Let’s face it: tunneling projects are inherently risky. The actual geology can harbor unforeseen conditions even with rigorous testing, and even when a highly-qualified consulting company creates the Geotechnical Baseline Report (GBR).
In TBM-driven tunnels, the machine and the crew operating it are key components of the risk management strategy. A TBM is not an isolated piece of equipment—with its mechanical action it can change the soil properties just in front of the cutterhead through injection of grout and additives. By improper operation the TBM can also over-excavate or under-excavate the ground around the tunnel.
So, how can we ensure that the operation is done effectively as often as possible?
Changes in the structure of contract risk management within North America would be a good start. All too often the owner, along with an over-zealous consultant, tries to pass unfair risk on to the contractor. In turn the contractor, who has won the bid based on lowest price, attempts to pass risk responsibility on to the machine supplier with the hope that this will reduce overall risk during tunneling. This risk can be passed by unreasonable payment terms and by forcing unreasonable liability clauses. In some cases the supplier must be willing to accept risks far disproportionate to what the business can sustain.
There must be a more objective way for owners and contractors to view risk, other than looking for the lowest equipment price and highest willingness to accept risk from a TBM supplier. In fact, a correctly designed TBM will be the key to a project’s success, and correct machine design, even with increased initial cost, is part of that formula to success.
Field results have shown, time and again, that a TBM built with “risk insurance”-type features (such as probe drills, shield lubrication, etc.) can have a huge impact on a project’s success in terms of schedule, cost and safety. It is far better to build features into the machine from the start as part of a comprehensive risk management strategy, than to add them in the tunnel after an unforeseen event has occurred or the machine has become stuck.
Soft and Mixed Ground Risk Management
In soft and mixed ground conditions, as in all geologic conditions, an accurate GBR is key to project success. As many projects in soft ground are in urban settings below building foundations, utilities, existing tunnels and other structures, the GBR becomes all the more important. It is also imperative to change the ground only as much as necessary to precondition the soil for pressurized tunneling, through additive injection and bentonite injection. Concurrently there must be a rigorous monitoring of sensitive structures.
Because of the finesse required, properly trained tunnel managers and TBM operators are the key to effective risk reduction. Most cost overruns in these conditions are caused by operator error or inexperienced management, not by machine component breakdown. To that end, Robbins and other companies are involved in developing training programs, together with universities such as the Colorado School of Mines, that will teach technicians and operators how to drive EPBs in a variety of ground conditions through computer simulations.
In mixed ground conditions, and in solid rock conditions under water pressure, a customized strategy is needed. At Mexico City’s Túnel Emisor Poniente (TEP II) Project, which has been written about in previous issues of TBM: Tunnel Business Magazine, a record-breaking Crossover TBM was equipped with features that enabled it to get through several known and unknown features. The hybrid-type machine could operate as a non-pressurized single-shield TBM or as a pressurized EPB TBM depending on conditions.
When the crew, operating in rock mode, detected an unforeseen large cavern of at least 90 cubic meters in size, the machine was stopped and the cavern was filled using a mixture of pea gravel, bentonite, and grout. Polymer was injected through the TBM cutterhead to consolidate ground directly in front of the machine, and by using the EPB features the TBM was able to effectively negotiate this section at a reduced advance rate with minimal delays. This is an example of what a customized machine, and a predetermined strategy, can do when obstacles present themselves.
Because of the customization required for mixed ground conditions, strategies that would work for soft ground, such as planned hyperbaric interventions, may not work well—particularly in very hard, abrasive rock that is water bearing. The current practice of designing cutterheads with a long distance from the face to the muck transport system inhibits smooth muck flow through the cutterhead, and should be reconsidered.
The cost for hyperbaric inventions on large diameter
tunnels in hard rock can be tremendous. These massive cutterheads with muck flow restrictions only increase cutterhead and cutting tool wear, and therefore increase the need for interventions. It is possible to reduce the need for such interventions by relying more heavily on modern grout materials and grouting techniques. To do this, however, we will need more balanced risk sharing between the owner, contactor and TBM manufacturer that supports the use of new and different concepts.
