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2013FallShotcreteEMag

Goin’ Underground Article reproduced courtesy of Tunnelling Journal, www.tunnellingjournal.com Where Are We Now with Sprayed concrete Lining in tunnels? By Andrew Pickett and Dr. Alun Thomas t prise a temporary primary lining of sprayed of shallow tunnels in soft ground or weak rock.functional requirements will be discussed in thispaper, along with examples from recent projects-raditional methods of sprayed concretelining (SCL) tunnels (in soft ground) com concrete with a sheet membrane inside and a per- Initial findings will also be reported from manent cast in-situ concrete lining, usually rein- preliminary testing with BASF exploring single- forced with steel bars. Even now, although it is shell tunnel lining solutions and bond strength widely accepted that sprayed concrete can be used between a sprayed membrane with permanent as a permanent material, the traditional methods lining to demonstrate a greater composite action. are applied to the majority of tunnels. This is This, coupled with some discussion on the most wasteful in terms of money, time, and materials. recent numerical modeling from a live project, Mott MacDonald is now providing design solutions will outline where SCL composite lining solutions for the use of permanent sprayed concrete with a are heading, expanding on the challenges that will variety of waterproofing solutions through its have to be met to handle different situations as involvement as designers on major projects in the well as satisfying functional requirements to UK—in particular, soft ground tunneling, where clients and the wider tunnel industry. the profile of the ground can be cut quite smoothly. The design solutions have ranged from Design Options permanent sprayed concrete, sprayed onto a sheet There are now several options for SCL tunnels membrane in a drained tunnel; permanent water- open to tunnel engineers to suit different proof sprayed concrete in generally impermeable geological and hydrological conditions and/or the ground; and permanent sprayed concrete, sprayed client’s functional requirements (refer to Fig. 11). in two passes with a spray-applied waterproofing The SCL options can be broadly categorized into membrane in between for cases where there is a three types. Double shell linings (DSL) involve a higher risk of water ingress. sacrificial primary lining, which takes the The focus of study has been on the last case in temporary loads, and a secondary lining to take recent projects and, having examined the the permanent loads (refer to Fig. 2). This has composite action, it has been found that significant significant pedigree, however, because the load sharing can be obtained even with modest primary is considered temporary while the bonding at the membrane interface. secondary is designed to take both long-term The issues related to the design of composite ground loads and hydrostatic, thereby providing linings and the range of suitability for different a robust design. It is a lot thicker than CSL types.2 44 Shotcrete • Fall 2013


2013FallShotcreteEMag
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