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2013FallShotcreteEMag

Goin’ Underground In one recent project, the first layer of sprayed Single Shell Linings concrete—the so-called sealing layer of 75 mm Single shell linings (SSLs) offer the most (3 in.) sprayed concrete—is regarded as temporary efficient lining design (in dry or largely dry and omitted from the design in the long-term. This ground) as they take both the temporary and long- was due to concerns over sulphate attack and poor term loads and the construction is very fast quality when spraying on to the excavated surface. compared to a double shell or composite lining Presently, there is further study and testing where there are both primary and secondary lining being undertaken to demonstrate a fully composite stages to the construction (refer to Fig. 5). SSLs lining as discussed later, and as shown in Fig. 4, have been widely used in the hydropower sector i.e. shear and bond strength at the interface of the and in all tunneling sectors in certain countries, waterproof membrane. Once this is ascertained, most notably Norway: further reductions could be achieved for the • No waterproofing membrane; thickness of the secondary lining. • Ground loading all on primary lining; Figure 4 shows composite action between • No hydrostatic load; linings by achieving shear capacity across • Watertight concrete design, but allows local membrane-concrete interfaces: seepage; and • Load sharing for the ground load and water • Optional drip trays provided outside architec- load (WL) in long term; tural cladding. • Full hydrostatic load applied to secondary The main disadvantage is that clients will tend to lining in the long term; opt for watertight tunnels, thereby avoiding operation • Bond strength on membrane interfaces to be and maintenance issues and drainage systems. Unless 1 MPa (145 psi); and the ground is dry or generally impermeable—such • Shear strength on membrane interfaces to be as London Clay—it is hard to achieve watertight 2 MPa (290 psi). tunnels with SSL. That said, this can still remain as The advantage, as discussed above, is the a design option for non-public tunnels where lower reduction to secondary lining thickness without levels of water tightness are acceptable. compromising the water tightness requirement. The main disadvantage is there is currently no composite Shell Lining— precedence for a fully composite lining with a spray-applied membrane. However, single shell Design Philosophy permanent sprayed concrete linings have been For recent projects, there has been a push to successfully used on a number of projects such mechanize sprayed concrete lined tunnel as Heathrow Terminal 51,4,5 and the design for construction as much as possible and thereby Hindhead considered both load cases—with and remove tunnel operatives from the face of the without full composite action. tunnel, decreasing the risk of death or injury as a result of tunnel collapse; being hit by falling sections of the newly sprayed lining (“sloughing”); or risks associated with fixing reinforcement, lattice girders, and sheet waterproof membranes at height. Therefore, with the precedent set from the A3 Hindhead tunnel construction, the lining design of sprayed primary and secondary linings with steel fiber reinforcement (SFR) and shape control techniques that remove the requirement for lattice girders and a sprayed waterproof Fig. 4: Fully composite shell lining membrane has been adopted for major SCL works in the UK where geological conditions are suitable. At present, little guidance exists on this subject so the features of this composite lining design are described in more detail as follows: Primary lining—The permanent primary lining is designed to take the full short-term applied ground load and any other loads, such as compensation grouting and surface surcharges, Fig. 5: Typical single shell lining expected in the 2 to 3 years prior to secondary lining 46 Shotcrete • Fall 2013


2013FallShotcreteEMag
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