheaper alternatives rather than undertake a total replacement facility. Most owners would want to avoid a situation where the entire program budget is dominated by mega size projects, with outsized impacts on annual cash flows to service bond debt and unpopular customer rate increases to cover the bond repayments — as may be the case of large capital improvement programs, including new, expensive tunnel projects.

Given a choice between constructing a new tunnel, or bypass tunnel, or simply a rehabilitation of the existing tunnel, owners would naturally first look at the rehab option. Rehab projects are significantly cheaper than building from new. It is estimated that the upfront capital outlay of a rehab can be 30 percent to 50 percent of the cost of comparable size and length of a new or bypass tunnel. Cheaper upfront capital investment in initial construction can mean increased financial capacity to invest in periodic maintenance of a rehabilitated asset for the long term.

Because the scope of work for a rehab is significantly less, the rehab may be completed in approximately half the time or significantly less time for comparable size facilities. This also translates to significantly less expense for project management and other soft costs. A relatively simple rehab project will not require large consultant services contracts for highly skilled and expensive design consultants. Nor would just-as-expensive construction managers and inspectors need to be hired on large, multi-year contracts to do contract administration and intunnel inspections.

There is significantly less cost and schedule associated with property acquisition for a rehab project. No permanent tunnel easements or rights of ways are needed for new tunnel alignment. Permitting for temporary construction surface access and staging areas for non-agency owned property takes less effort because either the rehab project does not requires them or require less of them. In the case of the new tunnel alignment proposed for Mountain Tunnel outside the tunnel right of way, there was the need to acquire tunnel easements and temporary surface rights for staging and access through private property and national forest lands. The property issues included concerns about noncooperative private owners, working with the U.S. Department of the Interior, and possibly with U.S. Congress for the final approval — a potentially long and daunting process.

Rehab projects can be done without the need for subsurface exploration. A new tunnel by comparison must conduct an extensive geotechnical investigation along any new alignment, at locations where tunnel portals, adit tunnels and vertical shafts are contemplated for the design. All this is dependent on gaining temporary access usually through private or other agency-owned property. The longer the new alignment, the more complicated and timeconsuming will be the effort. All such considerations are essentially cost and time savings for owners who elect to go with the rehab option.

Construction considerations

With any new tunnel, there is also an increased risk of cost overruns due to technical challenges, unforeseen conditions, large upfront costs for tunnel boring machines and other support equipment, constraints on the electrical grid for construction power, and environmental unknowns. Each new day of new tunnel excavation can yield a new set of unknowns and the potential differing site condition change order.

By contrast, tunnel rehab projects are somewhat repetitive and have fewer unknowns that could promote a lot of cost overruns. The tunnel repairs are typically classified into about four types ranging from patching of small holes to routing, cleaning and backfilling of much larger cavities with welded wire mesh and shotcrete (Fig. 3). The design details and the construction for these repairs are fairly similar from rehab to rehab project. It is fairly easy for contractor labor crews to get proficient after the first few repairs of each type of repair, gain efficiencies from lessons learned, and possibly make remarkable reductions in the unit cost and unit time of the repairs. This would make subsequent repairs of the tunnel more efficient and less likely to overrun.

The scope of a rehab project also lends itself to flexibility during construction. Tunnel rehab construction focuses primarily on repair of the concrete lining. Such repairs are discrete as opposed to continuous construction along the tunnel alignment that must be completed in blocks or sections before any return to service can be contemplated. Discrete repairs can be done in prioritized batches, or as much as the owner’s shutdown window constraints and budget allows.

Politics of new versus old

Sometimes, project leadership has been known to advocate for new replacement projects for reasons other than the practicality of cost and time, or even beyond the performance standards. New projects promise improvements that can be considered as a panacea for problems ranging from the old facility as allegedly beyond repair, to local hiring for a massive number of construction jobs, to having the opportunity to associate one’s name with a legacy project.

By contrast, there is less glamor and notoriety associated with a repair or rehab project. Tunnel rehab projects tend to be very similar and familiar. The main work is the patching or repair of various defects ranging from small holes to larger sections big enough for an inspector to crawl into. The repair is usually performed by shotcreting followed by systematic contact grouting through injection holes drilled through the repaired sections and into the native rock surrounding the tunnel in order to fill the annular spaces between the rock and the lining. In the case of the current SFPUC Mountain Tunnel, the repair details are almost a carbon copy of similar repair details developed 15 years ago for the East Bay Municipal Utility District’s Claremont Tunnel.

