Geotechnical Engineering

DHARM

790 GEOTECHNICAL ENGINEERING

The last three are the more important ones from the point of view of application. Of
course, the I.R.C. Method is based on the experimental investigations of a number of research
workers in the field and observed behaviour of models of well foundations. While Terzaghi’s
approach is the earliest and the simplest, Lazard’s method is one of the few approaches appli-
cable to cohesive soils.

In view of these comments, the last three alone are considered herein.

19.10.1 Terzaghi’s Analysis

Terzaghi’s (Terzaghi, 1943) solution for free rigid bulkheads may be used for the approximate
analysis of a well foundation. When a rigid bulkhead embedded in sand moves parallel to its
original position, the sand on the front side and rear side are respectively transformed into
passive and active states. Assuming that both the active pressure and the passive resistance
are fully mobilised, the net pressure at any depth z below the ground surface is given by

p = γz(Kp – Ka)
A free rigid bulkhead depends for its stability solely on the lateral resistance.
Let q′max be the horizontal force per unit length acting on the structure of total height
H 1 (Fig. 19.15). The pressure distribution on both sides of the bulkhead at incipient failure
may be represented as shown. The bulkhead rotates about the point 0 at a height of D 1 above
the base. As the soil around the well is usually submerged, the submerged unit weight is used.

q¢max

H

x

F

D

A Scour line

H 1

EB C

g¢D(K – K )p a g¢D(K – K )p a

O

D 1

S

Fig. 19.15 Terzaghi’s analysis (as for a free rigid bulkhead)

Considering unit length, and applying ΣH = 0,

q′max = Area ABC – Area FEC

=

1

2

1

2

γγ′−−′−DK K^2 ()()().pa 2 DK K Dpa 1