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Training torrents

Dramatic floods in September which again devastated Bangladesh underline the huge engineering challenges faced during the construction of the Jamuna Bridge.

June's official opening of the Jamuna Bridge marks the end of one of the last great civil engineering challenges of the century. The obstacles were formidable. The Jamuna (better known under its Indian name of Brahmaputra) is the world's fourth largest river in terms of flow, reaching over 65,000m3/s during the flood season. Sediment loads are the second highest in the world.

Two key problems had to be solved before the badly needed fixed linked could be built. First, foundations had to cope with scour and liquefaction. But more fundamental was the problem of persuading the river to flow under the completed crossing, as there was a significant risk that when the floodwaters receded one year the bridge would be left high and dry several kilometres from the wayward river.

The Jamuna is a braided river, flowing between a 15km wide strip of shifting islands of unstable micaceous sand. It is never less than 5km wide and in the wet season expands to between 30km and 40km. River levels vary by 8m over the year. During the annual floods, scour can cut channels up to 45m deep in the soft river bed, where the silt and sands are several kilo-metres deep.

UK consul-tant High-Point Rendel (then Rendel Palmer & Tritton), which had built an elec-tricity supply interconnector across the river in the 1970s, was asked to design a natural gas connector link-ing rich farm land in the north west to gas fields in the east. But it became ob-vious that a multi-purpose bridge carrying road and rail traffic as well as the gas pipeline and a second power line would be a more economical in the long run.

By 1986 the funders - the governments of Bangladesh and Japan, the World Bank and the Asian Development Bank - were convinced, and High-Point Rendel, in joint venture with NEDECO of the Netherlands and in association with Bangladesh Consultants, started preparing designs.

Raking tubular steel piles up to 75m deep took care of the foundation problem. The cure for the fickle nature of the river began with choosing a site 8km downstream from one of its rare fixed node points, the existing flood defences at Sirajgang on one bank and Bhuapur on the other. The idea was to build more than 2km long guide bunds on each bank upstream of the proposed 4.8km crossing.

But major floods in 1987 and 1988 caused the river's geometry to change and by 1992 the banks at the chosen site were 2.5km further apart.

To avoid extra delays and costs caused by necessary design changes, Rendel proposed to build the east bund first, during the dry season. The final position of the west bund would not be decided until the next dry season when possible positions could be investigated. This meant the final length and alignment of the bridge could not be fixed until October 1995.

Bunds could only be built during the six months of the dry season. Piling was planned to start at the east abutment once the length and alignment of the bridge had been determined. This meant it would not reach the western end until the west guide bund was finished, causing no interference between the separate contracts for the bridge and the guide bunds.

This latter contract went to HAM-Van Oord ACZ JV for $267M. A joint venture of Hyundai Engineering and Hyundai Construction was awarded the $247M design and construct contract for the bridge and approach viaducts, while local contractors won the smaller approach road contracts.

However, delays until spring 1994 meant there was not enough time to finish the east guide bund in the first dry season. Risking damage to the partially completed bund during the wet season and causing overlap problems with bridge construction, work was switched to the west bund in the second dry season and back to the east in the third.

Hyundai had already proposed an alternative method using an offshore piling barge to drive fewer but larger diameter piles. It deployed one of the world's largest piling hammers, capable of a 1,700kNm blow to drive up to 3.15m diameter, 83m long piles. Impact energy was high enough to cause considerable disturbance of the surrounding soft sands. The east bund was still unfinished when piling started but as construction approached the west bund, the trench in front was temporarily backfilled to stabilise it against the effects of the piling.

By now, bund designs had already changed. Basically a combination of trench and bund, the guide bunds' outer faces originally sloped at 1 in 3.5. However, a number of major and minor slips during construction of the west bund prompted the consultants to soften the slope to a more stable 1 in 5 or 1 in 6, with a total of 880,000m2 geotextile used to strengthen it (see page 34). In all, 26M.m3 of dredging was needed.

The completed bridge is 4.8km long, with 49 spans. The deck carries four road lanes, a railway and a 600mm diameter gas pipeline. Concrete pylons carry 232kV power lines and telecommunications links.

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