Building a D Wall

Some of the keener observers of the Commercial Bay construction site may have noticed the construction of a long trench against the south side of the HSBC Building.

The trench being built along the south side of the HSBC Building represents the initial enabling works for the northern Diaphragm Wall - colloquially known as the D Wall - a very rare form of construction in New Zealand.

The D Wall is designed to keep the groundwater out of the Commercial Bay site and carpark. Being built on reclaimed land and with the Waitemata Harbour immediately to the north, keeping water from entering into the site is a key priority. The D Wall is a concrete wall set into the ground up to 15 metres deep.

A D Wall will protect Commercial Bay on the northern and eastern boundaries. Traditional sheet piling will be used on the southern and western boundaries.

To give you a sense of what you will observe being built – here is a high level summary of the construction sequence:Firstly, in ground walls on either side of the trench are constructed. These are the guide walls that will ‘guide’ the excavation of the trench.

A special excavation ‘clamshell grab’ sinks down between these guide walls to begin excavating the D Wall trench. You will have seen this large yellow piece of equipment recently arrive on site.

As the trench is very narrow and deep, a Bentonite slurry is pumped into the trench to prevent the walls of the trench collapsing inwards as they are excavated.

Once excavated to the required depth and being full of Bentonite slurry, reinforcing cages are lowered into the trench through the Bentonite. You can see the reinforcing cages currently being assembled in the middle of the site.

Once the cages are in place within the trench, concrete is pumped to the bottom of the trench to begin filling the D Wall around the cages.

As the concrete rises in the trench, the Bentonite is drawn off and returned to the large tanks you see in the middle of the site.

We hope you enjoy watching this process over the coming weeks. It is a unique process and we are looking forward to it ourselves.

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