Flying 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 small cuttings, retaining walls are a more common solution, although the thick masonry required to construct these soon becomes expensive.

For extra stability, these arches were ballasted with high mounds of copper slag, a dense waste product conveniently available locally.

[12][13][14][15] Just west of Swansea, the 829 yards (758 m) Cockett Tunnel suffered a partial collapse in 1899, long after Brunel's death.

Some time after reconstruction, the Eastern end of the tunnel was opened out (reducing the length to 788 yards) and the resulting cutting supported by two brick-built flying arches.