Differences Between Saturated Unit Weight and Submerged Unit Weight

 

Differences Between Saturated Unit Weight and Submerged Unit Weight

Factor	Saturated Unit Weight	Submerged Unit Weight
Definition	Weight of soil when all voids are filled with water.	The effective weight of soil when submerged under water, considering buoyant forces.
Formula
Weight Consideration	Accounts for both soil particles and the water filling the voids.	Takes into account the reduction of soil weight due to buoyant forces.
Applications	Used in designing structures in fully saturated soils.	Relevant for underwater construction or soils below the water table.
Impact on Soil Strength	Soils tend to lose strength under saturated conditions.	Soils become lighter and more susceptible to instability when submerged.

Significance in Civil Engineering

Foundation Design:

Where the ground water table is deep or construction takes place under water, the saturated and submerged unit weight must be taken into account in soil bearing capacity determination.

If buoyancy/submerged unit weight is not accounted for, pressure on the foundation could be undervalued even to the point of failure of foundations.

Slope Stability:

For embankments and slopes, especially when submerged, lower weight of soil has to be considered to avoid landslide or soil collapse.

The calculation of submerged weight is very essential in designing coastal and river embankments.

Hydraulic Structures:

The dams, levees, amongst other hydraulic structures should be designed to acquire not only the saturated weight when water penetrates the soil, but the submerged weight when the soils are underwater as well.

Therefore, engineers should understand how soil behaves in saturated and submerged conditions because it impacts the system with possibilities such as piping and erosion failures.

Both saturated unit weight and submerged unit weight play fundamental roles in geotechnical and civil engineering, particularly for projects dealing with water-saturated soils or underwater construction. The understanding between the two conditions enables the design of safer structures, which would be more reliable considering the loss in soil strength in saturated conditions and the effective reduction in unit weight due to submersion.