I'm not so sure. You only need to consider once in a decade storm conditions, and the water wouldn't need to be up to ground level to build up a great deal of lift. Even at a couple of feet below the surface, you could be talking of several tonnes, depending on the size of the pit.
I'd certainly hope so. This is something I've deliberated about a great deal as I'm trying to work out the best way of tackling the problem myself. I've more or less settled on simply allowing the water to enter and then pumping it automatically from a sump built into one end as the consequences of failure are so much less serious. Incidentally, IIRC, Building Regs say that no pit can cut into a line taken at 45° downwards from the base of the structural walls. So a six foot deep pit has to be at least 6ft from the garage walls, which is a bit of a bugger is the garage is only 10ft wide!