Tunnel and underpass projects beneath operating highways and railroads are among the most sensitive and complicated underground projects in terms of design, construction, monitoring, and, most importantly, safety. They can potentially call upon many different tunneling techniques depending on the geometry, cover, geology and circumstances of what is, usually, a fairly compact project site for construction of what is not usually a large tunnel but its creation can be delicate and of vital economic importance.

In some cases, projects in this category have enough cover and fair geology to implement both conventional tunneling design and construction methodologies without any major concerns for the traveling public above. However, on many projects, design and construction teams come across challenging areas with a relatively low cover or sensitive ground conditions that require careful consideration to allow the unhindered, safe operation of existing railways and roads.

During tunneling, temporarily halting traffic or trains above an excavation can mitigate, through elimination, the risks that carry the highest safety consequences. However, in many cases, this is often not possible from a technical, financial, or public relations point of view. This means there are risks of substantial settlement, sinkholes, or damage. In such conditions, any misstep could result in accidents or, in the worst-case scenario, loss of life, not to mention significant disruption for the traveling public, urban economy, and reputation of heavy civil projects in the area.

While there are so many challenges, tunnel crossings have become a popular solution to crossing key transport corridors in busy urban environments. They bring their own design and construction challenges. Based on key lessons learned across several projects in the authors’ careers, the importance of geotechnical investigations and the selection of tunneling methods to execute major works adjacent to or below sensitive railways and highways are discussed, along with potential measures and techniques to deal with relevant risks, and the instrumentation and monitoring aspects required. Not least, the importance of effective communication channels with stakeholders to obtain permits and coordinate the works is highlighted.

WHY GO UNDER WITH LOW COVER

The main challenges of constructing above-ground infrastructure systems in busy urban environments are the associated safety and public impact issues. There is also a big economic challenge, as the above-ground space in busy urban areas is too expensive to justify being used for building elevated infrastructure rather than as space for human occupancy for residential and commercial buildings.

Ideally, traditional tunneling under major roads or railroads is accomplished with several tunnel diameters of good quality cover between the crown of the tunnel and the bottom of the pavement or railroad ballast. But the reality is that, for many projects, the cover thickness is often less than one tunnel diameter due to geometric constraints and to achieve minimum cost and earliest completion. This means there is little to no room for the team to go through the learning curve or make errors, as any collapse or ground loss can rapidly (if not instantly) translate into the surface with major consequences that could be catastrophic.

In most cases, track outages or lane closures are not acceptable to the owner of the facility to be crossed, and no disruption to their services is allowed; therefore, tunneling methodologies that provide ‘zero impact’ solutions (tunneling done while all surface facilities are in normal operation) become the desired option as they result in no disruption to the overlying facility. Of course, any construction activity will have some impact on the area, so the term ‘zero impact’ implies that the tunneling effort has no major effect on the transit corridor operation and traffic flow aspects of that crossing.

RISK EXPOSURE

Shallow tunneled crossings under live highways and operating railways impose more significant risks to the public and potential for damage to a range of third-party persons and property compared to most other tunneling projects. These risks include safety concerns for those who work on the project and thousands of commuters passing the project area at high speeds while tunnel excavation is underway only a few meters below them.

Other inherent uncertainties, including geology and groundwater conditions, could cause significant cost overruns, delay risks, and environmental incidents.

Finally, there is always a reputational and political risk that a problem caused by tunneling for major public transit corridors will give rise to public objection or protests, affecting the course of the project.

DESIGN CONSIDERATIONS

As it is for every tunnel project, understanding the environment in which the tunnel will be built is one of the most important factors before the design process starts. A number of aspects need to be looked at in this regard before or during the design process.

In general, what characterizes the nature of these types of crossings includes: having a relatively low cover (shallow tunnel crown to ground surface depth); the need to maintain face and crown stability to prevent surface deformations; the presence of compacted (non-cohesive) granular backfill material; and, the potential presence of artificial objects and natural obstructions.

There are also, always, serious concerns about safety and losing the face of excavation that could impact the operating surface structure.

Another common surprise in such areas is the problem with finding reliable ‘as-built’ information about details of the existing highway or rail asset to help establish the exact locations of elements of previous or historical projects or works in the crossing corridor of concern. This could result in unexpected encounters with different features, such as parts of existing or decommissioned structures, sheet piles, soldier piles, beams, support of excavation (SOEs), logs, etc.

Sub-Surface Studies

It is important to know the geology of the subsurface ground between the tunnel and the road, ensuring there is no artificial material that could either be in direct conflict with the tunnel alignment or affect the tunneling operation.

