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The Impact of Social, Economic, Political and Geological factors on the building of the Humber Bridge
The River Humber forms the boundary between Lincolnshire to the south, and East Yorkshire to the north. Historically, its broad waters have encouraged significant cultural and economic divisions, while at the same time people have always found ways to cross it. This essay explores some of the reasons as to why this structure has been at the centre of so many controversies.
A brief history of the river crossings
The Romans first crossed the Humber using a combination of ford and ferry. The Domesday Book mentions a ferry crossing at Barton, and Edward II allowed Hull merchants to operate a ferry between Hull and Lincolnshire. In 1725 Daniel Defoe described ‘a ferry over the Humber to Hull, where, in an open boat, in which we had about fifteen horses and ten or twelve cows mingled with about seventeen or eighteen passengers, we were about four hours tossed about in the Humber before we could get into Hull.’
By the mid-nineteenth century Hull industrial and commercial interests saw sufficient advantage in accessing Lincolnshire and Grimsby to champion a Humber crossing. In 1855 the Hull Chamber of Commerce discussed plans for a Humber rail bridge and in the 1860s plans for a ‘lofty viaduct, about a mile and a half in length’ between Barton and Hessle were suggested. In the late nineteenth century there were proposals to page link Hull at Brigg via a tunnel at Barton and for a 5,900ft long, 36-span rail bridge linking Barton and Hessle. However, Goole ship owners, arguing that the bridge would interfere with trade, strongly opposed the scheme.
Post-First World War, plans to build a bridge were revived by local commerce. In the1920s the Grimsby Chamber of Commerce and Shipping pressed for a bridge, and Hull Corporation requested advice ‘upon the most suitable economic method and geographical position for transport communication between the East Riding of Yorkshire and North Lincolnshire in the vicinity of Hull’. Government offered 75 per cent funding. Again, there was opposition from shipping interests but anyway the funding was cancelled following the economic downturn. Nevertheless, designs for a bridge were regularly updated, and when the Golden Gate Bridge in San Francisco was built it inspired the first suggestion that a suspension bridge should cross the Humber. Meanwhile, in 1933 the Humber Air Ferry service was introduced, taking passengers from Grimsby to Hull in 35 minutes.
After the Second World War economic pressures initially forced the Labour government to shelve plans to build the Humber Bridge. But in 1959 Hull Corporation persuaded neighbouring councils to support the establishment of a Humber Bridge Board entrusted with the construction, operation and maintenance of the bridge, the acquisition of the necessary land, raising loans and collecting the tolls. The Humber Bridge Act was passed and the Board established. In 1974 the county of Humberside was created under local government reorganisation; its viability depended on a page link across the Humber.
The bridge was opened on 17th July 1981. Since then, whilst widely acclaimed for its beauty and engineering, it has been financially problematic. This essay explores some of the reasons as to why this structure has been at the centre of so many controversies.
Political controversy
In 1964 Labour won North Hull with a very small majority of 1,181. This meant that at the next election, a voting swing of less than 1% would be enough to win the seat back for the Conservatives. In November 1965, Henry Solomons, MP for North Hull, died. Immediately afterwards the polls showed voters favouring the Conservatives. At this time the Labour majority in Parliament was less than 5, leaving the polls delicately poised. Labour launched a major election campaign to win the seat. During a by-election meeting on the 18th January, Barbara Castle, Minister for Transport, promised “You will have your Humber Bridge”. This seemed to have a strong persuasive effect on voters; Labour won the seat increasing their majority to 5,351 - the largest swing in a by-election since 1924. There was thus a strong sense that the construction of the bridge was achieved in return for a decisive Labour vote.
The problem of cost
In 1959 it was estimated that the construction cost was £15,750,000. At first there were strong hopes on Humberside that the government would largely fund the Bridge. Peter Shore, Labour Economics Minister, stated that no grants from the government would be offered as the bridge was not of national importance, and that the cost would have to be raised through loans.
In May 1971, the Conservative government announced a loan deal. The Humber Bridge Board would be loaned 75% of the total cost and the remaining 25% would be funded through the tolls once the bridge was open, also the Bridge Board would not repay the loan before 1994 and not after 2041. It was suggested that if the bridge were to open in 1976 at a cost of £20,000,000, within 16 years it should be making a profit. The size of the loan was estimated at £18,500,000.
A number of key financial issues became apparent. When reviewing the financial plan on any large scale construction project, it is imperative to study the economic conditions. The initial money owed levelled off at £24,000,000 in the very early stages of the project, but by 1981 this had spiralled to an extraordinary £145,000,000. £91,000,000 of this covered construction costs with a further £54,000,000 interest on the money borrowed. One of the main issues was inflation. During construction, inflation was very high and any delays meant further inflation. It was thought that these rising costs would be mirrored in the income from the tolls. Unfortunately, this was not the case.
