The North Frame Pedestrian Footbridge will span 32m across the Avon River in Central Christchurch, playing a key role in the total pedestrian experience of the Avon River Precinct
Redevelopment Scheme.

Supported by three, 20m deep concrete piles embedded into the dense gravel layer, the bridge construction required driving a 1.2m-diameter open-ended steel casing into the river bed,
excavating the soil from inside the casing, then lowering down a “plug” (concrete mass-filled steel mould).

The plug is driven into the ground by picking up and dropping a heavy hammer, 32m-long, steel cylinder, weighing 25T onto the top of the plug repeatedly, until the desired bearing
capacity is reached. Traditionally, a large piling crane would be used to pick up the hammer and drop it with freefall on the crane rope. However, the required crane became unavailable. Regional Technical Lead –Civil Phil Whilby developed an alternative design proposal to lift the hammer a height of 1m then release it in freefall, repeatedly, without the use of a piling crane.

Phil’s innovative solution involved a low-friction collar, clamping closed around the hammer using a small hydraulic ram. The hammer is lifted using a pair of vertical hydraulic rams, supported from the permanent steel casing. The clamp is then released, causing the hammer to freefall 1m onto the top of the plug. The vertical rams are then lowered, and
repeated. Advantages include improved safety by isolating the operators from the moving plant, reduction in staff required for the piling activity, increased control of the activity improving quality and reduced project costs.

The plug was successfully proven to capacity over two working days, and the ‘hammer dropper’ operated flawlessly.