Hawkins Infrastructure's work on the Hobbs Wharf seawall, creating an extension to the marina at Whangaparaoa's Gulf Harbour, illustrates the company's commitment to sound environmental practice; a commitment recognised at the New Zealand Contractors Federation 2008 Environmental Excellence Awards. The project involved the construction of seawalls to create a new boat harbour at the corner of Pinecrest Drive and the Anchorage in Gulf Harbour, Whangaparaoa. The site covered an approximate area of 3.8 hectares and included a basin of approximately 2.7ha.

Excavation works had been undertaken on the site in the mid to late 1990s to form the marina basin adjacent to the Gulf Harbour Marina. It had been left as an excavation area and had since filled with groundwater and storm water runoff. It was separated from coastal waters by two earth bunds.

The scope of works to create the new boat harbour in the basin included de-watering the basin, and excavating and shaping the profile to accommodate the proposed marina over approximately 2.9 ha. A further hectare of earthworks was carried out to form the seawall and promenade. The project involved 50,000m2 of earthworks, cofferdams, sheet piling and construction of keystone retaining walls, a wharf structure, and bearing walls (precast panel) for future development.

Mitigating environmental risks was critical, in particular preventing sediments from construction activity entering the harbour. Prior to earthworks commencing, erosion and sediment controls were installed in accordance with the erosion and sediment control plan and included perimeter bunds for storm water diversion, geo-textile lined diversion channels, silt fencing, sediment ponds and a decant system. An environmental assessment study was undertaken, resulting in the installation of a permanent dewatering system that stabilised storm water diversion channels, enabled clean water diversion and facilitated the successful relocation of protected trees. Habitat restoration planning presented a course of action for enhancing and re-establishing the natural environment of a Pohutukawa species and the ecological community.

To complete the project, the temporary bund had to be removed. The bund was constructed from sandstone with a thin clay core (50m long x 7m deep x 8m wide). To further restrict inflows and to provide inter-tidal stability a sheet pile wall had been constructed for the full length of the bund, using 105 sheet piles 10m deep and founded into sandstone. The tidal variation was 3m with a low tide depth of 4m, this meant that there was a maximum 7m differential in water height between the new and existing marina. This significant head differential restricted the potential removal methodologies.

The inundation of the basin was originally planned to be undertaken via natural means; however, it quickly became apparent that this method was going to be too slow, potentially putting the narrowed bund at risk. A decision was made to assist the filling process by directly pumping in seawater. This operation took a total of one and a half weeks.