Geotechnical Engineering

DHARM

CAISSONS AND WELL FOUNDATIONS 801

differs from the well foundation in the method of construction. Lazard gives empirical rela-
tionships for the limiting overturning moment, QH.

( QH)limit = K(27.45) MB2/3 ...(Eq. 19.61)

Here MB = (1 – εp) (K 1 ′′eNr + K 2 ′′γbD^3 )

In this

K 1 ′′ = 0 5136

0 175

054

..
(. / )

−

+be

K 2 ′′ = 28 96 5

68 5 3 375

10

.. 1045
..

− (./)

+

F

HG

I

KJ

F

H

G

G

G

G

I

K

J

J

J

J

+

N

eb

r

γbe a

where Q = Horizontal force applied at a height H above the ground surface

(QH)limit = limiting or peak value of overturning moment

K = Coefficient to take into account the configuration of the terrain, the direction

of applied pull, and the depth of embedment. It is taken as unity for flat

terrain and the direction of pull towards the fields (Values of K are given in

Table 19.7)

(1 – εp) = Correction factor for overburden

= 344 1 244 1

323

..+F ′

HG

I

KJ

L

N

M

M

O

Q

P

P

−+F ′

HG

I

KJ

R

S

|

T|

U

V

|

W|

D

D

D

D

Here D′ = depth of surface layer of terrain which has no cohesion and is dry.

D = depth of foundation

e = dimension parallel to the applied pull

b = dimension perpendicular to the applied pull

a = smaller of the two dimensions e and b

[Note: For cylindrical foundation with circular base of diameter 2R, a = b = e = 0.8 (2R)]

γ = unit weight of soil

Nr = total vertical load (weight of foundation, pole, fittings, etc.,)

Table 19.7 Values of K (Lazard’s approach)

Configuration of Terrain Direction of Pull

Towards Along the Railway Line

the

field i > 2 m i ≤ 2m

Embankment 0.85 0.95 1.50

Grade 1.00 1.30 2.00

Cutting 1.50 1.80 ≥ 2.00

Fig. 19.21 is the Key figure for Lazard’s approach.