Design
The trio of bridges are all similar in design, but they do have differing clearances, which were based upon the topography of the area. The bridges were designed by Robert Finley Laing of Laing and Douglas Construction Company; the company contracted for the ICRR from 1852-1853. In addition to designing the three bridges over the city streets in Dixon, Laing also designed the stone piered bridge that the ICRR took over the Rock River on its way to Freeport. The three stone bridges are set into a 15 foot (4.6 m) high embankment which stretches from West Seventh Street to the Rock River, the span includes the three stone arch bridges.
The yellowed limestone was cut to fit and is held together using no bond. Instead there are stone "keys" cut and notched into the rock which prevents the stones from slipping. The stone is of a plain ashlar masonry type and either cut or dimension stone. The stone's facing varies depending upon its location within the structure of the bridges. The arch stones are finished with a chiseled face while the wingwall and arch bases are finished with a rock face. There have been almost no alterations of the original structures besides a limited attempt at tuckpointing between 1912 and 1915.
Each bridge is of a different height. The bridge spanning First Street has 14 feet (4.3 m) of clearance, the Second Street bridge, also the tallest, has 15 foot (4.6 m) clearance and the Third Street Bridge has a clearance of 12 feet (3.7 m). Because of its short height the Third Street Bridge is sometimes referred to as "Little Sister." All three bridges have the same arch span, 28 feet (8.5 m), but each has a differing depth. The longest of the three bridges is the Little Sister Bridge which has a 38-foot (12 m) depth. The bridge at First has a 28 foot (8.5 m) depth and the bridge at Second Street a 34 foot (10.4 m) depth. The wingwalls on each bridge extends 33 feet (10.1 m) at their bases.
Read more about this topic: Illinois Central Stone Arch Railroad Bridges
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