SUMMARY

Practices for Controlling Tunnel Leaks

Water infiltration is a common problem in tunnels. Such infiltration accelerates the deterioration of the structure and elements within and creates hazards in the form of icicles and slippery roadways. The severity of water infiltration often reflects various factors, so determining the appropriate remedy is challenging and frequently requires an extensive investigation to understand the source and location of the leak, the structural details of the tunnel, and the geology of the surrounding substrate. Various methods exist to address water infiltration but understanding the appropriate mitigation to use in specific circumstances and what limitations accompany that mitigation are essential to success.

The objective of this synthesis study was to document the methods and practices used by state DOT tunnel owners to control tunnel leaks. This synthesis report documents the problems resulting from water infiltration, common practices for investigating and identifying the source of leaks, and methods for controlling and managing leaks to (1) prevent damage to the structure and (2) maintain the intended level of service. Elements of the synthesis include a literature search of relevant documentation, a survey of state DOT tunnel owners, and case examples with select DOT tunnel owners.

On the basis of the literature review, tunnels can be categorized as either (1) drained or open systems, where the liner is designed without hydrostatic pressure loading and water that infiltrates is carried to the drainage system; or (2) undrained or closed systems designed with full hydrostatic loading and the goal of keeping water out. The design intent is important to understand when remediating water infiltration; if water was intended to drain into the tunnel, efforts to prevent its inflow could build up hydrostatic pressures on the liner that were not originally designed for. Tunnels constructed in rock with low permeability often fall into the former category. Cut-and-cover tunnels and those constructed in soft ground, as well as submerged tunnels, fall into the latter category and are designed to prevent water infiltration.

For drained or open systems, remediation methods involve capturing the water and directing it to the drainage system. One method for accomplishing this is an umbrella system. With an umbrella system, a waterproofing membrane is installed on the inside of the tunnel; this allows the water behind the membrane to be collected and directed to the drainage system. Another common type of catchment system is installed at the joints when the primary leakage occurs only at joints. Gutters or drainage pipes are installed at each joint to collect infiltrating water and these pipes direct infiltration to the drainage system. Holes are sometimes drilled into the rock to attract water to the drain locations, thus reducing water in areas between joints.

For undrained or closed systems, methods to prevent water from infiltrating the liner include negative and positive side repairs. Negative side repairs are implemented on the inside of the tunnel (e.g., repair of joints, injecting grout into cracks and joints, and coatings to seal cracks on the surface of the liner). Positive side repairs are made on the outside of the tunnel and include repair of external waterproofing systems and joints by excavating to make the repair and then replacing the backfill material. Another positive side repair can be made from the inside of the tunnel but creates a perimeter barrier and seals cracks and fissures in the underlying rock. This method is referred to in this synthesis as curtain grouting. Redirection of drainage is also possible for undrained systems and can be used to control isolated locations of water infiltration.

The survey was distributed electronically to the voting membership of AASHTO (to each state’s bridge or tunnel manager). The survey was distributed to 50 states along with the District of Columbia and Puerto Rico. Representatives of 33 states (63%) completed the survey; of these, 26 respondents indicated they have state-owned tunnels in their inventory, and these 26 respondents provided the input summarized in Chapter 3. The tunnel inventory provided by the 26 responding states accounts for 206 of the 552 tunnels in the 2022 National Tunnel Inventory (approximately 37% of the U.S. inventory).

The survey also attempted to gather information on tunnel-like structures that may not be in the state’s tunnel inventory. Of the 26 states with tunnels, 17 indicated they also have deck-over structures, although it was not clear how many of these structures were in the tunnel inventory and how many were not. However, these respondents indicated they have 85 deck-over structures that have experienced water infiltration, and the joints are the location of greatest infiltration. This information is relevant given the significant number of these tunnel-like structures that exist today and that are planned for construction in states across the country.

The survey results from the 26 respondents with state-owned tunnels indicate that 121 of the 206 tunnels (59%) have water infiltration. Most agencies indicated the water infiltration in their tunnels results in accelerated deterioration of the structure (14 of 26), icicles (14 of 26), deterioration of functional systems and elements within the tunnel (10 of 26), and slippery roadways (8 of 26). Six DOTs (6 of 26) noted issues with mineral deposits in their drainage systems resulting from water infiltration. To investigate the source and location of water, six DOTs (6 of 26) have used non-destructive testing (NDT) methods, including light detection and ranging (LiDAR) scans, thermography, ground-penetrating radar (GPR), and photogrammetry; however, 73% of respondents have not used NDT methods. The six respondents who have used NDT methods had mixed reviews on the effectiveness of these methods.

DOTs provided specific tunnel information that has been categorized by tunnel construction and substrate. A total of 43 tunnels were entered in the survey, with all types of tunnel construction addressed. The sources of water most frequently noted were groundwater and surface runoff. Joints (18 of 43) and cracks (17 of 43) account for the most common location of leaks cited for the 43 tunnels, but penetrations in the liner (3 of 43) and transitions (3 of 43) were also noted.

The remediation methods used most often by state DOT tunnel owners are catchment type systems where water infiltration is redirected to the drainage system. Twelve of 26 respondents have used this remediation, followed by nine DOTs that have injected grouts to stop leaks and nine DOTs that have repaired tunnel joints. Other remediation methods have been used, but to a far lesser extent. The numbers dropped when considering effectiveness of these remediations over time. The most effective for at least 10 years and the most effective overall was redirecting drainage; it was most effective 64% of the time.

The second most effective in the long term according to DOT responses is injecting cracks and joints, but this method’s overall effectiveness was only 44%. DOT frustrations with the effectiveness of mitigation methods were shown by responses to questions about the challenges DOTs experience in mitigating water infiltration. Most (23 of 26) commented that the remediation was not effective in the long term, or the water moved to a new location, or the quantity of water infiltration was so large that it was difficult to control. Many DOTs noted they would like more information on effective remediation methods.

In addition to the surveys, four state DOTs were interviewed as case examples so the synthesis authors could learn more about the DOTs’ experiences with controlling water infiltration in tunnels. The four case examples were selected based on survey responses and to assess various tunnel types and conditions. The four case examples were as follows:

• Arizona Department of Transportation (ADOT)
• Colorado Department of Transportation (CDOT)
• Pennsylvania Department of Transportation (PennDOT)
• Washington State Department of Transportation (WSDOT)

Each DOT addresses unique challenges. ADOT discussed a deck-over structure with leakage through the joints and described challenges in repairing the joints given the depth of the post-tensioned box-girder superstructure and the limited access from above. CDOT described repairs for water infiltration using both chemical grouting and installing a drainage system and noted that neither have been effective in the long term. PennDOT described significant water infiltration remediation projects in the state being constructed by other toll agencies. These projects include an umbrella system and drains in each joint. The effectiveness of these drainage improvements will not be known for some time. WSDOT presented several materials that have been effective for them with state pontoon structures, which have no tolerance for water intrusion; these materials are used by WSDOT to grout leaking cracks and joints in their tunnels as well. WSDOT was the only responding agency that has acceptance criteria for new construction or tunnel rehabilitation contracts.

Further research is needed into leak remediation methods that are effective for the long term. A go-to knowledge base of specific materials and methods for various types of tunnel construction, along with information on effectiveness and limitations, would be most beneficial to DOT tunnel owners. This database of tunnel remediation would be valuable to tunnel owners if organized by type of tunnel construction and ground conditions and if it included details of the remediation method used and the method’s effectiveness over time.