Initial Design – Contract Preparations
Design work on the new crossing included further detailed studies, a hydraulic model to test pier positions, mathematical modelling, a bathymetric survey, and geotechnical and topographical surveys for the route corridors. Extensive research was carried out into wind shielding and also into climate change issues, which indicated a potential rise in sea levels. The viability of engineering concepts and innovations were confirmed, together with the buildability and quality of the scheme.
Extensive consultations were undertaken with all those affected to ensure that concerns were fully understood and positively addressed in the development of the scheme. Wide ranging studies were carried out into the existing environment, potential impacts were identified and removed where feasible, and proposals were developed for reducing remaining adverse impacts. The consultations also included navigation interests, industry, landscape advice, and the Royal Fine Arts Commission regarding the main bridge and other structures.
A series of public exhibitions was held in England and Wales in areas affected by the proposals, initially showing the results of the study and then the changes adopted as the design was developed to take account of local concerns and the results of surveys
In April 1989, tenders were invited for the main crossing and toll Plaza. The tender details included highly detailed technical requirements, contractual/financial issues, constructional aspects, and environmental monitoring. Separate bids were sought for two possible scenarios:
(a) to design, construct and finance the crossing, and to assume responsibility for operating and maintaining both it and the existing Severn Bridge during a concession period, in return for the toll revenue from both bridges during that period, and
(b) to design and construct the new crossing in return for staged payments from the government.
In 1990, following a rigorous assessment of the tenders, the Government accepted, in principle, the proposal of Severn River Crossing plc to design, construct, finance and operate the second crossing. Severn River Crossing plc was a consortium set up specially to bid for the project. It included major investment banks, a British contractor, John Laing plc, and a French contractor, GTM Entrepose.
Obtaining Parliamentary Approval 1990 to 1992
Authority to build the scheme was obtained through Parliament. A hybrid Bill was used to seek the powers required to construct the estuary crossing and the approach roads, to compulsorily purchase the land, and to charge tolls. The Severn Bridges Bill was lodged in November 1990 and, after thorough examination of the scheme by Parliamentary Committees, Royal Assent was granted in early 1992. The immense value of the extensive and detailed consultations, with over 40 affected parties, was shown by the small number of formal objections that were presented against the Bill. A Concession Agreement, between the Government and Severn River Crossing Plc, was signed and construction was started in Spring 1992.
Final Design for the Second Crossing. 1992 to 1993
The Second Crossing is comprised of a cable stayed bridge spanning the main navigation channel, with a two kilometre length of approach viaduct on either side. At 5 kilometres, it was the longest river crossing of this type in the country.
There are 20 spans of approach viaduct on either side of the main bridge and each span is made up of 27 separate units of hollow concrete box girder, tensioned together using high tensile steel strands.
The Shoots Bridge.
The centre-piece of this crossing of the Estuary is the cable stayed bridge over the main navigation channel, known as the Shoots. The main channel resembles a steep sided trench at this location and, although it is only about 300m wide at the base, the pylon legs had to be set back, well away from the top edge of the trench, to ensure stability. After careful consideration, a main span of 456m was agreed upon. At the time of its design, there were no cable stayed bridges operating anywhere in the world with a longer span, although the Pont de Normandie in France was well under construction with a span of 856m.
For more on the Design of the Shoots Bridge Pylons, Click Here
In parallel with these activities, the detailed design of the motorway approach roads was undertaken and tender documents were prepared. This work included the resolution of many issues affecting areas local to the roads, dealing with environmental, landscaping and community issues, and incorporating a wide range of mitigation measures.
Tenders for the approach roads were invited in October 1992 and contracts were awarded in time for construction to start in Spring 1993. The challenge was to construct the second crossing and the approach roads in time for an opening in 1996.