Britomart
A Bubble of Clean Air
Train arriving at Platform 5
Light cones above platforms
(click to enlarge)
Platform information
This is the first time anybody in the world has built an underground station for diesel trains. This claim to fame brings significant challenges for the ventilation system, which needed in any case to be fairly robust to deal with the possibility of fire. Opus lead fire engineer Roger Feasey (MIPENZ) explains that the worst-case scenario is covered – "For example, a diesel spill could catch fire. If this happens, automatic fibre-optic heat detectors, within 32 zones, will identify heat above a certain level and deluge the area with foam."
COP Outcome development and evaluation
A mixture of 49 unidirectional and reversible fans — including two 250kW extractors — have been installed to circulate air from east to west, venting through two 35-metre-high stacks. Dubbed the "Twin Tin Towers", they look fairly basic — the thinking is that future developments will surround them and effectively hide them from the public gaze. The ventilation's system's fans will use 85% of the station's electrical supply.
"In the platform area, air will be pumped in at passenger level, and air and fumes extracted from above the train and from around wheels," said Eric Hennephof. "The aim is to encapsulate people on the platform in a bubble of clean air." The volume of air involved? Well, it takes eight minutes to change the entire air mass at an extraction rate of 350 cubic metres/sec-you can do the maths.
The station itself is designed to be waterproof. This has primarily been achieved by limiting the widths of cracks in the concrete. Thermal cracking was not a problem as the walls and slabs are relatively thin. Construction joints were formed with hydrophilic waterbars, and additional hydrophilic material on the outside of the walls. The station's roof and external walls at the eastern end were tanked with bentonite waterproofing membrane, which is self- healing if damaged.
Being underground, the ceiling of the station effectively becomes the ground for the city above. The design requires parts of it to support roads, and in the future possibly buildings; four acres' worth of special pre-stressed concrete was provided by Stresscrete to meet these exacting demands. "We worked closely with the engineering design team to ensure that the product met the requirements of the job," said Peter Wyatt, Stresscrete's senior project manager. Highly-stressed 400 hollowcore units for the ceiling were designed and cut to be placed between the beams at 11-metre intervals.In all, Stresscrete provided 1,500 pieces of pre-stressed concrete from its Papakura factory, covering 8,500 square metres of the ceiling/ground level and about 3,000 square metres of the service/platform level.
Made from 2,500 square metres of glass on a steel frame, the imposing structure serves as an entry point to the station and dropping-off point for passengers in front. It will also act as a giant light source to the station platforms – and as a natural ventilation system, thanks to its 1,664 louvre panels (all easy-clean, by the way). Its roof structure spans 18 metres. Inside, a glass bridge connects the station to the CPO building at ground level. The space is decorated with representations of Auckland's landforms and geologies in the form of volcanic gardens with native plants, a curved basalt waterfall, and stainless steel nikau palms up to 26 metres tall.
After years of public and political debate, Britomart station was opened in July. On Monday 7 July the Britomart Express was the first train to roll into a central Auckland station for 73 years.
