Shallow Foundation Design:
Bearing Capacity & Settlement to EC7

1. Eurocode 7 Design Framework (EN 1997-1)

Eurocode 7 (EN 1997-1) introduces partial factor design for geotechnical structures, moving away from overall global factors of safety (e.g., FOS = 3.0). Footing designs must satisfy Ultimate Limit States (GEO & STR) and Serviceability Limit States (SLS).

Three Design Approaches exist across Europe:

• Design Approach 1 (DA1): Evaluates Combination 1 (Set A1 + Set M1 + Set R1) for structural failure and Combination 2 (Set A2 + Set M2 + Set R1) for geotechnical sizing (used in UK & Ireland).
• Design Approach 2 (DA2): Applies partial factors to actions and resistance simultaneously (Set A1 + Set M1 + Set R2) (used in Germany, France, Italy).
• Design Approach 3 (DA3): Applies partial factors to structural actions and geotechnical soil parameters (used for slopes and retaining walls).

2. Ultimate Bearing Resistance Calculation

The analytical equation for effective bearing resistance R/A' under drained conditions is derived from Meyerhof and Brinch-Hansen formulations:

Where bearing capacity factors depend on effective design friction angle φ'_d = arctan(tan φ' / γ_φ):

• N_q = e^{π tan φ'_d} · tan²(45° + φ'_d/2)
• N_c = (N_q - 1) · cot φ'_d
• N_γ = 2 · (N_q - 1) · tan φ'_d

3. Eccentricity & Effective Area

When subjected to moment M_Ed and vertical force V_Ed, the eccentricity is e = M_Ed / V_Ed. The effective foundation width and length are adjusted:

Ensure eccentricity satisfies e ≤ B/6 to maintain a fully compressed base without uplift.

4. Sliding Verification & Settlement Limits

Sliding resistance R_h,d at the footing-soil interface under drained conditions must satisfy H_Ed ≤ R_h,d:

Where δ_d is the soil-structure interface friction angle (typically 2/3 φ'_k to 1.0 φ'_k for cast-in-place concrete).

Geotechnical Verification Steps

1. 1

   Site Investigation

   Determine characteristic soil parameters (φ'_k, c'_k, c_uk, γ) and groundwater table depth.

2. 2

   ULS GEO Verification

   Check bearing capacity V_Ed ≤ R_d and sliding H_Ed ≤ R_h,d using DA1/DA2 combinations.

3. 3

   SLS Settlement Check

   Calculate immediate elastic settlement and primary consolidation settlement (ΔH ≤ 25 mm allowable).