During the slope stabilisation work on Ohau Point, GeoStabilization International’s engineers, on one of our regular inspection and monitoring helicopter flights, identified a large semi-detached column of rock on the Ohau Point’s lower south flank of the north face. The block was separating from the rockface along a steeply dipping joint, in places opening up to around 500 mm, particularly at the northern extent of the block. While the block remained embedded in the slope on its southern side and was not judged to be at immediate risk of detaching, it could become unstable in the short to medium term, particularly in response to future earthquakes or rainfall.
The block was comprised of slightly weathered greywacke sandstone with an estimated volume of 400 m3 (approx. 1000 tonne). After considering various options, including blast removal and ‘do nothing’, it was determined that stabilisation by the installation of passive, fully grouted steel dowels drilled through the block was the most appropriate solution. The design was intended to:
Before the permanent stabilisation work could begin, safe access was needed. The threat of immediate, direct rockfall from above and beside the work site, as well as the possibility of the block collapsing while working on it needed to be addressed. GeoStabilization’s Rockfall Remediation Technicians undertook a high-level assessment of the stability of the feature using helicopter inspection, high definition photos, and 3D images. While the overall stability of the block in its current state was deemed acceptable to work on, it was at high risk of collapse if any outside influences were introduced, i.e. ground shaking by either earthquake activity (high risk) or potentially drilling activity (lower risk). In order to protect our crews, we installed a number of monitoring systems which included,
The drilling operation involved a lightweight helicopter portable wagon rig using our proprietary low-carbon steel hollow bar. Voids up to 1.2m were encountered, so a double grout sock was used to avoid grout loss and risk of any hydraulic influence on the failure plane. Testing of the double sock on independent anchors installed at a site nearby, proved no loss of bond capacity using more than one layer of sock.
As one of the highest risk projects undertaken on the NCTIR network and requiring the closure of the state highway for the initial stages of the drilling, GeoStabilization International® is proud to have been involved in the mitigation of this highly complex feature.
After the Kaikoura earthquake in November 2016, the steep coastal slopes above the main State Highway (SH1), and the main trunk rail line, north and south of the town were severely affected by rockfall, landslide, and debris flow issues.