Ngaio gorge retaining walls

Webster Drilling installed 50 horizontal Macalloy 1050 MPa DCP anchors to depths of 26 m for an NZTA retaining wall. Overburden drilling required 200 mm temporary casing through 18 m of loose colluvium and scree containing clays, boulders and voids. A 150 mm Down-The-Hole (DTH) hammer was then used to extend the boreholes into solid rock for the anchor foundations.

The installation process utilised prefabricated anchor bars fitted with tremie tubes and Tube-à-Manchette (TAM) pipes. Primary grout was injected as the 200 mm casing was retracted to prevent borehole collapse. Once the grout had cured, a pneumatic packer was used to fracture the grout body at 1 m intervals. Post-grouting was carried out at pressures of up to 3,450 kPa, with CCTV and packer testing used to verify full structural encapsulation.

All drilling and grouting operations were carried out from below road level using proprietary man-portable and crane-portable rigs. This setup eliminated the need for large road-level machinery and traffic restrictions. As a result, full two-lane access was maintained along the arterial route, allowing full-day work shifts and reducing the overall project construction timeline.