Stand on loose sand. Your feet go deep into the sand beause [[1]].

Force exerted on the sand is the weight of your body 
When you stand on loose sand, the force is acting on an area equal to the area of your feet.
Pressure =$\frac{\mathrm{thrust}}{\mathrm{area} \mathrm{of} \mathrm{contact}}$
The effect of thrust, depends on the area on which it acts