Tunnelling professionals often face a world of unpredictability. This includes soil, rock, water and gas conditions that are not as expected or accurately investigated, political will that changes, property owners who become concerned about impacts to their buildings, and many other concerns.
They do not need more unpleasant surprises that can arise during cross-passage construction. Yet all too often these small headings turn out to be anything but small in terms of their ability to add to costs, leading to disagreements over who pays the increases. They can also delay completion dates, which leads to similar finger-pointing and blame.
Then there is the diversion of management time from the main task at hand – which is pushing the tunnels forward and completing the project.
One common way to add predictability into cross-passage construction is by modifying the ground conditions, to give tunnellers a better idea of what to expect. Often, this is achieved by drilling down from the surface and treating the ground at the location of the cross-passage or, in some other way that makes the ground easier to work.
This can pose several kinds of problems.
As a result, any methods for adding more predictability to the costs and timelines for constructing cross passages between twin tube road, rail and metro projects is welcome.
One possible solution to the cross-passage challenge relies on existing methods and technology, applied in a different way. We like to call it the Smart Cross Passage.
It avoids the operational issues involved in traditional cross-passage construction, which can come from bringing in excavators, excavator-mounted hammer heads, or drill+blast equipment.
Smart Cross Passage uses the rail tracks installed to service the TBM excavation of the main tunnel tubes. A small-scale TBM, mounted on the tracks, is positioned sideways so it can drill through the tunnel wall, and then excavate the cross-passage to the parallel tunnel (Fig 1). A standard one-pass segmental lining of steel or concrete segments fitted with sealing gaskets are installed during excavation. No subsequent waterproofing or inner lining is required.
The cross passage TBM is then disassembled and pulled back through the new cross-passage to be re-assembled and moved along the tracks to repeat the process for the next cross passage.
Smart Cross Passage depends on the fact that the ground conditions, as applied to TBMs, are already known, based on the experience of boring the main tunnels. This information includes the rock or soil type, the amount of groundwater, and the presence and type of gases that might be encountered. All of this information is used to design an appropriate cutterhead configuration for a TBM. An appropriate earth pressure balance is calculated in order to hold the cutterhead right against the face. This is not theoretical information. It has been ground-proven through the construction of the two main tunnels. It is information that can be used to plan how to use the smaller secondary TBM to excavate and construct the cross passages.
The secondary TBM must be compact enough to fit sideways into the existing tunnel, and still give some access to and from the face. It must also be designed for easy disassembly to the point that it can be easily retrieved through the newly built cross passage.
It should be possible to carry out the construction of a cross-passage heading in 72 hours, including propping, boring, installation of the liner, sealing the gaps, completing the junction between the lining segments to the lining of the main tunnels, and retrieval of the TBM unit from the completed cross passage.
Like any new idea, it takes time to master, but we are confident that Smart Cross Passage can develop into a production-line process, in which the needed equipment and crew can just move along the tunnel, drilling one cross passage after the other, to solve one of the most vexing problems in tunnel construction.
This method of constructing cross-passages is particularly effective in difficult conditions. These include:
Challenging geological or hydrogeological conditions: These have already been met and bested by the team constructing the main tunnels. If construction of the two main tunnels has been completed successfully, it should be relatively straightforward to apply the lessons learned to configure the cutterhead and excavation system of the small cross-passage TBM. Smart Cross Passage operates in all rock and soil conditions, above or below the water table, and in the presence of a wide range of gases.
Deep conditions: Far underground, the idea of modifying the ground conditions through freezing the soil or via other ground treatment methods, becomes more technologically challenging, time consuming, and costly. As a result, project owners and other interested parties may be more willing to consider alternatives that side-step the need to modify the soil conditions.
Challenging surface conditions: Many city centers have old historic buildings that are vulnerable to subsidence of the surface. Using TBMs for the cross passages minimizes risk to these and other surface structures. There is almost certainly the difficulty of access through city streets for truck-mounted drilling rigs.
Challenging political conditions: Political leaders can be under pressure from property owners, roads authorities, railroad operators and others to minimize surface disruption during subsurface construction works. They can also have a strong need to produce a completed project on schedule and on budget, with no surprises. Given the accepted reliability of a TBM operation beneath major urban areas, using the same technology to complete the cross passages can inject some valuable certainty into timelines and budgets.
Like any new idea, Smart Cross Passage must compete with existing methods, with which the industry is already familiar. It helps that TBMs are a mature technology and solutions have been found whereby this method of tunnelling applies within a wide range of circumstances.
We believe that Smart Cross Passage would be best introduced via a design-build contract environment, but with strong support from the owner and a process of risk sharing. While it is essential to mitigate risks as much as possible, the risk should be shared with the owner.
With experience of using the Smart Cross Passage concept, tunnelling interests should be able to industrialize the process of building cross passages, taking what has been a customized approach and making it predictable, in the same way that the early TBMs caused a revolution in tunnel construction.