If its your tank on your property you can do what you want with it. Can I enquire why you need to fill it to prevent its use ? Can you just not disconnect it. Just thinking of ease of future removal (if/when required).

Ok What do they want to fill it with ? Regards

...and of course you'd have to make sure it is empty and not leaking any of it's contents.

Ok If they are insistant on filling it you may want to raise the following issues with them (for what it may be worth). Depending on the size of the tank if they fill it with concrete and, at a later date decide/need to remove it it is likely to be very heavy indeed and will likely have to be cut up in situ and then the concrete broken out and removed. This will cost a lot more than just digging it out and loading it onto a wagon to be chopped elsewhere. If its filled with foam (and Im not sure about this but someone else on here will be) and its being cut up the foam may present an additional fire risk and also give off some toxic fumes. If they go down this route keep a record of the type of stuff it was filled with. Either way IMO false economy if they ultimatley end up having to remove it. Good luck. Regards

Could you fill it with something you can get back out ? Water ? If that is too much like coolant, how about sand ? When you say coolant, what do you mean; are you talking about the stuff in cars, the oil / water emulsification for machines or some other nasty ? Chris

The use of cement in "rendering safe" underground fuel tanks, that are no longer required, at petrol forecourts is a common / industry standard method of decommissioning. The Blue Book "Guidance for Design, Construction, Modification, Maintenance and Decommissioning of Filling Stations" makes reference and stipulates requirements for doing this safely - including the requirement to flush the tank before doing so. Although not the same substance, the principles are the same - your initial post appeared surprised at this method, so i thought I'd provide a different perspective that actually this is a common technique adopted (at least within the oil majors who operate forecourts). That said, it does not automatically mean concreting is the best way to decomissioning, it will depend on your situation and clearly the risk invloved are different to that of an underground fuel tank.

We recently decommissioned an underground tank and we finally went the fill it up with concrete method but it was not that simple especially the making it safe bit.

You'd fill it with 'foamed concrete'. This is nothing like structural concrete, isn't heavy (as a void filler you'd use a light grade, so potentially lighter than water, much lighter than sand) - it's more like shaving foam with cement mixed in. It's a very standard method for filling underground voids. It's used quite a lot for filling redundant basements, tanks etc and even re-filling roadworks trenches, because it self-compacts and though you do need to wait for it to set before surfacing over it, depending on your hole dimensions that might be quicker than filling with granular material and compacting. It's also actually easier to dig out than soil is (again, depending upon grade). The advantage of filling a void is that you then don't need to worry about the walls corroding and caving in. If the tank is redundant I'd fill it rather than leave it empty in situ.