Figure 1: Auckland’s reclamation zones are north of the original 1840’s shoreline (red). Map shows historic bays (light blue), headlands (yellow), paleo-river/stream channels (white arrows showing direction of flow). The surface represents the depth to unweathered ECBF based on subsurface site investigation data and geomorphic cues. Zones without contours did not have enough investigation data to develop surfaces.
The central Auckland City waterfront has been progressively modified since 1840, through a series of reclamation fills. Located on the northern edge of an isthmus, the reclamations served to increase the area of available land, catering for an increasing population. The aim of this study was to define the dynamic characteristics of the reclaimed land in the Auckland City Business District (CBD) using a combination of geotechnical, geological and geophysical data. The objectives were to (1) understand the history of reclamation and geology from historical ground investigations to provide constraints for the surface wave testing; (2) define shear wave velocity (Vs) of the deposits in the reclamation zones and; (3) define a range of site classification metrics across the region.
Reclamations began in the bays closest to the shoreline using local East Coast Bays Formation (ECBF) material from nearby headlands, with areas such as Commercial Bay, Brickfield Bay, Official Bay and Mechanics Bay (shown in Figure 1) using material excavated from Stanley Point and Britomart Point. From the early 1900’s, having depleted local sources, there was a shift to use hydraulic fill sourced from the Waitemata Harbour, including Wynyard Quarter and areas of Britomart Transport Centre. In the late 1980’s and early 2000’s, mudcrete was used to extend reclamations in the Viaduct Harbour and eastern areas of Wynyard Quarter. Across the reclaimed zones these fills overlie Tauranga Group Alluvium (TGA), which thickens in paleo-channels above the ECBF sandstone and siltstone bedrock.
Site Investigation data and geomorphic characteristics were used to develop representative soil profiles in each reclamation zone and develop surfaces for the top of the unweathered ECBF and the base of the reclaimed deposits. Historical subsurface data across the region from a number of sources was used in this process. In addition, over 100 horizontal-to-vertical spectral ratio (HVSR) measurements were made to provide an estimate of the site period of the reclaimed soil profiles above the ECBF, and identify contrasts between adjacent reclamation zones. At a selected number of sites, Multi-channel Analysis of Surface Wave (MASW) testing was performed to provide an estimate of the Vs of the deposits. The layering of the profiles developed in this process were calibrated using collated subsurface stratigraphy.
- Collated Subsurface Investigations
Subsurface investigations collated across the region revealed that the soils vary greatly across and within each of the reclamation zones. Sites proximal to the 1840’s shoreline generally contain shallow (0 – 4 m) fill and TGA deposits. However, collated subsurface investigation data and geomorphic cues show ECBF incised paleo-channels ~ >20 m depth across the reclaimed zones, as shown by the blue regions in Figure 1. In areas such as Britomart (previously Commercial Bay), the stratigraphy is highly layered and has sharp changes in both composition and stiffness relative to depth.
- Site Period and Shear Wave Velocity Estimates
HVSR and Vs profile derived site period estimates above the unweathered ECBF provided an estimate of the fundamental period of the soil profile above rock. In general, the longest periods were identified in areas furthest offshore from the 1840’s shoreline and within paleo-channels. In general, the site periods showed a good correlation to the depth to rock contours, as site period increases with increasing depth to rock. Additionally, there were no clear differences in the relationship between site period and depth across the different reclaimed zones using different reclamation materials.
MASW testing showed that the upper TGA deposits have lower Vs than most of the reclaimed materials, and only slightly higher velocities than the hydraulic fill. Given this is the dominant deposit, this seems to have the largest influence on the site period across this region.
- Site Subsoil Class
The focus of this study was the use of site period to classify site subsoil class according to NZS1170.5:2004, to delineate between site subsoil class C and D (using a site period of 0.6 seconds above the ECBF). The majority of the reclaimed region is defined as site subsoil class C, with site subsoil class D sites located proximal to paleo-channels in the area containing thickened deposits of alluvial material and in reclaimed areas that extended further out into the harbour. A more extensive site investigation programme is needed to identify potential site subsoil class E sites (with greater than 10 m of material with a Vs <150 m/s) and further assessment to identify areas susceptible to liquefaction.
- Across the highly variable reclaimed deposits in central Auckland site subsoil class C is the most prevalent based on site period, with site subsoil class D sites located proximal to paleo-channels and in reclaimed areas that extended further out into the harbour.
- Comparisons across all reclaimed zones suggest that the underlying young, unconsolidated TGA deposits have more influence on the site period than the various materials used for reclamations.
- The HVSR method was shown to be a valuable technique for providing a rapid assessment of the depth to the ECBF deposits in the region.
Gratitude and thanks go out to the following for their support and aid in field investigations: André Bellvé, David Elliott, Jamie Lai, David Bevan and Shannon Hawkins. For their patience and sharing of expertise: Simon Nelis and Jennifer Eccles. Additionally, thanks to Tonkin and Taylor and OPUS for allowing access to their investigation database and the New Zealand Geotechnical Database