During the planning stages of the improvement to the Dublin to Sligo/Galway road network it was established that an extension to the M4 and spur to the M6 motorways would be built by a Public Private Partnership initiative. The West Route joint venture of Siac Construction and Ferrovial Agroman identified that by developing a system of aesthetically pleasing, high-quality structures that could be built quickly and safely, the construction period could be kept to a minimum. This would result in an early opening of the toll route leading to a faster return on capital investment.

Optimising cost benefits

Five accommodation and public road underbridges (see Figures 1 and 2) were required, together with an archshaped river crossing beneath the new motorway, (see Figure 3). To increase the likelihood of time and cost benefits the consortium opted for a single supplier to carry out the design and construction of these structures. This simplified the management of the project by streamlining the co-ordination of the bridges sub-contract.

For the section of the M1 Dublin to Belfast motorway from Balbriggan to the Boyne Bridge crossing, the joint venture contractors Siac Construction/O'Rourke opted for an alternative to the original client-designed Lagavooran rail bridge. A 52 225m^sup 2^ Matire CM4 type precast concrete arch with VSL reinforced earth spandrels and wing walls was selected. This enabled the main contractor to avoid the use of extensive temporary works and saved on prolonged railway possessions.

Construction and installation

Using a selection of precast concrete units from the Matire range, the Ireland and UK licensee, ABM Construction, was able to achieve the design and manufacture required for all six structures for the M4 motorway within a 12-week period. Period precast concrete wing walls for each structure were also manufactured and installed by ABM within this time frame. At the peak of production the concrete units for three different structures were being manufactured simultaneously.

The Opti-Cadr open cell portals were chosen for the bridges spanning public highways beneath the motorway, using 'L' shaped wall units and an inverted 'U' wall/roof unit to achieve 10m spans and 5.3m height clearances. An in-situ concrete heel was added after site installation. The closed cell Opti-Cadr boxes of 'U' and inverted 'U' units were used for the 6m span accommodation underbridges.

Due to the highway layout and conservation issues, it was necessary for the 135m-long river structure to be divided into two sections beneath the M4 motorway and slip roads, with one section incorporating a light well to benefit fish stock. An adaptation of the typical Matire CM4 arch system formed the 14m span basic structural element for this structure, the arch section being supported by precast concrete padstones 'stitched' to an in-situ concrete continuous beam set on stabilised embankments.

Fewer rail possessions

The construction of a large span, precast arch (at 17.5m this was a larger span than initially envisaged) over the Drogheda to Navan industrial railway line (see Figures 4 and 5), provided many benefits to the overall progress of the construction of the relevant section of the Ml motor-way. Extensive temporary works, involving sheet piling close to the single track needed to build the original in-situ concrete abutments, were avoided. As the bridge was 105m in length, extensive rail possessions were also avoided. Since the arch walls were sited further from the tracks, the installation of the walls and roof caused minimum interference to the operation of the railway. The bridge structure was installed in four days using 82 'L' wall and 41 curved roof units. The use of off-site factory manufacture and the speed of installation resulted in beneficial programme changes enabling site resources to be employed on other construction operations that had become critical to the completion of the project. This provided the rail authority with the facility to upgrade permanently to twin tracks in the future.

Design considerations

Matire structures are configured with pinned (ball and socket) joints in the walls of the arch and portal/box so as to deform laterally under the effect of vertical loads. The associated horizontal deformations that occur subject the adjacent soil backfill to stress. The design takes into account the geotechnical properties of the granular back-fill. The walls and footings are modelled with spring supports to represent restraints provided by the interaction of the structure with the adjacent soil. The springs act lineally and tension is not permitted. The stiffness of the soil is determined based on the modulus of sub-grade reaction. By incorporating the lateral support of this soil block adjacent to the structure in the design model, some of the load from the structure is deflected to the soil to create an efficient design. The concrete sections are analysed as plane frames using the strip method in accordance with BS 5400-4(1) and the DMRB(2. Four geometric models were used to cover the range of structures within the package for the M4 and M1 projects.

Concluding remarks

By using an innovative construction process developed in France during the early 1980s, which has since been used successfully throughout the world for bridge and tunnel construction, the two projects and many more in Ireland have benefited from the early completion of high-quality construction built within budget.

The range of Matire structures are now available in the UK and before long this type of structure could be a feature of the highway network (see Figure 6). Continuing development tailored to client and user needs will provide a wide range of solutions to enhance quality and speed in safe construction leading to major cost benefits.