Rehabilitation of the Tuscarora Tunnel

Repairs are underway at Pennsylvania Turnpike’s Tuscarora Mountain Tunnels between the Fort Littleton interchange (Exit 180) and the Willow Hill interchange (Exit 189).

The construction of the eastbound tunnel began in the 1800s for railroad use, but it was never finished. The area was converted to roadway use as part of the original Pennsylvania turnpike construction started in 1940. The westbound tunnel, completed in 1968, increased the flow of traffic along the Pennsylvania Turnpike.

Typical deterioration has resulted in both tunnels experiencing water intrusion. Due to its advanced age and the construction technology available at the time, the eastbound tunnel has more significant systemic leakage throughout the structure than the westbound. The westbound tunnel has less systemic leakage, but locations of significant water leakage are still present, particularly at construction joints.

The current $110 million repair effort seeks to modernize the tunnels, improve safety and extend the structure’s design life. Once nicknamed “Lake Tuscarora” because of the degree of water infiltration, the tunnels will be repaired by multiple methods addressing the different leak regimes.

The westbound tunnel will have drainage installed at each construction joint and all water-bearing cracks will be repaired. An umbrella system to convey water into the tunnels’ existing drainage systems will be used in the eastbound tunnel.

This article focuses on the eastbound tunnel repairs as they represent the majority of the repair effort.

Design summary

Installation of the umbrella system in the eastbound tunnel will extend the lifetime of the tunnel by 100 years while also significantly reducing the amount of maintenance required. Given the systemic nature of the water infiltration in the tunnel, the repair of each leak individually would lead to chasing leaks throughout the structure. Therefore, a more systemwide solution was chosen.

In this method, water is captured by a waterproofing Rehabilitation of the Tuscarora Tunnel membrane and conveyed to the structure’s existing drainage system. This eliminates maintenance costs that would occur if more work would be needed to plug new leaks. With this design, only occasional standard maintenance is needed to ensure the drainpipes are flowing properly.

The Pennsylvania Turnpike Commission (PTC) tasked Gannett Fleming and subcontractor Gall Zeidler Consultants with developing a design for long-term remediation, with the goal of relieving PTC from excess maintenance costs and extending the life of the tunnel structures.

Therefore, the Tuscarora Tunnel rehabilitation design was developed with the idea that a tunnel with systemic, structure-wide leakage could not be successfully rehabilitated by treating individual leaks. Rather, the structure should be treated wholly by understanding the leakage regime, geology, hydrogeology and maintenance history.

After reviewing these factors, an umbrella system was chosen as the best option to accomplish the goals for the eastbound tunnel. The umbrella system consists of installing a polyvinyl chloride (PVC) waterproofing system on the walls and ceiling of the tunnel. The waterproofing system includes a PVC waterproofing membrane with remediation hoses and PVC water barriers. This system is more resilient than a PVC membrane alone, as it isolates any potential leaks, increasing the ease and effectiveness of a repair. In the event of leakage, the water barriers prevent the PTC from having to chase leaks.

The membrane is welded to a drainage angle installed at the roadway surface, allowing the water to flow through a newly installed porous concrete block into a slotted drainage pipe. This pipe then follows the existing slope of the roadway and is eventually routed to the existing tunnel drainage. New concrete barriers and a self-supporting reinforced shotcrete liner will protect the membrane.

The installation of the umbrella system requires the removal of the tunnel plenum ceiling. Since the existing ventilation design, which requires a plenum the length of the tunnel, would no longer be possible, an alternate design was developed using a Saccardo nozzle, which is a system that introduces a strong jet of air flow into a tunnel from an existing fan room.

To implement this system, the ceiling remained in place for a short portion at each end of the tunnel. Fans and ductwork were designed that would direct the air as needed to create the desired effect. This design decision allowed for the work to progress on the umbrella system.

Given the intricate nature of parts of the design, significant detail was put into the design documents. An embedded channel was designed to support tunnel lighting directly from the lattice girders prior to the shotcrete gaining full strength, and to eliminate the need for hundreds of anchors.

Mounting eyelets were precisely shown on the lattice girder geometry details for easy mounting of the embedded channels. The construction sequencing challenges were also addressed, with membrane protection details developed for lapping lengths that had to be exposed for months given the construction schedule.

Tunnel specific details were also developed for the sectioning, terminations and transitions in the portal sections.

Construction sequencing

Unlike metropolitan tunnels, the Turnpike’s tunnels have traffic peaks on the weekends, and thus construction is limited to weekdays only. Due to the nature of the repairs in the eastbound tunnel, the entire tunnel needed to be closed to ensure safe and efficient working conditions. As a result, traffic in both directions was reduced to one-lane in the westbound tunnel during weekdays. Based on the challenges posed by the eastbound tunnel needing to be open every weekend, it was critical that construction sequencing be well thought out to avoid delays and lessen any impacts to scheduling.

