Foundation Types: Spread Footings, Wall Footings, Mats and Deep Foundations

NCEES FE Civil spec: Geotechnical Engineering, H. Foundation types (e.g., spread footings, deep foundations, wall footings, mats). FE Reference Handbook: Civil Engineering chapter, Geotechnical section (bearing capacity, retaining wall toe pressure).

What the FE Civil Spec Covers

Section H asks you to choose and size foundations: spread (isolated) footings, wall (strip) footings, combined footings, mats, and deep foundations such as driven piles and drilled shafts. Expect questions on when each type is appropriate, sizing a footing from an allowable pressure, pressure under eccentric loads, and basic pile capacity from end bearing plus side friction.

Key Concepts

Type	Typical use
Spread footing	Single column on competent soil near the surface
Wall (strip) footing	Bearing walls; designed per unit length
Combined footing	Two columns close together or a column near a property line
Mat (raft)	Heavy loads, weak or variable soil, footings covering over about half the footprint
Piles and drilled shafts	Weak upper soils, high loads, scour, uplift or lateral loads; transfer load to deeper, stronger layers

Shallow foundations are sized with service (unfactored) loads against an allowable soil pressure. The concrete design of the footing itself (punching shear, one-way shear, flexure) uses factored loads and net soil pressure.

Choosing a Foundation

Find the depth of competent bearing soil from borings and SPT or CPT data.
If competent soil is shallow, try spread and wall footings; check bearing capacity and settlement.
If footings overlap or cover a large share of the plan, consider combined footings or a mat.
If weak or compressible soil extends deep, or loads are very high, go to piles or drilled shafts.

Deep foundation notes. Driven piles (steel H, pipe, precast concrete, timber) displace soil and their capacity can be verified by driving resistance or load tests. Drilled shafts are cast in place, carry very large loads and avoid driving vibration. Piles in a group can have less capacity than the sum of single piles in clay (group efficiency below 1). If soft clay around a pile settles under new fill, it drags the pile down, adding load instead of resisting it; this is negative skin friction (downdrag).

Core Formulas

Required area for a concentric load:

A_{r e q} = \frac{P}{q _{a l l}}, B = A_{r e q} (square footing)

Pressure under a footing with eccentricity $e = M / P \leq B /6$ :

q_{ma x, min} = \frac{P}{A} (1 \pm \frac{6 e}{B})

Deep foundation capacity (end bearing plus side resistance):

Q_{u l t} = Q_{p} + Q_{s} = q_{p} A_{p} + \sum f_{s} p Δ L, Q_{a l l} = \frac{Q _{u l t}}{F S}

Here $p$ is the pile perimeter and $f_{s}$ the unit side resistance over each length $Δ L$ .

Where It Is in the FE Reference Handbook

The Civil Engineering chapter, Geotechnical section, covers bearing capacity and the retaining wall toe pressure equation (the same trapezoidal pressure idea used for eccentric footings). Footing shear and flexure design is in the Structural Design section under reinforced concrete.

Worked Example: Square Footing with a Moment

A column carries a service load of 1,000 kN. The allowable net soil pressure is 180 kPa. (a) Size a square footing for the axial load alone. (b) The column also transfers a moment of 150 kN·m. Check the footing and resize if needed (round to 0.1 m).

Step 1: concentric sizing.

A_{r e q} = \frac{1000}{180} = 5.56 m^{2}, B = 5.56 = 2.36 m \Rightarrow use 2.4 m

Step 2: eccentricity.

e = \frac{150}{1000} = 0.15 m < \frac{B}{6} = 0.40 m (full contact)

Step 3: pressures with B = 2.4 m.

\frac{P}{A} = \frac{1000}{2. 4 ^{2}} = 173.6 kPa, q_{ma x} = 173.6 (1 + \frac{6 ( 0.15 )}{2.4}) = 238.7 kPa

238.7 kPa exceeds 180 kPa, so the footing must grow. Try B = 2.7 m: P/A = 137.2 kPa and $q_{ma x} = 137.2 (1 + 0.333) = 182.9$ kPa, still slightly high.

Step 4: try B = 2.8 m.

\frac{P}{A} = \frac{1000}{7.84} = 127.6 kPa, q_{ma x} = 127.6 (1 + \frac{0.9}{2.8}) = 168.5 kPa \leq 180

q_{min} = 127.6 (1 - \frac{0.9}{2.8}) = 86.6 kPa > 0

Use a 2.8 m square footing. The moment raised the footing area from 5.76 m² to 7.84 m², about 36% more.

Common Mistakes

Sizing with factored loads; footing area uses service loads against allowable pressure.
Forgetting footing and backfill weight when the allowable pressure is a gross value.
Applying the trapezoid formula when e exceeds B/6; the base then partly lifts off and the formula no longer applies.
Using area instead of perimeter times length for pile side resistance.

Practice Problems

Try these from the free PolyCalc FE Civil problem bank. Each one has a full worked solution.

A square footing carries a service load of 900 kN. If the allowable bearing pressure is 150 kPa, the minimum footing width is most nearly: See the worked solution or practice the Soils & Foundations deck
A square footing carries a total concentric service load of 800 kN, including the weights of the footing and overburden. The gross allowable soil bearing pressure is 200 kPa. Assume uniform pressure and no moment. What is the minimum theoretical side length B of the square footing? See the worked solution or practice the Concrete: Columns, Slabs & Footings deck
A strip footing has an ultimate bearing capacity of 160.5 kPa. With a factor of safety of 3, the allowable bearing pressure is most nearly: See the worked solution or practice the Soils & Foundations deck
A centered 400 mm square column applies a factored load Pu = 900 kN to a square footing with effective depth d = 200 mm. The footing extends well beyond the critical perimeter. The uniform net factored upward soil pressure is 100 kPa (footing weight and overburden already removed from it), balancing the column load; no moment is transferred. The critical square lies d/2 from every column face. What is the punching-shear demand Vu across that perimeter? See the worked solution or practice the Concrete: Columns, Slabs & Footings deck

Related Calculators and Examples

Frequently Asked Questions

When is a mat foundation used instead of spread footings?

A mat is used when individual footings would cover more than about half of the building footprint, when the soil is weak or variable, or when differential settlement must be limited. One thick slab spreads the load and bridges soft spots.

What is the difference between end bearing and skin friction in piles?

End bearing is the load carried by the soil or rock under the pile tip. Skin friction, also called side resistance, is carried by shear along the pile shaft. Friction piles in clay rely mostly on side resistance; piles driven to rock rely mostly on end bearing.

What is the kern or middle third rule?

If the resultant load stays within B/6 of the footing center (the middle third of the width), the whole base stays in compression and the pressure distribution is a trapezoid. Beyond B/6 part of the footing lifts off.

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