Flying arch
A flying arch is a form of arch bridge that does not carry any vertical load, but is provided solely to supply outward horizontal forces, to resist an inwards compression. They are used across cuttings, to avoid them collapsing inwards. Operation The conventional arch supports a vertical load downwards on the centre of the arch and translates this into forces both downwards and outwards at the base of the arch. In most cases, this sideways force is a nuisance and must be resisted by either strong foundations or a further 'bowstring' girder, in the form of a tied-arch bridge. In some cases though, originally for railway cuttings in loose rock, the sides of the cutting are unable to retain their own weight and tend to slide inwards. Flying arches may be provided to retain these side walls. Unlike the conventional arch, the vertical load on the arch does not carry a useful load, it is merely used to generate the side-thrust, which in this case is useful for restraining the side walls. Flying arches are not a common solution to railway cuttings. For large cuttings in soft earth, a gentle slope is self-supporting in most conditions. In small cuttings, retaining