Environmental considerations

New tunnel projects generally must undergo an extensive environmental review process before the project can be approved for construction. For projects in California, this involves the development, internal reviews and public review process of an environmental impact report (EIR) over several years. If a federal agency is involved as a project participant or for the approval of property for the tunnel easements and temporary construction staging areas, then an environmental impact statement (EIS) is also required. The EIS can be done concurrently with the EIR, but is typically completed with lag of an additional six months to more than one year after the EIR certification to allow the findings and related information in the EIR to be re-used in the EIS.

The environmental impacts of new tunnels are generally unavoidable and beyond simple mitigation offsets that are easily accepted by the communities that must host the construction of the tunnel facilities and staging areas. Neighborhoods must endure the visibility, dust and noise and share the streets and highways with the hundreds of truck traffic on a daily or weekly basis over many years of construction, and may want more mitigation for their endurance.

New tunnel construction projects of large diameter and long alignment have the additional issue of dealing with exceedingly large volumes of tunnel spoils. Such spoils must be transported often at long distances to commercial disposal fill sites, or shorter distances to agency owned fill sites. The longer haul to commercial sites often involves disclosure of the volume of regulated diesel emissions that have health effects on the public. The shorter haul to agency owned sites may involve less diesel emissions but requirements on the back end of the project for site restoration and native plant re-establishment.

Rehab projects generally have little to no significant environmental impacts that must be described, mitigated and publicly vetted before the project can be approved for implementation. Repairs, contact grouting and other work to rehab or improve an existing tunnel are completed underground and with relatively little surface visualization. The standard construction impacts of noise, traffic and environmental pollution can be avoided or mitigated by design to the point of insignificance. Rehab projects typically require smaller surface areas for temporary staging of construction trailers, equipment parking and storage of materials.

The environmental review process for a rehab project typically involves less documentation and less public review. Because the environmental impacts are so much less or can be avoided, it is often possible to publish a mitigated negative declaration for the rehab project. If federal agencies are involved, the comparable federal document is the environmental assessment. Both documents are easier and less time consuming to develop and may save about one year compared to the EIR/EIS process for the new tunnel (AECOM, 2017).

Environmental permits

Separate from the environmental documentation process is the issue of obtaining the environmental permits to allow construction to proceed. For example, in California, a biological opinion must be evaluated and obtained, and then California Department of Fish & Wildlife permits must be obtained. If the project involves federal participation or obtaining of easements, then U.S. Fish & Wildlife Service permits must be obtained. Such permits typically require special expertise either in-house or hired as consultants to interact with the permitting officials in order to work out conditions of approvals and details of the final project descriptions before the permit can be approved. It is not uncommon for such permits to consume more than a year of critical schedule after the environmental review process is completed.

If the project requires new tunnel easements and temporary surface easements for staging areas in national park land or forest preserves, as was the case for the new Mountain Tunnel options, then the permitting process is more complicated. The federal agency may pre-condition the granting of property easements on the approvals of the environmental review process, and in the process add more conditions of approval to the environmental permits.

Meeting performance standards

It is generally assumed that new tunnel projects can be designed to satisfy any set of performance standards for the project. While this is basically true, it overshadows the fact the most performance standards are derived from the design and operations of the existing facility, with perhaps a few upgrades.

In the case of the SFPUC Mountain Tunnel Improvement project, some of the performance standards were based upon the performance of the existing tunnel back when the tunnel was relatively new. A few standards were derived from the current operations of the tunnel. There were eight performance standards used as criteria for the selection of the preferred project during the project alternatives analysis (McMillen Jacobs, 2017):

