Load-bearing strength is one property which makes every Tubosider tank, including Twinstore, unique among stormwater management systems. And being load-safe is fundamental to any location of tank, be it a live load from vehicles overhead, a dead load in deep trench installation or a combination of both. The use of helically wound galvanised steel pipes which form Tubosider’s stormwater tanks grew decades ago out of our construction of highway culverts, manufactured with Highways Agency approval.
With the Design Manual for Roads & Bridges (DMRB) as a cornerstone, we still use BD specifications as the basis for each design. Unlike crate systems, for which there is no acknowledged standard or test, our materials have Highway Agency approval, each tank designed to the guidelines set out in BD12.
Not Tubosider’s, of course, but some stormwater tanks are liable to collapse under load unless the construction is not just of suitable material, but correctly calculated to match type of load and depth of cover. CIRIA, the Construction Industry Research & Information Association, is well aware of such failures.
To quote CIRIA C680, the four main contributing factors to failure are:
- Inadequate design, often not taking account of particular ground conditions on a site, or not allowing for creep of the units
- Lack of understanding of the performance of the tanks, leading to overloading, for example by running heavy plant across tanks that were not designed to carry such loads, or by using unsuitable backfill, for example containing boulders
- Lack of appreciation of the influence of groundwater levels or the effect of surface water flows into excavations during construction
- Inappropriate laboratory testing that overestimates the strength of the units
“Designing stormwater tanks to meet all the criteria is something of a science in itself, with adequate loadbearing a critical element,” says Tubosider’s technical manager Paul Rawcliffe.
“Using BD12 for Highways Agency approved culverts and as a design guide for calculating the load bearing capacity of the tanks is a great asset to a project. In design we need to account for the location of a tank within a site, the type of traffic during construction and for the intended lifespan of the development. We need to consider the depth of cover and the planned design life of the tank.
“Just take traffic and location, such as the new out of town shopping developments such as The Trafford Centre near Manchester. Traffic loads can be measured in either HA or HB units according to whether they are car related, heavy traffic or whatever.
“For the car parks in such developments, it would be wrong for the loading and tank strength to allow for just visitor traffic. It also needs to withstand the effect of delivery vehicles, fire engines etc for the 60 or 120 years of their planned life and as I say we are often asked to check construction traffic loading during installation.”