I live in florida, we have sand here that is sort of fine, and when I saw these cracks, the tank had been used in sand about three times maybe 1 hour each time. If you take top off, and put it on carpet, or any surface with good traction, you will see the twist when turning, it twists up on the powered side.

You probably need some horizontal brace for the hull for "torsional rigidity", that engineering lingo cos I am studying Mech Engineering.

The tank without the top is like a shoe box without its cover and it will be easier to twist.