Tunnel Business Magazine

JUN 2018

TBM: Tunnel Business Magazine is the market leader for North America. TBM is written for leading professionals in all aspects of tunneling and covers project stories, design elements, contracting strategies, legal issues, new technology and more.

Issue link: http://digital.tunnelingonline.com/i/986315

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Page 37 of 51

F E A T U R E S T O RY 3 8 TUNNELINGONLINE.COM TBM: TUNNEL BUSINESS MAGAZINE // JUNE 2018 There are a few variants to these main typ chanical Excavation Type," "Beam Type, ed Shield Slurry Type." Table 1, however, is sufficient to demon- strate the idea. As you can see from the Table 1, prior knowledge of expected ground conditions is critical. Hard rock may be indicated dur- ing feasibility studies based on limited core drilling. This would result in the choice of a Shield typ ber of faults are present in the rock this would mean much of the ground will be weak or variable, in which case it would have been better to choos type of mistake is costly. The choice of e a collaborative exercise be- tween the geotechnical engineer and the procurement/tender team. As the project matures, the amount of information will increase, however as decisions are made and capital is spent the ability to change will decrease. Both geotechnical engineers and project managers need to understand this dynamic and the pressures it creates. TBM DESIGN STAGE After sele e then the geotechnical engineer should still be actively engaged during the design and manufacture as there are many variables that will affect the performance of the tter- head, maximum trust and torque, over- boring, probing and ground improve- ment capabilities. Of secondary importance is the fact that during this stage any further geo- technical assessment will allow for opti- mization of the TB ease performance. There will always be pressure to re- duce costs however understanding con- sequences of each compromise must be appreciated in each instance. Saving money on a probe drill may make it im- possible to conduct adequate probe drill- ing ahead of the cutterhead during main- tenance periods, creating the situation where you need to either mine without sufficient cover, delay mining to achieve the required cover or purchase and fit better drills during excavation. The third choice may not always be possible. The question should not be, "Are all re- quested designs in place?" but, "Are all re- quested designs in place and optimized to the predicted ground conditions?" TBM OPERATIONAL STAGE ody likes surpris like surprises. Project managers do not like surprises. Any geotechnical engineer who wants a long career in tunnel boring would do well to remember this. Conditions in the tunnel can change quickly transitioning from minute to min- ute or week to week in conditions ranging from very wet to very dry, and from soft ground to hard rock. To prevent surprises, the geotechnical engineer needs to be present at the job site. Working remotely is not an option. The advantages of having a geotechnical engineer on site at all times are: • Prediction of ground conditions ahead of the cutterhead both good and bad along with expected im- pacts on advance. • Providing solutions to allow ad- vance rate to be maintained. • Advise as to when mining can or cannot be stopped and advise if this will require a contract variation from the client. • Analyze performanc and provide understanding to the project manager as to why variation is occurring. • ound conditions and re- sponding to mining and updating the geotechnical model to enable better predictions. Project managers are specialists in their field they are not geotechnical en- gineers. The jargon used by geotechnical engineers is often confusing and does not provide the knowledge required to make decisions. Project managers constantly have to weigh up conflicting priorities and advice as per Figure 3. Geotechnical engi- neers need to understand this and provide information in a way that makes sense. Recommendations must be free of jar- gon and must be given in context. For ex- ample, the engineer may detect increased cutterhead torque due to mining through a fault and is aware tha e- quired to stop for maintenance within the next few hours. Do not tell the project manager the torque is high so he cannot stop. This does not provide context and will create a direct conflict with the mechanical en- gineer and contract requirements so the advice will probably be ignored. If the geotechnical engineer explains that stopping now would result in the cutterhead being stuck and that if the stop was delayed for 50 m or so when better ground conditions are expected, ould safely be stopped for maintenance. A variation would need to be obtained from the client and the maintenance schedule would need to be adjusted. The geotechnical recommen- dation would be far more likely to be fol- lowed and this would produce a better result for the project. Figure 3 - Considerations Figure 2 - Knowledge vs Ability to change over time.

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