Circular piers Forth Bridge

1 circular piers

1.1 inchgarvie
1.2 fife
1.3 queensferry

circular piers

a completed caisson on inchgarvie granite pier

the 3 towers of cantilever each seated on 4 circular piers. since foundations required constructed @ or below sea level, excavated assistance of caissons , cofferdams. caissons used @ locations either under water, @ low tide, or foundations built on mud , clay. cofferdams used rock nearer surface, , possible work in low tide.

six caissons excavated pneumatic process, french contractor l. coisea. process used positive air pressure inside sealed caisson allow dry working conditions @ depths of 89 feet (27 m).

these caissons constructed , assembled in glasgow arrol brothers, namesakes of unconnected w. arrol, before being dismantled , transported queensferry. caissons built large extent before being floated final resting-places. first caisson, south-west pier @ queensferry launched on 26 may 1884, , last caisson launched on 29 may 1885 south-west pier @ inchgarvie. when caissons had been launched , moored, extended upwards temporary portion in order keep water out , allow granite pier built when in place.

above foundations each of different suit different sites, tapered circular granite pier diameter of 55 feet (17 m) @ bottom , height of 36 feet (11 m).

inchgarvie

the rock on 2 northern piers @ inchgarvie located submerged @ high water, , of other 2 piers, site of eastern 1 half submerged , western 1 three-quarters submerged. meant work had done @ low tide.

the southern piers on inchgarvie sited on solid rock slope of around 1 in 5, rock prepared concrete , sandbags make landing-spot caissons. excavation carried out drilling , blasting, no blasting done within 1.5 ft (0.46 m) of caissons, , remaining rock quarried within 6 in (150 mm).

fife

once positions of piers had been established, first task @ fife level site of northernmost piers, bedrock of whinstone rising level of 10 20 feet (3.0 6.1 m) above high water, height of 7 feet (2.1 m) above high water. south piers @ fife sited on rock sloping sea, , site prepared diamond drilling holes explosive charges , blasting rock.

queensferry

the tilted caisson



the mode of sinking queensferry caissons

the 4 queensferry caissons sunk pneumatic method, , identical in design except differences in height. t shaped jetty built @ site of queensferry piers, allow 1 caisson attached each corner, , when launched caissons attached jetty , permitted rise , fall tide. excavation beneath caissons carried out @ high tide when caisson supported buoyancy, , when tide fell air pressure reduced in order allow caisson sink down, , digging begin anew.

the north-west caisson towed place in december 1884, exceptionally low tide on new year s day 1885 caused caisson sink mud of river bed , adopt slight tilt. when tide rose, flooded on lower edge, filling caisson water, , when tide fell water did not drain caisson, top-heaviness caused tilt further. plates bolted on divers raise edge of caisson above water level, , caisson reinforced wooden struts water pumped out, pumping took place , water pressure tore hole between 25 , 30 feet (7.6 , 9.1 m) long. decided construct barrel of large timbers inside caisson reinforce it, , ten months before caisson pumped out , dug free. caisson refloated on 19 october 1885, , moved position , sunk suitable modifications.