Recent earthquakes in Christchurch had highlighted the need for earthquake strengthening across the board and Cargills Monument was identified as being at high risk and given priority for stabilisation work. The limited space around the monument and the complexity of locating the drill at the desired micropile anchor location in the cornerstones next to the vertically rising monument meant that a small, highly mobile rig was most required for the project. Large equipment that caused excessive vibration was also not a suitable option due to the risk to the structure of the monument.
Self drilling anchor micropiles were chosen as the only practical and cost effective solution to anchor the monument to the existing substrata. The small size of the Pacific Fox drilling attachment allowed it to be mounted on a small 5.5 tonne digger with the drill mast able to be easily manoeuvred around the monument for micropile installation with precision. The rotary percussive drill head produced minimal vibration while advancing the self drilling anchor micropile and this posed no threat to the monument integrity.
Corner stones were first removed and the flat topped corner blocks then cored to allow the installed anchors to be hidden completely within the structure. Four Minova Weldgrip galvanised R38 Self Drilling Anchor Micropiles each six metres long were installed using a Pacific Fox Cub 250 drilling attachment. The angled top corner stones were then re-positioned.
The simplicity of the self drilling anchor micropile system meant that it was a much more cost effective implementation than traditional cased hole anchoring. The solution was implemented smoothly and on time and the whole project came in on budget. Spiire were pleased with the final result.