Challenges in Construction of Sharavathi Backwater Extra-Dosed Bridge
Er. Rakesh Kumar Chowbey - General Manager (Structrures) Dilip Buildcon LimitedCase Study
Construction of Extra -Dosed major Bridge across Sharavathi back waters and approaches between Ambargodu and Kalasavalli of NH-369E in the state of Karnataka.
Date of Commencement : 12-12-2019
Date of Completion : 22-11-2024
Project Cost : 370.87
Salient features:
Total length of Bridge : 2125 M
Clear carriage way : 11.00 M
Width of footpath : 1.5M on both side
Over all Deck width : 16.00M
Extra -Dosed Bridge Portion : 1x104.5+3x177+1x104.5=740M
Balance Cantilever Bridge : 3 modules of (90+135+135+90) =1350M
End Span : 35M cast in situ Box Girder
Foundation : Cast in situ Pile Foundation with rock socketing, Pile Dia -1800 mm
Substructure : A shaped Pylon for Extra dosed bridge, Twin circular Piers – Balance cantilever Bridge
Super Structure : PSC Segmental with variable depth single cell box girder
Upper Pylon : Rectangle shape, 24.5 M long & 4-Degree outside inclination with six stay cables connected to deck with two planes and semi fan fashioned
Picture Courtesy: Dilip Buildcon Limited
Challenges
Unable to cast bridge foundations due to high water levels in the reservoir.
The Proposed bridges was located at the upstream side of the Linganmakki Dam where the depth of standing water varies from 20m to 40 m, and the low water level is 540 m, which is always maintained for power generation by KPCL (Dam Authorities).
The pile cap level is fixed at 540.00 m based on the average level over the last 20 years so that the contractor gets the maximum working period and can work on piling and pile caps for at least 5-6 months in a year.
Accordingly, the contractor planned to complete all the pile caps during the period from April to June 2021. Unfortunately, the water level during the period from April to June 2021 was above 542 m and did not go lower than 540 m. The sacrificial beam and slab method was adopted for construction of the pile caps in the river bed, which requires the water level of the reservoir to be below 540 m.
For a continuous period of 650 days (from 19.07.2020 to 30.04.2022) the water level is above 540 m, due to which the contractor was unable to cast the pile caps work.
Picture Courtesy: Dilip Buildcon Limited
Overcoming high water level issues
The Dam authorities agreed to maintain the water below 540 m in the year 2022 for a period of only 40 days instead of 3 months. Accordingly, the contractor prepared a resource loaded micro works schedule for casting of pile caps in 40 days. The contractor casted the sacrificial beams, slabs, pile caps pre-fabricated reinforcement cages in advance and kept them ready at the river bank, and also mobilized additional resources likes workmen, cranes, barges, pile axile cutters etc for removal of excess concrete in the piles.
Finally, the reservoir water level reached below 540 m on 01-05-2022 and the level was maintained up to 19.06.2022. During this period, the contractor had completed the all the pile caps successfully.
Picture Courtesy: Dilip Buildcon Limited
Pile cap reinforcement fixing and concreting
Minimum water level required for erection of segments
As per the Bridge Construction Methodology, the super structure is Precast Segmental variable depth single cell box girder. Precast segments are cast at a casting yard and after achieving the strength, they will be shifted to the river location using low bed trailers and barges.
For lifting the first segment, the minimum water level required is 545m as shown in the schematic drawing below. The segment barges have to come up near the pier and above the pile cap. This is only possible when the river water levels maintain above 2m of pile cap to flow the barge, which is 545 m.
During the period October 2023, the EPC Contractor was making arrangement for erection of the segments of spans P3, P5, P6 and P7. Unfortunately, the river water levels were are less than 545m and will touch 545 m after the monsoons i.e August -2024. As per the current situation with low water levels, the contractor informed that it was unable to start the segment erection works for the spans of P3, P5, P6 and P7 until water levels maintained 545m.
Picture Courtesy: Dilip Buildcon Limited
How to overcome the situation
To overcome the above critical situation and enable the segment erection works at P3, P5, P6 and P7 spans when the water level was below 545 m, the contractor’s design team developed a scheme for pushing / sliding of the segments to above the top of pile caps from segment barge and later segment will be lifted from pile cap.
