Pushing the Limits of U Shaped Viaducts

The Pragati-Maidan viaduct, is an extradosed railway bridge with a main span of 93 m. The deck cross-section has a U shape, which permits a perfect integration of the metro system in the superstructure. The extradosed cables are covered by a concrete beam that allows to consider them as internal prestressing. This beam also increases the stiffness of the main span. This bridge is the first of its kind to be erected using the cantilever construction method, and it is the first extradosed bridge ever built in India.

Rajan Kataria, Chief Engineer (Design) Delhi Metro Rail Corporation Ltd.

Introduction
For the extension of the U shaped viaduct of the 3rd Line of Delhi Metro (India), DMRC was confronted to the crossing of 5 railway tracks with very high constraints: sharp plan curvature, railway vertical clearance, impossible location of intermediate piers, no possible interruption of the railway traffic, and minimum span length of 93 m… In addition to the technical difficulties, time was not an ally: the line opening date was scheduled in less than one year.

All these constraints have not allowed DMRC to use the typical structures of the line 3 viaduct, which had been designed by Systra. Systra has then proposed a very innovative and economic solution, which will allow a very fast construction thanks to the use of the same cross section as for typical viaduct, and by means of only conventional materials.

The segmental extradosed bridge has been erected using for the lateral spans the same construction method than for typical viaduct (span by span), and cantilever construction for main span. (Fig. 1, 2).

General elevation of extradosed bridgeFigure 1: General elevation of extradosed bridge

Extradosed bridge plan viewFigure 2: Extradosed bridge plan view

Design

Foundations
All the piers and pylons are founded on deep foundations, with cast in situ concrete piles. The impossible relocation of some utilities and place constraints, has led to design large pilecaps with 20 piles per pylon, as shown in the Fig. 3.

Some utilities, which it was not possible to shift vertically, were embedded in the 3m depth pilecap. The remaining lateral piers are founded on 4 piles of 1.5m diameter. Piles length was limited to 30m to save the time of pile testing (30m is the longest length for which the pile testing was already done).

pylons pilecap

Substructures and pylons
The bridge pylons are supported on a portal with vertical circular shafts of 2.4 m diameter. The rectangular transverse beam is 2.5 m depth as shown in Fig. 4. Above this transverse beam, the pylon segment is cast in situ to be rigidly connected to the pylon.
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NBM&CW June 2012

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