BOTTOM DRIVEN PILING

Bottom Driven Piling : Float Viaduct Bridge renewal, Carstairs, Scotland, 

Description of the works : As part of the West Coast Main Line upgrade the Float Viaduct bridge needed to be replaced. Carillion Infrastructure Management Limited were appointed as main contractors on the £5 million project with a brief to demolish and rebuild the bridge within a five day possession.  The success of the project depended on having the foundations completed prior to the bridge being demolished and reconstructed.  Conditions were among the most challenging Branlows bottom driven piling teams have ever encountered. By adopting an innovative approach to the problems, both practical and theoretical, all were overcome to provide Railtrack with a successful solution.

 
250 x 273mm diameter steel cased concrete Cast-in-situ bottom driven piles installed to depths of between 15 to 28 metres to support three bridge support piers.  Branlows were actively involved in the feasibility and detailed design stages of the project right through to the installation of the piling works.

• Particular site issues : Headroom restrictions : All the driven piling works were carried out underneath the existing live railway bridge with headroom restrictions of less than 4 metres.
   
• Environmental : The foundation works were carried out on either side and in the centre of the River Clyde which is considered to be one of the best game fishing rivers in Scotland. Consequently the piling works had to have minimal impact on the river.
   
• Programming constraints : The new abutments had to be in place before the critical possession period and due to land procurement issues and ground obstructions the piling works were accelerated.  Seven-day, 24-hour working with up to four rigs were required.
   
• Restricted access : The site is approximately I km from the nearest country road and access was created for our bottom driven piling teams by the formation of a temporary road through a farmer’s field.  River levels had been known to rise by up to 5 -6 metres.  Our tracked rigs had to be removed on occasions for short periods of time.
   
• Ground problems : Major obstructions were encountered, both natural and unnatural, at near surface and at depth. Standard driving systems had to be modified to increase pile robustness.
   
• Testing : Involved static, dynamic and integrity testing of the piles.  A strict testing schedule determined the SWL of 425kN. Average settlements of 1.6mm and ‘l.3mm were recorded at working load and twice working load respectively.  On removal of load, defection recovery of around 90% was recorded suggesting the majority of movements were elastic compression.
   
• Economy and practicality : Other methods were considered at the feasibility stage of this project but were ruled out as impractical or uneconomical.  The only other method that was seriously considered was to utilise a drilled mini pile but this would have nearly doubled the cost of the project and would have had a major environmental impact due to the use of grout and air/water flushing techniques.  One of the main advantages of using this system is that it is a dry method of piles that utilises a closed ended permanent steel cased lining for the full piling bore.
   
• Ground conditions : The ground was found to comprise of loose sands in the upper 12 metres (alluvium) but became more dense below 15-16 metres.  Significant obstructions were encountered in the upper 3-4 metres and also in a band of ground between 8-12 metres due to substantial boulders arid cobbles.  High water levels were present throughout and on numerous occasions flood levels rose significantly above the drillers platform level.