For someone with some knowledge of science, this is a surprisingly wrong statement.Sand liquifaction[sic] would result in anything denser than the sand-air mix (e.g. marble) sinking to the floor. You have described a process which might remove marble fragments from the middle of the fill, but would not put them into the middle of the fill.
Nothing could be further from the truth ! There is a well known phenomenon called gravitational convection, or more popularly the "Brazil nut effect" or "Muesli effect" whereby in a mix of different sized particles subject to vibration, the larger and heavier ones ( somewhat counter-intuitively ) rise to the top.In fact a popular children's science experiment involves placing pebbles in a jar, filling it with sand, and then shaking the jar and watching the pebbles rise. It can be seen in farmers fields with certain types of soil subject to liquefaction, and in glaciers. In the case of Amphipolis it will have been seismic activity over the centuries that has provided the vibration, causing the stones to rise..........
Speaking of seismic activity, I wonder if all Pothosians are aware of the fact the reason that most ancient buildings, particularly those of the Eastern Mediterranean, are 'ruins' is seismic activity/earthquakes, and not due at all to man-made causes/demolition. In fact most 'stone robbing' or re-use of building materials occurs after the original building has been shaken down.
A classic example is that of Masada, because it was largely undisturbed by human activity since the Romans took it in AD 74 [not AD 73]. In pristine desert conditions preserved items included food of the defenders, leather sandals, wooden items, human remains with the hair still intact in plaits, clothing and many other fragile items - and much of it in the open rather than preserved through burial. Yet of Herod's vast palace complex, not one building remains, and not even walls over a metre or so high, save here and there. The whole complex is completely flattened into rubble. This is because the area is particularly earthquake prone and there have been literally hundreds of severe earthquakes down to the present time.
[digression: What many people think is the Roman siege ramp is in fact a natural feature which gave access to the plateau. It fell short of the top of the plateau, and the Roman ramp was built on top of it. Not a skerrick of the ramp remains, save for some timber re-inforcements shaken to the base of the ridge, preserved by the pristine conditions]
Now Greece does not suffer so much, but being one of the countries on the edge of the Aegean plate,does have frequent earthquakes and there can be little doubt that the Amphipolis tomb has suffered some seismic damage at the very least. The degree of this, and what damage is attributable to human activity remains to be seen.
Therefore the blocks found embedded well above ground level are there because that is where they fell. In other words the chamber had been partly filled with sand at the point that the doors were broken. 
