Water treatment tanks are used for storing, treating, and processing water, and SWJ Consulting work closely with local water authorities and their specialist sub-contractor to design these structures and/or the foundations they sit on. Like any tank, the ultimate goal is that is won’t leak, can withstand various significant forces and be sufficiently durable for the environmental conditions. As well as the initial design we are looking to ensure the tank’s stability and longevity. Some key factors to consider include:

Withstanding the load

The calculations used to design water treatment tanks are not dissimilar to the ones we use to design swimming pools, as they are both water-retaining concrete structures, albeit they serve a very different purpose.

Wastewater Tanks must support significant hydrostatic (fluid at rest) and hydrodynamic (fluid in motion) forces exerted on the tank’s wall and the base and we need to look at both. When it is full the weight of the water makes the tank invariably heavier and so it naturally wants to ‘sink’ into the ground, we design suitable foundations to prevent this from happening.

If you are building a tank underground, we need to consider the buoyancy forces to keep the tank in place when it is both full and empty. It’ll want to sink when full and rise when empty.

An empty underground tank will be lighter than the surrounding soil, and it will want to rise. We design the tank to withstand any hydrostatic uplift because this pressure can cause various issues such as cracking of the tank, or in extreme cases, manhole covers rising out of the ground.
There are also additional loads to consider including soil pressures (for buried tanks) and wind loads (for above-ground tanks). The surrounding structures, machinery, conveyor belts and loading bays at the treatment plant will also make a difference to the forces the tanks need to withstand, not only increasing soil pressures but creating vibration that we will need to account for.

Material Selection and Stress Analysis

To ensure longevity we will select the appropriate materials based on strength, durability, and resistance to corrosion. We need a material for the tank and it’s supports that will be able to ensure the tensile and compressive stresses, where it will be pulled apart by the water inside and compressed by surrounding soil or wind loads.

We need to ensure that the tank will be able to withstand the effects of thermal expansion and contraction, and design movement joints if we feel this movement will cause an issue.

Finally, we want to reduce the risk of fatigue in the materials used. The tanks will have to handle cyclic loading from filling and emptying operations. If the material used ‘fatigues’ you will end up with lots of microcracks and if they propagate this can cause a sudden failure.

Wind load considerations for above-ground tanks

Above ground water treatment tanks must be designed to withstand wind forces, particularly in areas prone to strong winds or heavy snowfall. Wind strength can differentiate depending on the direction it is coming from, and whether it is coming from an open field or through a more sheltered site. We use the latest computer software to calculate the design wind pressures for that site and the impact on tank stability. We can then establish if the tank needs additional means of stability such as anchorage and bracing systems.

We are currently designing an above-ground storage tank for a local water treatment centre. In this case, to resist the wind loads, the tank will be nailed to a concrete plinth. The original design was to use a braced steel frame but this solution would have been hard to implement on a congested site as well as the material not being suitable in terms of withstanding the reasonably aggressive conditions, so it would have needed ongoing reinforcement treatments. Securing the tank to a concrete plinth was the best option for installation and durability.

Foundation Design

The foundations need to support the weight of the tank and its contents, preventing settlement and instability.

The bearing capacity of soil will determine the type of foundation we recommend and the likely settlement of the tank will be the governing factor in its design, and limiting differential settlement in particular. If all the tanks sink, or settle, the same the angles of the connecting pipes will remain the same. But if there is differential settlement this could lead to the significant issues where the fluid either remains motionless in the pipes due to a change in angle or flows backwards.
We also have to consider other hazards in the ground, such as buried services or areas of contamination which may affect where we can place our foundations and thereby fundamentally influencing the foundation layout.

SWJ will liaise with specialist cub-contractors to ensure that any site investigation work is as thorough as it can be to allow us to de-risk your project.
If you would like to speak with us about water treatment tanks or foundations, please give SWJ Consulting a call on 01993 225085 Oxfordshire office, 02381 920656 Southampton, or email mail@swjconsulting.co.uk