The Humber Bridge (Debts) Act was passed in 1996 to reorganise the debt so that maintenance work on the bridge could be carried out safely. A significant proportion of the debt was suspended; however there was no “write off” of any debt. In July 1998, the government responded to increased concern from local MPs and the public about the escalating debt. A writing off order was passed whereby £62 million would be cleared from the debt in an attempt to restructure the loan repayments and reduce the interest rates from 12% to 7.75%.
Design
There were two main reasons as to why the suspension bridge design was preferable. Firstly, the Humber estuary has what is known as a “shifting bed” and the path which ships can navigate is always changing. A suspension bridge has no support piers in the middle of its span and therefore would not obstruct ships passing through the estuary.
The main bridge deck is suspended by two large steel cables which pass over towers on each bank and are then fixed to the ground. The bridge was the first suspension bridge to use reinforced concrete instead of steel for its 2 towers. The bridge deck is made up of 124 hollow steel boxes, 18 meters long, which have been welded together. Each box consists of stiffened steel panels, welded together to form a hollow box section 22 meters wide and 4.5 meters deep. The top side of these hollow boxes forms the bridge’s main carriageway. These boxes are suspended by smaller cables attached to the main cables.
Secondly, due to the complex geology of the Humber estuary and its surrounding area, the construction costs of building a tunnel would have been huge.
Geotechnical and Geological Problems
Site investigations were undertaken in 1967 and 1970 to ensure the geotechnical materials on location were of suitable strength to support the massive structural loads of the proposed suspension bridge. The piers would provide foundation for the main towers and the anchorage would support the main cables. Some practical problems surfaced during the design and construction.
The Hessle pier foundation is situated on the edge of the riverbank and had to carry a vertical load of 45000 tonnes. It is founded 8m below the river bed. The Anchor system is situated 21m below the bed and supports a total horizontal pull of 38000 tonnes from the main cables. The north bank consists of a shallow layer of soft silty clay with unsuitable strength capabilities on top. This overlies a 40m thick layer of good quality chalk suitable for supporting the vast foundations and anchorage required for a suspension bridge. The allowable bearing pressure on the chalk was 1250 kN/m^2. This is more than high enough to allow the construction of the tower foundation and the anchorage in the chalk. Under the chalk is Kimmeridge Clay, a hard, over-consolidated, fissured (cracked) clay. The Kimmeridge Clay is strong in-situ, but is significantly weakened by removing it from site or disturbing its settled form. On this bank the clay’s strength issues were not a problem, as the chalk layer was deep enough to support the foundations. However, the contractor encountered problems during construction due to rock falls. Rock falls were prevented by dry-spraying the rock face with a gunite/poly-fibre mix to stabilise the surface.
The Barton Pier foundation is located in the River Humber itself, about 500 m from the south bank; this created difficulties as water had to be kept out during construction. On the south side of the river, there is no strong chalk to support the foundations. Here there is about 30m of soft clay, boulder clay, sand, gravel and alluvium overlying the Kimmeridge Clay which was either too weak or too variable to carry the heavy loads from a major suspension bridge. Consequently, the foundations were taken down into the clay. This added to the construction costs. At the pier location, the clay is 28m below the river bed. As this clay was known to be heavily fissured (liable to crack) the construction process was carefully monitored. This was to prevent the fissures from filling with water, which would critically reduce the strength of the clay. The net bearing pressure on the clay is 400 kN/m^2, significantly lower than that of the north bank’s chalk. However, this value was enough to support the foundations. Difficulties were encountered when sinking the supports for the pier foundation, and the pier had to be re-designed to provide extra weight. The south anchorage was located on the river bank behind the river flood bund. It had to support the same horizontal load from the two main cables of 38000 tonnes. As the horizontal pull is exerted at the top of the anchorage system a moment is created. This moment would cause uneven pressure on the base of the anchorage and cause it to tip forward, towards the bridge. To counteract this, weight was added at the rear of the anchorage block.
Mathematical Analysis
In theory the curve of the main cable is a parabola. This is due to the uniformly distributed load from the bridge deck. This theory relies on a few assumptions: the suspension cables are equally spaced and relatively close together, the weight of the bridge is much greater than the cable’s self weight, and the cable is flexible. However, in practice the main cable forms a shape that is close to a parabola, but not perfect. The parabolic curve is formed naturally when the main cable is put under a uniform dead load. This ensures all parts of the cable are acting in tension.
During construction the unloaded main cable will hang under its own self weight in the shape of a catenary, which can be described by the following equation:
Where ‘a’ defines the rate of curvature of the catenary. For flat curves seen over large spans the ‘a’ value would be high.
As the weight of the bridge deck is added the catenary curve will be drawn out into the shape of a parabola, defined by the following equation:
Where ‘a’ again defines the rate of curvature or sharpness of the curve. For the Humber Bridge, the value of ‘a’ can be defined using its geometry. We know the long central deck section spans 1410m, and the cable is attached to the towers at a vertical distance of 155.5m from the deck