Geotechnical Engineering

DHARM

BEARING CAPACITY 593

Skempton’s equation:

qnet ult = c. Nc, where Nc = 5 F 102 +

HG

I

KJ

.

b

L

(1 + 0.2 Df /b) for Df /b ≤ 2.5)

∴ Nc = 5 1021

2

F +×

HG

I

KJ

. = 5.5, since Df = 0.

∴ qnet ult = 5.5 × 50 = 275 kN/m^2.
Since Df = 0, qult = qnet ult = 275 kN/m^2.
Example 14.17: What is the safe bearing capacity of a rectangular footing, 1 m × 2 m, placed
at a depth of 2 m in a saturated clay having unit weight of 20 kN/m^3 and unconfined compres-
sion strength of 100 kN/m^2? Assume a factor of safety of 2.5.

Rectangular footing:

b = 1 m L = 2 m Df = 2 m qu = 100 kN/m^2 γ = 20 kN/m^3

Df /b =

2

1

= 2 b/L =

1

2

c =

1

2

qu = 50 kN/m^3

Skempton’s equation:

qnet ult = c. Nc, where Nc = 5^102 +^102

F

HG

I

KJ

+

F

HG

I

KJ

..b

L

D

b

f for D

f /b^ ≤ 2.5

Since Df /b = 2 < 2.5,

Nc = 5^102

1

2

FHG +×. IKJ (1 + 0.2 × 2/1) = 7.7

∴ qnet ult = 7.7 × 50 = 385 kN/m^2

qnet safe =

qnet ult

η =

385

25.

= 154 kN/m^2

qsafe = qnet safe + γDf = 154 + 20 × 2 = 194 kN/m^2.

Example 14.18: A steam turbine with base 6 m × 3.6 m weighs 10,000 kN. It is to be placed on

a clay soil with c = 135 kN/m^2. Find the size of the foundation required if the factor of safety is

to be 3. The foundation is to be 60 cm below ground surface.

Skempton’s equation:

qnet ult = 5c GFH 102 + IKJ 102 +

F

HG

I

KJ

..

b

L

D

b

f for D

f /b^ ≤ 2.5.

Df = 0.6 m

For φ = 0°, Nγ = 0 and Nq = 1 Assume γ = 18 kN/m^3.

Adopt b/L = 0.6, same as that for the turbine base.

Df /b = 0.6/b

Area, A = bL =

bb^22

06

5

3.

=

F

HG

I

KJ

m^2

∴ qnet ult = 5 × 135 (1 + 0.2 × 0.6) 1

02 06

+

F ×

HG

I

KJ

..

b

= 756 1

012

FHG + IKJ

.

b

kN/m^2