Construction of bridges in the Northeast has always been a great challenge for construction agencies owing to the terrain and weather vagaries. With over 900 bridges constructed by the Border Roads Organisation (BRO) along the border areas in Northeastern states in the past six decades, the organisation and its men have gained significant experience in building bridges in these challenging high-altitude areas of the great Himalayas. Recently, in 2022, one such signature bridge was constructed in Arunachal Pradesh, which was inaugurated on January 3, 2023 by Defence Minister Rajnath Singh. Majestic location Pangin is a small town and the district headquarters of Siang district in Arunachal Pradesh, located about 60 km from Pasighat. The settlement is at the junction point where Siyom River meets Siang River (Brahmaputra). It is also the home of the Adi tribe. The mighty Siyom is a tributary of the Siang. It rises in the south of the main ridge of the Himalayas, not far from the border with Tibet. Initially, it flows in a southerly direction and then meanders through the west Siang district. Saje River is the most prominent of its several tributaries. Mouling National Park is located on the east bank of the river. It passes through district capital Aalo and finally flows into the Brahmaputra, 50 km above Pasighat. Siyom River has a length of about 170 km. Dazzling crossover Anyone travelling on the Along-Yingkiong road has to cross Siyom River. Therefore, it is important to have a suitable crossing place on the river to reach the areas ahead of Pangin. In 2021, BRO constructed a 300-ft Bailey suspension bridge of load class18 R, which was temporarily erected at the site after the accidental collapse of the existing bridge caused by an overloaded vehicle attempting to cross. The existing bridge was unfit for the movement of heavily loaded vehicles carrying construction material, heavy machines, artillery guns and other operational equipment of security forces. Thus, the local administration and local army formations proposed the construction of a robust and a permanent bridge that could allow the passage of vehicles carrying heavier loads. Therefore, BRO commenced the construction of this arch bridge over Siyom River, which was completed in December 2022 and opened for traffic on January 3, 2023. Design components This is the first steel arch bridge built by BRO on the Siyom with a load classification of 70R. It has a total length of 100 m and consists of steel superstructure (deck type) on open foundation and 200 mm deck slab along with 75-mm wearing coat. The deck level has a total width of 8.9 m with a 7.8-m-wide carriageway. The deck level has been provided with crash barriers for protection and drainage spout for faster drainage of water. The abutment has been supported on open foundation and steel arch has been supported on anchor foundation. Weathering different storms The bridge construction was executed at a location with perennially heavy rainfall, which persists from April to October; the annual rainfall in the region is about 3,200 mm. To overcome the challenges posed by incessant rains, construction work was executed using tarpaulin sheets and huts of CGI sheets. Stage forwarding of logistics was another challenge because the existing bridge did not allow the movement of heavy loaded vehicles. The approach roads leading to the bridge site passed through landslide-prone areas, which further led to poor traffic conditions. Formation cutting and road-widening works, which were necessary, only worsened the conditions owing to the soft soil. The new bridge site was very close to the existing old Bailey suspension bridge on the downstream. This bridge was a lifeline for the people and troops to the areas ahead of the bridge. Therefore, construction had to be speeded up to keep the communication on this strategic road open. A success story The construction of such a complex bridge structure in a remote area under adverse weather and difficult site conditions, though challenging, has brought out many lessons for the Karmayogis of BRO. It has been a great success story and the study of its features by interested engineers will provide them great insight into bridge construction and design. The completion of bridges accrues many advantages to locals, tourists and armed forces alike. The forward areas in upper Siang Valley ahead of Siyom River are known for their scenic beauty and boast rich flora and fauna. These places remained unexplored by tourists and travellers in the absence of good connectivity. With the completion of required infrastructure, these virgin areas will now be explored by the adventure seekers. In addition, the higher load classification of this bridge facilitates the speedy induction of men, material, equipment and vehicles including heavy equipment, thereby assisting in the rapid buildup of forces in case of any escalation along India’s northern borders. Fact file Project: Siyom Bridge Location: Arunachal Pradesh Year of completion: January 2023 Superstructure length: 100 m Developer: Border Roads Organisation (BRO) Contractor: Project Brahmank, BRO Material used: Steel