Historically, many highways and railroads have used a fair amount of granular backfill material underneath the road base structure or around buried utilities, which can act as a flowing ground trench and cause a range of issues for the tunneling process. Therefore, a detailed look at the previous data, existing logs, and historical information is critically important.

In some cases, the tunneling crew might find manmade materials like steel or concrete that have been left in place or mixed with backfill. These materials are usually left in the ground unintentionally or due to a lack of proper restoration and cleanup. There is also a chance of encountering natural obstructions such as a nest of boulders or large tree trunks. These types of obstructions could halt or slow down the tunneling operation, depending on the method of mining and size/type of obstruction. Losing time is not always the biggest concern, rather it is when the excavated ground remains open for longer than anticipated, considerably increasing the chance of surface effects.

One of the simplest ways of investigating the ground below the surface, assuming temporary lane or track closure is achievable for a few hours, is to dig test pits and undertake similar methods, such as potholed trenches or a pattern of shallow boring. Equipment needed may only be a small excavator or bucket augers. However, while they are quick investigation methods in themselves, there needs to be consideration of the time needed to properly restore the track or road to the highway/rail owner’s requirements.

Undocumented and undiscovered manmade fill can present challenges to excavations when encountered in shallow cover crossings. A project in Canada, in Ontario–Kirkland Lake area for the replacement of an existing highway culvert, saw such challenges. The original culvert was constructed in a backfilled embankment and the geotechnical investigations had been unable to fully detect the presence of many boulders and cobbles. Contract documents did not adequately provide information on the magnitude of the rock fill within the embankment.

‘Zero Impact’ Methodologies

It is always a big challenge to do geotechnical investigations around live rails and roads for the same limitations, safety issues, and operational concerns apply to an underground tunnel as would be the case if an open-cut construction method were used to cross the transport corridor. This is so even if the financial cost is higher due to the social cost of stopping the transport services during the construction duration. It is a proximity risk. Therefore, other ways of completing geotechnical investigations that could also provide ‘zero impact’ solutions need to be considered.

Horizontal Subsurface Probing

An example of ‘zero impact’ methods to assess the ground under live transitways would be probing the ground horizontally using methods similar to horizontal directional drilling (HDD). Using this technique, particularly with some modifications such as optical sensors or geophysical instruments, it would be possible to gather different types of investigation data. Even simple drilling data combined with a visual inspection of excavated material could, at a minimum, give some understanding of the type of ground, the density of soil, saturation, and many other indications that could help with the design of the best method of ground support and choice of construction methodology.

Ground Penetration Radar

Mobile ground penetration radar (GPR) is another ‘zero impact’ method of ground investigation. GPR equipment could be attached to the rear of typical cars/trucks and rail vehicles, respectively, being pulled to gather data on the ground and any artificial material below the surface.

The antenna of a GPR system emits a high-frequency pulse of energy into a material. The pulse reflects off the boundary of materials with different dielectric constants and the strength and the time required for the return of any return signal is recorded. The survey data are presented in the form of average asphalt and granular thickness over a defined interval. High-resolution GPR technology can help produce a pseudo-cross- section of the upper meter of the subsurface, closest to the road pavement or rail ballast.

Pilot/Probe Tunnel

Another way of investigating the ground below the road or rail level is with a pilot/probe tunnel, which involves drilling a smaller cased tunnel that is being planned for the full-size arrangement. The pilot/probe tunnel would allow at least one person to enter the excavation and investigate the ground at any point under the road or rail surface.

A popular way of using this pilot/probe method is to combine it with the pipe roof methodology (see below), utilizing the first few bores/pipes which helps to optimize the excavation for the following pipes as well as the excavation of the final tunnel profile. Provisions to use pilot/probe tunnels can be built into the limited notice to proceed section of construction contracts for such projects.

Road and Railbed/Track Sub-Structures

When tunneling under live loads with low cover, the type, thickness, and strength of elements that constitute the road or track structures are just as important as the geology of the ground below.

In the case of highways, for example, the design of the road base, thickness of asphalt, and strength of the concrete slab (if one exists) are among the many items the design team needs to consider. On the other hand, for railroads, the design of the track slab, the characteristics of the compacted soil below it, maximum weight, and frequency of trains transiting over the proposed crossing corridor are among the key factors to study.

Utilities, Instrumentation, and Contaminated Soil

In looking into the structural elements below the surface, it is also essential to conduct a confirmation survey of all the installed utilities and monitoring instruments in the area. Some old and abandoned utilities and monitoring apparatuses are not shown on municipal and reference drawings, so subsurface utility engineering investigations are needed to consolidate all the available and missing information before the tunneling work takes place.

