DHARM

600 GEOTECHNICAL ENGINEERING

Example 14.26: A footing, 2 m square, is founded at a depth of 1.5 m in a sand deposit, for
which the corrected value of N is 27. The water table is at a depth of 2 m from the surface.
Determine the net allowable bearing pressure, if the permissible settlement is 40 mm and a
factor of safety of 3 is desired against shear failure.

Settlement criterion:
N = 27 b = 2 m Df = 1.5 m
Using Teng’s equation for the graphical relationship of Terzaghi and Peck (Fig. 14.20)
for a settlement of 25 mm,

qna = 34.3 (N – 3)

b

b

F +

HG

I

KJ

03

2

.^2
Rγ. Rd

qna is in kN/m^2 b = Width in m

Rγ is the correction factor for the location water table.

Rd is the depth factor.

zγ = (2 – 1.5) = 0.5 m ∴ zq > Df

∴

z

b

γ = 0.5/2 = 0.25

Rq = 1.0 (limiting value)

Rγ = 0.5^1 +

F

HG

I

KJ

z

b

γ

= 0.5(1 + 0.25) = 0.625

Rd = 1 +

D

b

f = 1 + 15

2

. = 1.75

∴ qna = 34.3 × 24 ×

23

4

F.^2

HG

I

KJ × 0.625 × 1.75 kN/m

(^2) ≈ 298 kN/m (^2).
Since this is for a settlement of 25 mm,
qna, for settlement of 40 mm = 298 ×
40
25
≈ 476 kN/m^2.
Shear failure criterion:
For a factor of safety of 3, Teng’s equation for Terzaghi’s bearing capacity equation is:
qns =
1
18 [2N
(^2) b R
γ + 6 (100 + N
(^2) ) D
f Rq],
neglecting (2/3) γDf. (for square footing)

1
18
[2 × 27^2 × 2 × 0.625 + 6(100 + 27^2 )1.5 × 1.0] ≈ 516 kN/m^2
Hence settlement governs the design and the allowable bearing pressure is 476 kN/m^2.
(Note: This is conservative and if Bowles’ recommendation is considered, it can be enhanced
by 50%; in this case shear failure governs the design, and qsafe will be 516 kN/m^2 ).