Dynamic Pile Testing at the Five Mates Bridge-Paper

Paper

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The Five Mates Bridge in New South Wales was founded on driven, 550 mm octagonal prestressed concrete piles. The long abutment piles comprised a 24 m section and a 12 m extension joined by a glued steel box splice. Driving through stiff clay required high impact energy and produced substantial tensile stresses.<br /><br />Dynamic testing of the single-piece pier piles showed high tension and signs of microcracking, but no confirmed structural damage. After the abutment piles were spliced and driving resumed, testing indicated progressive damage near the splice. Excavation confirmed open cracking and spalling about 100 mm below the bottom of the steel box. Similar damage occurred in multiple piles, which were ultimately replaced with steel tubes.<br /><br />Subsequent WEAP analysis indicated that the massive splice increased pile impedance, raising stresses below it. The damage was likely associated in part with tensile stress acting within the prestress transfer length, where the concrete was not fully protected by prestress. Reflections from the splice were assessed as a minor contributing effect.<br /><br />The paper proposes leaving the lower pile section unglued in the splice. This would make the splice transfer compression but not tension, reducing tensile stress at the vulnerable section. However, such a connection would not provide tensile or moment capacity and could cause alternating small and large pile sets during driving; a cushion may be needed to prevent concrete impact.<br /><br />Where tensile capacity must be maintained, the author recommends further testing of a longer glued splice or considering an end plate and wedges on the prestressing wires to reduce the prestress development length.

Keywords

Five Mates Bridge

New South Wales

prestressed concrete piles

glued steel box splice

dynamic testing

microcracking

pile driving

tensile stress

WEAP analysis

prestress transfer length