Humber Geology
The design and construction of the foundations for the Humber Bridge were dictated by the geology of the site which differed greatly between the North (Hessle) and South (Barton) banks of the Estuary.
On the North side, a bed of sound chalk was close to the surface making the solution for creating the foundations relatively straight forward.
On the North side, a bed of sound chalk was close to the surface making the solution for creating the foundations relatively straight forward.
However, on the South side the chalk was significantly eroded by glacial action leaving a 30 metre deep bed of sand, silty clay, boulder clay, gravel and saturated soil. These deposits overlie a thick bed of over consolidated Kimmeridge Clay. This was established by trial boreholes and subsequent laboratory tests.
Hessle Anchorage
Construction of Hessle Anchorage began by digging a 20 meter-deep hole into the chalk on what was previously Hessle Golf Course.
Hessle Anchorage was then built up in reinforced concrete sections to a height of 41 metres above the foundation level.
Inside this massive structure, there are chambers where the main cable wires are anchored into position.
The purpose of the Anchorage is to take all of the weight of the Main Cable) Road Deck and all of the traffic. The Anchorage is 65.5 metres long and 36 metres high.
Hessle Tower
The foundation for Hessle Tower was sited on the highwater line at the foreshore. Due to the good chalk layer in the area, only a reinforced concrete slab was needed to evenly support the load from the two Tower legs.
"The innovation of building the towers in concrete meant Humber was ahead of its time. Most big bridges prior to this were generally built in steel." Douglas Strachan, Resident Engineer
A technique called ‘slip forming’ was used to pour the concrete Towers. The concrete is continuously poured until the Tower reaches its maximum height. Hessle Tower was completed in 19 weeks working around the clock in 12-hour shifts.
In May 1974 Hessle Tower reached its maximum height of 155.5 metres (above pier). Next, the portal beams would be added to complete the structure.
In July 1976 the heavy cranes were being prepared for eventually lifting the saddles to the top of Hessle Tower.
Barton Anchorage
Before any excavation could be completed on Barton Anchorage, the ground water level had to be lowered by adding a ring of wells around the perimeter of the site.
Barton Anchorage is located just behind the sea wall on the south side of the river and, like the Barton Tower, is founded in the Kimmeridge Clay.
Barton anchorage is 14m deeper than Hessle Anchorage, at 35m depth and sunk into Kimmeridge Clay, rather than chalk. Although machines were used, at times the hard clay had to be dug by hand.
Barton Jetty and Cofferdam
Due to the complex geology on the Barton side, the decision was made to build Barton Tower on its own island. This meant that a number of temporary works had to take place to prepare the area for construction. The first was a temporary pier and staging in the proposed location for Barton Tower.
The jetty was 500 metres long with a staging area for cranes, equipment, and facilities.
Considerable problems had to be overcome before the foundation was successfully completed. The steel shortage of 1973 caused a delay to building the pier and in placing the cofferdam.
A figure-of-eight steel cofferdam was placed and filled with sand to form a temporary island. Scouring around the cofferdam proved to be worse than had been envisaged and 12,000 tonnes of chalk had to be dumped outside of the cofferdam to overcome the problem.
Barton Caisson
Now that the cofferdam had been created and emptied, the Caisson could be sunk into the bed of the Humber estuary by open dredging.
Each Caisson is comprised of two concentric concrete cylinders joined by six radial walls so dividing the Caisson into seven cells.
The sinking process involved the excavation of the river bed and concreting the Caisson walls which had a lubricant to facilitate.
The top of the pier had to be redesigned due to the issues of sinking the Caisson. This caused a significant delay to the construction of the bridge.
The sinking process proved to be slow and challenging - particularly when the West Caisson hit a pocket of water pressure. The inflow of the water then flushed out the lubricant on the outside of the Caisson. This greatly increased the friction and challenged the load needed to sink the Caisson.
Multiple unsuccessful attempts were made to reinstate the lubricant. The sinking of the Caisson was finally achieved by adding 3,000 tonnes of temporary steel ingots from Scunthorpe and 4,000 tonnes of permanent concrete, to create extended outer walls of each Caisson.
Because of the difficulties, the overall rate of sinking the Caissons was much slower than programmed and the bottom was finally reached in September 1975.
Barton Tower
The Barton Tower is located 500 metres into the Humber Estuary on the South (Barton) side.
The Pier below Barton Tower is a concrete structure about 42 m deep which is supported by the Caissons.
Barton Tower is built with reinforced concrete and there is a significant amount of reinforcing bar (steel) to facilitate movements.
The same technique of slip forming the concrete was used. Using 12-hour shift patterns around the clock meant that the structure was completed in ten and half weeks.
