Many railroad tunnels in the US experience severe seepage and icing in winter. The icing can restrict clearances, interfere with rail traffic, cause deterioration of the existing tunnel lining, and require significant maintenance and manpower to keep rail traffic moving. Ice shields are an effective method for insulating the tunnel to solve these issues. Design considerations, construction issues, and lessons learned from case studies in different parts of the US are discussed.
Reasons for ice management
Groundwater seepage into railroad tunnels is a common problem; water can find its way through discontinuities in the rock mass and find a pathway into the tunnel, whether it is unlined or lined with shotcrete, brick, or concrete. In severe winter conditions, the seepage may freeze, resulting in ice buildup (Figure 1). The icicles can encroach on the clearance available for passing trains, which can lead to speed restrictions. Large icicles may even result in such hazards as damage or penetration of railcars. The time required to remove icicles and the hazards that railroad personnel face during ice removal are also causes for concern. Where ice builds up over the track structure because of poor drainage, the risk of derailments increases.
Icicles also have a long-term impact on structural components within tunnels. Such impact includes the loss of integrity of the tunnel lining, ballast contamination, and unplanned additional weight on utility lines. Over time, freezing and thawing of water in the lining can cause subsequent cracking, whether the lining is shotcrete, brick, or concrete. Additional cracking of the lining allows not only more water to seep through the lining, but reduces the structural integrity. This allows for additional and increasing deterioration, which can increase the chances of shotcrete and rock fall in shotcrete-lined tunnels, brick fallout in brick linings, and deep concrete spalls in concrete linings. Icicles can also introduce waterlogging and clogging of the track ballast, which complicates track maintenance. Utilities within the tunnel may experience heavier loads than expected and could be damaged or collapse from the weight.
Types of icing shields
Each icing shield is required to fit in the limited space between the railcar clearance envelope and the existing tunnel lining so that the shield will not interfere with rail traffic. Depending on the clearance available in each tunnel, icing shields are designed either as self-supporting structures or are attached directly to the tunnel lining (a direct fixation system). They can also be either active or passive, depending on whether a power source is available to the tunnel.
An active system uses a heat source to supplement the insulation to keep the water from freezing, while a passive system relies solely on the insulation. If a power source is available, active systems are a more certain method of preventing ice. Active systems permit thinner insulation because external heat is being added to the system; however, this may be offset by the cost to provide and run the power source.
An available power source at a railroad tunnel is also a relatively rare occurrence. Therefore, in our experience, passive systems are more common.
The concept that McMillen Jacobs Associates has developed and proven effective is a multilayered system consisting of waterproofing, insulation, and protection. This system is easily modified for the conditions at any particular tunnel, including the available clearance and whether the system will be active or passive.
Self-supporting icing shield System
A self-supporting icing shield typically consists of evenly spaced steel sets anchored into concrete footings with a layered arrangement of components located between, and supported by, the steel sets. A typical section is shown in Figure 2. This arrangement, going from the field side to the track side, typically consists of a drainage mat, multiple layers of insulation planks, and a protective layer for the insulation, which consists of expanded metal (such as galvanized Stay-Form®) or c-channel lagging. The track side of the steel sets and protective layer are then covered with a thin layer of fiber-reinforced microsilica shotcrete (Figure 3). A waterproofing admixture may be added to the shotcrete or a waterproof coating may be applied to the track side of the protective layer to make the system more robust and prevent any leaks.
The shotcrete layer provides additional protection to the insulation planks from locomotive exhaust gas and also helps form an airtight system, contributing to the effectiveness of the icing shield system. This airtight system can be compared to the insulation of an attic or basement of a house — air does not circulate but maintains heat and acts as an insulator.
The steel set supported system is not meant to offer structural support to the tunnel ground or ice loads, but rather to inhibit the formation of ice by maintaining the temperature of the seepage above freezing until the seepage enters the tunnel drainage system.
Direct fixation icing shield System
When clearance is limited, direct fixation icing shields may be used. These icing shields are attached directly to the existing tunnel perimeter. From the field side to the track side, these shields typically consist of a drainage mat followed by a single or double layer of insulation planks and a protective layer of expanded metal (Figure 4). Each layer is carefully anchored to the tunnel perimeter to prevent groundwater infiltration. Polyurethane caulking is placed around the edges of the insulation planks and within the anchor holes to further limit seepage from entering the tunnel, since any leaks may form ice and accelerate the deterioration of the system. Expanded metal is then attached to the insulation planks for protection, and a protective shotcrete coating may be applied if clearance and budget allow.
