Hydraulic Failures of Bridges: A case study of Mahad Bridge on Savitri River on Mumbai-Goa National Highway [NH-66] on 3rd August 2016

         1. Railways and Roads

Railways and roadways form an integral part of the infrastructure of a country and play an important role in the economic growth and development of the country.  One can correlate the pumping of blood from the human heart to the different cells in the body employing arteries, in a similar manner the goods, fruits, and vegetables, raw material for various industries, and many more are being transported from the agricultural fields, industries, etc. to the central market in a city through roadways and railways.



    

Fig. 1: Circulatory system along with road network in the country (Source:https://www.britannica.com/science/circulatory-system)

Railways are the cheapest mode of transportation after the waterways. It is an important mode of transportation which is being used for the conveyance of people and goods. Indian Railway is the predominant operator for the operations of railways in India. The national rail network comprises approximately 26,400 km of track over a route of 67,400 km along with 7,325 railway stations. It is the fourth largest railway network in the world.

Fig. 2: A passenger train from Indian Railways

(Source:https://timesofindia.indiatimes.com/travel/as77548525.cms) 

    The railway is one of the biggest industries in the country, which not only enables jobs in the field of engineering but also from the maintenance and service,  management, catering and bedding services, time-table and schedule monitoring,  cleaning, and much more.  

Also, it provides opportunities to the daily wagers and hawkers to earn through different businesses. These people are very much dependent on the railways for survival and livelihood.


Fig. 3: A representational image of construction work on the Indian Railways (Source:https://theprint.in/india/blow/445551) 

Bridges form an integral part of the roadways and railways, bridges are built to span a physical obstacle, such as a body of water, valley, or road, without closing the way underneath. It is constructed to provide passage over the obstacle, usually something that can be detrimental to cross otherwise.

2. Bridges

 Bridge is a structure constructed to cover a physical obstacle created across a stream, a water body, valley, road, or rail without blocking the way below it.  Bridge, spans horizontally between two or more supports, whose function is to carry vertical loads. The conventional form of a bridge is quite simple, it consists of two supports holding up a beam, yet the engineering involved behind in the construction of every bridge requires great precision and higher accuracy, the supports must be strong enough to hold the structure up, and the span between supports must be strong enough to carry loads of heavy automobile. Spans are generally made as short as possible; long spans are justified where good foundations are limited for example, over estuaries with deep water.


    Fig. 4: Seto Great Bridge over the Inland Sea, linking Kojima, Honshu, with Sakaide,  Shikoku, Japan

(Source:https://www.britannica.com/technology/bridge-engineering)

3. Failure of a bridge

The most common causes of bridge failure are

1. Structural and design deficiencies

2. Corrosion

3. Construction and supervision mistakes

4. Accidental overload and impact

5. Scour

6. Lack of maintenance or inspection etc.

    But amongst these causes, hydraulic failure is often neglected or ignored by the  design engineers, who are responsible for the safety and stability of the structure  of the bridge. This article focuses on the importance of the hydraulic failure of a  bridge involving a case study. 

    A bridge on Savitri River on NH-66 in Raigad collapsed on 3rd August 2016. Several vehicles, including two buses, were swept away in the river after the incident took place. From the point of view of a water resource engineer following are the probable reasons for the bridge collapse:-

Fig. 5: Failure of the old bridge on Savitri River on old Mumbai-Goa National Highway (NH 66)

(https://www.dnaindia.com/india/2241023)


Fig. 6: Bird's eye view of the failure of the old bridge on Savitri River (https://www.dnaindia.com/india/2241032)


        From Fig. 5 and Fig. 6 it can be seen that the two bridges are very adjacent to each other, the bridge that collapsed was from British tenure and the other one was constructed recently. Here, the distance between the old bridge and the new bridge is very less, which must have reduced the cost for geological investigations which are prerequisites for laying the foundation of the new bridge. But this resulted in the reduction of the area of flow of water approaching the old bridge which is towards a downstream of the new one. Because of this reduction in the area of flow the velocity of water approaching the piers of the older bridge has risen up. This velocity is quite higher than the designed velocity. The piers were unable to handle such high velocity, and have failed in horizontal direction i.e. in shear failure.

        Also, it is evident from these images that the bridge deck, beams, and part of the bridge pier have been swept away due to high velocity and a portion of the pier is standing in the river, which suggests the foundation of the bridge is sound and it is not a structural failure, rather it is a hydraulic failure.

Fig. 7: Location of the old bridge on Savitri River

(https://www.dnaindia.com/india/2241024)

        From Fig. 7 it is observed that the bridge is constructed on a meandering bend i.e. a curve. It can be noted that at a curve the pressure is more on the bends with larger radii in comparison with bends in smaller radii and thus only half of the older bridge i.e. located on the bends with larger radii has collapsed.

Conclusion

From the present case study, the following points can be derived: 

1. The hydraulic properties of a river play a very important role while designing and constructing a bridge. 

2. If there is an existing bridge in the river the new bridge should be planned at some distance away from the existing one. 

3. Proper maintenance and rehabilitation work should be executed on the older bridges, if not then they should be either demolished or should not be used.

4. Construction of a bridge on a meander should be avoided.


Want to know more about MIT College of Railway Engineering and Research Barshi, MH.

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