A promising development in this is area is what is being designed and tested at New York’s Delaware Aqueduct Repair, where a Robbins single-shield TBM has the capacity to hold plus 20 bars of water pressure while grouting and pressure reduction occurs. The TBM can be sealed against potential water inflows and high water pressures, and this, combined with extensive grouting capabilities using down-the-hole hammers for drilling under pressure, can stem the tide of water ahead of the machine. Once this has been shown effective the project can be used as a case study for more cost-effective risk reduction in rock tunneling under water pressure.
Reducing Risk in Hard Rock
In hard rock, a competent GBR is equally necessary. However, in mountainous conditions, and in remote locations, an accurate GBR can be impossible to obtain. When an adequate GBR is lacking, a push for continuous probe drilling should be made by all parties involved. Writing continuous probe drilling into the contract can and has effectively reduced risk—but we need more buy-in from the industry. Through continuous probing, crews can generate an in-tunnel GBR concurrent with advance. This GBR could be used to analyze trends and predict upcoming transition zones. The requirement for an in-tunnel GBR would effectively force contractors to take the time to analyze what is ahead of them—a small price to pay when a big feature is detected in time to save the tunneling operation.
Even when risks are considered low, it is still better to equip the machine from the outset with the tools needed to get through unforeseen conditions. These tools have been tested in the field and can mean the difference between project success and failure. Robbins currently is equipping several shielded hard rock TBMs with Difficult Ground Solutions (DGS)—a suite of options that can prevent a machine from becoming stuck and can enhance visualization of the ground around the TBM.
For example, two-speed gearboxes allow a rock machine to shift into a high torque, low RPM mode to get through fault zones and collapsing ground without becoming stuck. Shield enhancements such as external shield lubrication using bentonite, and emergency thrust systems, can be deployed when ground convergence occurs. These are just some of the options available.
The effects of DGS may be most evident when they are not in use—for example, a decade ago at the Pajares High-Speed Rail Tunnel in Spain, a 10 m diameter single shield TBM met with many unforeseen obstacles for which it was ill-equipped. The machine encountered blocky ground from the outset that blocked the cutterhead multiple times, requiring several weeks of time over a period of months to clean the front and back of the cutterhead so it could continue on.
Every time the cutterhead became blocked, the contractor would spend weeks stabilizing the ground with geofoam, then cleaning the cutterhead before attempting to move forward. Steve Chorley, Robbins Field Service Director, who was at the jobsite, commented that “If two-speed gearboxes had been installed, the contractor could have saved at least two months on the overall completion time of the tunnel. They would have provided the breakout torque needed to get the machine freed up a lot quicker.”
Indeed, we have seen the two-speed gearboxes advance machines through similar ground in Turkey at the Kargi Hydroelectric Project with great success. Such design improvements can be added to a machine prior to launch for a fraction of the cost that two months’ delay would be likely incur.
Yet another development in risk reduction for hard rock tunnels involves the final tunnel lining. Shifts in the rock mass and rock support are minimized the faster a lining is put into place after excavation—this is a well-accepted concept in soft and mixed ground tunnels with their specified segmental linings, but we see it less often in hard rock tunnels.
Concurrent, continuous concrete lining with tunnel boring operation for hard rock has been proven, but the process is often complex and only efficient for larger tunnel diameters due to the required space for concrete forms. However, there have been developments in the process, and studies are underway to accommodate concurrent lining and boring efficiently in smaller tunnels.
Contractors would do best to arm themselves with knowledge and case studies of what has and hasn’t worked on projects similar to theirs. Planning ahead for unforeseen events should be a key component of their strategy. But not all of the change needs to occur in contractor viewpoints of what risk is: risk should also be distributed more fairly between suppliers, consultants, contractors and owners.
Contracts should be awarded by owners after giving equal weight not only to cost, but also to technical merit and risk mitigation strategies in the proposed bid. We have seen this strategy applied successfully in other markets abroad, such as the U.K. The Crossrail project developed a risk management strategy before contracts were awarded, and thus equipment in the procurement stage was designed with an eye toward risk reduction.
Innovative projects like Crossrail show that successful risk sharing and management can be done, even on a “mega-project” with multiple machines in operation. If they can do it, so can we. It’s time that we, as tunnelers in the North American market, ask for a much-needed change: one of sharing risk more equally, especially on complex projects.
Lok Home is owner and president of The Robbins Company. Desiree Willis is public relations manager of The Robbins Company.