We've just acquired a 3000l water tank, to counter the ever-drier summers. (Our existing 1200l of water butts is no longer sufficient to water the garden properly). It's 1.4m diameter base, and of course will weigh 3 tonnes when full.

The supplier tells me to install a 150mm deep reinforced concrete base (600mm wider than the tank - i.e 300mm all round). Which is fine, but I'm wondering if there is a lower-carbon way of doing this? Can one make a strong-enough base to avoid settling and potential damage to the tank base without just pouring 0.6m3 of concrete?

Concrete is about 380 kg/m3 so this would be 228KgCO2e, or approx one-way flight Stansted to Barcelona. It feels like there should be a better way.

Maybe the way driveways are made with an MOT subbase, and a solid perimeter to avoid spreading.
The point loading is very much lower with the tank (1.54m2 vs 0.09m2 for the tyres of a van of similar weight - i.e 1/17th of the load). I'm not sure what surface material to use to avoid risk of puncturing over time.

As well as the load the tank imposes on the concrete, you also have to think about the load the concrete imposes on the ground. Where we are with clay + trees = possibility of heave we had to build the garage as a raft essentially because the ground might disappear (or at least cease providing support) under any part of it. And dig a 2 m deep hole under the house for similar reasons.

But if your ground is fairly solid and rigid then I'd think that a foot of sharp gravel would be as much use as a foot of concrete. And reinforcement, seriously? Some sort of membrane + sand to prevent puncturing the tank base?

djh, I can see them when I sign in, but nowadays I generally don't. Nick

Hoping that all of us who dislike the software, have chipped in to the replacement kitty!

https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18341

As was said, the pressure on the ground (2tonnes/m2) is not an issue here, much less than a human footprint. It's more about the risk of differential settlement where one part of the base gets compacted over time more than another part, as the tank fills and empties, and maybe as rainwater moves underneath it. Then the tank weight might end up on uneven ground and it would tilt, or even split.

The proposed slab is to act as a ground beam and transfer the loads from any unsupported points, to well supported points (which is why it has reinforcing)

The tank manf hasn't seen your site ground condition so has to give CYA guidance that works anywhere. (We're in the same position, and know less about how the tank is built and how accommodating is base is!)

If you're willing to take some limited risk of it tilting/splitting/leaking (what damage could that cause, where it is located?) then dig a hole down to undisturbed subsoil, fill it back up with well compacted hardcore, install the tank and keep an eye on it. If you can find some old paving slabs, they would be good to go on top of the hardcore layer and stop any pokey-up stones from poking the tank.

If your soil is clay based then you can't rule out shrinkage or heave in the future.

I would go with the manufacturers recommendations.

Posted By: djhBasically dig a deep enough hole and fill with hardcore and the problem goes away. Piles and beams are for if you can't dig for some reason (TPO).

It is important that when digging down you do not disturb the ground at the level of where the hardcore meets the bottom of the hole. The ground will be compacted over the years and will form an excellent subbase for well compacted hardcore. Around here the BCO is very particular that the base of excavations prepared for concrete pours are flat and solid and they don't tolerate digger bucket tooth grooves. Might be overkill for your tank but the principle holds well. Topping the hardcore with flags is a good idea, helps distribute the load, look better more a finished look and inhibit weeds that would otherwise grow around the base of the tank.
I use a redundant 3600 l silage tank to collect rain water and I used to pump it up (ca 7m) to our growing area, that is until I worked out that it cost a bit more in electricity to drive the pump than the cost of domestic water.