Unit Weight of Soft Clay

Soft clays hold enormous amounts of water — moisture contents of 40–80 % by dry weight are ordinary, and some marine and organic clays exceed 100 % — so their unit weight is low despite the mineral grains having a specific gravity of 2.7. Very soft organic clays and peats fall to 11–14 kN/m³, barely above water itself.

The unit weight is rarely the parameter that governs. What controls foundation design in clay is undrained shear strength and consolidation behaviour: soft clay settles for years under load as water is squeezed from the pores, and total settlements of hundreds of millimetres are routine. Unit weight enters as the overburden that sets the pre-consolidation stress and effective stress profile, and errors there propagate into every settlement estimate. Use published figures for preliminary work only — soft clay is the soil type where site investigation pays for itself most often, and where a value taken from a table has caused the most damage.