● Central – Wan Chai Bypass – Central Interchange: Cost Saving Design of Bridge A Foundation

WT Chan & Associates Limited is appointed by Leighton Contractor (Asia) Ltd. to carry out the cost saving design of Bridge A foundation for Central – Wan Chai Bypass (Contract No. HY/2009/18). Bridge A is a two lanes access ramp connecting the existing Rumsey Street Flyover Extension in front of Four Seasons Hotel.

The conforming design of the Bridge A foundation comprises of 14nos. of 1800mm diameter bored piles penetrated into subsurface stratum with SPT-N value greater than 200; and the piles length are between 52m and 76m. The 1800mm diameter bored piles are difficult to construct due to limited working space. Alternative schemes such as minipiles, socket H-piles and driven piles are considered to be not feasible. The shemes of minipiles and socket H-piles are rejected due to the deep rock head level (>100m below ground level). The scheme of driven piles is rejected because the installation method is not allowed in this project.

Our design team finally comes up with the best solution for the cost saving design (CSD) of the foundation system. The CSD is to replace the bored piles with small diameter friction bored piles and post grouting technique at the pile toe. The small diameter friction bored piles used steel H-piles as the reinforcement instead of steel cages to provide higher lateral strength. The final scheme of the CSD contains 30nos. small diameter friction bored piles penetrated into the stratum with SPT-N value greater than 100; the piles length are approximately 50m.

Statics pile load tests were conducted on 3nos. of piles; the results indicated that the behavior of the small diameter friction bored pile with post grouting technique at the pile toe can compare to an end bearing bored pile. The total settlement and the residual settlement are much smaller than BD requirement for the end bearing bored piles.