July 5, 1986 is important date in history of pre-stressed concrete bridges built with Russian design and French Freyssinet structural construction technology. Our hundred year old house at Bondir, Calapur with thick walls of mud, located three kilometres away from Old Mandovi bridge shook for a minute that day, like it was a mini earthquake. We also heard the loud ‘thud’ clearly, reverberating with the collapse of the two spans of the bridge. The shock waves had definitely travelled fast through soft sandy soil. “Did you hear it?” I asked my mother. “Yes, may be a big banyan tree has come down in the village,” she replied.
This article is a grim technical reminder of 30th anniversary of collapse of Mandovi or Nehru Bridge on July 5, 1986. It was a major engineering disaster in India. Science of building bridges has undergone a revolutionary change in last 30 years. But no lessons were learnt in Goa – except changing the design from T-shaped cantilevers to ‘simply supported’. Government, people and engineering professionals have forgotten the report of late Justice Devidatta Mangesh Rege (1923-2013) commission appointed by government of Goa in 1986 to conduct a judicial inquiry into the causes of the disaster.
A Amtrthanathan, former assistant executive engineer, TWAD Senior Civil Engineering Nagercoil, India later made a study of Mandovi Bridge collapse. He observed: “The main reason for the corrosion thin layer of deck “in-situ” concrete wherein the pre-stressed High Tensile steel cables were embedded and the fact the most of the cables were not cement grouted to protect the wires and seal of the voids between the individual HT wires (a bundle of 12 wires of 7mm diameter from a cable) and the sheathing pipe through which the cables were threaded. Sixteen years of water seeping through the porous in-situ concrete deck, past the pre-stressed cable, corroded them leaving only rest powder in many places and thin corroded HT wire sections to hold the precast concrete segment in place. There were complains in areas under the bridge during the rains just like roofs leaking in modern RCC buildings”.
What were the causes of collapse: “The most portable collapse mechanism was that of the pre-stressed cable above pier 1 (Panaji side) were totally corroded. This could be seen in the inspection at the top of the inclined deck, as well as cable that were lying below – adjacent to the pier 1. The five precast concrete segments on the Porvorim side cantilever attached to pier 1 collapsed. Therefore on the Panaji side, the precast concrete segment along with the massive counterweight concrete block caused an unbalancing force on pier 1 and the pier bars snapped at one lift below the cantilever topping the unbalanced load onto the ground, crushing amongst people in a boat below.
While the Porvorim side cantilever precast elements of pier 1 came down, they also pulled down the adjacent precast segment of pier 2 on the Panaji side cantilever, as they were linked by means of the deck concrete wearing coat and bitumen coat and bitumen top. Obviously, the pre-stressed cables above pier 2 were also corroded. This resulted in unbalancing pier 2 that also snapped at one lift below the cantilever which topped and somersaulted into the water below on the Porvorim side.
“Rege commission had asked Central Electrochemical Research institute to examine the bridge material for corrosion. They were shocked to find that saline beach sand was used in the concrete mix and salty river water was used for curing. One of the most important finding of Rege commission was taking utmost care in correct RCC mix, grouting of steel cables and prevention of corrosion.
The bridge was inaugurated without any mandatory ‘load testing’. When the concerned PWD chief engineer was questioned by Justice Rege, he had admitted that he was afraid that load testing could have caused collapse of the bridge. Still it was a miracle that the bridge certified without load testing gave service for 17 years. The Zuari Bridge built with similar design is much more corroded and somehow has survived despite indicating high stress and vibrations.
Bridges in Goa are subject to a highly humid, tropical, salt rich, corrosive environment and are not built in accordance with science of deteriology. Bridge deteriologists examine the causes of wear, tear of bridges due to physical, chemical, atmospheric and biological factors and suggest the remedies. Zuari, Borim bridges are already endangered.
Any RCC bridge in Goa needs to pass the standard deteriological tests. Preventive measures against salinity induced corrosion need to be built in. Corrosion resistant cement needs to be used. Novel corrosion inhibitors need to be incorporated. The water used for RCC mixture and for curing of structures need to be totally free from dissolved salts and chlorine. Curing period should not be shortened so that final structure becomes rock hard without any micro-cavities. Bridges including all metal joints need to be protected with outer thick coating of anti-corrosion paints. But 30 years after Mandovi bridge collapse, it is sad to see that the Rege commission report still remains without implementation.