Well Foundations for Bridges are Obsolete

S.A.Reddi, Fellow Indian National Academy of Engineering

Introduction

Bridge 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.

Well Foundations for Bridges are Obsolete
Figure 1: Brahmaputra bridge foundations No. 17 & 18
Pre-cast RC bored piles of 2.5m diameter was first successfully realized in India for the old Thana Creek Bridge constructed in 1960s. For the 13 Km long Saudi–Baharain Causeway large diameter pre-cast pre-stressed bored piles were adopted. By 2005, piles of 2-3 m dia, upto 120m deep are extensively used in the rest of the world. These new techniques eliminate complicated weather-dependent and risky operations in water. They have reduced the delays considerably and minimized the technical and financial risks. The development of modern techniques has considerably reduced quantities of materials used for foundations as well as energy consumption and environmental impacts.

Well Foundation Problems at Brahmaputra Bridge at Tezpur

The 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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