Construction and Verification of Large Bored Pile for Bridge Foundation Through Deep Weak Alluvial Soil Layers and Artesian Aquifer-Paper

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The Mangaonua Stream Bridge project required 20 large-diameter bored piles through up to 14 m of weak alluvium, multiple artesian aquifers and deeper peaty soils. The principal construction challenges were providing a stable working platform, controlling artesian pressure, preventing bentonite loss and completing pile-base grouting.<br /><br />A 2.5 m-high temporary platform was built over the soft gully deposits. Its stability and the effects of crane and piling-rig loads were assessed using staged Plaxis 2D modelling, calibrated against monitoring data. Although substantial settlement and ground movement occurred, predicted pile deflections remained within design limits, so further ground improvement was unnecessary.<br /><br />Artesian pressure was controlled by maintaining bentonite inside casing extended above ground. Bentonite leakage occurred when the permanent casing ended in permeable alluvium and the elevated bentonite column imposed high lateral pressure. Extending the casing into a low-permeability layer resolved the problem.<br /><br />Base grouting was specified to improve the variable soil beneath the piles. When the grout-tube valves could not be opened in two piles, alternative injection holes were drilled—through the pile for one and via an inclinometer pipe for the other. Both piles ultimately met the specified grouting criteria. The experience highlighted the need to crack the pile base within 24 hours of concreting to avoid more difficult remedial drilling.<br /><br />The paper emphasizes staged construction analysis, careful monitoring and close geotechnical supervision, including checks of bentonite levels, soil samples and pile-base cleanliness. It concludes that preloading weak ground, correctly terminating casing in low-permeability material and timely base cracking are key to reliable pile construction in complex ground conditions.

Keywords

Mangaonua Stream Bridge

large-diameter bored piles

weak alluvium

artesian aquifers

peaty soils

temporary working platform

Plaxis 2D modelling

bentonite leakage

casing termination

pile-base grouting