These direct fixation systems may be constructed as either passive or active systems. If active, heat tape is installed directly to the tunnel lining prior to installing the drainage mat and the rest of the system.
Self-supporting systems are more robust and have a longer design life than direct fixation systems. However, selfsupporting systems are also more expensive. Because each system has different pros and cons, the severity of the problem, the quantity of seepage, and the condition of the lining are also considered when selecting the appropriate ice-shield solution. For example, even if there is adequate clearance for a self-supporting system, if a power source is present, the owner may opt for an active direct fixation system to save construction costs, since with a heat source less insulation is required for an effective system.
Premanufactured panels
In the 1980s and 1990s, passive icing shields consisting of premanufactured panels were installed in tunnels on the Alaska Railroad and in the Allegheny Tunnel in Pennsylvania (Figure 5). The panels consisted of 3in-thick (75 mm) panels constructed of two thin galvanized steel sheets with a core of polyurethane insulation planks between them. The steel sheets acted as protection for the polyurethane insulation planks, while also providing structural rigidity to the system. Individual steel sheets were connected to adjacent steel sheets with camlock devices. Like the self-supporting systems, the steel shields were not meant to offer structural support to the tunnel ground or ice loads, but rather to inhibit the formation of ice by maintaining the temperature of the seepage above freezing.
These premanufactured panels were constructed as either self-supporting or hanging systems. The panels were rigid enough to form a self-supporting arch within an existing tunnel, or the entire shield structure could be suspended from the existing tunnel lining with hangers attached to the shield panels with screws. Similar to the steel set self-supporting system, the panels are founded on formed concrete footings. For new ice shields being constructed, the steel set-supported system described above has replaced this system as the steel set system tends to be more robust. In addition, the premanufactured panels were specially made and are no longer easily available, but may be available from some international manufacturers.
Design Considerations
Clearance
The importance of clearance in the design of icing shields cannot be overstated. A limited amount of clearance is available in any tunnel. In an unlined or shotcrete-lined tunnel, there may be plenty of available clearance for a selfsupporting icing shield. In a brick- or concrete-lined tunnel, the available clearance is usually much more limited, unless the tunnel previously contained two tracks and is now operated as a single-track tunnel. For brick- and concrete-lined tunnels with very limited available clearance, active systems offer the significant advantage of lower insulation requirements because an external heat source is provided and a thinner insulation system can be effective.
Live-track work
Since these installations are in active rail tunnels, the designs have to be implemented in work windows of two to eight hours’ duration. The length of available work windows will depend on the quantity of rail traffic and not on the nature of the design. Therefore, the design has to accommodate the track time available.
On a busy main line, a design that requires eight- or 10- hour windows to accomplish key tasks will likely not be effective since the track time will simply not be available. A design that can be built incrementally is much more efficient, particularly if it can take advantage of short track windows and allow for as much preparatory work as possible to be completed away from the tracks. Where tunnels have two tracks, as much work as possible should be performed on one track at a time to minimize the track outages required.
The time and space constraints for construction have an overriding impact on the design of these systems.
Drainage
To be effective, a design also has to incorporate seepage and drainage improvements. The drainage system is critical for the icing shields to be effective for three reasons: (1) the water has to be transported out of the tunnel before it freezes; (2) blocked drainage could impose hydraulic loads on the shields; and (3) effective drainage and seepage control are necessary to ensure that no water gets through the icing shields and to the track side of the tunnel, where it will be exposed to freezing temperatures and can form icicles.
Calculations
In order for the icing shields to be successful at inhibiting ice formation, adequate thermal resistance (R-value) and a proper drainage system are required. Structural calculations are also needed for self-supporting systems to ensure that adequate structural support is provided during construction and for the life of the system.
Thermal resistance (R-value)
The R-value is a measure used to determine the required insulation parameters for specific structures in a climate location. The minimum required R-value for a project is a function of the geographic area climate zone.
For icing shield design, the minimum required R-value for a project can be determined from the International Energy Conservation Code (IECC, 2009). The IECC minimum R-values are intended for aboveground structures.
Because tunnels are more confined, using these values is considered conservative.
