The initial ground support is provided by shotcrete in combination with fibre or welded-wire fabric reinforcement, steel arches (usually lattice girders), and sometimes ground reinforcement (e.g. soil nails, spiling). The permanent support is typically a cast-in-place concrete lining placed over a waterproofing membrane. There is a quick closure of the invert, that is, the bottom part of the tunnel, to create a structural ring that takes advantage of the rock or soil arc naturally created on the top part of the tunnel section.