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