Extraction of Precast Piles and Secant Piles up to 27m-Paper

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The paper describes removal of driven precast piles and secant pile walls from a former concrete batching plant site in North Melbourne, ahead of construction of an underground rail tunnel. The piles, installed about 15 years earlier, extended 23–27 m to siltstone. Their high shaft friction made extraction difficult, while the pile joints had an estimated tensile capacity of only about 500 kN, so extraction forces had to be controlled to avoid breaking them.<br /><br />Initial trials using augers to pre-drill beside the piles were unsuccessful. The adopted method used a 610 mm steel tube fitted with high-pressure bentonite jets. The tube was vibrated down over each pile, with bentonite helping to loosen and lubricate the surrounding soil, before the pile was lifted using welded lifting lugs. Forty piles were extracted, although some could not be removed because of obstructions, pile breakage, or inability to drive the tube to full depth. Secant pile walls were removed by casing and coring; high torque sometimes sheared the piles into sections.<br /><br />Inspection of the extracted piles provided information on their condition after prolonged burial. Most of the 68 exposed joints showed negligible corrosion. Two had significant corrosion to non-structural collars, with isolated pitting of up to 2 mm on pins. Concrete carbonation was approximately 1 mm, with no visible reinforcement corrosion or concrete deterioration. The findings suggest the pile concrete and steel generally performed well in the site conditions.<br /><br />Other observations included piles tending to curve toward their rougher face and good verticality, estimated at around 0.6% or less. Overall, pre-drilling combined with vibratory installation and bentonite jetting proved an effective extraction approach.

Keywords

precast piles

secant pile walls

pile extraction

bentonite jetting

vibratory installation

shaft friction

pile joints

corrosion inspection

concrete carbonation

North Melbourne