Introduction

A foundation must carry the structure's load safely and economically. The major criteria are:

1. Location and depth: the foundation must be placed below the zone of seasonal moisture change, frost, scour, swelling/shrinkage and made ground, and safe from adjacent excavations.
2. Safety against shear failure (bearing capacity): the contact pressure must not exceed the net safe bearing capacity of the soil, i.e. a factor of safety of about 2.5-3 against shear failure.
3. Tolerable settlement: total settlement and especially differential settlement must stay within permissible limits so the superstructure is not damaged.
4. Structural adequacy: the footing/raft/pile itself must be strong enough in bending, shear and punching.
5. Durability and economy: it must resist chemical attack and groundwater, be buildable with local skill and materials, and be the cheapest solution that meets criteria 1-4.

Foundations are chosen by comparing the bearing capacity and compressibility of the soil layers with the building load. They fall in two groups.

Shallow foundations ($D_f \le B$)

Used where a firm layer is near the surface.

Deep foundations ($D_f > B$)

Used where the upper soil is weak, compressible, or scour-prone and a good layer lies deep.

Selection rule: try the cheapest shallow type first; if the footings would cover more than about half the plan area or settlement is excessive, go to a raft; if still unsafe, use piles or wells.

Definition

A foundation is the lowest part of a structure, in direct contact with the ground, that transmits the load of the superstructure (and its own weight) safely to the soil or rock below.

Major design criteria

• Foundation depth must be adequate (below frost, scour, weak and swelling soil).
• Safe against shear failure (bearing capacity with proper factor of safety).
• Total and differential settlement within permissible limits.
• Structurally strong, durable and economical.

Two important things considered

1. Bearing capacity: the soil must not fail in shear under the foundation pressure.
2. Settlement: the soil must not settle more than the tolerable total and differential limits (for example about 25 mm isolated footing in sand, 65 mm for clay as per IS 1904).

The choice of foundation depends on the following factors.

1. Soil profile and properties: bearing capacity, compressibility, depth of good stratum, groundwater level, swelling/collapsible soils.
2. Type and magnitude of load: heavy or light, vertical, lateral, uplift, dynamic loads.
3. Type of structure: building, bridge, tower, machine; sensitivity to differential settlement.
4. Depth of foundation: frost depth, scour, seasonal moisture change, adjacent foundations.
5. Groundwater: need for dewatering, buoyancy, aggressive chemicals.
6. Seismic and site conditions: liquefaction, slope stability, nearby buildings, access for equipment.
7. Construction factors: available materials, labour, equipment, time.
8. Economy: cost of the foundation against the whole structure; the simplest safe type is preferred.
9. Codes and local regulations, such as NBC and municipal rules.

Factors influencing selection of foundation

• Bearing capacity and compressibility of soil, and depth of the firm layer.
• Magnitude and type of loads, and the allowable settlement of the structure.
• Groundwater level, frost, scour and seasonal moisture change.
• Seismic condition, adjacent structures and site restrictions.
• Availability of materials, equipment and skilled labour, and total cost.

Combined footing

A single footing supporting two or more columns. It is used when:

• Columns are so close that their separate footings would overlap or touch.
• An exterior column is at the property line, so an isolated footing cannot be centred under it.
• The soil is weak and a larger area is needed.
• Differential settlement between neighbouring columns must be reduced.

Rectangular combined footing is used when the loads are similar; trapezoidal when the loads differ much or the space is limited at one end.

Strap (cantilever) footing

Two isolated footings joined by a rigid beam (strap). It is used when:

• The exterior column is on the property line and its footing cannot extend outside.
• The distance between columns is large, so a combined footing would be uneconomical.
• The strap takes the eccentric moment of the edge footing and transfers it to the inner footing, so both footings get uniform pressure.

A shallow foundation transfers load to the soil near the surface ($D_f \le B$); a deep foundation transfers load to a deeper, stronger stratum ($D_f > B$, usually $D_f/B > 4$ to 5 for piles).

Types

• Shallow: isolated (spread) footing, wall (strip) footing, combined footing, strap footing, raft (mat) foundation.
• Deep: pile foundation (driven, bored, friction, end-bearing), pier foundation, well (caisson) foundation, drilled shaft.

Why foundations are used

• To spread the structural load over a large soil area so the pressure is less than the safe bearing capacity.
• To keep total and differential settlement within permissible limits.
• To place the structure on a stable level below frost, scour and weak surface soil.
• To resist uplift, overturning, sliding and lateral/seismic forces.

Types of foundation

Shallow foundations ($D_f \le B$)

• Isolated (spread) footing: one footing under each column; square, rectangular or circular.
• Wall (strip) footing: continuous footing under a load-bearing wall.
• Combined footing: common footing for two or more columns.
• Strap footing: two footings joined by a strap beam.
• Raft (mat): one large slab under the whole structure; used on weak soil or heavy loads.

Deep foundations ($D_f > B$)

• Pile: slender driven or bored member; end-bearing or friction pile.
• Pier / drilled shaft: large-diameter column, bored and concreted.
• Well (caisson): large hollow masonry or concrete cylinder sunk by dredging, used for bridge piers in rivers.