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 34 of 47

TUNNELINGONLINE.COM F E A T U R E S T O RY 3 5 TBM: TUNNEL BUSINESS MAGAZINE // APRIL 2018 RIGHT: Hybrid System with PVC sheet and sprayed membrane. not migrate along the interface between mem- brane and concrete. Potential groundwater paths can be eliminated, and the risk of water ingress into the tunnel is considerably miti- gated. Additionally, the same strong adherence be- tween the membrane and the secondary lin- ing provides a second barrier against ground- water ingress into the tunnel. Thus, to reach the tunnel a groundwater inflow has to pass through three aligned failure zones, i.e. a crack in the primary lining, a defect in the mem- brane, and a crack in the secondary lining. An eventual leak would be localized and could be easily treated with injection measures. "Our goal at BASF is to promote a different design method for tunnel linings using the composite effect of the membrane, which actually glues together two concretes as a waterproofing product," said Frank Clement, Global Te branes for BASF's Underground Construction division. "This will allow de- signers to reduce the total thickness of the lining which results in: smaller excavation diameter; less handling of spoil, including transport and disposal; more efficient tunneling as a result of fast- er advance, less labor and time savings; and a more ecofriendly way of building a tunnel. Design models have shown that a pos- sible reduction of the tunnel lining of 20 to 30% is feasible. A simulation for a new build tunnel showed a savings of 20% of the consumed concrete volume resulting in an increased efficiency" Spray-applied membranes were introduced late 1990s and the first successful applica oject in Br e recently, they were utilized in the landmark Cross- rail project in London. EXPERIENCES: CROSSRAIL PROJECT Crossrail is among the most significant infrastructure projects ever undertaken in the United Kingdom. This new metro line will improve the way people travel around the capital by optimized journey times and connections. The new railway will be a high- frequency, high-capacity service to 40 stations linking Reading and Heathrow in the west, to Shenfield and Abbey Wood in the east via 21 km of new twin-bore tunnels under central London. Construction flexibility Flexibility during the construction of the stations, elevator and ventilation shafts was a key to the success of the use of the spray-applied membrane due to the complexity of the project. Timing and logistics were crucial and coordination between the different contractors was essential and any disruption could cause delays for the project. The waterproofing system to be used should respond to this flexibility. The spray-applied waterproofing system chosen by the contractors is applied by simple, readily available dry spraying equipment that provides the required flexibility. The system uses compressed air for conveying the material. Water is added at the nozzle, creating a creamy consistency of the membrane. The equipment is suit- able to be used even in small tunnel diameters. This application is a simple stop-and-go application reducing any losses since no material will stay in the lines when spray- ing has to be stopped. Additionally, the contractor can resume application at any time by overlapping the already applied membrane. Based on the polymer chemistry, newly sprayed membrane will bond to previously applied membrane, result- ing in a continuous waterproofing membrane, even after sev- eral months. Due to the complexity of the project, BASF was asked to confirm that not only a new layer of polymer would bond to already cured membrane, but also concrete would bond to the membrane even when it was exposed for several months to the tunnel environment. To confirm this, microscopic investi- gations were performed on how the mechanism of the bond between the membrane and concrete was created. The bond between the first concrete layer and the membrane is based on a mechanical fixation and interlocking. During the curing of the membrane, the polymers penetrate into all the pores and irregularities of the concrete substrate, creating the required bonding of ar a after 28 days. With a microscope the polymer interlocking onto the con- crete pores can be seen. The second bond to the concrete ap- plied on the membrane is a different mechanism. During the curing of the concrete, cement crystals from the hydration process are penetrating into the upper layer of the membrane. This provides the required second bond, resulting in a double- bonded membrane. Since these mechanisms are independent of the age of the primary lining or the age of the spray-applied membrane, a certain flexibility in the construction sequence is

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