My fear is that because the brick are wet laid, the water which will seep through the surface has nowhere to really go, nowhere to escape. So after a rain, the sand base under the bluestones would be completely saturated and would remain saturated for quite some time. Is that a problem? The sand base also would have nowhere to go, so it's not as if the sand would be washed out. So the question becomes, is saturated trapped sand problematic? Would it heave like soil? Would it even matter if it heaved a little, since there is no mortar to crack?