Ice shields are less likely to have weather-related issues if they are designed to have an R-value that meets or exceeds the minimum required. The R-values for an insulation system can be determined from the Colorado Energy Organization values for material properties. The different R-values assigned to the different layers making up the ice shield are added together (Figure 6). The single biggest contributor to the R-value is air space.
Drainage System
For self-supporting systems, the drainage path should be designed to allow water to seep either down the tunnel lining or the back of the drainage mat, be captured in a trough or pipe on the concrete footing, and be piped out of the icing shield to tunnel invert drains that will take it out of the tunnel. For direct fixation systems, the system should be designed to permit water to flow between the tunnel lining and the drainage mat into invert drains.
In the authors’ experience, thick PVC pond liner (40 to 45 mil) is preferred for the drainage mat. Thinner pond liner is much easier to damage during construction. Dimple board may also be used; however, it is stiffer than the pond liner and more difficult to place. For added redundancy to the system, two layers of pond liner may be used. The costs associated with installing a second layer of drainage mat at the time of shield construction are relatively small, especially considering the difficulties involved in repairing a leak in an icing shield after it has been constructed — which likely involves ripping out and replacing the leaking section.
To reduce inflow of water between seams in the drainage mat, it is recommended that seams overlap by at least 12in (305 mm) and a continuous drainage mat be placed over the top of the arch. When two drainage layers are installed, they should be offset by a half-width longitudinally to ensure the seams do not line up.
In direct fixation systems, the pond liners are pinned directly to the tunnel sidewalls and arch with structural concrete roofing anchors. In self-supporting systems, the liners are attached to the steel sets by an adhesive and are often kept in place during construction with weights, such as sandbags.
Structural Calculations
Icing shield systems are designed to inhibit ice formation. However, for self-supporting systems, the steel sets are typically designed for varying loading conditions, including ice formation along various segments of the icing shield.
Steel set geometry is designed to comply with the tunnel clearance requirements and to facilitate ease of construction. For example, steel sets consisting of two column sections and two arch sections with a single radius of curvature spaced on 4 to 5ft (1.2 to 1.5m) centers are often used. All of the steel elements should be corrosion protected.
Case Studies
NS Allegheny Tunnel, Pennsylvania
Severe seepage and resultant winter icing were an ongoing problem in the Allegheny Tunnel, restricting clearances and at times covering the track structure with ice for hundreds of feet, interfering with traffic, and requiring significant maintenance.
In the mid-1990s, the tunnel was enlarged to a double track/double stack structure. To control ice formation in the tunnel, a hanging ice shield consisting of premanufactured interlocking panels was installed at that time. After 15 years of use, many of the panels were seriously deteriorated, mostly because of locomotive exhaust blast to the underside and rusting of the rock bolt hangers that connected the panels to the adjacent rock (Figure 7). With extreme winter conditions and record freezing temperatures in 2009 and 2010, large icicles developed from existing seeps within the tunnel. NSR personnel were required to remove them every other day to limit hazards to rail traffic.
To remedy this situation, McMillen Jacobs Associates designed a self-supporting structure consisting of steel sets lagged between the webs with closed-cell polyethylene insulation, a drainage mat over the back of the structure for seepage control, and expanded metal and shotcrete covering the underside to protect the structure from locomotive blast.
Some excavation of the existing shotcrete lining and bedrock was required to obtain adequate clearance to install the steel sets. Unfortunately, the rock excavation was slow and time consuming, and track time was limited.
Only a portion of the work was completed by the end of the 2010 construction season.
Based on observations during the 2010 construction season, several modifications were made to the icing shield design to improve constructability and performance. These modifications included thicker pond liner for the drainage mat to make it heavier and less susceptible to damage; altering the shape of the steel sets to minimize the amount of rock excavation required; adding a waterproofing additive to the shotcrete; and increasing the thickness of the shotcrete in the crown of the icing shields. The design modifications were effective, and construction proceeded smoothly with the only major issue being limited track time availability, which caused the construction schedule to be extended. (Figure 8 shows successful installation of a shield.)
Key lessons learned during this project include the importance of good survey data, understanding the effects of material substitutions, and the benefits of good coordination between the contractor and the flaggers. A clearance survey was not performed during the design phase in 2010 because of time and budget constraints. The need for rock excavation during the 2010 construction season was at least partially due to the combination of a lack of good survey data and the irregular tunnel perimeter. At the start of construction in 2010, the contractor requested substitution of the thinner plastic for the pond liner drainage mat because it was more readily available and easier to work with. Unfortunately, the plastic was easily torn and, with the limited track time available, was often placed behind the steel sets but left unprotected, causing it to be blown against the rock surface and damaged. The pond liner used in 2011 was far more successful. Since track time was always limited, a great deal of the work was performed on one track at a time, which required careful coordination and good communication (Campbell et al., 2012).
