Nepal's roads department turns to Bailey bridges on the Trishuli corridor

What happened

The flood on the Bhotekoshi and Trishuli rivers cut road links between districts, towns and villages. According to the Department of Roads, 41 motorable bridges were destroyed and four more damaged between Rasuwa and Chitwan. The concrete bridge at Devighat in Nuwakot was swept away, as was the Furke Khola bridge that linked Dhading's district headquarters to the Prithvi Highway. The department put damage to roads and bridges at about Rs 30 billion and rebuilding at about Rs 50 billion.

The department owns 11 Bailey bridges, each up to 48 metres long. Sushil Babu Dhakal, a joint secretary at the Ministry of Physical Infrastructure and Transport, told the Post that moving them is hard because so many roads were washed away. He said seven Bailey bridges of 30 to 70 metres are urgently needed between the Rasuwagadhi border crossing and Syafrubesi alone. The Nepali Army was separately installing an Acrow bridge, a similar modular steel bridge, at Devighat.

Neighbours offered help. China sent two 100-metre Bailey bridges to the Korala border crossing in Mustang. Together they weigh about 320 tonnes, need 17 trucks to move, and can carry loads above 40 tonnes. China also sent two technicians to help install them. India offered Bailey bridges as well, but none had arrived by 8 September. Officials said the 100-metre spans could only go where the ground allows.

On 21 September, Ratopati reported a briefing by Department of Roads Director General Bijay Jaisi to a parliamentary committee. At Bidur, where the river washed away two bridges, the department planned a 60-metre Bailey bridge within 15 to 20 days, after narrowing the river's flow. Where possible, it planned to contract permanent bridges to be built within six months to one year.

The engineering behind it

A Bailey bridge is a kit. As general knowledge, it is made of standard steel truss panels, about 3 metres long, that workers pin together by hand or with light cranes. Panels can be stacked side by side and in layers to make the bridge stronger. This is why the Post notes that a Bailey bridge's size can be adjusted to the site. The trade-off is that longer spans and heavier traffic need more panels, more steel and more time to build.

Span is the main limit. In general, as a simple truss gets longer, the bending effect from its own weight and from traffic grows quickly. A 48-metre bridge from the department's stock cannot cross a 70-metre gap without a middle support, and a pier in a fast, debris-filled river is hard to build. This is why the 100-metre Chinese spans matter, and also why they need suitable ground at both ends to sit on.

Narrowing the river at Bidur is a river engineering step. In general, engineers can build embankments or place rock to push the flow into a narrower channel, so a shorter bridge can cross it. This is cheaper and faster than buying a longer bridge. But a narrower channel makes water flow faster and deeper, so the banks and the bridge seats, called abutments, must be protected from scour, which is the washing away of soil around foundations.

Load class is the other decision. A temporary bridge must carry the vehicles that reconstruction needs, such as trucks with rock, cement and steel. The Chinese bridges are rated for more than 40 tonnes. In general, engineers check each bridge's rating and post load limits, because a temporary bridge that is overloaded can fail suddenly.

What it means in Nepal

The work runs in two stages. First, temporary bridges and new road tracks reopen access. The Post reported, for example, that crews were opening a track from Dhunge Bazar to Betrawati in Nuwakot, with a Bailey bridge planned at Betrawati once the road reached it. Ratopati reported that traffic resumed on the Galchi-Benighat road after crews cleared debris and landslides, using temporary arrangements on the damaged Kolmu Khola bridge. Nepal has also asked China to let the workforce building roads on the Chinese side of the Rasuwagadhi border help repair the roads on the Nepal side.

Second, permanent bridges must be designed and built. Each will need a site survey, a hydrological study of the new river channel, foundation design and a contract. At Ghatibesi, Ratopati reported that engineers were still comparing two options: repairing the damaged bridge's piers and adding a new deck, or building a new truss bridge. Finance Secretary Ghanshyam Upadhyaya told the Post that budget is available once a request is made.

For civil engineering students, this is applied structural and river engineering at full scale. The choices are practical: which span to use, where to place supports, how to protect abutments, which vehicles a bridge must carry, and how to deliver heavy steel on broken roads. These are the same calculations taught in class, made under time pressure and with real consequences for the people who depend on the road.