Under it, in work that lasted 11 hours, about 20 construction workers poured 340cu m of cement - equal to about 70 truck loads - over 100 tonnes of reinforcing steel, to form the basement floor for a new four-storey building to house electrical equipment and a control room for the new "Pole 3" line.
The Benmore HVDC (high-voltage direct-current) convertor substation is the southern link in the feed to the Cook Strait cable and North Island, emerging at the Haywards substation (in the Hutt Valley, just north of Wellington).
Transpower is spending $672 million at Benmore and Haywards to increase the HVDC link from its present maximum of 700MW to 1000MW in 2012 and eventually 1400MW in 2014.
Siemens was awarded part of the contract last year to replace the 44-year-old Pole 1 equipment at Haywards and Benmore substations with new, modern thyristor (four-layer semiconductor) converters.
Preparatory work had already been done at Benmore, but yesterday marked a major milestone in starting the project, which is due for commissioning in April, 2012.
Yesterday, between 40 and 50 people in total were on site for the pouring of the concrete.
At its peak, when electrical equipment starts arriving from early next year, up to 100 people will work on the upgrade.
They will be accommodated in motels, hotels and rented homes as far away as Twizel and Kurow.
A mobile batching plant set up on the east bank of the Otematata River, just off the SH 83 bridge, by Allied Concrete is producing the concrete for yesterday's pouring and will remain for future pourings.
Transpower site manager Alan Bulling said a constant stream of trucks ferried the concrete from the plant to the Benmore site, where it was fed into a long-reach concrete pump through an open section of the marquee to be poured to form the 45m-long, 15m-wide and half-a-metre-deep foundation.
The marquee was needed to protect the fresh concrete from extreme climatic conditions, such as the heavy frosts the area can experience.
A heater unit kept the marquee at 12degC ambient temperature to allow the concrete to cure at a surface temperature of about 10degC.
Once the concrete had partially set, a team started to float (smooth off) the surface, which was expected to take until early today.
The foundation poured yesterday included large reinforced blocks, which will form part of earthquake proofing allowing the building to withstand a one-in-2500-year seismic event.
On top of those blocks will go "shock absorbers" (lead-rubber bearings and sliders), which will allow the building to move during an earthquake.
Once they are in place, a concrete floor will be laid before transformers, three new, modern thyristor converters to convert electricity from alternating current to direct current - which is more efficient to transmit - a control room and other equipment are installed.
A new switch station, about half the size of the existing one, will also be added at Benmore for Pole 3.
The Pole 3 project is a major component of Transpower's wider investment plan to reinforce the inter-island grid.
The link is critical in balancing energy use between the islands and will allow for electricity to be transported to where it is needed, regardless of where it was generated.
