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Boyne Bridge

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  • The Institution of Engineers of Ireland

    The Design and Construction of the Boyne Bridge Joe ODonovan BE CEng FIEI MICE MConsEI Managing Director, Roughan & ODonovan Ltd Keith Wilson MA CEng MICE Chief Bridge Engineer, Roughan & ODonovan Ltd Pat Maher BE MEngSc MSc Dip Highway Eng CEng MIEI MICE MIStructE Bridge Manager, National Roads Authority (formerly Chief Bridge Engineer, Roughan & ODonovan Ltd) Paper presented to the Institution of Engineers of Ireland Dublin March 4th 2003 Cork March 11th 2003

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    ABSTRACT The Boyne Bridge carries the M1 Northern Motorway over the River Boyne approximately 3 km west of Drogheda. The paper briefly describes the environmental issues which influenced the preliminary design before describing the bridge as finally designed. The bridge has a number of innovative features, such as wind shielding and unpainted enclosed steelwork, and these are described together with the reasons for their inclusion. Other significant design aspects which arise as a direct result of the form of structure are discussed. The procurement strategy for the bridge is described including the development of a supplemental agreement in which the contractor assumed a much greater degree of risk in return for a lump sum payment and the opportunity to develop a deck design which maximised his fabrication and erection expertise. The main differences between the two designs are described. Finally, the key aspects of the pylon construction and the erection procedure for the deck are outlined.

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    1. THE NORTHERN MOTORWAY (GORMANSTON TO MONASTERBOICE)

    The Boyne Bridge is located on the Gormanston to Monasterboice section of the M1 Northern Motorway. This 22km long section of motorway connects the Balbriggan Bypass to the Dunleer Bypass and is a key element in the major road network of the country. In February 1998, Northconsult, a consortium of consulting engineers was appointed by the client to carry out the detailed design and to supervise the construction stages of the motorway. The consortium consisted of:

    M C OSulllivan Ltd Atkins Ltd Roughan & ODonovan Ltd.

    The Boyne Bridge was designed by Roughan & ODonovan Ltd as part of the above consortium.

    2. THE CLIENT The proposed Northern Motorway involved a number of Local Authorities Meath County Council, Drogheda Corporation and Louth County Council. The latter two authorities, however, under a section 59 agreement of the Local Government Act (1995) confirmed that Meath County Council would exercise all their functions in relation to the motorway and its associated link roads.

    3. SITE OF THE BRIDGE The Boyne Bridge is located about 3km west of Drogheda, see Figure 1. The bridge crosses the river approximately at right angles in a north-south direction, see Figure 2. The River Boyne is approximately 150m wide and consists of two channels separated by Yellow Island. The river is tidal at the crossing. At the bridge, the river valley has an asymmetrical cross-section with a steep escarpment to the south of the river and a gently rising slope from the river edge going northwards.

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    Figure 1 Bridge Location

    Figure 2 Bridge Site

    4. MOTORWAY AT THE BRIDGE The motorway is about 20m above the normal water level of the river and the distance between the points where it departs from ground level on either side of the valley is approximately 400m. The bridge cross-section will accommodate two carriageways, each with a road paved width of 11.5m in the short term and 12.75m in the long term, see Figure 3. The short term width of 11.5m comprises a 3.0m

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    hard shoulder, 2 x 3.75m wide lanes and a 1.0m wide shoulder on the median side. In the long term, provision is made for an additional 3.75m wide lane by reducing the width of the median, reducing the hard shoulder width to 0.5m and adjusting the lane positions.

    Figure 3 Motorway Cross-section at Bridge

    5. PRELIMINARY DESIGN OF THE BOYNE BRIDGE

    The preliminary design of the Boyne Bridge was completed in June 1994 and in April 1995 the Environmental Impact Statement (EIS) was published. The EIS for the Boyne Bridge was separate from that done in respect of the motorway and was the first formal bridge EIS to be carried out under the Roads Act 1993 and Part 5 of Roads Regulations 1994. A paper on the preliminary design of the bridge was presented to the North-East Region IEI in November 19961. That paper detailed the environmental issues and the constraints which influenced the bridge design ultimately selected and these may be summarised as follows: (a) Historical and Heritage The Boyne Valley is one of the most historically significant sites in Ireland. The ancient monuments of Newgrange, Knowth and Dowth lie within 3-5km of the crossing. The three main Williamite crossings of the river at the Battle of the Boyne in 1690 took place adjacent to the bridge location. (b) Flora and Fauna The EIS concluded that the site for the proposed bridge represents the most ecologically important area in the proposed motorway. Eight

