The completion of required infrastructure by BRO allows the adventure seekers to explore the virgin area of the North East.Construction of bridges in North Eastern region has always been a great challenge for any construction agency due to terrain and weather vagaries. With more than 900 bridges constructed by BRO along the border areas in North Eastern states in last 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 by BRO in Arunachal Pradesh, which was inaugurated on 03 January 2023 by the Hon’ble Raksha Mantri Rajnath Singh.Majestic locationPangin is a small town and the district headquarters of Siang district in Arunachal Pradesh located at a distance of about 60 km from Pasighat. The settlement is at the junction point where river Siyom meets river Siang. It is also the home of the Adi tribe of Arunachal Pradesh.The mighty Siyom river is a tributary of the Siang (Bramhaputra). River Siyom 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. The Saje river is the most prominent of its several tributaries. The Mouling National Park is located on the East bank of the river. The river passes through the district capital Aalo and finally flows into the Brahmaputra, 50 km above Pasighat. The Siyom river has a length of about 170 km.Dazzling crossoverAnyone travelling on the Along-Yingkiong road has to cross the Siyom River. Therefore, it is important to have a suitable crossing place on the river to reach the areas ahead of Panging. In 2021, BRO constructed a 300 ft long Bailey suspension bridge of load class-18 R which was temporarily erected at the site post an accidental collapse of the existing bridge caused by an over loaded vehicle attempting to cross the bridge.The existing bridge was unfit for move of heavily loaded vehicles carrying construction material, heavy machines, artillery guns and other operational equipment of security forces and thus, the local administration and the local army formations proposed the requirement of constructing a robust and a permanent bridge that could allow passage of vehicles carrying heavier loads. BRO thus commenced the construction of this arch bridge over the Siyom River, which got completed in December 2022 and was opened for traffic on 03 January 2023.Design componentsThis is the first steel arch bridge built by BRO on the Siyom river with load classification of 70R. The bridge 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. Deck level has been provided with crash barriers for protection and drainage sprout for faster drainage of water. The abutment has been supported on open foundation and steel arch has been supported on anchor foundation.Weathering different stormsThe bridge construction was executed at a location which has a perennial problem of heavy rainfall. The heavy rainfall persists from April to October, and the annual rainfall in the region is about 3,200 mm. To overcome the challenges posed by incessant rains, the 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 moving of heavy loaded vehicles on it. The approach roads leading to the bridge site passed through heavy landslide prone areas which further led to poor traffic conditions. Formation cutting and road widening works which was a necessity only worsened the conditions due to the soft soil.The new bridge site was very close to the existing old Bailey suspension bridge on the downstream. This existing bridge was a lifeline for the people and troops to the areas ahead of the bridge. Therefore, the construction had to be speeded up to keep the communication on this strategic road open.ConclusionConstruction of such a complex bridge structure in a remote area, under adverse weather and difficult site conditions, though challenging, has brought out a large number of lessons for the Karmyogis of BRO. It has been a great success story for BRO and study of its features by interested engineers will provide them great insight into bridge construction and design.The completion of bridges accrues a number of advantages to the locals, tourists and armed forces alike. The forward areas in upper Siang Valley ahead of the Siyom river are known for their scenic beauty and boast of rich flora and fauna. These places remained unexplored by the tourists, travellers and adventure seekers in the absence of good connectivity.With completion of required infrastructure by BRO, these virgin areas will now be explored by the adventure seekers. In addition, the higher load classification of bridge facilitates speedy induction of men, material, equipment, vehicles including heavy equipment thereby assisting in rapid build-up of forces in case of any escalation along northern borders. The Siyom bridge is a special project and will continue to be a source of inspiration for BRO Karmyogis for years to come.(Contributed by: Border Roads Organisation)