The Tiers Subdivision

The Tiers is a residential development comprising 13 units over five building platforms on a steep and challenging site along Frankton Road.

The initial assessment of the site raised concerns around tension cracks and bedrock shears when excavating the roads, carparks and building platforms. Restrictions on excavations including pilot cuts and limits on cut heights were imposed to help mitigate issues as they arose, and the steepness of toe slopes and reinforced earth retaining walls was reduced from the original plans.

Due to the steep nature of the site, the order of construction was dictated by temporary access requirements, meaning the suitability of building on the site had to be assessed in stages as rock was exposed.

Each building platform had the potential for different geotechnical risks. During the construction of one platform, significant amounts of low quality rock and glacial shearing distress was found with a number of open joints and voids found in the rock that could undermine the site. This could also lead to high rockfall danger to the properties below. Once a detailed assessment had taken place it was found the increased risk could be mitigated with the installation of rock dowels and pinning the foundation of the unit block to bedrock.

Other hazards included rock regression once building had been completed which was mitigated by specific foundation design and pinning the foundations to bedrock, and also the risk of slide debris from slopes above and rock weathering over time. To deal with this it was recommended the units be redesigned to allow rock catchment areas behind them, slicing and shotcreting rock faces behind the units.

Our analysis and mitigation factors of the site allowed the subdivision to safely progress without compromising the design and original vision of the developer.




Alpha Properties NZ

Completion Date