I have not done a sand filling yet in any of my project. sand is non cohesive type so does not have idea about how the compaction is done.
For murrum or a soil available in your area need to be used as backfilling material in plinth area.
Before proceeding to back filling work, the test need to be carried out in third party laboratory for its properties like MDD, OMC, Plastic Limit etc. and the report should also confirm in writing that, the material can be used for back filling purpose.
The thickness of layer is depend on the type of machinery use for doing compaction.
Plate compactors are effective up to depth of 150 to 200 mm.
2 MT rollers (Mini Rollers) are effective for depth of up to 600 mm.
Again it also dependent of the material properties.
Back filling is generally done in layers and each layer is tested for achieved compaction ( denoted as compaction factor). upon getting above 95% of compaction factor the work of next layer is done.
In case when there is no work is to be carried out above the fill for almost a year (considering 4 months of monsoon) the filling may be done in uncontrolled way i.e. - without doing compaction.
During monsoon due to rain water the fill get saturated and settle down on its own which leads to maximum compaction.
To achieve the 100% saturation, normally the water is poured in fill area by tankers.