What Tango said about Great Stuff needing air to cure is true. It works good to seal up exposed cracks but isn't good for back filling hollow areas. I call it "So So Stuff".
Drifting a bit, While serving as Range Master at the local Police Academy, One of the instructors got aggravated because the Cadets kept spilling the sand out of the weapon clearing tubes when they set them up. He "solved" the problem one lunch hour by topping off the upper third of the clearing tubes with Great Stuff so the sand wouldn't spill out. Needless to say, when the Cadets came back from lunch and loaded their weapons, not a one spotted the gooey foam in the tube and like good little Cadets did what they had been taught to do--end of range day. That particular Commander has since retired and now is a member of the City Council. Just figures.