Abstract:
Naveen Govindaraju*, Afcons Infrastructures Limited, naveen.govindaraju@afcons.com
Sunil S. Basarkar, Afcons Infrastructures Limited, sunil.basarkar@afcons.com
Ramesh Kumar Shrivastava, Afcons Infrastructures Limited, ramesh.shrivastava@afcons.com
ABSTRACT
Reservoir across Volta River located at Ghana, West Africa is one of the largest man-made waterbody in the world, measured in terms of area. This reservoir stretches from Akosombo Dam in the south to the northern part of the country and it is a major source of Ghana’s Electricity.
Authors’ firm have executed probably, the deepest Bored Cast-In-Situ Raker Pile (70m) in the world. The design and construction of 300m long single standard gauge railway bridge was never an easier task. The Volta bridge was designed as 5-span simply supported structures between abutments A1 and A2. Span between piers was 60m. Each pier was earlier planned to be supported on vertical piles of 2.0m diameter given the fact that the peak ground acceleration (PGA) was 0.16g. However, during designing, new version of code came into force, making design PGA value of 0.35g mandatory. Entire piling philosophies underwent complete revamping, and it was decided to go for raker piles to counter very high lateral forces arising from seismic event. The piles were revised to 1.6m diameter with raking of 1:5. Abutments A1 and A2 were supported on 6 vertical piles, while piers P1, P3 and P4 with 4; and P2 with 6 raker piles, respectively.
Raker piles supporting piers were modelled in Midas Civil© software to obtain second order effect arising from lateral deformation of piles. Pile sections were checked for its ultimate capacity in flexure and shear at ultimate limit state. Stresses and crack width were checked at serviceability limit state using Oasys Adsec© software.
Carefully worked out lateral soil/rock stiffness values were assigned to the piles. While evaluating the seismic forces, the site is classified according to Ghana Building Code Ed. 2018. The bridge, under the revised seismic records of 2018 was located at Seismic Zone 3 with horizontal design peak ground acceleration (ag) of 0.35g and soil profile S1.
The importance factor (I) of 1.0 corresponding to importance class of II was considered as per Ghana Building Code. The site coefficient of 1.0 corresponding to soil profile type S1 was considered since piles at all the pier & abutment locations were to be terminated at hard rock comprising metamorphic gneiss with UCS in the range of 20 to 70MPa. Shear keys were provided as an additional safety measure in the event of failure of bridge bearings. Hence, shear keys were also designed for PGA of 0.35g with consideration of behaviour factor (K) of 2.0 as per Ghana Building Code.
Construction of Raker pile system was a surmountable challenge by itself. Planning of activities for temporary working platforms, driving of inclined liners, drivability in sand, mounting and boring by Reverse Circulation rig, cage lowering, concreting, etc., had to be executed with planned logistics and execution methodology. All these activities were to be performed while maintaining stringent safety standards in marine environment.
The Volta bridge now stands completed and is an epitome of design and execution challenges and how they were overcome.
Contents of the proposed paper is planned to provide a background of the project and factors that lead to Raker pile foundation as a viable option. The paper is also planned to cover analyses of the raker piles, construction methodologies and how critical bottlenecks during their execution were overcome.
*Corresponding Author