Tunnel Business Magazine

OCT 2018

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TUNNELINGONLINE.COM 2 6 TBM: TUNNEL BUSINESS MAGAZINE // OCTOBER 2018 The flow chart shown below represents what the author believes is the nine-step process that must be implemented if you want to produce successful tunneling projects. A detailed discussion of this topic was presented in a paper titled Intelligent Tunnel Design in the August 2017 issue of primary purpose of this paper is to empha- size the four steps as listed below where geotechnical professionals have the most input and the most responsibility for the successful outcome of a tunneling project: • Performance of the Subsurface In- vestigation, • Assessment of Ground Behaviors, • Preparation of the Contract Docu- ment, and • Prov - ing Tunnels are radically different as com- pared to above-ground buildings primarily because they come into contact with the ground for 100 percent of all construction activities. Above-ground buildings come into contact with the ground for maybe 10 percent of construction and the contrac- tor working on an above-ground project is always happy when he finishes with the foundation and basement portions of the project and is able to start work on the su- perstructure. Tunneling contractors do not have that luxury. In addition, the project schedule for a tunnel is largely dependent on how fast the face of excavation can be ad- vanced. If adverse ground behaviors inter- fere with face advancement, then the en- tire project can be subjected to significant delay. Finally, most tunnels are built in densely populated urban areas and come into contact with a huge number of third- party facilities such as existing utilities, highway and railroad rights-of-way, and building foundations that can be negative- ly impacted by ground movements caused by tunneling. All in all, it is probably safe to say that you need to be on your toes if you intend to design and/or construct tun- neling projects – and that includes having the assistance of experienced and knowl- edgeable geotechnical professionals. An evaluation of the risks associated with a tunneling project can also be di- vided into four parts as listed below; each of which is associated with the input pro- vided by geotechnical professionals. • Risk Identification – is closely related to the subsurface investigation, • Risk Minimization – is largely asso- ciated with the evaluation of ground behaviors, • Risk Allocation – is heavily related to the geotechnical portions of the con- tract document, and, finally, • Risk Management – is associated with the observation and monitoring of ground conditions and ground be- haviors during construction. Risk Identification/ Subsurface Investigation Successful tunneling can be defined as producing a satisfactory finished facility for no more money and in no more time than is required to deal with the existing ground condition. Hence, the question be- comes, "How well do you know the exist- ing ground condition?" Successful tunneling is also a four- step process that includes excavating the ground, controlling the ground during the process of excavation, supporting the ground in a safe and stable manner as the tunnel advances, and, finally, constructing the finished facility – and all of the activi- ties associated with excavating, control- ling and supporting the ground are related to input provided by geotechnical profes- sionals. It is also during the accomplish- ment of these three activities that the tun- nel comes into contact with the ground and encounters most of the risks associ- ated with tunneling. The subsurface investigation is the only process whereby tunnel designers can ob- tain an accurate description of the existing ground condition. It is beyond the scope of this paper to explain all of the techniques and procedures available to geotechnical engineers for investigating the ground, but this is one of the most important aspects of successful tunneling. Some tunnel owners are primarily concerned with the cost of a subsurface investigation and seek to find the lowest cost provider of those services. This is a very bad idea. The subsurface in- vestigation for a tunneling project must be thought of as an investment in successful project completion and the owner's pri- mary objective in obtaining the services of geotechnical professionals must be fo- cused on obtaining the most knowledge- able and experienced people for that role. Geotechnical engineers working on a tunneling project are almost always asked to produce two reports: a Geotech- nical Data Report (GDR) and a Geotech- nical Baseline Report (GBR). The GDR is defined as an accurate and thorough factual description of all the informa- tion collected as a result of the subsur- face investigation. As the police officer on the old television show Dragnet used to say to the eyewitness; "Just give me the facts." Similarly, the GDR should not contain any interpretation of the data. The GBR is the report that is intended to provide all parties working on the proj- ect with the geological and geotechnical interpretations required both to design and to construct the project and a brief description of how one goes about pre- Geotechnical Risk Assessment for Tunneling Projects F E AT U R E S T O R Y BY GARY S. BRIERLEY

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