UPRR Moffat Tunnel, Colorado
The Moffatt Tunnel was opened to railroad traffic in 1928 and has a variable lining that includes sections of concrete lining, unlined tunnel, gunite (precursor to modern shotcrete), and steel sets with lagging. The icing shield was installed in a 70ftlong (21.3m) section of the tunnel that was partially unlined and partially lined with gunite. There were existing drainage ditches on both sides of the tunnel at the invert, which extended out both portals.
Similar to the Allegheny ice shield, this shield is self- supporting. Prior to installing the ice shield, existing drip shields had to be removed and a reinforced concrete footing poured. The steel sets were pinned to the concrete footings and attached to the tunnel sidewalls with anchors. The insulation system consisted of two layers of drainage mats placed on the field side of the steel sets, two layers of insulation (Ethafoam planks) installed between the steel sets, channel lagging with additional Ethafoam on the inside of the channel, a sprayed waterproofing membrane applied to the track side of the channel lagging (Figure 9), and a 3- to 4in-thick steel fiber reinforced microsilica shotcrete (FRMS) layer over the track side of the shields.
Drain pipes were placed on either end of the icing shields, permitting drainage into the existing drains at the invert of the tunnel.
The system is more robust than the Allegheny system and incorporated the lessons learned at the Allegheny Tunnel. Differences from the Allegheny ice shield included channel lagging rather than expanded metal to protect the insulation, two layers of drainage mat rather than one, and the addition of a waterproofing membrane between the channel lagging and the shotcrete. The waterproofing membrane eliminated the need for a waterproofing additive in the shotcrete mix. The waterproofing membrane worked well, but was expensive. A larger steel set was also used so that an additional 1.5in-thick insulation plank could be installed behind the channel lagging.
BNSF Stampede Pass, Washington
Stampede Pass Tunnel #3 is located west of Easton, Washington, at an elevation of approximately 2,700ft (823m) above mean sea level, an area that receives more than 30ft (9.1m) of snow between November and May in a typical snow year. Temperatures hover below freezing both day and night during that period. Snow accumulation is so great during storms that a concrete snowshed extends for more than 80ft (24.4m) outside both portals of the tunnel.
Tunnel inspections completed by McMillen Jacobs Associates in 2011 revealed that seepage and icing were concentrated in the first 100-200ft in from the original portals (not the snowsheds), and was especially troublesome at the interfaces of the tunnel and snowsheds.
Wall ice up to 24in thick was observed on the sidewalls, with similar thickness on the track structure. It was also noted that icicles up to 4ft (1.2m) in length were suspended from the tunnel quarter-arches.
During the coldest portions of winter, double crew shifts were needed to keep up with ice removal.
The objective of the project work was to install a low-profile de-icing system that would capture, insulate, and convey water that seeped through the tunnel lining before it could freeze and form icicles or mound on the track structure. Because of the limited clearance in the tunnel, a direct fixation icing shield was required.
Construction was started by filling large spalls in the concrete lining with mortar to create a smooth surface for the icing shield. Slotted PVC drain pipes and heat tape were then installed at 12ft (3.7m) and 2ft (0.6m) above top of rail, respectively, on both sides of the tunnel. Heat tape was placed in the PVC pipes, and the pipes were sloped downward from the east portal face into the tunnel. The pipes were then connected to the existing trench drains.
Following the installation of the drain pipes and heat tape, a drainage mat (45 mil pond liner) was attached to the tunnel walls with structural concrete anchors. Two-inch-thick, 4ft by 9ft (1.2m by 2.7m) Ethafoam insulation planks were then installed on the pond liner inside the tunnel and attached with all-thread bolts drilled into the existing liner and epoxied in place (Figure 10).
Polyurethane caulking was applied around all the edges of the insulation planks and within the anchor holes to limit seepage from reaching the insulation.
Once the epoxy cured, scribe wire was installed, and a nut and two washers (plastic and steel) were used to hold the Ethafoam planks in place.
