Refurbishing and extending Hughes Bridge, Sligo
23 February 2016
Michael Hughes Bridge links the N4 National Primary Route with the N15 and the N16. Construction of the bridge commenced in July 1987 and it was opened to traffic in December 1988. It carries the N4 national primary road over the Garavogue River in Sligo city. The general topography around the bridge is level.
The surrounding areas are urban, with Sligo city centre to the southeast of the site and Sligo harbour immediately west of the bridge. The carriageway was 13.42m wide with footpaths approximately 3m either side.
Prior to the widening works there were two traffic lanes in each direction crossing the bridge, with a right turn pocket provided for northbound traffic wishing to turn onto Markievicz Road. There is a large right turn movement southbound over the bridge to Ballast Quay, and a dedicated right turn lane was provided to accommodate this.
The bridge severely restricted traffic flow, especially that travelling in a southern direction. Lane swapping by traffic approaching the bridge was common and led to collisions. Tailbacks for traffic travelling southbound occurred daily with traffic backing up and beyond the N15/N16 junction, causing significant delays at peak times. The N4 at this location currently carries a traffic volume in excess of 27,000 AADT (3.4 per cent HGV).
A considerable investment was spent on the construction of the N4 Sligo inner relief road in recent years through Sligo city and environs. However, this investment was not being fully utilised at the northern end as a result of the restrictions on traffic imposed by the bridge structure. The widening works have helped to alleviate the difficulties with traffic congestion and have maximised the benefits of the inner relief road scheme.
The concept of widening Hughes Bridge originated with a project entitled N4/N15 Sligo to the county boundary, which was progressed through various planning stages from 2004 to 2010. The proposed route commenced at Hughes Bridge and tied into an existing improved section at the Co Leitrim boundary ‘Bunduff Bridge’.
Formed part of the Atlantic route corridor – an objective of the Transport 21 initiative
It formed part of the Atlantic route corridor, which was an objective of the Transport 21 initiative. A constraints study and route selection report were prepared by Sligo County Council and the preferred route was adopted by the elected members of Sligo County Council and Sligo Borough Council in 2006. Ryan Hanley of WSP Ltd was appointed in 2007 to progress the preliminary design and planning for the proposed project.
All deliverables including the business case were forwarded to Transport Infrastructure Ireland TII (formally the National Roads Authority or NRA) in 2010. However, due to budgetary constraints, funding was not allocated to proceed with the planning process.
The rural section of the project was then suspended and it was decided to progress with the urban section from Hughes Bridge to the Sligo borough boundary. This project was 1.6km in length and involved widening the existing road to six lanes and upgrading the at-grade junctions.
The statutory documents were developed for this section and submitted to the TII in 2012. However, as with the larger project, due to budgetary constraints, funding was not available to progress to the next stage. Sligo County Council then prepared a proposal to develop the N4 Traffic Improvement Scheme and forwarded it to the TII for approval. Funding was allocated for this project in 2012.
In July 2012, Sligo County Council appointed Arup to develop the scheme from preliminary design to detailed design and to take it through tender to construction.
Preliminary design proposed incorporating three lanes of traffic in either direction
A preliminary design of the scheme had been carried out and proposed incorporating three lanes of traffic in either direction across the existing Hughes Bridge by relocating the existing footpaths onto two new pedestrian and cycle bridges.
A review of the preliminary design, including a value engineering exercise in consultation with Sligo County Council (SCC) and additional topographical survey and site investigation, led to some amendments to the preliminary design. These included a revision to the footbridge deck and foundation types and the incorporation of two lengths of cantilevered boardwalk on the northern side of the Garavogue river.
The scheme is located on the Cummeen Strand/Drumcliff Bay Special Area of Conservation (SAC) and an Environmental Appraisal Report (EAR) was prepared prior to submission of a Part 8 Planning application. The purpose of the report was to determine whether there were any environmental impacts associated with the proposal. An Appropriate Assessment Screening was also carried out and was appended to the EAR on submission.
The report concluded that with the avoidance of sensitive habitats and application of best practice management during construction there would be no significant impacts on qualifying interests or conservation objectives as a result of the scheme.
The planning package was submitted in December 2012 and planning was granted in January 2013.
The detailed design of the scheme included the design of two new footbridges taking the existing pedestrian and cycle traffic off the existing Hughes Bridge, two lengths of piled cantilevered boardwalk, retaining walls, existing deck strengthening, junction realignment and upgrade works, including service diversions onto the new footbridges.
Decks consist of a W3 precast beam with reinforced concrete deck slab
The footbridges are three-span with a main span of 25.99m and two back spans of 18.75m each. The decks consist of a W3 precast beam with a reinforced concrete deck slab. The overall deck width is 3.97m, with a 3.0m walkway. The structure has bespoke pedestrian parapets with built-in lighting and corbels to support public lighting columns relocated from the main deck. It is integral at all supports.
