Bearing Capacity of Shallow Foundations

CIVL 451 – Foundation Engineering – Classroom Notes

General Bearing Capacity Equation

• The general bearing capacity equation is suggested by Meyerhof (1963) for modifying bearing capacity theory for the following aspects;

– Shape of the foundation

– Load Inclination

– Depth

– Shearing resistance along the failure surface in soil above the base of the foundation

General Bearing Capacity Equation

General Bearing Capacity Equation

General Bearing Capacity Equation

General Bearing Capacity Equation

• Solve Ex. 3.1 using general bearing capacity equation and comment on the results.

• Ex. 3.3 A square foundation has to be constructed as shown in the figure. Assume that the soil unit weights above and below the water table are 17kN/m3 and 19kN/m3 respectively. The internal angle of friction is 34° and the design column load is 500kN. Determine the size of the foundation considering general bearing capacity equation.

• D1=0.5m and Df=1.5m

Pressure Distribution at Foundation Base

• The foundation design in practice requires that the foundation dimensions are calculated for a given design column load, hence the bearing capacity calculation is used to check that the applied pressure at the base of the foundation can be resisted by the ground, i.e.

qapplied ≤ qallowable

• Another way of presenting the above check is to indicate the ratio of the applied pressure to allowable pressure as the ‘utilisation’ of the allowable bearing capacity in percentage.

% utilisation = qapplied / qallowable x 100

Pressure Distribution at Foundation Base

• The pressure distribution at the base of an elastic foundation is assumed to be;

– Uniform, if the applied load is vertical and applied at the centre of the foundation.

– Irregular, if the applied load is eccentric, inclined and/or a moment is applied.

• The magnitude of the maximum and minimum pressure applied at the foundation base is dependent on the eccentricity caused by inclined loads and/or moment.

Pressure Distribution at Foundation Base

• One-way eccentricity; For e < B/6

For e > B/6

qmin = 0

Note that B is used to compute shape factors in general bearing capacity equation.

Pressure Distribution at Foundation Base

• However, shallow foundations are traditionally designed in accordance with the ‘Middle-third Rule’, which is based on the restriction that the eccentricity should be within the middle third of the foundation width or in other words;

e ≤ B/6

Middle third

B/3 B/3 B/3

Eccentricity should be within

this zone

B/6 B/6

Pressure Distribution at Foundation Base

• Two-way eccentricity;

In order to evaluate effective width and length the following cases should be considered.

Pressure Distribution at Foundation Base

Pressure Distribution at Foundation Base

Pressure Distribution at Foundation Base

Pressure Distribution at Foundation Base

Pressure Distribution at Foundation Base

• Note that B is used to compute shape factors in general bearing capacity equation.

• Always consider the ‘Middle-third Rule’ when sizing the foundations.

• Meyerhof’s simplified approach can be used for a quick bearing capacity estimation considering one or two-way eccentricity.

Exercises

References

• B. M. Das, Principles of foundation engineering.