Contract drawings initially specified the installation of galvanized Stay-Form, but scribe wire was proposed by the contractor and approved for installation because of material availability and a reduction of the insulation section thickness by more than 1in, which was critical for clearance. Two inches of FRMS wet-mix shotcrete were then applied over the wire.
Because it started late in the season, construction was completed in two phases. One hundred linear feet (LF) (30m) of the 300 LF (91m) of icing shield were completed by the end of November 2012. The remaining construction work resumed in mid-June 2013 and was completed in early September. All work was performed under live track conditions, with work windows averaging five hours per day depending on the train schedule.
Because of the late start to the construction season and corresponding below-freezing temperatures, additional measures had to be taken during construction. Water had to be heated prior to mixing shotcrete, so heating cables were used in the water tanks overnight. Where groundwater was seeping into the tunnel, it formed ice that needed to be chipped out to ensure a safe working environment. Low temperatures contributed to machinery seizing up overnight and also created a difficult work environment for laborers.
In addition to constructing the ice shield, other drainage improvements were made to decrease the amount of water entering the tunnel. There is a waterfall located just south of the tunnel portal, and water cascades down the rock face directly onto the top of the tunnel portal. An existing drainage trough was present above the portal, so the contractor cleaned off debris and temporarily installed a pond liner to divert the water and keep it from seeping into the tunnel. Maintaining better drainage above the snowsheds will help prevent water from entering through the showshed/portal interface and causing icing issues during the winter months.
UPRR Tunnel 16, Colorado
Concrete-lined Tunnel 16 is located in the Rocky Mountains west of Denver, Colorado. The east end of the tunnel has a long history of icing issues. Insulation was first installed in the tunnel in the late 1980s by railroad personnel and worked well, except that the insulation did not cover the entire tunnel arch. This allowed ice to develop and led to bulging of the insulation planks. Full perimeter insulation was initially installed near the east portal in the early 1990s, but it deteriorated over time and was replaced by a contractor in the mid-2000s. By 2011, this insulation already needed replacement.
Similar to the Stampede Pass ice shield, the Tunnel 16 shield is a direct fixation system; however, the system is passive. Prior to installing the new ice shield, the existing installation planks and pins were removed. Once removed, the contractor filled in any voids and insulation pinholes with a waterproof mortar to create a smooth working finish. The drainage mat (40 mil pond liner) was then pinned tightly to the tunnel sidewalls and crown. One 2in layer of insulation planks was then installed on top of the pond liner inside the tunnel. Galvanized threaded anchor bolts were drilled into the existing tunnel lining and epoxied in place.
Polyurethane caulking was applied around all the edges of the insulation planks and within the anchor holes to limit seepage.
Galvanized Stay-Form was then installed tight to the front of the insulation planks for protection. Because of limited clearance and budget considerations, shotcrete was not applied to the track side of the shield. The railroad reported that the system was very effective during the harsh winter of 2013/2014.
Relative costs
Costs are highly dependent on the size of the project, project logistics including remoteness of the tunnel and available access, and available daily track time. The pay item for icing shields is usually per linear foot of tunnel and includes everything but the shotcrete. It is the authors’ preference to pay the shotcrete by the cubic yard, but it can be included in the linear foot price.
Claims are minimized with the use of extra work clauses to cover changed conditions, and delay time clauses to handle the uncertainty related to available track time. The delay time clauses account for the available track time compared to the track time promised in the bid documents.
The bid documents provide an average number of hours of track time per shift, with a bid item for compensation on an hourly basis for any loss of average track time.
For example, if six hours per day of track time are guaranteed in the contract and an average of five hours is actually received over the length of the contract period, the contractor would be compensated for 1 hour x number of shifts x the bid rate for delay time. The delay time can be calculated monthly or over the length of the contract.
Based on the icing shields installed in the last four years, a self-supporting steel set icing shield can cost USD 10,000 to USD 12,000 per linear foot of tunnel (including mobilisation). Direct fixation icing shields can vary from USD 2,500 to USD 5,000 per linear foot of tunnel, depending on a number of factors, including tunnel access, remoteness, and the size of the tunnel.
Conclusions
Icing is a very common problem in railroad tunnels. The limited clearance and track time available create unique design and construction challenges.
The multilayered insulation systems described in this paper provide a flexible design framework that has been proven effective and can be modified as needed for specific tunnel conditions.
With each project, new lessons are learned based on the experience during construction and can be incorporated into future designs.
