Atherley Narrows Piers 3 & 4

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Mini Pile Project

Upon installation of bridge caissons, a karst formation was discovered in the underlying rock. To develop the required structural support in the caissons, a mini pile support system was specified. Six newly constructed bridge caissons were affected by this situation.

To access the work over the water, ACT designed and installed a complete access platform system utilizing barges to encompass the work areas. Following the work platform installation, cofferdams were installed around each caisson location. With the cofferdams in place, ACT was able to drill the mini piles through the till into the limestone and through the karst formation into competent limestone.

24 mini piles, 70 feet deep, 9-1/2” diameter were installed with 2 No. 16 (2” diameter) high strength anchor bars 70 feet long inside each mini pile. Prior to installation of the mini piles, the karst zone was pressure grouted and sealed with a low mobility grout.

A reinforced concrete pile cap was constructed around each caisson to connect the mini piles to the caissons.

All the work was performed on a watercourse in a shipping lane. The Coast Guard only allowed a 12 foot encroachment into the shipping lane.

All drill cuttings and spoils were pumped through geotextile filter bag and into a settlement pond.

The river bed contained historic aboriginal fish weirs over 5,000 years old. Extreme care and caution was exercised during construction to protect these historical artefacts.