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2013FallShotcreteEMag

Goin’ Underground Composite shell linings (CSL) involve the primary lining taking the temporary loads and a proportion of the permanent load through composite action with the secondary lining. Single shell linings (SSL) are one lining taking the temporary and permanent loads—although this one lining may be built up in several passes. In most cases, a waterproof membrane is employed to provide a watertight structure (in CSL solutions this is generally between the primary and secondary linings). composite Shell Linings Through recent projects, such as A3 Hindhead road tunnel3 and Thames Water Hampton shaft, all in the UK, the use of sprayed waterproof membranes have given engineers an opportunity to explore the benefits of a composite shell lining, i.e. a sprayed permanent primary lining, sprayed Fig. 1: Design options for linings and waterproofing1 waterproof membrane, and a sprayed secondary lining, where the primary lining acts compositely and takes a proportion of the long-term ground loads. A key step that had facilitated this leap forward has been omission of lattice girders and the use of laser profiling systems to control the shape of the tunnel during construction.3 Lattice girders are usually not regarded as structural members, but they have been seen as essential in controlling the shape of the tunnel. They are notoriously difficult to spray around and leaks— and therefore corrosion—often occur at the Fig. 2: Typical double shell lining location of the lattice girder. Removing girders removes both a corrosion problem and also reduces the need for men to work at the face when the full support is not in place. Composite linings are now being incorporated into major UK projects, typically under the following design conditions, as shown on Fig. 3: • 100% ground and hydrostatic loads applied to primary lining in the short term; • The option of load sharing for the ground loads in the long term; • Full hydrostatic load applied to secondary lining in the long term; and Fig. 3: Typical composite shell lining (partial • No bond or shear capacity between linings is composite with no shear or adhesive bond) used in the structural design. This design methodology has resulted in some it varies depending on the loading behavior of the reductions to the thickness of the secondary lining ground. In materials such as clay, there is a distinct when compared to conventional DSL, but this is short- and long-term behavior, while in others fundamentally limited by the assumption that the there may be little or no change in the loads over water pressure acts on the membrane. For a the lifetime of the project from the loads generated shallow tunnel in soft ground, the water load is during the construction period. In other words, similar or even exceeds the ground load. The without some consolidation or rheological percentage of ground load on the secondary lining behavior in the ground, the secondary lining may is usually determined from numerical models and not experience much of the ground load. Shotcrete • Fall 2013 45


2013FallShotcreteEMag
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