• Service life: This standard requires the typical tunnel design for 100 years of service life. Although the best way to meet this standard is to construct a new tunnel, the rehab option can also achieve a 100-year service life. The Mountain Tunnel design consultant’s solution was to fix all the defects in the concrete lining with welded wire reinforcement and shotcrete and perform contact grouting to fill all the annular spaces between the lining and the surrounding rock (Fig. 4). When completed, the lining should be as structurally sound as a new lining and the rehab tunnel should last another 100 years with normal, periodic maintenance.
• Water quality: This standard limits the overall turbidity from Mountain Tunnel to occurrences of more than 1 NTU to no more than twice per year, and occurrences of more than 100 NTU to no more than once every five years. During normal operations, ground water intrusion is the main culprit for degrading water quality. For both new and existing tunnels, a way has to be found to limit this intrusion. For concrete tunnels, the best way to cut off the intrusion seepage pathways is to do an adequate job of grout injection of the native ground surrounding the tunnel, or contact grouting. For the rehab project, the entire 18 km (11 miles) of concrete-lined section will be aggressively contact grouted, essentially sealing the rehab tunnel from seepage. As an improvement, Mountain Tunnel will also install new large control valves at the downstream portal to keep the tunnel full of water when the tunnel is not running. With the tunnel full and pressurized, there would be little to no hydraulic gradient for the initiation of ground water intrusion. In addition, a short section of very leaky tunnel, upstream of the South Fork Siphon crossing underneath the Tuolumne River, will be replaced by a new 137-m (450-ft) long bypass tunnel section. This will eliminate the one worst section where ground water intrusion occurs the most.
• Water conveyance capacity: This standard requires a hydraulic capacity of 740 cfs (478 MGD). Advantage goes here to a new tunnel, in that a new tunnel can be sized to accommodate any flow capacity. However, in the case of the Mountain Tunnel rehab, flow capacity will be enhanced by the complete repair of wall defects, and invert paving and possibly smoothing, to improve hydraulic efficiency. The rehab project should be able to recover 706 cfs of initial capacity, or more than 95 percent of this performance standard. The SFPUC found this sufficient and efficient when the consideration of budget cost and schedule savings over the new tunnel are factored in. Also, the addition of downstream control valves to keep the tunnel flow at full volume will eliminate the erosive effects of the current tunnel operations, with intermittent surges and turbulent transitions between full flow and open channel flow inside the tunnel on a daily basis.
• Minimum flow: This standard requires a minimum flow rate of 300 MGD be available at all times outside of planned and unplanned outages. This is actually a fairly easy criterion to satisfy for a new tunnel and a rehab tunnel. For an existing tunnel, the key is to do the repairs of lining defects competently so that lining fallout does not occur and block flow capacity. This is accomplished by routing the defects back to structurally sound concrete, and backfilling the cavity with welded steel reinforcement and high strength shotcrete. The resulting repair would be as structurally sound as new lining.
• Operational flexibility: This standard includes four key operations. Mountain Tunnel must accommodate reductions in demand such that the tunnel may operate in open channel flow for extended periods. The tunnel needs to operate at full portion to meet water supply needs. The tunnel needs to accommodate power generation and local recreational needs, such that the tunnel may operate with substantial fluctuations in daily and hourly flows to the extent possible. The tunnel needs to accommodate full dewatering every five years for 100-day shutdowns for needed inspections of the Hetch Hetchy Aqueduct. The new tunnel and rehab tunnel can both be designed to handle all of these operational needs. In the case of the Mountain Tunnel rehab, downstream control valves will be added to maintain full volume flows so the erosive effects between full flow and open channel flows can be significantly avoided. With the downstream control valves, keeping the tunnel full of water, flows can be ramped up and down relatively quickly without developing the vacuum or surge pressures that promote erosion of the lining. A related operational consideration during construction is the owner’s requirement for emergency return to service. The implementation of tunnel repairs can be done in finite prioritized batches. Such repairs are discrete as opposed to continuous construction along the tunnel alignment that must be completed in large units before any return to service can be contemplated. This is a very important consideration for any owner that encounters an event that requires an emergency return to service. Such emergencies usually require the curtailing of construction and return to water services over a few days. The 2017 Mountain Tunnel inspection and interim repair contract had a three-day requirement for the contractor to return the tunnel back to the owner for emergency return to service.
• Planned outages: This standard requires the reliable operation of the tunnel with an inspection frequency of 20 years with outage durations limited to 30 days, and major repairs at no more than once every 20 years with outage durations limited to 100 days. This is fairly easy for a new tunnel or a rehab tunnel to satisfy. After completion of both types of projects, the key is to not ignore the periodic maintenance required to keep the tunnel lining in good physical condition, and eliminate the need for major maintenance that often results from neglect. In the case of the rehab tunnel, the repairs need to done competently so that lining fallout does not occur and require major maintenance. For Mountain Tunnel, simple planned inspections would only require outage durations of less than 10 days. A 30-day outage would allow time for some patchwork repairs of the lining. These short duration outages should be conducted concurrently with the five-year periodic outages for the Hetch Hetchy Aqueduct inspection interval under the operational flexibility performance standard. Periodic inspections every 20 years with outage durations of 100 days for the Mountain Tunnel should only be planned if major repairs are needed. Again, the goal of the inspection outages should be to catch the incipit defects when such defects are still small in size and fairly easy to repair.
• Unplanned outages: This standard limits the interruption in water delivery from a catastrophic event to no more than 90 days. Although there is uncertainty with any catastrophic event, the new tunnel and rehab tunnel can both be designed to make the lining as robust as possible to withstand shakeout from the forces of remote earthquake faults, or inadvertent damage from man-made events. Such is the case with the Mountain Tunnel rehab. The 2017 inspection found the existing tunnel lining to be an average of 35 cm (14 in.) thick. The rehab will structurally repair all the defects and the entire 18 km (11 miles) of lined section will be contact grouted to make sure the lining is in intimate contact with the surrounding granitic rock. By doing so, any need for repairs after a catastrophic event will be mitigated and the forecast interruption for repairs should be less than 90 days.
• Seismic reliability: This standard requires the reliable delivery of the minimum flow without interruption following a near tunnel seismic event. This is the easiest of the performance standards to satisfy in that the tunnel does not cross any active earthquake faults and the Sierra foothills location of the tunnel is in a region of low seismic activity.

