That was exactly what he used to do if he could. Once the safe is tied to the rebar in the slab any effort upwards was communicated to the safe as a downward force. A bit like standing inside a big bucket and trying to lift the bucket by the handle!

A very effective way is to weld some big RSJ or similar on the bottom of the safe and cast the floor around it.

I saw a big Chubb, once, that had been attacked. I am not sure which model it was but it had the coffee and cream paintscheme and the isolator bolt work. Anyway I suspect there wasn't much to choose amoungst the bells used in the entire series. This one had been centre base anchored with Chubb's patent base anchor. The bad guys had brought along a piece of equipment used on the railways for heavy lifting. They had applied so much force that the base of the tdr safe was actually convex, but the anchor hadn't slipped. Frankly if someone had told me I doubt if I would have believed it. The floor quality and the fitting of the anchor must have been absolutely perfect.

The worst job I ever saw was where a customer had paid for base anchoring. Inside the safe I could see the big bolt, with a big nut and washer. In fact that was all there was. The installer had just dropped a piece of threaded bar, complete with nut, into the fixing hole. It was not attached to anything!