Hertfordshire Bridges

Photo:Digswell viaduct

Digswell viaduct

Talk by Richard Lavender, on 22nd September 2009

By MPT

At a recent meeting of the Local History Society, Mr Richard Lavendar spoke about ‘Hertfordshire Bridges’  As a mechanical and civil engineer, he drew on his professional experience in Herts, Suffolk and London of designing and maintaining structures we generally take for granted.

An impressive slide collection illustrated the immense variety of style and use of construction materials in bridges county-wide.  Detailed drawings and cross-section plans showed how geographical and logistical challenges in construction had been met.

From the earliest times, arches proved to be the strongest structures.  Even Victorian brick-layers copied the same style of building with Norman arches. Often a main arch is flanked by two smaller arches, particularly spanning rivers.

Some bridges dated from the 1770’s – intended for little more traffic than loaded carts – remain in use now, coping with far greater loads.  Loading Standards for bridges were first introduced pre – WW1 specifying a load-bearing capability of 12 (Imperial) tons.  By the mid 1950’s, the increase in road traffic and size of lorries caused the figure to rise to 22 tons.  The Bridge Assessment Standard of 1984 standardised the capability across the EEC to 38 tons.  Current regulations specify 40 (Metric) tons.

Statutory regular inspections take place to check for wear and tear.  Stocks of assorted old bricks in Imperial sizes are held to effect visible repairs, wherever possible.  Occasionally ‘scouring’ occurs, when the force of water under a bridge erodes its support system, necessitating urgent action

All bridges under 16’ 6” have to be signed.  Where accident blackspots are identified, steel girders are used to protect the bridge, acting as buffers.  Our own Station Road railway bridge is an example.  Bridges are also numbered, though the Highways Agency, Railway Boards and British Waterways Board all employ their own numbering system!

Bridges in the county worthy of note include the bridge at Watton at Stone , built in 1830 at a cost of £215 14s 7d!  A feature of this attractive bridge is the use of special interlocking bricks in its construction.  Five thousand navvies were employed to build the Digswell Viaduct , first digging a ten foot trench- the clay from which went to make some of the five million bricks used.  There are forty spans of 30’ and at the highest arch the bridge stands 98’ above the ground.  However the red bricks were found to be porous and it took 14 men a period of five years to re-face the bridges with blue bricks.  In 1947 a buttressed bridge at Much Hadham, dating from 1776, was replaced by a single concrete span.

At Bishops Stortford the bridge arches were used as air-raid shelters during WWII (these have since been filled in).  In the same period, ‘tank traps’ were sited under a railway bridge on the Cuffley to Hertford North line.  The concrete blocks are still in place!

Mr (later Sir) Donald Bailey, a Royal Engineer, designed the innovative Bailey bridge at Winston Churchill’s request in 1941.  Pre- constructed sections could be transported, erected and dismantled quickly by only six men.  Although intended as a temporary structure, Bailey bridges are still in operation in the county.

More recently, thirty pre-cast concrete stands were used in the construction of the viaduct at Ware, opened in 1976, which carried the A10 across the source of the New River.  The wide span of the bridge necessitated a contraction allowance of 9”.  In 1981 a box-built aqueduct was constructed over the M25.  It diverts the New River across the motorway, and Mr Lavendar highlighted the difficulties posed when leaks developed and leaks had to be sealed.

Members felt that they would look anew at bridges they cross as a result of this very interesting talk.

   

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