Detailed history of the M4 Second Severn Crossing

Panoramic view of the M4 Second Severn Crossing
Panoramic view of the M4 Second Severn Crossing

The philosophy of the design and construction was to reduce the amount of work affected by the river with its massive 14.5m tidal range and notorious currents. Precasting and prefabrication methods were used wherever possible together with a large, sophisticated fleet of 10 floating craft and 7 back-up barges.

The foundations of the bridge were made out of 37 precast Concrete Caissons weighing up to 2,000 tonnes each, 53m long. These were then filled with concrete taking 12 to 20 weeks to cast.

Placing the Caissons involved 2 of the largest vessels in the marine fleet. The SAR3 was used to move the Caissons into position. It is a specially modified flat barge, powered by 4 Jimecal Thrusters, and a computer controlled navigation positioning system. Once this positioning system is started it will hold the barge stationary, maintaining a stable position to within 0.5m of tolerance.

While the DPS held the barge "on station" the LISA A lowered its hydraulic, four legged lifting frame to close under the Caisson. When all was stable the LISA A lifted the caisson off the Barge allowing it to return to its berth. The whole operation had to be completed within a 2 hour tidal window.

The caisson was held by LISA A until the tide had gone out, by which time the bed of the construction area was dry, when it was placed on the river bed.

Once positioned using the satellite and land based systems, the 300mm joint between caisson and bed was sealed and the inside filled with a mass concrete plug followed by reinforced concrete to form the completed foundation. With the sealed caisson acting as a "cofferdam" the filling operation was free of interruption from normal tides.

Example of the cranes used
Example of the cranes used

Pier Construction Precast concrete units were vertically stressed together to form the piers for the '2.1 km long approach viaducts. The system of 'Match Casting', meaning that each unit is cast against its adjacent unit, ensured a perfect fit when erected on its completed caisson foundation. Units were transported from the casting yard to the SAR3 barge. They were then transported to the desired location in the estuary and lifted into position by the crane barge JAY ROBERTSON.

Each 'Match Cast' set of Pier Units contains ducts into which the vertical stressing cables were placed. On completion of stressing, these ducts were filled with wax to preserve the cables and facilitate inspection and renewal when required.

Viaduct Segments

Purpose built launching gantries were used to erect the 2,300 precast concrete segments forming the deck. Each segment weighed approximately 200 tonnes. The design of the viaduct is of precast, reinforced concrete segments, placed into position at each end of a double cantilever, glued and stressed in balanced construction. The span between Piers is generally 98.11 metres. Each span has 27 precast segments forming each carriageway. The segments were again 'Match cast' in purpose built casting sheds on both the Welsh and English sites. Precast segments were transported to the storage area by straddle carriers and then by heavy duty trailer to thee Viaduct Launching Gantry. Both decks are joined by an insitu concrete stitch which forms the central reserve of the three lane mnotorway carriageway.

Pylon Construction

To construct each of the 137m high, hollow, rectangular pylons with tapering shafts, a special PERI "Jump-form" climbing framework system and insitu reinforced concrete were employed. The precast lower and upper crossbeams were introduced as work proceeded.

After concreting the connection of the lower crossbeam to the shafts, construction continued. When it was 3 lifts clear of the beam the upper crossbeam was transported from the precast yard, lifted, placed on top of the lower beam and left.Some months later, when the shafts were high enough, the beam was lifted by strand jacks and fixed to the sides of the shafts in its final position. The joints were completed and the stressing carried out while the shafts were raised to their full height.

Deck construction work

Bridge deck construction
Bridge deck construction

The Bridge Deck Units are structural steel with reinforced concrete roadway. Each unit weighed 180 tonnes and was 34.6 metres wide and 7 metres long. Units were assembled in the construction yard with each unit being "match bolted" to its neigbour to ensure perfect fit and line, then separated before removal from the yard. A pair of special Double Shear Legs (DSL) were designed to stand on the road deck and lift each unit as work proceeded. To give sufficient deck area for the DSL's to stand each side of the Pylon the first five units were placed by the LISA A.

Deck units were transported from the yard by heavy duty, extended trailer onto the SAR3 Barge and floated out into the estuary. By means of the Dynamic positioning System the SAKY held position while the deck unit was lifted by the DSL's, maintaining a balanced construction each side of the Pylon.

Deck units were transported from the yard by heavy duty, extended trailer onto the SAR3 Barge and floated out into the estuary. By means of the Dynamic positioning System the SAKY held position while the deck unit was lifted by the DSL's, maintaining a balanced construction each side of the Pylon.

Once lifted into position the DSL held the units while bolting took place, followed by concreting the deck stitch and the installation of the cable stays. The DSL then released the unit and was ready to move forward to repeat the cycle.

Bridge Statistics

Diagram of the Cable Stay Design
Diagram of the Cable Stay Design
  • Overall length of Crossing Structure: 5,128m
  • Length of Shoots Bridge: 948m
  • Length of Main Span: 456m
  • Navigation Clearance: 37m
  • Length of Welsh approach Viaduct: 2,077m
  • Length of English Approach Viaduct: 2,103m
  • Height of Main Span Pylon Towers: 137m
  • Number of Approach Spans - Welsh Side: 24
  • Number of Approach Spans - English Side: 25
  • Maximum height of Approach Piers: 48m
  • Number of foundations in Caissons: 37
  • Total volume of concrete required: 320,000m cubic
  • Weight of reinforcing steal: 30,000 tons
  • Total length of pre-stressing steel: 150,000m
  • Number of Stay Cables In Shoots Bridge: 240
  • Total number of Concrete Deck Units: 2,434
  • Approximate number of men employed: 1000
  • Construction Period: 1992 to 1996