Tunnel Business Magazine

AUG 2017

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Page 27 of 47

TUNNELINGONLINE.COM TBM: TUNNEL BUSINESS MAGAZINE // AUGUST 2017 negatively impacting either the tunneling operation itself and/or any overlying or adjacent, existing third party structures such as util- ity lines and building foundations? 3. What is the best method for sup- porting the ground after the re- quired openings are made in a manner that is safe for the work- ers and stable for the third parties? Each of the three topics listed above involves a mind-boggling evaluation of innumerable construction techniques that might be appropriate for any given underground project. Depending on the size and shape of the various tun- nels and shafts and the various ground conditions in which those structures will be built, the tunnel Designer can call upon a vast array of construction methods to excavate, to control and to support that ground. In addition to the actual tunneling methods themselves the Designer can also call upon various ground improvement methods such as dewatering, freezing, and/or grouting in order to make the ground easier to control and support. In order to accomplish the ground behavior vs. ground control brain- storming effort, it is necessary to as- semble a team of highly experienced individuals from different aspects of the industry such as engineers, engi- neering geologists, estimators, suppli- ers, and construction personnel. One of the most important concerns associ- ated with Intelligent Tunnel Design© is the fact that the best way to know how to design an underground opening is to know how to build it. In no other area of civil engineering expertise are the design and construction processes so intimately intertwined; i.e. the design will guide construction, but the pro- posed construction methods will also have a significant impact on how the project should be designed. Project Design – As stated above, the primary focus of this report is on the design procedures required to produce the openings inside of which the fin - ished facility will be constructed. With that goal in mind, it is important to note that all of the temporary structural elements required for tunnel support come into direct contact with and in- teract with the ground. Steel ribs, rock bolts, and shotcrete are used most often to support the rock, and steel ribs and boards, steel liner plates, and jacked pipe can be used to support the soil. Sometimes, the temporary support can be augmented such as by providing ad- ditional layers of shotcrete in order to provide a final lining. And, finally, tun- nel linings such as bolted and gasketed concrete segments can be used in con- oth to support the ground and to provide the final lining. However, and this is a big however, project designers must be careful when specifying various construction proce- dures that they believe are "necessary" in order to create the underground openings because this is where the in- terface between design and construc- tion can become highly problematic. Having been involved with the design and construction of literally hundreds of tunneling projects the authors of this paper have become well aware of two of the most critical aspects of the down- side of tunnel design as listed below: 1. Lots of tunnel designers are simply not aware of all of the construction techniques and procedures avail- able to highly experienced con- tractors for both controlling and supporting the ground, and 2. Lots of tunnel contractors do not seem to fully appreciate the al- most unlimited ability for adverse ground reactions to cause trouble for their proposed construction procedures. Without doubt, the appropriate and successful combination of the two considerations listed above is the key to Intelligent Tunnel Design©. Tun- nel Designers should not specify more than is required to build the proposed openings and tunnel Contractors should not do less than is required to build those openings in a safe and stable manner. In the final analysis, an appropriate balancing of tunnel design requirements with tunnel construc- tion procedures is the primary goal of Intelligent Tunnel Design©. ds – Tunnel con- struction is a complicated combination of the work that needs to be performed in order to advance the tunnel heading as quickly as possible and the logisti - cal issues associated with accomplish- ing that task. Working from inside the space created for the tunnel and actu- ally accomplishing something in that restricted space at the tunnel heading is not a simple task. Over time, and based on actual on-site experiences, success- ful tunneling contractors have become very good at performing these pro- cedures, but if the ground introduces unanticipated problems for those pro- cedures then the cost of completing the tunnel can quickly spiral out of control. In addition, the contract documents can require work to be performed in the tunnel that might not be necessary and might add considerably to the cost of building the project. For instance, many tunnel design- ers like to specify the requirement for probe hole drilling and grouting from the face of a tunnel exca- vated in rock. This approach might provide some comfort for the tunnel designer that he is covering all of his bases and reducing project risk, but is it necessary? Probe hole drilling and grouting means that the entire tun- neling operation must be halted and all of the costs associated with the en- tire project must be dedicated to this one activity. For some ground condi- tions that might be required, but more often than not that is not the case. Hence, and as discussed above, an ap- propriate balancing of design risk and construction methods is what is re- quired for "successful" tunneling. Third Party Impacts – Overlying all of the above design and construc- tion considerations is the issue of third party impacts. Sometimes design and construction procedures must be spe- cifically tailored to the protection of existing, overlying and adjacent third parties. For instance, if you are tunnel- ing directly below an existing 6-foot- diameter sanitary sewer or adjacent to an historic church, it is necessary to avoid doing anything that will cause those structures to be damaged. For the sewer, it might be possible to grout the ground surrounding the sewer in order to make certain that that ground will not be destabilized during tunneling. For the church, it might be necessary to underpin the church foundations prior to tunneling so that the tunnel will not cause detrimental settlements and cracking of the church. As above, this interface between design requirements and construction methods can be dif- ficult to accomplish relative to existing third parties, but, when done is a prop - er manner, it is a critical component of successful tunneling. The Contract Document – The con- tract document for a tunneling proj- ect is radically different as compared to a vertical building project for three important reasons: the contact with the ground, the amount of temporary construction, and the complexity of the finished facility. For a vertical building, maybe 10% of the structure comes into direct contact with the ground as compared to a tunnel where 100% of every portion of the structure has a ground/structure interface. For a tunnel, anywhere from 2/3's to 3/4's of the cost and the risk of the project F E A T U R E S T O R Y 28

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