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Creative Solutions to as Steep Problem

When original plans included tanks that would be too long to be safely transported, and buried so deep there was risk of tank damage, APD had the right solution. Nga Tapuwae School needed to detain up to 80,000L of storm water but the site they had available was long and narrow with a very steep slope.

Our Task

APD were asked to provide tanks for the school but there were two issues:

  • The original plan called for two 36m long tanks – these couldn’t be safely shipped in one piece and welding onsite would take a lot longer and cost more.
  • The site for the tanks had a 3 meter drop off – one of the tanks would end up 1.2m underground and would face pressures that would damage a standard tank.

Our Solution

The best answer was to build four 18m long tanks and connect them onsite using an electrofusion join to become two 36m tanks.
This would make it far easier to transport to site and would ensure safety of the tank when being raised and placed into the ground. One of the tanks was designed to have additional external strengthening to withstand the ground pressure at the deeper depth. A concrete manhole was installed above the manway access to the deeper tank. We worked closely with the drainage contractors throughout the installation. This solution was far more efficient than welding an entire tank onsite or constructing a concrete tank.

Side View


  • The client saved money and avoided costly fixes with a good installation first time round.
  • The tanks were delivered early to site at the cost quoted.
  • An APD team member was available to consult throughout design and installation.

The original engineer had designed something that met the technical requirements but APD were able to give a practical perspective that ended up saving costs and time on site.


  • Custom built.
  • Tanks too long to transport.
  • Deep tanks facing prohibitive pressure.
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