The unit weight above the water table should certainly not be specified as the dry
unit weight. In the field, the soil is seldom completely dry except for perhaps a
relatively thin layer near the ground surface.
A situation where the difference in unit weight above and below the water table
may be significant is when very coarse rock fill is placed in water to create a water
break. For this type of situation, assume that e is 1.0, G is 2.7 and γw is 10 kN/m3. Below
the water line where S is 1.0 the unit weight is 18.5 kN/m3. Above the water line
where S is zero, the unit weight is 13.5 kN/m3. This is a significant difference and
consequently worth the effort to specify two different unit weights.