The new structures were designed in accordance with Eurocodes and detailed in accordance with the NRA Design Manual for Roads and Bridges.
On the southern end the abutments are founded on pad foundations sitting on made ground. The site, adjacent to Ballast Quay and Custom House Quay, was excavated previously in the late 1980s for the construction of Hughes Bridge and therefore there was minimal concern that there would be an archaeological find; however, the TII archaeologist was kept informed of all works.
The northern abutments are designed to be supported on piled foundations. The original abutment is founded on rock, however additional site investigation to both sides showed that the rock armour surrounding the existing abutment was unsuitable for bank seat abutments and that rock was at a lower level in these areas.
Each of the northern abutments was designed to be supported on three 600mm diameter reinforced concrete bored piles. The piles were to be encased in a 20mm thick sacrificial steel casing. The casing offers protection to the concrete pile in the marine environment and is necessary to achieve a 120-year design life. The piles are end bearing and have a 2m rocket socket.
The footbridges are integral with the piers of the existing structure. Both decks act independently and movement is not transmitted between the two. The existing Hughes Bridge is fully articulated. The leaf piers supporting the existing precast beam and slab deck are particularly wide and they extend beyond the footprint of the existing deck.
Existing piers incorporated into new bridge design to provide intermediate supports to new decks
The existing piers were incorporated into the new bridge design to provide intermediate supports to the new decks. This benefitted the scheme as it is providing an intermediate support without the need for excavation in the SAC.
Hydro-demolition was proposed to break out the existing piers and expose the existing reinforcement. Additional stainless steel rebar was designed to be drilled and grouted into the existing pier and a new corbel detail cast to support the W beams. An insitu concrete diaphragm was designed to provide an integral connection.
The existing Hughes Bridge supported a large number of services within the wide footpaths, including ESB, Eircom, public lighting, water, UPC and eNet. The removal of the existing footpaths necessitated the relocation of the services contained within the footpaths.
The shallow RC deck of the new footbridges was not deep enough to contain the services and the use of the W beam as a trough for the services was not considered best practice. It was decided to provide a steel utility support beam. The beam comprises two I sections welded together with 12mm deep steel plates.
The beam is supported directly off the footbridge supports at the abutment and piers and spans between these locations. The beam is painted to provide durability. At the request of SCC Water Section two watermains have been retained on the existing deck contained in the 600mm wide rubbing strips.
At the southern end of the scheme the footbridges ramp neatly down to meet the existing footpaths at Customs House and Ballast Quay with the assistance of a curved reinforced concrete retaining wall. At the northern end of the scheme it was established that it would not be possible to support the footway off the rock armour and shrubbery beyond the extents of the deck.
Based on a review of the site and the additional site investigation it was decided to provide a piled cantilever boardwalk. The boardwalk is designed for pedestrian loading in accordance with IS EN 1991, vehicle access is restricted. The insitu reinforced concrete slab was designed to be supported off 600mm diameter bored piles, again protected by a sacrificial steel casing.
As part of the original preliminary design the existing Hughes Bridge had been assessed for the increase to three lanes of traffic in each direction. It was shown that when assessed for 40 tonnes of HA loading the structure failed in hogging over the piers for the outer precast beams.
In agreement with the NRA it was decided that Carbon Fibre Reinforced Polymer (CFRP) would be used for the deck strengthening works and a solution was designed incorporating 1.4mm thick CFRP strips applied with a thin layer of adhesive to a primed surface. Four areas were strengthened, extending for 2m in each direction over the centreline of the pier for a 3m width. The CRFP was then protected with a 10mm deep layer of non-shrink grout.
The contract was tendered under the Public Works Contract for Civil Engineers Works designed by the Employer and was awarded to L&M Keating Ltd. Works commenced on site in October 2014 and were completed in August 2015.
Bernard Scott, Chartered Engineer
Bernard is currently a senior executive engineer with Sligo County Council in the Sligo County Council Road Design Office where he has been involved in the planning, appraisal, design and construction of regional roads as well as design and appraisal of National Road Projects. He is the project engineer for the N4 Collooney to Castlebaldwin Road Project. Bernard is a graduate of IT Sligo and a Chartered Engineer. He previously worked with Irish Enco on Coastal works before joining Louth County Council during the construction of the M1 motorway project.
Ann-Marie Gallagher, Chartered Engineer
Ann Marie is a senior engineer with Arup Dublin managing the civil structures team. She has over 16 years’ experience in the design, procurement and construction of highway and rail structures, working on traditional and D&B and PPP schemes in Ireland and the UK. She is currently managing a bridge rehabilitation scheme in Donegal and is responsible for the team delivering the bridge and civil structures updates to NRA DMRB. She assists TII in co-ordinating the CEDR research programme for Europe. Ann-Marie is a graduate of TCD.
The authors would like to extend their sincere thanks to the Road Design Office of Sligo County Council, ARUP Consulting Engineers, the employers’ representatives on site during construction and to L&M Keating Ltd for constructing the works