S.A.Reddi, Fellow Indian National Academy of Engineering
IntroductionBridge foundations are the most complicated and difficult to construct. Unexpected difficulties cause delays, extra costs and revision of designs due to altered situations. Loss of human lives was normal rather than exception. In the Sixties, more than 50 lives were lost due to the accident during pneumatic sinking operations for well foundations of Mahanadi bridge in Orissa. For Kali Bridge at Karwar in Karnataka, pneumatic sinking was required for inspection of the founding surface of the wells. Apart from delays and extra costs, large number of workers suffered caisson disease. Due to difficulties in well sinking, two contractors left the job and the third took more than 5 years to complete the well foundations. Alternative construction techniques and equipments have emerged. With the introduction of advanced piling equipment, large diameter piles up to 3.5 m dia are easily realized at a fast rate, with a significant reduction in cost and material quantities. Bridges elsewhere are now constructed with pile foundations.
Figure 1: Brahmaputra bridge foundations No. 17 & 18
Well Foundation Problems at Brahmaputra Bridge at TezpurThe bridge was more than 3 km long with 26 spans of 120 m and 2 shore spans of 70 m each, founded on 12m dia Wells. Sinking well No.2 to full depth was not possible due to bouldery strata Despite best efforts, the well only went down by 35.25 m after three seasons and at extra cost! The well was plugged at RL 32.075 m and 5 nos 1.5 m dia RCC bored piles (25-35m) were provided to anchor the well, one in the middle through the dredge hole and 4 outside at the four corners. Further a launching apron of crated boulder 3 m thick was laid making a circle of 60 m dia around the well, at RL 61.00. These extra works caused further delay in the comp- letion of the substructure works.
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