Tunnel Business Magazine

APR 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.

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Page 41 of 47

TUNNELINGONLINE.COM F E A T U R E S T O RY 4 2 TBM: TUNNEL BUSINESS MAGAZINE // APRIL 2018 CAN SINGLE-BORE TUNNELING TRANSFORM URBAN SUBWAY CONSTRUCTION? By Dennis Ratcliffe T he Santa Clara Valley Transportation Authority (VTA) is evaluating a tunneling methodology that would use a single, large diameter tunnel to construct its next BART extension through the downtown core in San José, California. This evaluation of the single- bore method has revealed a number of unique advantages and opportunities for urban subway construction. The single-bore method compartmentalizes the large diam- eter tunnel into separate trainways. In this method, the tunnel and boarding platforms are constructed completely indepen- dently of the station structures. This configuration is made pos- sible by recent innovations in large-bore tunneling equipment and related control technologies. The single-bore proposed for San José would use one 45-ft diameter tunnel b onstruct a 5-mile long circular tunnel structure. The boarding platforms will be constructed one above the other inside the tunnel bore with- out construction disruption at the surface. The station entrances (ticketing halls) which include ticket- ing, fare gates, station agent booth, and vertical circulation el- ements are constructed off-street (two entrances per station). The connection between the tunnel and the ticketing halls can also be constructed without disruption at the surface by min- ing an access corridor (adit) between the tunnel and the bottom of the ticketing hall. WHY IS THIS IMPORTANT? Conventional subway construction often entails the use of cut-and-cover techniques that involve removing the street sur- face, relocating and protecting existing utilities, and excavating down from the street level. This method is highly disruptive to auto and pedestrian traffic, and has significant negative impacts on existing businesses. The conventional subway construc- tion approach attempts to reduce these impacts by using two onstruct the trackways in separate running tunnels between stations, but this method still requires cut- and-cover methods to construct the stations. For the subway planned in San José, using cut-and-cover techniques would re- quire the downtown station excavations to be approximately 1,800 ft long, 60 ft deep, and would extend from curb to curb. The single-bore method avoids most of these in-street im- pacts because the tunnel, trackways and boarding platforms are all constructed below ground without disruption at the surface. Also, because the ticketing hall facilities are construct- ed offstreet, the construction disruption for these facilities is limited and comparable to construction of any high-rise build- ing with a basement. The underground connection between the ticketing halls and the boarding platforms can also be con- structed without disruption at the surface through the use of mining techniques in conjunction with ground improvement technologies such as ground freezing. San José is typical of urban revival now being experienced all across the country e leading a trend toward urban living for both work and home, and cities are responding by creating housing, workplaces, and entertainment destina- tions in their urban cor e seeking a lifestyle that is not dependent on the automobile, especially without long commutes behind the wheel. Convenient public transit is a key attribute of their ideal urban lifestyle, and in the urban setting this means constructing a subway. HOES DOES SINGLE BORE ADDRESS MAJOR IS- SUES? Disruption – The principal problem of subway construction in the downtown core stems from the disruption to the auto and pedestrian infrastructure caused by cut-and-cover con- struction methods. In most cases the subway station is needed at the most vibrant area of the city – the very location that will be most negatively affected by its construction. Because the single bore method can be constructed almost entirely under- ground, it avoids this problem. To understand the advantages, one must simply compare five years of construction disrup- tion using the conventional methods to five years of continued business and economic development while subway construc- tion continues underground relatively unnoticed using the single-bore approach. Costs – t are considering transit subways tend to be older with aging utility infrastructure beneath the streets, with little as-built information. This poses significant construction risks when using cut-and-cover methods, and

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