Unique Soakage Design for the Three Kings Quarry

Fletcher Residential Ltd own the former Three Kings scoria quarry in central Auckland and are turning the quarry into a large scale residential development – a place of 1200 homes. The development is directed by a comprehensive masterplan which includes a range of different housing typologies, cultural recognition, creation of new sports fields, retaining elements of the quarrying and geological heritage and the filling the quarry pit by about 70% of its former depth.

The Three Kings quarry sits within a unique environment. The geology is complex, with a 1km diameter surrounding tuff ring (with a breach to the north), a series of scoria cones and basalt pushing up through the scoria in dykes and forming lakes. Much of the geology is disturbed and overlain by subsequent eruptions. The quarry sits at the top of the Meola Catchment where there is little separated stormwater infrastructure and only an overloaded combined sewer nearby. The planned development retains part of the former quarry pit, which means the development floor sits at about 15m below Mt Eden Road at RL62m to RL65m - with no way of providing piped gravity drainage or overland flow paths. Groundwater pumping is also to cease and the groundwater levels will return to the pre-pumping levels of RL56.5m.

Approximately 39% of the area of Auckland City is underlain by volcanic lava flows and associated scoria which has been used for soakage and stormwater disposal on the Auckland Isthmus. What differentiates the Three Kings development from others in the Auckland region is the discarding of the capture and pump stormwater system in favour of the design and construction of a unique large scale stormwater disposal system to soakage via the aquifer within the Three Kings basalt lava flow.

A companion paper by Messrs. Brunton & Seyb describes the development of the overall stormwater management approach and how the challenges were addressed related to management of stormwater in relation to the proposed dwelling floor levels, storage and soakage of stormwater, the local mounding of groundwater, protection of soakage systems and the overall operation of the system.

This paper focusses on the soakage and groundwater aspects of the design. The approach adopted for the conceptual design is discussed and the practicalities of implementation and construction utilizing materials available are covered. The assessment of the soakage capacity of the volcanic aquifer around the Three Kings quarry was done by modelling using the insitu test results

The implementation of the design objectives and the insitu testing carried out to achieve and confirm the required design criteria are described together with the approaches and methodologies adopted. Challenges associated with the construction of the soakage systems as part of the quarry filling are described and the solutions presented.