In many soil mechanics problems, it is necessary to determine the net intergranular weight, or effective weight, of a soil when it is below the groundwater table. (In this context, intergranular refers to the weight or force that acts at the point, or on the surfaces, where soil particles are in contact.) Effective soil weight is used to determine effective stress in a soil deposit, a value that influences factors such as soil shear strength, soil compressibility and settlement, and slope stability—topics discussed in later chapters. For this “underwater” condition, the soil solids are buoyed up by the pressure of the surrounding body of water, and the submerged soil weight becomes less than for the same soil above water. The effective soil weight then becomes the unit weight of the soil material when it is weighed under water. The water in the voids has zero weight (when submerged, all voids can be assumed to be filled with water), and the weight of the soil solids is reduced by the weight of the volume of water they displace. Therefore, a submerged soil weight (W
sub) equals the soil weight above water minus the weight of water displaced, or:
Since unit weight is total weight divided by total volume:
Similarly, in terms of density:
For a given soil (the subject soil) the effective unit weight when submerged (or the submerged effective density) will be the same regardless of depth below the water surface.
For the condition where the soil is 100 percent saturated and the wet unit weight is known, the equations for submerged soil unit weight reflect that the weight (or mass) of both the soil particles and the voids water are for the buoyant condition, to become:
The equation indicates that an accurate determination of the submerged unit weight requires that the specific gravity of the soil solids and the void ratio be known. Unfortunately, in terms of time and expense, some testing or physical analysis is required to determine the specific gravity, which is then used to calculate the void ratio. Also, soil properties such as particle size distribution, void ratios, and soil weights vary somewhat over even relatively limited distances (including areas identified as “uniform deposits”). Commonly for foundation studies at building sites, representative unit weights are selected on the basis of values determined from tested soil samples (samples obtained from borings, test pits, etc.) and assumed for areas between locations of known conditions (borings, etc.). Because of the various practical aspects, the effort to make highly accurate determinations of submerged soil weights is seldom undertaken when studies and designs are done. Instead satisfactory estimates, which can be made from knowing a wet weight, are frequently utilized. For many soils, and fortunately for ease of computation, the submerged unit weight is on the order of half the wet soil unit weight above the water table (related to the relatively limited range of specific gravity values for particles and typical void ratios), see the Eqs. below; a notable exception to this condition is soils containing significant decomposed vegetation or organic material. For many practical applications, the effects from adopting the simplification are negligible (but where accuracy is required, the Eqs. above for γsub or ρsub should be used).
