Chapter 5 — Spread Footings & Mats¶

5.1 Footing types¶

Shallow foundations distribute structural loads over a wide area at or near the ground surface:

Isolated (pad) footings — single columns.
Combined footings — two or more columns sharing one footing.
Strip (wall) footings — continuous under a wall.
Mat (raft) foundations — a single slab under the whole structure, used where loads are large, soils are weak, or differential settlement must be minimized.

5.2 Proportioning for bearing and settlement¶

Footing size is set by the more demanding of:

bearing capacity — the base pressure must not exceed the allowable bearing pressure (Chapter 3); and
settlement — the computed settlement must not exceed tolerable movement (Chapter 4).

For eccentric loading, proportion the footing so the resultant lies within the middle third where possible (no tension at the base), and use the effective area method for capacity.

5.3 Depth of embedment¶

Footing depth is chosen to:

reach a competent bearing stratum below weak or organic surface soils;
be below the frost depth (frost heave) and below the zone of seasonal volume change in expansive clays;
provide adequate cover and resistance to scour, erosion, and uplift;
avoid disturbance from adjacent construction.

5.4 Mat foundations¶

A mat is used when:

spread footings would be too large or would produce excessive differential settlement;
the soil is weak and variable;
loads are heavy and closely spaced;
uplift or hydrostatic conditions favour a large, stiff base.

Mats spread load and equalize settlement, but they are analysed as a soil–structure interaction problem, with the mat stiffness and the soil modulus both important.

5.5 Construction and quality control¶

Subgrade preparation — proof-roll, compact, and protect the bearing surface; do not disturb or soften it.
Water control — keep the excavation dry; water softening of the base is a common cause of settlement.
Backfill and drainage — provide free-draining backfill and positive drainage away from the structure.
Concrete cover — adequate cover for durability in aggressive ground.

5.6 Key takeaways¶

Footings are sized by the more demanding of bearing capacity and settlement.
Keep the resultant within the middle third where possible; use effective area for eccentricity.
Set embedment depth for bearing stratum, frost, expansive soil, and scour.
Mats equalize settlement but are a soil–structure interaction problem.
Protect the subgrade — construction water and disturbance are common causes of poor performance.