Shallow Foundations — Types, Proportioning & Rafts

Requirements of a foundation

A foundation must:

1. be safe against shear failure of the soil (adequate bearing capacity with a factor of safety);
2. keep total and differential settlement within permissible limits;
3. be placed at a depth safe from seasonal moisture changes, frost, scour and the effect of adjacent construction;
4. be structurally strong and durable against aggressive soil and water;
5. be economical and practical to construct.

Types of shallow foundations

Depth of foundation

Factors governing depth

• Stratum of adequate bearing capacity and acceptable settlement.
• Seasonal volume change zone in expansive clays (often 1.5–3.5 m in black cotton soils).
• Frost penetration in cold regions.
• Scour depth for bridge foundations near water.
• Ground water level, adjacent foundations and future excavations.
• A minimum depth of about 0.5 m (practice often uses 0.8–1.0 m for buildings) to remove topsoil and vegetation and protect against weathering.

Rankine's minimum depth (cohesionless soil)

For a footing with pressure $q$ on soil with $\gamma$, $\varphi$:

Proportioning footings for uniform settlement

Footings of different sizes carrying the same pressure settle differently (larger footings settle more on clay because the pressure bulb is deeper). To reduce differential settlement, footings may be proportioned on the basis of the dead load plus the average (sustained) part of the live load rather than the full design load: the footing with the highest ratio of live to dead load is sized for the allowable pressure under full load, and the same "service" pressure is used for all others.

Combined footings

For two columns with loads $P_1$ (exterior, at property line) and $P_2$, spaced $s$ apart:

1. Locate the resultant: $\bar{x}=\frac{P_{2}s}{P_1+P_2}$ from column 1.
2. Make the centroid of the footing coincide with the resultant so pressure is uniform.
3. Rectangular footing: length $L=2\times$ (distance from property line to resultant); width $B=\frac{P_1+P_2}{q_{a}L}$.
4. If the length is restricted at both ends or loads are very unequal, use a trapezoidal footing (wider under the heavier load).
5. Design longitudinally as an inverted beam (upward soil pressure, columns as supports) and transversely for local bending under each column.