The Malta Independent 15 November 2018, Thursday

A Link to the North – Manwel Dimech Bridge

Malta Independent Sunday, 4 May 2008, 00:00 Last update: about 6 years ago

Now that the bridge is open for traffic and with only rehabilitation works on Wied Ghomor left, it is worth taking stock of what work went into rebuilding this bridge. A commentator recently criticised the time it took to reconstruct the bridge, saying that he “recently witnessed an avenue of the same length and width being taken apart and resurfaced in one night!” It is perhaps legitimate to criticise the time it took to deliver this project but it is obtuse to compare it with a road constructed on a flat surface rather than a bridge on stilts.

In 2004 a review of Malta’s bridges on the main road network raised the red flag. Manwel Dimech Bridge was damaged and deteriorating and needed immediate attention.

One of the first remedial steps to ensure the safety of motorists and pedestrians was to impose a 45km/hr speed limit and a 35-tonne vehicle weight restriction. Then a full engineering inspection was conducted that gave the Authority three options for a way forward:

a. to carry out rehabilitation work on the existing structure;

b. to carry out rehabilitation work on the substructure (the piers and abutments/columns) and the replacement of the superstructure (decks or viaducts);

c. to totally replace the structure.

The technical advice given was to go for the second option, that is to carry out rehabilitation work on the substructure (piers and abutments/columns) through an additional reinforced shell of concrete and to adjust the piers and abutments to accommodate changes in bearing position while reducing the amount of debris falling in Wied Ghomor. The original time frames, as defined in the Tender document following recommendations by the Authority’s consultants, anticipated that the project would take 52 weeks to complete.

The superstructure (deck) has been designed as a continuous, post-tensioned concrete box girder (one cell) bridge fitted with galvanized steel gratings which allow the drainage of rain water run off through galvanized steel pipes that run within the superstructure deck which eventually discharge into the valley. Unlike the old bridge, which was constructed between 1967 and 1971, the new bearing pads have been designed to enable replacement during the lifespan of the bridge.

Work on the eastern and western viaducts was carried out in two-phases. Phase 1 commenced on the eastern viaduct in late September 2006 with the removal of all street furniture – crash barriers, street lighting poles, traffic signs together with the erection of the scaffolding. The eastern viaduct was completely closed with northbound traffic being re-directed to the western viaduct in December 2006. The eastern viaduct was open to northbound and southbound traffic on 26 November 2007. The second Phase on the western viaduct started a day later.

Work on the eastern and eventually on the western viaducts commenced with the erection of the scaffolding structure. The scaffolding, which was imported from abroad specifically for this project, was intended to collect rubble material rather than risk it falling into the valley during the demolition stage. It was also used to replace a 3cm thick concrete layer around the sub-structures and to support the formwork that took the shape of the bridge deck structure.

The actual demolition of the bridge involved the removal of the old structure, which was made up of reinforced concrete and pre-stressed concrete beams. Following the completion of the demolition phase, the placing of the formwork for the new bridge deck structure commenced. This involved cutting timber sections to shape the new bridge.

Due to the curved shape of the bridge structure on the eastern viaduct, the placing of steel reinforcement and concrete casting was carried out in two stages primarily in the bottom slab and webs on the sides. Once the steel was placed and the concrete casting completed, the formwork for the top slab was erected so that it could handle the steel reinforcement of the top structure.

The concrete casting of the top slab had to be carried out not more than 14 days after the cast of the bottom slab and the side webs. During the placing of steel reinforcement, ducts were placed in the webs of the bridge for the laying of cables. The ducts were placed in the webs vertically in a curved profile through which tendons (cables) were fed and later tensioned so as to accommodate forces generated through the use of the bridge. This being a post-tensioned structure, these cables, of course, play a crucial role in ensuring the structural performance in the lifespan of the bridge.

Once the formwork was removed, and following the hardening of the concrete surface of the top slab, the waterproofing of the bridge deck commenced. The waterproofing consisted of primary preparation work involving spherical blasting of the concrete surface to eliminate unevenness. In some areas epoxy resin strewed with sand was laid to obtain a smooth surface. Following the treatment of these localised areas, a primer layer was applied on the whole bridge and then an asphalt sheet was welded onto the surface using asphalt strips laid in a longitudinal direction. At both ends of the bridge, expansion joints were installed so that the structure could accommodate thermal movement and shrinkage of the concrete deck. These were designed in a way to transfer the vertical loads of two wheels with 140 kilo newtons each at a 2-metre distance and for a braking force of 40 kilo newtons at each wheel.

The scaffolding was removed after a specified number of days following the concrete casting operations and post tensioning of the bridge.

Another aspect of the project was the installation of new bearings that were placed at the connection between the supporting abutments and columns (the sub-structure) and the new bridge deck. These bearings have a very important function in the transfer of loads from the bridge deck down the sub-structures ensuring the full lifespan of the bridge.

Expansion joints were designed to provide continuous waterproofing of the deck structure. Over the layer of asphalt sheets, a 3.5cm layer of “gussasphalt” was applied by hand as part of the waterproofing system. This material is made by heating a mixture of aggregate and bitumen to very high temperatures. The high temperature and the mix characteristics produce a flowable mix that has very good sealing characteristics even though it is laid without compaction thus making it more effective. A 4cm asphalt wearing course layer was finally laid using imported aggregate materials making the surface skid-resistant in accordance with international standards.

This was the first time this high-spec waterproofing system was applied to significant structures in Malta and is in full conformity with recognised international codes. The installation of street furniture, crash barriers, railings on the sides of the bridge complemented with road markings completed the work on the decks primarily designed to ensure the safety of road users.

A strict quality assurance programme was set up before the work started with engineers supervising thoroughly the operations of the contractor throughout the projects. Although various tests have been carried out, final tests are currently underway to compare performance of the new structure with performance criteria and specifications established in the original design. All of the testing was carried out in accordance with international standards.

With the Manwel Dimech Bridge being a major arterial road, and with an estimated 44,000 vehicle crossings per day, one of the challenges of the project was the implementation of a nation-wide traffic management scheme. The scheme involved the erection of temporary area-wide directional signage clearly indicating the diversion routes, the installation of Variable Message Signs at key junctions providing real-time information to commuters, the continuous liaison and co-ordination with Traffic Police, and information to road users so that they are able to make informed decisions about their journeys. The ultimate aim of this scheme was to reduce the impact of traffic congestion.

The project has been part-financed by the European Union under the European Regional Development Fund 2004-2006. The original estimate prepared by the consultants for the project amounted to €5.8 million. The tender was in fact awarded to the contractor for €4.6 million. An additional €46,000, which primarily catered for additional traffic management costs and extended waterproofing work were recommended by the Authority for acceptance by the Director of Contracts as per standard practice.

With regular maintenance, the re-constructed 130-metre bridge (deck) has a lifespan of 80 years while the supporting structures have had their lifespan extended by a further 100 years.

Both viaducts of the bridge were open to traffic on 21 April and this section road of M.A. Vassalli Street has been reclassified with a 60km/h design speed.

Mr Buhagiar is an architect and Director (Roads) at Network Infrastructure Directorate

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