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 33 of 47

TUNNELINGONLINE.COM F E A T U R E S T O RY 3 4 TBM: TUNNEL BUSINESS MAGAZINE // APRIL 2018 T ypically waterproofing of conventionally excavated tunnels has been done by installing prefabricated sheet membranes between the primary temporary support lining and the secondary permanent inner lining. The membrane acts as a separation or sliding layer between the linings, which is often desirable. The geotextile installed behind the sheet membrane provides the required drainage and protection of the system. The design of the tunnel lining is strongly influenced and limited by the use of waterproofing sheet membranes since primary and secondary linings are separated from each other, forming a double-shell lin- ing system. In that case, the secondary lining must be designed for all permanent loads, although the sprayed concrete primary lin- ing continues resists ground and water loads for a long time after construction. Significant progress has been made in sprayed concrete tech- nology over the last two decades, with advanced admixtures, as well as the improved application technology using spraying ro- bots. The development of new admixtures like superplasticizers based on poly-carboxylates, alkali-free accelerators and silica fume, enables designers to use permanent sprayed concrete lin- ings (SCL) increasingly for long-term service life. When designed and applied correctly, sprayed concrete can be considered as permanent. Conventional waterproofing systems with sheet membranes don't allow the use of SCL as a final lining. With the introduction of spray-applied waterproofing membranes, the tunnel lining can consist of a total sprayed system: sprayed con- crete as the primary support lining, a spray-applied waterproof- ing system and a spray concrete secondary and final lining. The use of spray-applied membranes has seen increased use over the last decade in some countries, and demonstrated functionality and cost-effectiveness in several situations. This paper describes the use of the sprayed waterproofing mem- br A polymer-based product, and the practical experience of its use on the Crossrail project in London. BACKGROUND The sprayed waterproofing membrane is polymer based and has been specially developed for underground structures. The membrane is applied between primary and secondary concrete linings and develops high bond strength on both sides of the con- crete (doubled-bonded). It is applied onto a concrete substrate usually with an average of 3 mm in one pass, and covered later on by a secondary concrete lining or a protective non-structural concrete layer, depending on the design requirements. Due to its double-bonded function, it provides the tunnel lin- ing system with unique mechanical properties and waterproofing features. The bond is unaltered by the concrete placement tech- nique, be it sprayed or cast in place, or by the presence of fibers. The use of this membrane is particularly advantageous in geo- metrically complex areas such as in lay-by niches, cross passages, turn-outs and crossover caverns. In such cases, the installation of conventional sheet water- proofing membranes is difficult and locating of possible leaks and repairs are challenging. The use of a sprayable waterproofing membrane could be a solution. Additional advantages to owners, contractors and designers result from: Flexibility in the design and application Fast application and easy logistics Flexible work programming/sequencing Significant program and cost savings Reduced long-term maintenance costs The waterproofing concept with the sprayed waterproofing membrane is based on both impermeability and bonding proper- ties of the membrane. Due to the high bonding of the membrane to the primary lining, any groundwater inflow through a crack in the primary lining stops as it reaches the membrane, and can- SPRAY-APPLIED WATERPROOFING MEMBRANES

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