Timber Pile Preservative Injection System& Earth Bonding of Reinforcing Cageson Older Concrete Poles

Updated: Sep 10, 2020

PART A: Timber Pile Preservative Injection System

A1: Background

There are many bridges, transmission towers and other structures built on driven timber piles. As they age, it is often difficult to maintain the piles as they are costly to excavate or in some cases excavation is impractical. Transpower has a number of 1930-1960’s era transmission line towers built on timber piled foundations. Generally these are located at or close to river or other waterway crossings. The piles are invariably Australian hardwoods, typically ironbark, black butt or spotted gum. The foundations usually consist of three timber piles on each corner of the tower, a total of 12 piles per tower. Each group of three piles is capped with a concrete block approximately 2 m deep, which in turn is bolted to the bottom of the tower leg.

PART B: Bonding of Reinforcing Cages on Older Concrete Poles.

B1: Background

All new transmission strength concrete poles are manufactured with “integral earthing systems”. This means the pole is built with an earthing system within the pole wall connected to ferrules on the pole surface, to enable crossarms and earthwires to be bonded into the earthing system. The earthing system runs the length of the pole, and ensures the pole reinforcing cage and all accessories on the pole remain at a uniform potential at all times. This ensures a reliable operating and a safe working environment. However some older transmission concrete poles were not fitted with an integral earthing system. To ensure operational reliability and prevent damage to the poles and crossarm bolts, Transpower have retrofitted these poles with an external earthing system. This system consists of a copper XPLE cable connecting the pole top earthwire and crossarms to a ground driven earthing electrode. It does not however connect to the pole reinforcing cage. While an external earthing system has ensured operational integrity, safety issues have emerged during portable earthing. This paper looks at methods to achieve a bond between the external and internal earthing systems.

-Wal Marshall

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