Unit Weight of Loose Sand

Geotechnical work uses unit weight — force per volume, kN/m³ or pcf — not density, because every calculation that follows is a stress calculation. Loose sand runs 14–17 kN/m³ moist, dense sand 18–21, and the difference is entirely void ratio: the same quartz grains packed loose or tight. That is what compaction buys, and why relative density rather than mass density is the parameter of interest for granular soils.

Three distinct unit weights exist for the same soil and mixing them is the classic error. Dry unit weight excludes pore water; moist (bulk) unit weight includes the water currently present; saturated unit weight assumes all voids are full. Below the water table you use the buoyant (effective) unit weight — saturated minus 9.81 kN/m³ — which for sand is only about 10 kN/m³, roughly half the total. Effective stress, and therefore bearing capacity and lateral earth pressure, is computed on that buoyant value, so a rising water table can cut effective overburden nearly in half. Always get site-specific values from a geotechnical report; these figures are for feasibility arithmetic only.