BAUER Foundations Canada (BFC) was contracted by Shell to construct a 40m diameter secant pile wall 18m deep. The circular arrangement of the secant pile wall will form a shaft which after excavation will serve as an Emergency Dump Pond (EDP). This new design for the EDP reduces the footprint required compared to a conventional EDP with relatively shallow excavation. Another advantage is that problems occurring for cleaning of the embankments are eliminated by the vertical walls of the secant pile. wall.
As the design of the excavated shaft is based on “hoop” stresses, accurate alignment of the piles and pile verticality are very critical to the stability of the excavated shaft. To ensure accurate positioning of the piles, a reinforced concrete circular guide wall was constructed by Bauer prior to commencing piling.
The piles were 1000mm in diameter and were cased to toe level (i.e. 18m below ground level) to ensure plumbness within 0.5% and also provide support during boring. Two BG28 drill rigs were used for installing the piles.
The ground conditions consisted in general of a layer of loose to very loose Sand fill, underlain by a loose to compact Pleistocene Sands containing gravels and frequent cobbles and boulders which in turn were underlain by very dense sand of the McMurray formation. Top of McMurray formation varied between 15 to 20m below existing ground level.
The secant piles were installed in a sequence of primary and secondary piles. Secondary piles were installed through concreted primary piles and were reinforced over their full length. Due to the circular shape of the wall, the primary and secondary piles had the same concrete strength of 35MPA.
Drilling of the piles was performed under water to stabilize the pile base and prevent any inflow of sand material into the pile bore during drilling. Concreting of the piles was by means of tremie pipes.
The project was completed 2 weeks ahead of schedule.