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Hexham Bridge, site of
River Tyne, Hexham, Northumberland
Hexham Bridge, site of
associated engineer
John Smeaton
date  1777 - 1780
era  Georgian  |  category  Bridge  |  reference  NY939646
ICE reference number  HEW 536
photo  courtesy Cornwall Centre
In 1780, John Smeaton completed a nine-arch masonry bridge over the River Tyne at Hexham. Sadly, only two years later, several arches were brought down in a flood. The bridge we have today at this spot is a reconstruction of Smeaton's design.
Smeaton's bridge replaced an earlier one by William Gott, sited just upstream. It too had been destroyed by a flood, in November 1771. The new bridge had nine segmental arches consisting of eight 15.5m spans and one span of 11.3m. The arch thickness was 700mm. The spandrels were hollow, with ornamental circles at the centre of each spandrel panel, picked out in black rubble masonry. Smeaton used this detail on several other bridges.
Site investigations led him to believe that the gravel riverbed would support the bridge's masonry piers without piling. He designed a cast iron diving bell for working on the foundations, as he couldn't manage to make a single-wall cofferdam watertight. Five of the eight piers were constructed in caissons laid directly over the undisturbed riverbed. However, as work proceeded it proved expedient to drive timber piles for the foundations of the three central piers. Soon after completion, one of the non-piled piers settled and needed rebuilding. The original drawings show the abutments and bankside piers to have been on piles too.
The building of bridge piers in rivers brings with it the dangers of scouring, which undermines the piers. Smeaton had his views on how to overcome this problem but he was to come unstuck at Hexham. Scour protection here was provided by a girdle of stones and rubble at the foot of each pier, anchored by timber sheet piles.
An unusually violent Tyne flood in March 1782 caused serious erosion of the riverbed. Four of the gravel-founded piers and two of piled piers were undermined. Some of the arches were swept away. Smeaton later calculated that the velocity of the floodwater was some 19km/h, although it may have been as high as 22km/h.
In October the following year, Smeaton was asked to if he would undertake the rebuilding of the bridge but he declined, asking that another engineer "who has not got the Horrors of the River Tyne painted upon his imagination" be appointed. The reconstructed Hexham Bridge was completed in 1795 by Robert Mylne, founded this time on a piled timber platform that stretches from one bank to the other.
Resident engineer: Jonathan Pickernell
Research: ECPK
reference sources   CEH NorthJS
Location

Hexham Bridge, site of