Buoyant unit weight (submerged unit weight): The buoyant (effective/submerged) unit weight of a fully saturated soil equals the saturated unit weight ________.

Introduction / Context:In seepage and stability problems, the effective (buoyant) unit weight governs the submerged weight contributing to effective stress. Understanding the relation between saturated and buoyant unit weights is fundamental to piping checks and bearing analyses.

Given Data / Assumptions:

• Soil is fully saturated.
• gamma_sat is the saturated unit weight.
• gamma_w is the unit weight of water.

Concept / Approach:Buoyant (or submerged) unit weight gamma′ represents the reduction of total unit weight due to buoyancy when a soil is submerged: gamma′ = gamma_sat − gamma_w. This follows directly from Archimedes’ principle and the definition of effective stress sigma′ = sigma_total − u.

Step-by-Step Solution:

Verification / Alternative check:For typical sand with gamma_sat ≈ 20 kN/m^3 and gamma_w ≈ 9.81 kN/m^3, gamma′ ≈ 10.2 kN/m^3, which matches common reference values.

Why Other Options Are Wrong:

• Multiplication or division by gamma_w has no physical basis here.
• Addition would increase weight, contradicting buoyancy.

Common Pitfalls:Using gamma_d (dry unit weight) by mistake; forgetting temperature-dependent variations of gamma_w are minor in most geotechnical calculations.

Final Answer:minus the unit weight of water