Unit Weight of Soil

There is no single unit weight of soil: dry, moist, saturated and submerged are four different numbers for the same ground, and every geotechnical calculation cares which one it is fed.

Notes for design and site

Unit weight is solids plus whatever water rides along, so one soil owns several numbers. Dry unit weight counts the grains alone, moist adds field moisture, saturated fills every void, and submerged subtracts the 9.81 kN per cubic meter of water that buoyancy gives back. Feeding the wrong one into a calculation is the most common unit weight mistake there is.

The rule that keeps checks honest: above the water table use moist, below it use saturated for total stress, and let the effective stress equation do the subtracting rather than doing it by hand twice. Lateral earth pressure scales directly with unit weight, so a wall designed for dry backfill at 16 and built with saturated fill at 20 sees a quarter more soil pressure, plus the water pressure it was never designed for.

Compaction reporting uses dry density on purpose: water adds weight to a test but no strength to a fill, so the specification chases the solids. That is why a proctor result and a design unit weight are related but never the same number.

Related Calculators and Reference

• Effective Stress: where moist, saturated and submerged earn their definitions.
• Retaining Wall Pressure Calculator: lateral pressure scaling directly with these values.
• Void Ratio, Porosity and Unit Weight Converter: move between void ratio, porosity, moisture and unit weight.
• Proctor Compaction Calculator: the dry density target for engineered fill.
• Friction Angle of Dense Sand: matching strength values, one page per soil type.
• Trench Excavation Calculator: the bank volume of a trench, which you can turn into a weight with a soil unit weight.
• FHWA Geotechnical: reference manuals for soils and foundations.