Benefil RG22 hardfoam.

Most commonly used for filling old pipelines like sewers, gas or water mains that’ve been taken out of use.
Also available for filling old tanks at petrol stations or household fuel tanks.

Projects that require a higher density foam / higher structural strength, should look at a RG30 or RG50. Big voids like mineshafts, or lifting sunk floors require higher density foams.