EN 1997-1 §6.6.1 covers serviceability limit state (SLS) settlement for shallow foundations. It requires calculation of immediate (elastic) settlement, primary consolidation settlement using Terzaghi 1D theory, and secondary compression. The total settlement must remain below the allowable value for the structure.
Immediate (elastic) settlement is calculated using the Boussinesq stress distribution method. The vertical stress increase σ_z at the center of a loaded rectangular area at depth z is computed as σ_z = q_net · B·L / ((B+2z)(L+2z)). The settlement is then Σ(σ_z · H / E_s) per compressible layer.
Terzaghi 1D consolidation theory assumes that excess pore pressure dissipates only vertically (one-dimensional) and that the soil is saturated. The primary consolidation settlement is δ_c = Σ [C_c / (1+e₀)] · log₁₀((σ′_v0+Δσ) / σ′_v0) · H. The time factor Tv = c_v·t / H_d² drives the degree of consolidation U(Tv).
Secondary compression (creep) occurs after primary consolidation and is due to viscous adjustment of the soil fabric under constant load. It is calculated as δ_s = C_α · log₁₀(t₂/t₁) · H, where C_α ≈ C_c/5 to C_c/10 for most clays. It becomes significant for organic soils and peats.
What do t50, t90, and t100 mean?
H_d is half the thickness of the compressible layer for double-sided drainage (most common), or the full thickness for single-sided drainage. Double drainage means water can escape both upward and downward from a clay layer, roughly halving the consolidation time compared to single drainage.