The composite system of stone columns reinforced soil overlaid by granular mattress (GM) has been given more attention as a hybrid improvement technique. This work presents 3D finite element analyses using PLAXIS 3D software for a composite system of floating stone columns group constructed in a deep soft clay deposit in Port-Said in Egypt. The limited area of loading was investigated in conjunction with the system key parameters, i.e., length, diameter, and the GM main properties. Results validated the effectiveness of the GM in settlement reduction and system behavior enhancement. The GM thickness results in considerable settlement reduction up to value of 1.5d where d is the stone column diameter. The thickness increase can be an economical substitution instead of increasing stone column length where a new factor named “Mattress improvement factor: Ƞgm” is defined to represent the occurring improvement due to GM existence. Results also show notable reduction of settlement with increasing the length to a limiting value of L = 1.8B where B is the footing width.