For reclamation by hydraulic filling with sand, it is commonly observed that the density of sand varies significant with depth. Why?

In reclamation works, the density of filled sand varies with the method of placement. For example, bottom dumping is adopted for sand as the method of placement and this results in higher density of filled sand. For placement by pipeline discharge, the density of filled sand formed by this method is lower than that by bottom dumping. Hence a weak zone is formed for sands placed by pipeline discharge and hence these loose sand fill (relative density about 30%) may cause settlement when subjected to loading or vibrations.

In general, the filled sand above water table is found to be higher in density as they are well compacted by bulldozers and traffic of constructional plant.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.