Similarly, the issue of the potential for contaminated soils in the area needs to be looked at. Many old highways and railroads have residual contamination from when the road/rail was being constructed. Some others have been built above contaminated areas that were unknown or not deemed critical at the time. Environmental engineers initially evaluate this risk by studying the historical information and specifying additional tests and investigations to ensure all environmental requirements are met, and best practices have been utilized.

Communication with Stakeholders and Obtaining Permits

Close and timely coordination with regional municipalities and transportation departments is a must. The successful approach consists of sessions to review existing data with the stakeholders to optimize tunneling configurations and monitoring methodologies. A typical tunnel project in an urban area that crosses a live transit corridor might require more than a dozen types of permits and several types of agreements. Some of the material that needs to be submitted in the applications could take months to be prepared and months to be reviewed. Therefore, it is never too soon to start coordinating and having discussions.

The instrumentation and monitoring operations around the transit corridors generally have their own particular challenges regarding the location, type, and reading frequency of each instrument. Therefore, it is important to confirm and communicate all details of needs on instrumentation and monitoring to the transportation departments early on while at the same time leaving room in the design and specification for the contractor to come up with modern instruments and cutting-edge methods of monitoring.

COMMON METHODS OF TUNNELING WITH LOW COVER

Generally, tunneling methods for low cover crossings are characterized by short length drives where the geometric design must often ‘thread’ the new tunnel structure into an available window and perform installation while the overhead surface facility stays in operation. This introduces safety and reliability challenges.

Such projects typically encounter highly variable ground conditions and manmade unconsolidated material (due to the previous construction history in that area) that could act very differently to undisturbed natural material and show highly variable water tables with significant seasonal variations. Much attention must be paid to robust methods that include readily implemented, in-place contingency plans. Since surface movements can occur within a very short duration relative to construction activity, construction must include remote monitoring systems with real-time reporting, interpretation, and distribution of data.

Pressurised Face TBMs

Earth pressure balance, or EPB, is a mechanized tunneling method in which the excavated material is used to support the tunnel face while it is being plasticized using foams and other additives to make it stable and transportable. The excavated material is moved through the tunnel boring machine (TBM) via a screw conveyor arrangement, which allows the pressure at the face of the TBM to remain balanced.

A slurry machine (STBM) removes excavated material by hydraulic means with the flow velocity and pressure carefully regulated to enable the slurry chamber pressure to effectively counterbalance the groundwater pressures. The soil to be removed is pumped out along with the slurry to a separation plant situated outside the tunnel, where the slurry is separated from the muck for recirculation.

EPB has historically ‘competed’ with the slurry balance method in mechanized tunneling.

Box or Deck Jacking

Box or deck jacking is a tunneling method that involves the jacking of typically rectangular structures simultaneously with excavation. Prefabricated tunnel sections are advanced horizontally using high-capacity hydraulic jacks.

This concept requires a jacking frame and equipment to be set up on one end of the tunnel alignment and the delivery of large concrete segments to the site. The benefit of jacking a precast box or deck is having initial and final ground support immediately in place while excavation is ongoing. This allows excavation of the ground and construction of the tunnel lining to be performed safely and simultaneously under a safe cover.

Risks involved in box or deck jacking include more difficult steering and the chance that the jacked structure deviates from the design alignment. Also, the short distance between the top of the jacked element and the road base (or rail slab) could destabilize the existing structure and cause deformation or settlement.

Project Example: King Road at CN Grade Separation

Canadian National Railway’s Oakville Subdivision at King Road is integral to its Great Lakes Region. Located within the jurisdiction of the City of Burlington, King Road is at Track Mile 33.31 along the CN Oakville subdivision serving CN’s freight traffic, commuter traffic on GO Transit’s busiest corridor, and VIA Rail’s passenger service.

King Road is a two-lane arterial road that intersects four CN tracks and grade separation was principally accomplished by erecting a concrete reinforced deck structure, constructed adjacent (south) to the site and then sliding it into place using deck jacking/sliding design and technology.

The innovation was selected primarily because the existing mainline tracks could not be taken out of service for the duration required to construct the bridge in place, and as such, an alternative method had to be engineered to minimize conflict with the daily operations of the Oakville Subdivision. Hatch designed the structure to be built adjacent to the tracks. During a weekend shutdown, CN’s three mainlines were taken out of service, and the structure was slid, using hydraulic jacks, into place. The tracks were then restored in time for regular commuter traffic.

