Tunnel Business Magazine

AUG 2017

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39 TUNNELINGONLINE.COM TBM: TUNNEL BUSINESS MAGAZINE // AUGUST 2017 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 condi- tions, 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 pressur- ized tunneling, through additive injection and bentonite injec- tion. Concurrently there must be a rigorous monitoring of sensi- tive structures. Because of the finesse required, properly trained tunnel man- erators are the key to effective risk reduction. ost overruns in these conditions are caused by operator error or inexperienced management, not by machine compo- nent breakdown. To that end, Robbins and other companies are involved in developing training programs, together with univer- sities such as the Colorado Scho t will teach tech- nicians 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 neede o City's Túnel Emisor Poniente (TEP II) Project, which has been written about in previous issues T e- cord-breaking Crossov quipped with features that enabled it to get through several known and unknown features. The hybrid-type machine could operate as a non-pressurized essurize ending on conditions. When the crew, operating in rock mode, detected an unfore- seen 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 tterhead to consolidate ground directly in front of the machine, and by using the EPB featur ble to ef- fectively negotiate this section at a reduced advance rate with minimal delays. This is an example of what a customized ma- chine, and a predetermined strategy, can do when obstacles pres- ent 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—particu- larly in very hard, abrasive rock that is water bearing. The cur- rent 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 pos- sible 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 be- tween the owner, c er that sup- ports the use of new and different concepts. A promising development in this is area is what is being de- signed and tested a elaware Aqueduct Repair, where a R has the capacity to hold plus 20 bars of water pressure while grouting and pressure reduction oc e 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 tunnel- ing 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 lack- ing, a push for continuous probe drilling should be made by all parties involved. Writing continuous probe drilling into the con- (LEFT) The recently completed Tunel Emisor Poniente (TEP) II project in Mexico City using a Robbins Crossover TBM is a good example of what a customized machine and prudent planning can accomplish when unforeseen obstacles occur. (RIGHT) 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.

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