Geology of Auckland: Evaluation for Tunnel Construction

This report investigates the geology of Auckland. It provides information about the tectonic setting, rock and soil types, present-day processes, weathering and geological hazards. Some engineering aspects of the most common soils are discussed. A detailed ground profile is included in support. Additionally, the possibility of constructing a tunnel under central Auckland is investigated. Potential construction materials and methods were researched as part of this exercise.

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Auckland is situated on an active basaltic volcanic field made up of over 50 volcanoes disappeared around the gentle rolling hill landscape between the Hauraki Gulf and Manukau Harbour. At the deepest level, it is made up of Mesozoic basement rocks (greywacke), that are hard and high in strength. These belong to the Waipapa group, which is made up of thin-bedded alternating fine-grained sandstone and argillite. There is also massive, jointed greywacke sandstone and massive laminated argillite in beds of up to tens of metres thick. Minor forms of rocks they may be found are chert and green spilitised basalt. The Waipapa group weathers to a soft, white to yellow-brown sandy clay, to depths of 20 m.  These are covered by Early Miocene sedimentary rocks and more recently, largely marine sediments and volcanic deposits. The general geological makeup is illustrated in Figure 1.

Below central Auckland, the Waipapa group gives way to Murihiku terrane, a layer of ultramafic and mafic igneous rocks comprised of fossiliferous conglomerate, lithic sandstone, siltstone and volcanic tuff. Over the last 200,000 years, approximately 3 km2 of volcanic material has been deposited in Auckland with the majority of it being olivine basalt. Significant deposits of associated material including scoria cones, ash and lapilli mantles, and tuff ring deposits, especially concentrated in central Auckland, see Figure 1.

Oxidic soils are found in Auckland.  They are clayey soils formed by weathering over extensive periods in volcanic ash or dark coloured volcanic rock such as basalt. Oxidic soils have a stable structure, slow permeability and moderate to rapid infiltration rates.  Granular soils can be found in South Auckland regions,  and are clayey soils formed from material derived by strong weathering of volcanic rocks and tephras mostly older than 50 000 years. Ultic soils can also be found in Auckland. They are strongly weathered soils with high clay content. They are prone to erosion and known to Auckland gardeners as the “yellow clay” soil.

Figure 1. Volcanic materials from 48 volcanos in the greater Auckland area

Renewed volcanic activity is a geological hazard facing Auckland. An eruption in the Auckland area would be devastating. The current assessment of volcanic activity shows that a new one erupts every 1.5 to 2.6 kyr (Roberts & Leonard, 2017). Given the last eruption was 600 years ago, and the difficulties in forecasting volcanic activity is not feasible to reduce the risk to Auckland by managing land development based on potential volcanic sites.

In terms of seismic vulnerability, Auckland sits on the Australian tectonic plate, approximately 400 km from Hikurangi subduction zone, where the Pacific plate is being subducted by the Australian plate. (Roberts & Leonard, 2017). Therefore it is unlikely Auckland will face any seismic activity and combined with its hard ground conditions, it is classed in the lowest hazard category in New Zealand earthquakes.

Additionally, Auckland faces hazards from slope instability. This is a current process in which hillsides may fall. This is determined by the strength of rock material, weathering, slope angle and ground conditions. Auckland is also at risk from ash deposits from volcanoes further south and tsunamis.

This section details a concept study of constructing an underground tunnel through central Auckland from Auckland Airport to connect to the City Rail Link to provide an alternative, faster transport option for residents and tourists (Figure 2). The tunnel will be located 30 m below ground level. Although there is a range of soil types, at this depth it will predominately be Waipapa group of soils that will have to be cut through. The construction method would utilise both the cut and cover method and a TBM due to the rolling hill nature of Auckland. This project would require a large amount of reinforced concrete due to the strong sandstones and argillite that would be pressing against it. The hard material the is mined could be recycled and incorporated into the tunnel wall aggregate.

Figure 2. Proposed tunnel location, Auckland Airport to Mt Eden station.

Some design decisions ] require more investigation. These include the exact route, which will be dependent on cost, exact soil conditions (preferable to avoid high concentration of tough rock). It will also depend on the existing transport infrastructure.

In summary, Auckland has a diverse range of soils due to its unique past. The Waipapa group that consists of sandstones and argillite is a common soil group in Central Auckland. It is vulnerable to processes such as slope instability and especially volcanic eruptions, which are difficult to forecast. The seismic hazard is low. Constructing a tunnel underneath central Auckland would be difficult due to the strong and varied soil types that make up Auckland’s geology.

Edbrooke, S., Mazengarb, C., & Stephenson, W. (2003). Geology and geological hazards of the Auckland urban area,New Zealand. Wellington: Institute of Geological and Nuclear Science.

Hewitt, A. (n.d.). SURVEY OF NEW ZEALAND SOIL ORDERS. Lincoln: Landcare Research.

Roberts, R., & Leonard, G. (2017). Volcanic hazard from the Auckland Volcanic Field. NZGS.

Figure 3 shows a detailed soil profile of central Auckland.

Figure 3. Detailed soil profile of Central Auckland.