Course · lesson 1 of 7

Soils for structural engineers

Every foundation design starts with the ground. This lesson separates the allowable bearing pressure from the bearing resistance, explains drained and undrained soil, the partial factors of SANS 10160-5 and the effect of water, and shows what the STATIX foundation calculators ask for.

1. Learning outcomes

After this lesson you can:

tell the allowable bearing pressure (SLS) from the bearing resistance (ULS) and know where each comes from;
say when soil behaves drained or undrained and which strength applies;
apply the SANS 10160-5 partial factors to the soil and work out a bearing resistance by EN 1997-1 Annex D;
allow for the founding depth, the water table and eccentric loads;
give the STATIX foundation calculators the soil values they need.

2. Before you start

RC Design 1, lesson 7 (pad footings: bearing, bending, shear, punching) and Loads 1, lesson 6 (the column take-down).
Your copy of SANS 10160-5, the free pad footing calculator, and the soil values from a geotechnical report if you have one.
A pocket calculator with trigonometry, and about 50 minutes.

3. What the soil must do

What the soil must do. Under a footing the soil must not fail in shear (a wedge under the base pushing soil out sideways) and must not settle more than the building can take. These are two different checks with two different inputs.

Two checks. The allowable bearing pressure q_all from a geotechnical engineer is a service value: it keeps settlement acceptable and is checked with characteristic loads (RC Design 1, lesson 7). The bearing resistance comes from the soil's strength and is checked at the ultimate limit state. The STATIX pad footing calculator does the first always and the second when you enter the soil strength.

4. Soil behaviour

Drained and undrained. Water drains quickly from sand, so it always behaves drained and its strength is the friction angle φ′. Clay holds its water: when loaded quickly it is undrained, with the shear strength c_u, and only slowly becomes drained. A clay footing is checked both ways; the calculator runs whichever you give.

5. Bearing resistance

The drained bearing resistance (EN 1997-1 Annex D, which SANS 10160-5 Annex C.2.2.5 accepts as a recognised model): a cohesion term, an overburden term from the soil beside the base, and a term from the soil's own weight under the base. The factors N depend only on φ′; s are shape factors. STATIX takes no depth factors, as Annex D gives none.

The undrained bearing resistance (Annex D, D.1) is simpler: (π + 2) times the design c_u, with a shape factor, plus the total overburden. It is usually the short-term case that governs on clay.

Partial factors on the soil. In the GEO set SANS 10160-5 divides tan φ′ and c′ by 1.25 and c_u by 1.4; the soil weight keeps 1.0. The STR sets use the unfactored soil with the STR load factors. STATIX checks every set and reports the worst.

design strength: tan φ′_d = tan φ′_k / 1.25; c′_d = c′_k / 1.25; c_u,d = c_u,k / 1.4 drained (D.2): q_Rd = c′ N_c s_c + q′ N_q s_q + ½ γ′ B′ N_γ s_γ N_q = e^{π tan φ′} tan²(45° + φ′/2); N_γ = 2 (N_q − 1) tan φ′ undrained (D.1): q_Rd = (π + 2) c_u,d s_c + q check: V_d (with the footing's weight) ≤ R_d = q_Rd A′

6. Depth, water and eccentricity

Overburden and water. The soil beside the base, down to the founding depth D, holds the failure wedge down. Below the water table the soil is buoyant and weighs only γ′, so a high water table can cut the resistance sharply. The calculator asks for D and the water depth d_w.

Eccentric loads. When the load is off-centre by e, the GEO check uses an effective width B′ = B − 2e centred under the load, with the full load on it. Lesson 2 goes further with moments and uplift.

7. Settlement and what the calculators ask

Settlement in outline. Sands settle almost at once; clays settle slowly as water is squeezed out. A geotechnical engineer predicts it; STATIX does not compute settlement from soil properties. The raft calculator models the soil as springs, with a modulus k, or with q_a and the settlement it causes (k = q_a/s).

What the STATIX foundation calculators ask for. Every one needs some description of the ground; most take the allowable pressure from the geotechnical report, and some take soil strengths directly. Know which values you have before you start.

8. In STATIX

In the STATIX pad footing calculator: the loads and the allowable pressure give the SLS bearing check; the optional “Soil strength (GEO bearing)” group adds the ULS bearing resistance with every factor shown in the result.