The preliminary step was to demolish the existing tunnel ceiling and sidewall barriers. After the barriers had been demolished, the new drainage system was installed at the base of the walls. Waterproofing was installed over the new drainage system and up to a height of approximately 2.4 m (8 ft). New barriers were then constructed over the waterproofing, measuring 1.5 m (5 ft) high. Metal flashing was installed over the final 1 m (3 ft) to protect the waterproofing, which was not covered by the new barriers. The new barriers were installed on each side of the tunnel prior to moving on to the next phase of construction.

Once the new barriers were in place, the waterproofing membrane was installed radially from barrier to barrier in 9 m (30 ft) panel sections. Following the membrane installation, lattice girders were installed along with the welded wire mesh. Embedded lighting channels were also installed on the lattice girders to allow for easy light installation without installing anchors. A 20 cm (8 in.) shotcrete liner was then installed in 10 cm (4 in.) lifts. This process is currently being completed panel by panel for all 180 panels in the eastbound Tuscarora tunnel.

While it was originally assumed that this phase of work would occur with panels being completed consecutively, that was not ultimately the case. To lessen the impact of waterproofing reinforcement installation and shotcrete operations on each other, the contractor had its crews working at spaced out locations along the tunnel.

Innovations and challenges

Innovations in sequencing: Originally, only limited sections (lapping length of under 0.6 m (2ft) of the waterproofing membrane were to be exposed leading into the weekend of the tunnel opening. However, it was determined that this requirement would make construction difficult, especially on the proposed schedule. The PTC permitted the contractor to leave exposed waterproofing in the eastbound tunnel to ease schedule concerns.

Before shotcrete and the lattice girders are installed, the membrane is carefully inspected for damage. If there is damage that has occurred to the membrane, the area is repaired prior to the work continuing.

In addition to opening every weekend, there had to be contingencies for the tunnel to open as soon as possible for weather or traffic emergencies. If an accident or blockage occurs in the primary transit tunnel, the contractor must be ready to quickly move out of the worksite to allow traffic to flow through the tunnel.

This means that all equipment and supplies must be mobile and ready to clear out of the tunnel within defined time periods set in the contract. Additionally, a minimum shotcrete liner strength is specified prior to opening the tunnel to traffic. It is a cardinal project goal from the PTC that there is as little inconvenience to drivers as possible.

Optimizing shotcrete means and methods: When construction began, the contractor proposed several alternates to the shotcrete liner installation shown in the design drawings. The design drawing prescribed two 10 cm (4 in.) lifts in both the vertical and overhead applications. The contractor requested that instead of spraying shotcrete through one layer of mesh in 10 cm (4 in.) lifts on the walls, they spray through two layers of mesh in 20-cm (8-in.) lifts on the walls.

After a proof of concept during the construction mock-up phase of the work, it was determined that the contractor could safely spray through two layers of mesh in a 20-cm (8-in.) lift on the walls of the tunnel. With this improvement, additional float was allowed during the weekly construction schedule. This in turn allowed for work to proceed more efficiently without compromising worker safety.

Embedded channels: One major challenge faced during the design phase was how to install lights without having anchors in the new shotcrete liner. A solution was devised to use lattice girders with eyelets to which embedded channels could be affixed. These channels were installed after the first shotcrete lift was sprayed.

Since the tunnel lights have two points of attachment, two embedded channels had to be installed in parallel wherever lights were planned for installation. The contractor developed an innovative solution whereby the embedded channels were preconnected in parallel with curved steel supports. This allowed for two embedded channels to be connected to the lattice girders as one unit, resulting in easier installation of the embedded channels.

Waterproofing installation means and methods: When the project first began, the waterproofing membrane installation was set by manlifts that raised the membrane into place. This process was tedious as only two manlifts could be operating in one panel at the same time. Additional coordination was required for vehicles or personnel to safely pass under the manlifts while they were in operation.

Custom scaffolding was ordered to allow an entire panel to be scaffolded while vehicles and personnel could pass underneath. This helped to alleviate worksite congestion and make construction more efficient. The scaffolding was determined to be much safer to use than the manlifts in the tunnel, as the scaffoldings’ fixed structure reduces potential hazards inherent with high variable lifts.

Conclusion

The PTC set out to bring the Tuscarora Tunnels to current design standards and address the leak infiltration problems that had plagued the structures for years. The design team reviewed the existing conditions, evaluated possible remediation options and after consultation with PTC, chose the remediation option that most completely remedied the tunnels going forward.

Collaboration with the contractor before and during construction has allowed this design to come to life more efficiently, meeting an ambitious design schedule and goals while maintaining quality workmanship. With the state-of-the-art umbrella system, the lifespan of the Turnpike tunnels will be extended, and anyone transiting the tunnels will take notice of the effort that has been put into improving these structures.