Kolia Bhomora Setu
The bridge helps the development of the North-East India. The Brahmaputra not only splits the state of Assam along its length but also cuts off the seven sister states of the North-East from the rest of the country.
The river having a catchments area of 280,000 km2 flows in the most unpredictable manners, occupying 12 km to 16 km of space at places and poses a challenge to the bridge engineers. Despite the pioneering efforts of the Railways at Saraighat, the Northeast continued to suffer from the long-felt lack of transportation network. Around 25 years later, Indian Railways have turned yet another dream into reality. The 2nd Brahmaputra Bridge has greatly accelerate march of the North eastern states towards 21st century.
Brahmaputra, one of the longer rivers in Asia with a total length of 2880 km – 920 km of which lie in India – had been bridged only at one place, the only link between the North Eastern states with the rest of the country. This 1.3 km rail-cum-road bridge, completed in 1962, was also constructed by Indian Railways. Road Bridge, Guwahati.
Out of 27 foundations, 25 are open wells. The North abutment and pier No.1 are on raft foundations. The wells have been sunk to 56 m below L.W.L, mainly from scour and seismic considerations. The strata encountered were dense sand mixed with pebbles. During sinking, obstructions in the form of wooden logs and boulders were overcome; this was possible with armoured well curb having shoe-brackets and 12 mm thick steel plates.
With elaborate monitoring at each stages of sinking, it has been possible to position all the wells within permissible limits of tilt (1 in 80) and shift (150 mm) each well costing of Rs. 8 million and involving 5,000m of concrete was completed within an average period of 90 days.
Two of the well foundations required launching of Caissons in depth of water ranging from 8 m to 12 m 290 t of steel was required to be fabricated for these caissons. Water-ballasting of sinking-pontoons. To ferry across the construction materials, for simultaneous working on a number of foundations, a large fleet of marine craft was deployed. it consisted of tugs, 20 barges and a number of country boats. For concreting, batching plants of 950 m3/day capacity were installed. nine SLD cranes, three crawler cranes and stand cutter – dredger pumps were used for sinking of well foundations.
Pier No.1 founded on a 2 m thick R. C. C raft, which is held down with 96 rock-anchors on North and 6 R.C.C bored piles of 1.5 m dia on South. These piles were driven with CASSAGRADE pile-driving equipment.
Substructure is hollow circular R. C. C piers of 12.5 metre height constructed with concrete of grade M-300. It being a navigable river, the height of the piers has been determined to cater to the navigational clearances. R. C. C feeders have been provided to protect the piers from any possible damage due to impact of stray vessels. The pier-cap is made 6.5 m square, to facilitate the construction pier-head.
While sinking the well for pier No.2 near Bhomoraguri hill, boulder stratum at a depth of 26 m below L.W.L was met with. Services of deep-sea divers were utilized for sinking the well through this stratum, by under-water blasting.
This well has been secured by 1.5 m diameter R.C.C bored piles along with protection work in the form of a 60 m diameter boulder apron.
The distance of 120 m between consecutive pier-center is spanned by two 52.5 m long balanced cantilever from the Piers and a 15 m long suspended span in the middle. The Cantilever portion is a single-cell P.S.C, Box-girder in concrete of grade M-425 using special cement, post- tensioned with 90 nos. of 24x 8 mm dia, H.T.S cables. The cantilever arm has been constructed in 11 segments of different lengths. Diaphragms have been provided at pier head and 11th segment to impart tensional rigidity to the cantilever arm. Considering the unpredictable nature of the river and short working season available, cast-in-situ method of segmental construction by Traveler from-work has been preferred to pre-cast segment launching system. The entire cantilever construction having 275 segments, has been constructed at and average rate of 7 m/day which favorably compares with internationally accepted standards.
For the construction of the cantilevers, the BRIDGE_BUILDER system has been imported from Norway for the first time in India. An average cycle time of five days has been achieved here for segment construction, as against the accepted international average of seven days. FREYSSINET system has been adopted for pre-stressing. Careful detailing of reinforcement and a cable profile was necessitated to achiever good quality and to minimize construction time. In order to place concrete through dense reinforcement and cable assembly, use of super plasticisers has been made. For manufacture and placing of large volumes of high grade concrete, electronically controlled automatic ELBA batching plants along with concrete pumps, were imported from West Germany. The total installed concreting capacity was of the order of 1000 m3/day.
The suspended span consists of two R.C.C girders placed on the tips of cantilevers along with a cast-in-situ R.C.C deck. The purpose of providing the suspended span is to minimise damage to the pier during and earthquake, by structurally isolating the piers. The R.C.C girders of suspended spans are cost on the cantilevers and launched in position.
The bearing at the interface of a cantilever and a suspended span is specially designed to cater to complex seismic movements. These consist of a combination of spherical knuckle and P.T.F.E stainless steel sliding surface for free end bearings and spherical knuckle cum electrometric pad for fixed end bearings. Electrometric type of expansion joints are provided for to cater to large seismic movements and to give a smooth riding surface. The bridge deck is provided with 25 mm thick layer of Mastic Asphalt.
Guide-Bund and South approach Bank
In the absence of any firm control point on the South Bank at the bridge alignment, the river had a tendency to shift towards the south. To regulate this tendency and to guide the flow of river within the waterway provided, a massive Guide Bund of 2 km. length along with a 1.7 km. long approach bank was constructed after extensive hydrological studies. The Guide Bund has a 3 m thick launching apron made with G.I.Wire crates filled with man-size boulders. Its width varies from 78 m at mole head to 54 m in the shank portion. The total volume of work involved 820,000 m3 boulder work and 1.9 million m3-earth work using local river sand.