The Maasvlakte II project is a joint venture between Boskalis and Van Oord. During the construction, they have measured the relative density for different kind of work methods, by making cone penetration tests. The result of those measurement is, that the measured relative density for the Maasvlakte II project is higher than the values that can be found in literature for the different kind of work methods. Thus the objective of this MSc-thesis is to understand the influencing and determining processes for the relative density of dumped or sprayed sand underwater. This MSc-thesis can be divided in four parts. The first part describes the characteristics of sand and the relation between the density and the relative density. Laboratory testing is executed to determine the relevant parameters of the sand, such as the minimum and maximum density and the particle size distribution. The second part describes the used work methods, such as dumping, rainbowing, back filling through the suction pipe and spraying with a spray pontoon, that are used by the construction of the Maasvlakte II project. The third part describes the analysis of the CPT data. The CPT data is linked to the work methods and the relative density based on the correlations of Baldi and Lunne and Christoffersen. Also, a statistical analysis is made. The statistical analysis is based upon different kinds of statistical tests, such as Kruskal-Wallis and Wilcoxon Rank Sum Test. The last part consist of physical hypotheses that explain the potential influencing and determining processes for the relative density of dumped and sprayed sand underwater. The work methods used for the hydraulic placement, the discharge conditions, the layer thickness, the water depth and the soil properties of the borrow area are all important in the achieved relative densities. From statistical testing, with the Kruskal-Wallis and Wilcoxon Rank Sum Test, the used work method has an influence on the achieved relative density.