The Government of Bangladesh sees bridge building across the country’s major rivers as vital in its drive for better road access and economic stability.
One of these important crossings is the Bhairab Bridge, which carries the Dhaka-Sylhet trunk road over the flood plain of the Meghna River, and provides a direct connection between the towns of Ashuganj and Bhairab. Prior to its construction, the only way for road vehicles to make the crossing was by ferry. Halcrow was appointed by the Government of Bangladesh to prepare design and build tender documents for the construction of the new fixed link, and subsequently went on to supervise construction, acting as the Employer’s Representative for the project.
The central section of the bridge is 929m long and consists of seven 110m-long spans and two 79.5m spans, all built of post-tensioned concrete box girders constructed as in situ segmental balanced cantilevers. These are supported on reinforced concrete piers at 18m centres, with elastomeric bearings connecting the substructure and the box girder superstructure. The piers are supported on reinforced concrete pile caps, which transfer bridge loads to six bored cast in place reinforced concrete piles at each pier location.
At each end of the main bridge there are viaduct spans - 141.5m in length at Bhairab and 123.5m at Ashuganj - which terminate at abutments, where the road continues on earth embankments. There are two reinforced concrete underpasses for pedestrians and rickshaws, one combined road-rail underpass and two major reinforced concrete drainage culverts.
The total length of the project is 2.6km, made up of the 1.2km long bridge and a further 1.4km of new roads.
One of the main problems of constructing a bridge of this scale over such a vast flood plain was to secure the foundations and protect them from scour when the river is in flood. In the dry season the river reduces to a depth of 1.5m, but during the monsoons the water level can rise by 6m. The soil of the river bed is made up of silty sand and clayey silt, and the upper layer of loose material will scour and shift as the flow changes, so it was important to sink the foundation piles deep into the very compact sands below. The river banks have been protected by dry stone pitching, sand bags and grass covering, and the bed of the river was reprofiled with sand bags.
Halcrow’s initial commission was from the UK government’s Department for International Development (DfID), which part-funded the new crossing. Our role was to prepare and issue tender documents, and to evaluate tenders prior to contract award. We also undertook surveys and studies, including bathymetric and topographic surveys, ground investigations, utilities survey, and mathematical and physical modelling of the river, and prepared a social action plan and an environmental management plan.
Construction started in autumn 1999, with Halcrow acting as Employers Representative, reviewing all the contractor's designs and administering the project on behalf of the Government of Bangladesh’s Roads and Highways Department. The £65 million scheme was completed in autumn 2002.