Q1: If the water table reaches the ground level, the unit weight of soil for bearing capacity calculation is taken as:
📝 Detailed Explanation
Soil possesses different unit weights depending on its moisture state, and choosing the correct one is critical in a bearing-capacity calculation. When the water table rises all the way to the ground surface, every soil particle below that level is submerged in water. A submerged particle experiences an upward buoyant force (per Archimedes’ principle) equal to the weight of water it displaces, which directly offsets part of its own weight. Because bearing capacity depends on the effective (particle-to-particle) contact pressure transmitted through the soil skeleton, engineers must use the submerged (buoyant) unit weight, γsub = γsat − γw, rather than the dry, bulk, or fully saturated unit weight, since those do not account for the buoyant reduction.
Using a unit weight that ignores buoyancy (such as the saturated or bulk unit weight) would overestimate the effective stress in the soil and, consequently, overestimate the bearing capacity — an unsafe error in foundation design.
💡 Key Concepts for Students
- Submerged/buoyant unit weight: γsub = γsat − γw = (G−1)γw/(1+e), always less than the saturated unit weight by exactly γw.
- Why buoyancy matters for bearing capacity: Terzaghi’s bearing capacity equation uses the effective unit weight of soil below the footing/water table; ignoring buoyancy overstates the safe bearing pressure.
- Bulk vs. saturated vs. submerged: bulk unit weight applies to a soil with any degree of saturation, saturated unit weight applies when all voids are filled with water but no buoyant correction is made, and submerged unit weight applies when the soil sits below the water table.
- Effective stress principle: this scenario is the practical face of Terzaghi’s principle — total stress minus pore water pressure equals effective stress, and it is effective stress that governs shear strength and bearing capacity.