Generally you can drive over the sea ice for up to 9 months of the year. A quick look on Wiki tells us the Ross Ice Shelf, which surrounds Ross Island (where the huts are located) is an area of roughly 487 000 square km, and about 800 km across: about the size of France. So in theory, as long as its safe to do so, you could drive for a pretty long time. As for “has ice ever given way under a vehicle”. The short answer is yes. However, safety is a number one priority for Antarctic Government programmes and so, we are unaware of this happening in recent times.