Frequently Asked Questions
What is Design Approach 1 (DA1) in EN 1997-1?
EN 1997-1 §2.4.6 Design Approach 1 applies two sets of partial factors to the same underlying soil parameters. Set 1 (A1+M1+R1) uses γ_G=1.35, γ_Q=1.50 on actions and γ_Rv=1.0 on resistance. Set 2 (A2+M2+R1) reduces action factors to γ_G=1.00, γ_Q=1.30 but applies γ_φ=1.25, γ_c=1.25 to soil strength parameters. The governing utilization is the worse of the two sets.
How are bearing capacity factors N_c, N_q, N_γ derived?
For φ=0 (undrained clay): N_c=5.14, N_q=1.0, N_γ=0 (Terzaghi). For φ>0: Prandtl-Reissner N_q = e^(π·tan φ)·tan²(45+φ/2), N_γ = 2(N_q+1)·tan φ, and N_c = (N_q−1)/tan φ (Caquot-Kerisel). These are from EN 1997-1 Annex C.
What is the effective area method for eccentric loads?
When load is eccentric (M ≠ 0), Meyerhof (1953) reduces the footing area: B′ = B−2·e_B, L′ = L−2·e_L. The bearing check uses A′ = B′·L′ and verifies e ≤ B/6 to ensure no tensile contact. EN 1997-1 §6.5.2 Eq. (6.1) confirms q_Ed ≤ R_d/A′.
How is immediate settlement calculated?
The Janbu modulus method gives s_i = (q_net/E_u)·B·(1−ν²)·I, where ν ≈ 0.3 is Poisson ratio and I ≈ 1.0−0.33·sin φ is an approximate influence factor. E_u is the soil modulus from oedometer or triaxial tests. The result is in mm and should be added to consolidation settlement.
What are the typical allowable settlement values?
EN 1997-1 §6.6.2 Table 6.1 gives indicative values: total settlement ≤ 50 mm for isolated footings, ≤ 25 mm for rafts. Maximum angular rotation ≤ 1/500 for isolated foundations (sensitive structures) or ≤ 1/250 for non-sensitive. Consult the National Annex for the applicable limits in your country.
When is consolidation settlement relevant vs. immediate settlement?
Immediate settlement (Janbu method) applies to all soil types and occurs as load is applied. Consolidation settlement applies only to fine-grained soils (clay, silt) with low permeability, where pore pressures dissipate over time. For sand and gravel, consolidation is negligible and immediate settlement governs. The tool lets you enable/disable consolidation via the soil layer inputs.