[willyt]

I looks like they tether it to the ground with tension cables to weights or piles when they are lifting very heavy loads. If this was replacing a standard construction crane that would be economic as these things often have their own significant foundations. If it's a piled foundation for a building it's not uncommon to put extra piles in where you will have cranes. Maybe smaller loads can be done with bladders as others have suggested. You only have to lift 25 tonnes to beat the MI-26 which is the biggest heavy lift chopper and hard to get hold of.