Umbrella Roof/Wall system

This method supports the ground ahead of the excavation by installing – and possibly grouting in place – a canopy of parallel pipes above and following the excavation profile. An adequate cover above the excavation profile is required for this technique to work.

The method allows the tunneling crew to observe ground response every few meters and select the proper ground support from the range of pre-designed options and pre-planned parameters as excavation progresses. This approach results in speeding up the advance rate when conditions are more favorable or increasing the amount of support when the ground is less stable than initially anticipated.

Sequential Excavation Method

Characterized by the progressive yet sequential removal of ground material followed by installation of support, the sequential excavation method (SEM) includes a thorough investigation of the ground and adjacent structures to create functional classifications for support and advance lengths (maximum unsupported excavation length). The subdivisions of the tunnel cross-section that are sequentially excavated and supported are the heading, bench, invert (top, center and bottom) and the side wall drifts (sides).

Tunnel and geotechnical engineers use these classifications in combination with direct ground observations on site to assess the result of the latest tunnel advance and recommend a new round length and class of support system for the excavation operation ahead.

In SEM, the strength of the ground around the excavation is purposely mobilized to the maximum extent possible. This is achieved by allowing controlled deformation using initial primary support with load-deformation characteristics appropriate to the ground conditions.

Rather than using stiff support members that attract high loads to fight the ground deformation, flexible but strong support measures (like shotcrete lining) are used to redistribute loads into the ground by deflection and allow the ground itself to become an integrated part of the tunnel support system. In addition to shotcrete and, additional support can include rebar mesh, lattice girders, bolts, or dowels in rock. Different arrangements might be used for the heading, bench, invert, and side wall drifts.

In many projects, pre-support measures must be used to protect the next advance before excavation and installation of support measures occur. Among the favorite techniques to improve the ground ahead are spiling, forepoling, and pipe arch/canopy.

Pre-excavation Ground Improvement Techniques

Geological and site characterization enable successful pre-construction mitigations and design of pre-excavation ground improvement to be achieved. Where ground improvement is necessary before excavation for low cover tunnels at road/rail corridors, two of the commonly used methods are grouting and ground freezing.

Grouting (Cementitious or Chemical)

The successful design of grouting operation requires a full analysis of the ground, groundwater, and tunnel design to specify the appropriate method of grouting.

In addition, consideration must be given to parameters such as pressure, flow, and volume tested in similar geology to mitigate the risk of grout damaging the road or track base or causing channelling out (‘frac out’) to the surface. In many cases, existing boreholes and water monitoring wells complicate this process by becoming a path of least resistance to the ground surface.

Usually, an early trial ground improvement program is performed in similar ground conditions around the site to demonstrate that the product/material selected can adequately permeate the fill so that the design intent can be achieved.

The trial aims to test whether the injection materials will consolidate the soils after being introduced directly into the pores or crevices. Through a chemical reaction or physical change of state, the injected material hardens and retains its form and location after the injection. The injected soil area will thereby solidify and simultaneously seal.

The injection process and pressure selected are performed according to the geological and physical conditions of the ground, as well as the anticipated water and other relevant conditions.

Ground Freezing

Ground freezing is an environmentally friendly construction technique to provide temporary excavation support and groundwater control during tunnel construction. The method is extremely helpful when tunnels need to be built in difficult geological and hydrological conditions. The additional cost (in most cases) is justified by the wide range of soil and ground conditions where it can be used effectively.

The usual freezing system consists of a refrigeration plant that chills a brine solution. This is then pumped down the center of an annular freeze pipe and subsequently returns through the outer annulus that is in contact with the ground that needs to be supported. The returning circulation of brine that has extracted the latent heat from the surrounding ground is then returned to the refrigeration plant to chill, and the cycle continues.

In most projects, arrays of freeze pipes are installed to cover the entire zone that needs to be supported as a frozen mass block. Initially, circulation takes more energy to freeze the zone around each pipe and to overlap with other pipes. However, after that stage, the refrigeration only needs to maintain the frozen area.

Creating a freeze zone stronger than necessary may result in lower advance rates, larger ground movements, and additional costs.

Project Example: Central Artery Tunnel I-90/I-93 IC

The project was designed to cure massive traffic congestion in the heart of Boston by replacing the elevated central artery section (opened in the 1950s) with a new eight- to ten-lane highway running mainly underground. The consultant was brought in to review the preliminary design and site constraints where the highway crosses beneath eight heavily-trafficked railway tracks. Interruption of rail traffic was inevitable under the preliminary engineering design. The consultant proposed a tunnel jacking solution that allowed construction without interrupting train services, saving millions of dollars of railway operating revenues.