Barton Tower was handed over to the superstructure contractor with only the two upper portal beams in position. This was to stabilise the legs from wind movement.
The lower two beams were added after the catwalk was established for the parallel cable spinning.
Deckbox Construction
17,000 tonnes of steel panels were delivered to the assembly yard two miles downstream of the Bridge site.
124 steel boxes each weighing about 120 tonnes were assembled after being painted on site.
Three boxes were assembled longitudinally to facilitate matching when erected. All joints were welded.
The boxes were generally 18.1 metres long, 22 metres wide, and 4.5 metres deep and weighing 120 tonnes.
The boxes include provision for pedestrians and cyclists.
Whilst work was progressing on site the 124 boxes were ready for loading onto the temporary Gantry and Barge for transport to the bridge for erection.
Catwalk Construction
To enable the construction of the Main Cable and the subsequent Deck Box erection, a temporary catwalk/footbridge was built.
This created the first link between Yorkshire and Lincolnshire across the Humber estuary.
The Catwalk was made up of 20 steel ropes and panels of welded mesh. There were five crosswalks (mini bridges) in between the two Catwalks to allow access.
In addition to walking access there, was also a tramway system implemented to facilitate cable spinning, lighting, and communications.
Cable Construction
11,000 tonnes of steel wire was used to construct the main cable. Spinning operations were conducted from the Hessle Anchorage.
The cable is made up of 37 strands containing 404 5mm wires, each wire is the thickness of a pencil.
Once the cable has been completed it is compacted and cable bands are clamped in position.
From these cable bands, the hangers to the road deck are suspended.
Spinning the cable is a misleading description. The criteria for the Main Cable is that the wires are parallel. Spinning refers to the circular motion of the wheel taking the two loops of wire across the Humber Estuary.
The spinning wheel slowed down at the saddles placed at the Tower tops and Anchorages. This is where the wires were placed into the grooves of the saddles.
Cable Construction (continued)
Spinning was conducted in two shifts from 6am to 10pm. An adjusting shift operated from 10pm to 6am when temperatures were constant.
Cable spinning was very weather-sensitive as the 5mm wires could be blown over the side.
The adjusting shift was responsible for lowering the completed strands into their final position in the cooler temperatures.
Completed strands were secured and spinning wires were adjusted at mid-span by control of winches at the Anchorages and Tower tops.
Once the load from some of the erected Deck Boxes was in place the wrapping of the Main Cable could commence.
The Cables were wrapped by using red lead paste on the outer perimeter, before being wrapped with galvanized wire and painted with six coats of paint.
In accordance with Health and Safety Regulations, the people involved in applying the lead were monitored for exposure to lead.
The 37 individual strands were compacted to form the Main Cable.
Deck Box Erection
From the Priory Yard assembly area two miles away from the Humber Bridge) the boxes were offloaded onto a purpose-built barge to float them up the estuary.
Tides determined when boxes could be erected and on occasions, two boxes were erected on one tide.
In November 1979 the public could see a major change to the skyline as the first main span deck box was erected.
As the Deck Boxes were erected) the Main Cable deflected. The boxes had temporary connections at the joints during the erection phase. When sufficient boxes had been erected the welders could start making the permanent joints to create the Road Deck.
The heaviest Deck Boxes were swung into position between the Tower legs and weighed 168 tonnes. This allowed the standard closing box to be erected.
One of the main problems to overcome in the design of a very long span bridge is not the traffic loads, but the issue with the bridge moving under certain wind conditions.
The Deck Boxes are designed to be like an upside-down aircraft wing to keep the Road Deck stable during high winds.
Humber Bridge is a living, moving structure subject to winds, loading and temperature. The Humber road deck section was developed to overcome the problems of wind experienced by the Tacoma Narrows Bridge in America. The Tacoma main span finally collapsed in 40-mile-per-hour (64 km/h) winds on the morning of 7th November 1940.
An Interesting Friday Night........
The Deck Boxes were lifted using two Gantry Beams that traveled along the main cable.
To move the Gantry Beams they were winched up the cable from winches on the ground at Hessle Tower.
One Friday night in March 1980 when moving the Gantries near the top of the Tower, a hauling rope failed and the two gantries plunged 120 metres down, smashing into the previously erected boxed and breaking the temporary deck connections. Two men were seriously injured on the catwalk.
It was announced at halftime at the local rugby league match that an incident had occurred at the Humber Bridge Project. Many of the workers attending the match were called back to site to help.
By the following Monday, the two butterfly wing boxes had been rotated back onto position and the temporary connections had been reinstated.