Recommended inspections and maintenance

Comprehensive and competent repairs and contact grouting during the rehab construction should produce a tunnel whose lining is free from defects and with a renewed service life that compares with a new tunnel lining. It is important for owners to support the renewed tunnel with proper, periodic inspection monitoring, water quality testing and maintenance. The inspection should be conducted at reoccurring intervals of between five and 20 years, as required by tunnel condition. The inspections may have to be conducted at shorter intervals if it is noted during the initial inspection that the erosion is occurring more aggressively than anticipated. The key is to catch any new defects in the lining while they are still incipiently developing. Such defects should be small in scope and more easily addressed in subsequently scheduled tunnel shutdowns that are well planned and budgeted in advance. If the repairs can be scheduled periodically at intervals of no more than 20 years or concurrent with the inspections, then the scope of repairs will be less significant, cost less per shutdown, and be able to be accomplished in fewer shutdowns of shorter duration, with better control of the scheduling and costs of the work—all good considerations for budget and operation minded owners.

Conclusion

Done right, the rehab tunnel project can result in a renewed tunnel that can match the 100-year service life and other performance standards of a new tunnel but at a fraction of the cost and schedule. The new tunnel can be a more glamorous project to design and construct, but the renewed tunnel will typically require a less complicated and time-consuming environmental review process and fewer environmental permit conditions for completion. In the case of Mountain Tunnel, the rehab project can almost match the required flow capacity of the new tunnel with the same accommodations for operational flexibility, planned and unplanned outages, and seismic reliability. As with any new or renewed tunnel, the key to facility longevity, without major headaches, is the attention and commitment to performing periodic inspection and maintenance. A well-designed and executed periodic repair program with the newest engineering and construction methods can yield similar results in a fiscally responsible way. The rehab option will successfully preserve the SFPUC’s Mountain Tunnel and meet its needs for many years to come.

References

McMillen Jacobs Associates, 2017. Mountain Tunnel Improvements Project Alternatives Analysis Report, prepared for the San Francisco Public Utilities Commission, July, Draft Report.

McMillen Jacobs Associates, 2017. Mountain Tunnel Improvements Project Inspection Report, prepared for the San Francisco Public Utilities Commission, June, Final Report.

McMillen Jacobs Associates, 2017. Mountain Tunnel Improvements Project Condition Assessment Report, prepared for the San Francisco Public Utilities Commission, June, Draft Report.

McMillen Jacobs Associates and Black & Veatch, 2017. Mountain Tunnel Improvements Project Hydraulic Analysis for Conceptual Improvement Alternatives, prepared for the San Francisco Public Utilities Commission, June, Final Report.

AECOM, 2017. Opportunities and Constraints Report for the Mountain Tunnel Improvements Project, prepared for the San Francisco Public Utilities Commission, July, Environmental Report.