Padma Multipurpose Bridge Project

6.5.2 Design of Transition Structures23. In the transition section, the railway alignment is required to be separate from the road alignmentin order to minimise the length of the viaduct structures. Two alternative options have been developedto achieve this separation. Option H1 considers separating the road into two carriageways andpassing one of these carriageways below the railway viaduct. Option H2 considers passing the wholeroad carriageway over the top of the railway viaduct to separate the alignment. A comparison of thesetwo alternatives is presented in Table 6-4.View of transitionstructureTable 6-4: Comparison of Transition StructuresOption H1Option H2AlignmentOption H1 separates the northboundand southbound carriageways into twoseparate viaducts which split outwardson a horizontal curve to a point wherethe extents of the road viaduct structureare clear of the extents of the railwayclearance envelope.Structural Arrangement Option can accommodate differentstructural bridge formsGeometrical designClearancerequirementsCostIssues with sight distances due to tighthorizontal curves and the underpassOption H2 carries both roadcarriageways over the top of therailway viaduct to separate thealignment.Complex pier and headstockarrangementsNo geometrical design issues.No Restrictions Restrictions. Clearance requiredabove the railway corridor for beamand slab structure and the supportingpier headstock. This may restrict theutilisation of the space above therailway corridor in the main bridge forservices and utilitiesLess, due to minimum length of viaductstructureLand Acquisition Both road and rail right-of-wayrequirements will be more, about 12 to13 hectares.Environment/AestheticsVery pleasing.The viaducts are also located over landand are readily visible to thesurrounding areas and the localcommunitiesMoreLess right-of-way requirements, 5hectaresObstructed view.The road user is unable to get a viewof the main bridge as they approachtowards the bridge6.5.3 Modifications of Feasibility Design: Bridge End Facilities & Navigation Clearance24. The proposed bridge length in the FS was 5.58 km, while in the detail design the bridge length isincreased to 6.15 km to accommodate the present river width, which was widened due to the river6-18