Geotechnical Engineering

DHARM

BEARING CAPACITY 585

Rγ and Rq = correction factors for the position of the ground water table, defined in Eqs.
14.102 and 14.103.

With a factor of safety of 3, the net safe bearing capacity qns, is given by

qns =

1

18

[3N^2 b Rγ + 5(100 + N^2 )Df Rq] –

2

3

γ. Df ...(Eq. 14.125)

(for continuous footings)

qns =

1

18

[2N^2 b Rγ + 6(100 + N^2 )Df Rq] –

2

3

γ. Df ...(Eq. 14.126)
(for square or circular footings)
If γ is not known with any degree of confidence, the second term may be ignored al-
though this may introduce some error in the value of qns. The smaller of the two values qna and
qns will be used for design.

14.14 BEARING CAPACITY OF CLAYS

For pure clays, φ = 0°.

qult = cNc + γDf = 5.7c + γDf
∴ qnet ult = 5.7c, for continuous footings.
qnet ult = 1.3 × 5.7c = 7.4 c ...(Eq. 14.127)
(for square or circular footings, c being the cohesion.)
These are from Terzaghi’s theory. Alternatively, Skempton’s equations may be used.
(Eqs. 14.19 and 14.92).

Skempton’s equations are preferred for rectangular footings in pure clay (Eqs. 14.91
and 14.92).

Correlation of cohesion and consistency of clays with N-values is not reliable. Unconfined
compression test is recommended for evaluating cohesion.

Overconsolidated or precompressed clays might show hair cracks and slickensides. Load
tests are recommended in such cases.

Settlements of footings in clays may be calculated or predicted by the use of Terzaghi’s
one-dimensional consolidation. Long-term load tests also may be used but they are highly
cumbersome and time-consuming.

The bearing capacity of footings in clays is practically unaffected by the size of the foun-
dation.

14.15 RECOMMENDED PRACTICE (I.S)

The safe bearing capacity may be obtained from the relevant table given in IS: 1904–1986
(Revised). Where data for characteristics of a soil (cohesion, angle of internal friction, density,
etc) are available, the safe bearing capacity may be calculated from consideration of shear
failure (Terzaghi’s theory). A factor of safety of three shall be adopted. Safe bearing pressure
for sands may be obtained from the standard penetration resistance values (corrected for the
presence of ground water and for overburden pressure), from both considerations of shear