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    distinct habitats were identified within the environs of the river and all show a diversity of flora and fauna species. (c) Amenity Walkers and anglers make considerable use of the southern bank of the river along the Oldbridge Road. (d) Physical Within the river channels, on Yellow Island and in the tidal reed beds to the north of the river, the presence of soft alluvial conditions, mudflats and fluctuating water levels would make the construction of bridge substructures very difficult irrespective of environmental considerations. (e) Aesthetic Given the outstanding natural beauty of the Boyne Valley, the scale and height of the bridge at the crossing, the aesthetic merit of any design was regarded as probably the single most important factor to be considered. Accordingly the design team took the view that the bridge design should as far as possible achieve the following aims:

    be compatible with the river valley, make a positive visual statement, cause minimal interference with flora and fauna, cause minimal intrusion onto the Battle of the Boyne Site, cause minimal ( even if temporary) loss of amenity during

    construction, and be the design which best meets the design constraints and not

    necessarily the cheapest design.

    6. GENERAL DESCRIPTION OF BRIDGE Span Arrangement The elevation of the bridge is shown in Figure 4. The deck spans, centre to centre of bearings, are as follows:

    Back span 42.5m Main (cable stayed) span 170.0m

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    Side Span 1 45.0m Side Span 2 40.0m Side Span 3 30.0m Side Span 4 25.0m Total Length 352.5m

    Figure 4 Elevation of Bridge

    Pylon The pylon is an inverted Y in elevation, extends some 93m above the top of the pile cap and is constructed in reinforced concrete. The legs and the pylon head are hollow being generally rectangular in section and tapering in both front and side elevations. The pylon legs vary in section from 5.0m x 4.8m at the base to 4.2m x 3.865m at the underside of the pylon head. Back stays and fore stays are stressed and anchored from the inside of the pylon head. Full access for both personnel and equipment is provided to the pylon head through the hollows in the pylon legs and head. There is no access from the pylon into the deck void. The legs of the pylon are connected with a concrete crossbeam at underside of deck level. The cross beam is rectangular in section 4.0m wide x 3.9m deep, is hollow inside and supports the deck at two points transversely through mechanical bearings. The 7m x 7m x 3.5m deep concrete plinths at the bottom of each leg of the pylon are connected across with a 4m wide x 2.5m deep concrete tie beam. Each plinth is supported by a 17.5m x 16m x 2.5m deep reinforced concrete pile cap while each pile cap sits on 16 no. 1.5m diameter concrete bored piles.

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    South Abutment The south abutment provides a gravity anchorage structure for the back stays of the pylon, see Figure 5. It is a reinforced concrete multi-cellular structure 32.6m x 41.0m x approximately 7.5m high. Many of the cells are fully or partially filled with Class 10/40 concrete. Between the structural concrete and the infill concrete some 17,000 tonnes of

    Figure 5 South Abutment

    concrete is provided in the abutment. However, permanent access is provided to the interior of the abutment so that the anchorages to the stays can be inspected. The abutment is founded on a spread footing at an average depth of 5m below existing ground level and in service conditions exerts a lower ground pressure than the original insitu condition. Access into the bridge deck is provided from the abutment. Northern Approach Span Substructures The intermediate supports for the northern approach spans consist of pairs of 2m diameter reinforced concrete circular columns. The columns generally decrease in height as the ground gently rises going north, column heights varying from 14m to only 5.5m. Columns are supported on individual reinforced concrete pile caps. The northern abutment is a simple reinforced concrete cantilevered abutment and wing walls which is supported on bored piles. Access into the bridge deck and inspection gallery for bearings and the movement

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    joint are provided at this abutment. All piles for the northern approach span substructures are 750mm diameter. Bridge Deck The cross-section of the deck as designed, shown in Figure 6, is basically the cross-section used for all spans of the bridge. The deck structure consists of a 230mm thick reinforced concrete slab acting compositely with a steel sp

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