Geotechnical Engineering

DHARM

598 GEOTECHNICAL ENGINEERING

qsafe =

2542

3

,

≈ 847 kN/m^2 (for failure against shear)

From settlement consideration,

S

S

p = b b

pbbp

(.)

(.)

+

+

L

N

M

M

O

Q

P

P

030

030

2

Sp = S

bb

bb

p

p

(.)

(.)

+

+

L

N

M

M

O

Q

P

P

030

030

2

= 40

075 2 030

200 075 030

2

.( .)

.(. .)

+

+

L

N

M

O

Q

P = 40

075 23

2105

2

..

.

×

×

F

HG

I
KJ mm = 27 mm
Pressure for a settlement of 27 mm for the plate (from Fig. 14.23) = 550 kN/m^2.
Allowable bearing pressure is the smaller of the values from the two criteria = 550 kN/m^2.
(iii) Design load = 2,000 kN
From Part (ii), it is known that a 2 m square footing can carry a load of 2 × 2 × 550
= 2,200 kN.

Therefore, a 2 m square footing placed at a depth of 1.5 m is adequate for the design
load.

Example 14.23: A loading test was conducted with a 300 mm square plate at depth of 1 m
below the ground surface in pure clay deposit. The water table is located at a depth of 4 m
below the ground level. Failure occurred at a load of 45 kN.

What is the safe bearing capacity of a 1.5 m wide strip footing at 1.5 m depth in the same soil?
Assume γ = 18 kN/m^3 above the water table and a factor of safety of 2.5.

The water table does not affect the bearing capacity in both cases.

Test plate:

qult = Failure load

Area of plate

=

×

45

03 03..

= 500 kN/m^2

γ = 18 kN/m^3

For φ = 0°, Terzaghi’s factors are Nc = 5.7, Nq = 1, and Nγ = 0

∴ qult = 1.3 c Nc + γDf Nq

= 1.3 × 5.7c + 18 × 1.0

= 7.4 c + 18 (kN/m^2 , if c is expressed in kN/m^2 )

∴ 7.4 c + 18 = 500

∴ c =

482

74.^ ≈ 65 kN/m

2

Strip footing:

qult = c Nc + γDf Nq = 5.7 c + γDf in this case.

qnet ult = qult – γDf = 5.7 c

∴ qnet ult = 5.7 × 65 = 370.5 kN/m^2

qnet safe =

370 5

25

.

.^ ≈ 148 kN/m

2

qsafe = qnet safe + γDf = 148 + 18 × 1.5 = 175 kN/m^2

However, the net safe bearing capacity is used in the design of footings in clay.