e-mosty September 2017: Mersey Gateway Bridge. Arenales Bridge. Mersey Gateway Bridge. Arenales Bridge. BLWT. - Page 9

3. DESIGN AND THE CONSTRUCTION 3.1 General description The design is based on a cable-stayed structure with three pylons. The 80m high Central pylon is shorter than the two outer pylons, which are 110m (North pylon) and 125m (South pylon) high. sandstone. Piled foundation options were considered but direct (spread footing) foundations using an in- situ reinforce concrete foundation were selected as the preferred solution. This design for the bridge was selected from a wide range of options and it brings maximum benefits for users and local people and has a minimal impact on the estuary and its surrounding environment. Each footing is 4.5m high and up to 22m in diameter. For their construction double skinned cofferdams were built using around 300 steel piles driven into the riverbed. Including the approach viaducts on each side the bridge is 2,250m long with a river span of 1,000m. Inside each cofferdam a mass concrete base slab was cast onto which a cage consisting of 190 tonnes of steel reinforcing bars was assembled. 3.2 Temporary Access Bridge Work started on the bridge in mid-2014 with two temporary access roads being built across the salt- marsh. A 1km temporary trestle bridge with a precast concrete deck around 9m wide was built across the estuary to provide construction access. Up to 140 steel piles were driven to a depth of around 16m to support the temporary access bridge. 3.3 Cable-Stayed Bridge Construction 3.3.1 Pylon Foundations Vertical steel reinforcement bars were then fixed into the centre of the cage to form the base of the pylon shaft. 1,400m 3 concrete was poured to form the pylon foundations. The weathered rock provides a high bearing capacity although careful assessment was required to validate the strength and stiffness at each foundation location. Therefore, during construction of the spread footings and subsequent bridge pylon and superstructure construction, the settlement and rotation of the foundations and ground movement was monitored. The ground conditions comprise a relatively shallow thickness of alluvium overlying variably weathered Figure 7: The North cofferdam with its newly completed pylon base Figure 6: The concrete pour on the North cofferdam 3/2017