Construction Information

The Caldecott Fourth Bore Project is being constructed with an approach called the New Austrian Tunneling Method (NATM) or sequential excavation method. In this method, short sections of tunnel are excavated, then supported with shotcrte (a form of concrete sprayed from a pressurized hose), steel girders, and rock dowels.  The strength of the surrounding rock determines the length of each excavated section and the amount of support that is installed.

The majority of the excavation has been performed using roadheaders, large machines with a rotating cutter head located at the end of a hydraulically controlled boom. The cutter head spins rapidly, excavating the rock as it is pushed against the tunnel face. The cutting head is moved around and ahead of the tunnel face until the required section of the tunnel has been excavated. Some tunnel excavation through harder rock might be performed using controlled blasting.

The support for the tunnel  includes rock dowels, lattice girders and shotcrete. Rock dowlels consist of steel rods that are grouted into holes drilled in a radial pattern around the perimeter of the tunnel. Shotcrete consists of a special concrete mix that is sprayed into place on the roof and sidewalls of the tunnel. The lattice girder is made of steel rods bent into an arch-shaped structure, which is put into the excavated section and then sprayed with shotcrete.  Once the tunnel has been excavated, a waterproof membrane will be installed on the roof and sidewalls of the tunnel. Following installation of the waterproof membrane, the final cast-in-place concrete lining will be installed. Tiles will be installed in the lower portion of the tunnel.

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