Flying arch

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 flying 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 walls are a more common solution, although the thick masonry required to construct these soon becomes expensive. Flying arches were often used, as at Llansamlet, where an initial cutting of gentle slopes was later considered to be unreliably stable and the arches were then added as a safety measure.

Possibly the first flying arch bridges over a railway were on the London and Birmingham Railway (opened 1838), of which Mile Lane Bridge in Coventry survives.{{NHLE|num=1431090|desc=Mile Lane Bridge|accessdate=29 December 2022}}{{cite book|last=Biddle|first=Gordon|title=Britain's Historic Railway Buildings: A Gazetteer of Structures|edition=second|publisher=Ian Allan|location=Hersham, Surrey|year=2011|isbn=9780711034914|page=377}} Other early examples can be found at Chorley, on the Bolton and Preston Railway, built in 1841.The Flying Arches at Chorley The Railway Magazine issue 633 January 1954 pages 65/66{{Cite book|title=Brunel in South Wales|last=Jones|first=Stephen|volume=II: Communications and Coal|publisher=Tempus|year=2006|isbn=0-7524-3918-9|ref={{harvid|Jones|II}}|pages=140–142}} These were a series of narrow {{convert|25|ft|m|adj=on}} long, strut-like arches between two masonry retaining walls. The retaining walls constrained the side forces such that they could only act axially along the columns; between earth banks, such narrow arches would otherwise have been at risk of collapse from off-axis forces. In 2008 the original stone arches were replaced by steel during work to lower the running lines in order to create clearance for electrification work.{{Cite news|url=http://news.bbc.co.uk/1/hi/england/lancashire/7528223.stm|title=Rail link closes for maintenance|date=2008-07-28|newspaper=BBC News|access-date=2016-03-27}}{{YouTube|Q23aj3vZzCw|Stobart Rail - Chorley Flying Arches - Time Lapse – March 2012}} The stone arches were subsequently restored atop the new steel structures in 2014.{{Cite news|url = https://www.bbc.co.uk/news/uk-england-28372218|title = Chorley flying arches: Historic railway feature to be rebuilt| work=BBC News | date=20 July 2014 |access-date = 2016-03-27}}Chorley arches replaced Today's Railways UK issue 155 November 2014 page 19 The 16 arches have been grade II listed since 1984, and Historic England's listing uses the term "strainer arches".{{NHLE|num=1072648 |desc=Series of 16 strainer arches in railway cutting at SD 581 192|access-date=21 April 2024}}

{{anchor|Llansamlet}}The South Wales Railway at Llansamlet,{{coord|51.66106|-3.877965|type:landmark_region:GB-DUR|display=inline|name=Llansamlet arches}}, Llansamlet arches near Swansea, runs through a cutting designed by Isambard Kingdom Brunel. After a landslip in the opening year of 1850, Brunel then designed four {{convert|70|ft|m|adj=on}} flying arches to hold the cutting walls apart. For extra stability, these arches were ballasted with high mounds of copper slag, a dense waste product conveniently available locally.{{Cite book

|title=The Life of Isambard Kingdom Brunel, Civil Engineer|author=Isambard Brunel Junior|orig-year=1870|year=2006|publisher=Nonsuch Publishing|isbn=1-84588-031-5|url=https://books.google.com/books?id=7kbGklaeTQQC&pg=PA176|page=136}} The four arches are now individually Grade II listed.{{Cite web|website=British Listed Buildings|title=First arch over South Wales Railway at Peniel Green, Llansamlet|url=http://www.britishlistedbuildings.co.uk/wa-82382-first-arch-over-south-wales-railway-at-pe}}{{Cite web

|website=British Listed Buildings|title=Second arch over South Wales Railway at Peniel Green, Llansamlet|url=http://www.britishlistedbuildings.co.uk/wa-82383-second-arch-over-south-wales-railway-at-p}}{{Cite web|website=British Listed Buildings|title=Third arch over South Wales Railway at Peniel Green, Llansamlet|url=http://www.britishlistedbuildings.co.uk/wa-82384-third-arch-over-south-wales-railway-at-pe}}{{Cite web|website=British Listed Buildings|title=Fourth arch over South Wales Railway at Peniel Green, Llansamlet|url=http://www.britishlistedbuildings.co.uk/wa-82385-fourth-arch-over-south-wales-railway-at-p}}

Just west of Swansea, the {{convert|829|yd|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.{{harvnb|Jones|II|pages=153–154}}{{coord|51.633655|-3.966907|type:landmark_region:GB-DUR|display=inline|name=Cockett tunnel arches}}, Cockett tunnel arches{{Cite web|title=Cockett Bridge and tunnel near m.p. 216 and a quarter|url=http://www.rcts.org.uk/features/mysteryphotos/show.htm?img=B-93-01|publisher=Railway Correspondence & Travel Society}}

• Flying buttress
• Counter-arch
• Inverted arch

{{Reflist}}

Category:Arches and vaults