How to build a water tank base over a drainage trench?

Sopwith

Hello

I want to install a water tank next to my bush block shed. It's for garden water, not potable water.

The problem is this. The open space, where there was a tank previously but long gone, and where I would like to install the replacement tank is about 1850mm from the edge of the shed to a open drainage trench. The tank I'd like to install is 5000l / 1830mm dia - a tad larger than the old tank.

I intend to construct a base from treated sleepers, @75mm thick. After I take into account the "apron" of 175mm from the edge of the tank to the near edge of the sleeper (manufacturer advises 150mm min) and the thickness of the sleeper (75mm) I will need a min of 1830mm (tank) + apron and sleeper (2x 250) = 2330mm. The diagram below might help understanding my word salad.
This scheme runs well into the drainage trench.

My question is this: could I extend the box into the drainage trench, and back fill and compact the soil before filling the box with FCD? Or must I use undisturbed ground? I would excavate the far side of the trench to restore its width, so the trenach could keep functioning.

I would be grateful for any thoughts and advice.

JacobZ

Hi @Sopwith,

While I understand what you are saying, and the diagram does help, a photo of the space would certainly help. If you have one you could share, it would be good to see it in physical form so we can better understand things.

My first question is how deep the trench is and how much water flows through it? Does it fill up during a heavy downpour?

On the surface, it is a simple question, but there are a few factors that make it complicated. There are three main factors here: the weight of the tank when full, the ability of the ground to support that weight and the likelihood of erosion from water in the drainage trench.

The tank alone is likely to weigh somewhere in the area of 100kg, and when full with 5000L of water, it is going to weigh in excess of 5 tonnes. That is a fair amount of weight.

If you were on undisturbed ground, you wouldn't have any issues, but with introduced material, settlement is going to be a factor. If you used fine crusher dust and compacted it in layers, ensuring it is extremely solid, there may still be some minor settlement, but it would be manageable.

The final factor that really makes it a complex question is that you have, what I assume based on your description, an active water channel, so the risk of erosion is greatly increased. This could be managed through the use of a retaining structure, though the volume of water flowing through that channel would need to be considered.

Ultimately, it is not something I can really give you a definitive answer on, and it is likely you would need to speak with a structural engineer who can assess these factors and give you some more certainty.

In saying this, the height of the trench and depth of fill are major factors. If you were looking at introducing, let's say, 200-300mm of soil depth, and you retained it with a solidly anchored structure, using deep concrete piers, then I'd say you could do this with relative safety. If you were looking at introducing upwards of 300mm of depth of soil, I would be consulting with a professional.

I hope this helps in some way. I'm sorry that I can't be more definitive.

Jacob