Geotechnical Engineering

DHARM

CAISSONS AND WELL FOUNDATIONS 793

Kp′ =

K

FS

p

..

In view of this, equations 19.28, 19.32, and 19.33 get modified with Kp′ in place of Kp.

Total Safe Lateral Load on a Well

Since equations 19.28, 19.32, and 19.33 for q′max give only the lateral load per unit length of
the well, this value should be multiplied by the length of the well, L, parallel to the water flow
in order to obtain the total lateral load on the well. Since the bulkhead equations are derived
based on the assumption that the length is very much larger than the width, in practice, the
error is considered to be not appreciable if the wells are rectangular in shape. A multiplying
factor, less than unity, called the ‘Shape Factor’, has to be applied for circular wells. This

factor is taken to be π

4

.

However, the shape factor for circular wells with a diameter larger than 4.5 m, is taken
to be unity, as for rectangular wells. The safe lateral load, Qa, for the well would be got by
applying Kp′ in place of Kp in the relevant equation for q′max, and multiplying by the length and
the shape factor as applicable.
Base Pressures
If Q is the actual applied transverse (horizontal) load and Qa is the allowable equivalent resist-
ing force, the unbalanced force (Q – Qa) acting at a height H above the scour level would
produce an overturning moment MB about the base.
MB = (Q – Qa) (H + D)
The maximum and minimum pressures at the base will then be

qmax =

W

A

M

b Z

B

b

+ ...(Eq. 19.34 (a))

and qmin =

W

A

M

b Z

B

b

− ...(Eq. 19.34 (b))

where W = net vertical load on the base of the well, after making allowance for buoyancy and

skin friction,

Ab= Area of the base of the well,

and Zb = Section modulus of the base cross-section of the well.

The maximum pressure should not exceed the allowable soil pressure. The minimum

pressure should not be negative, that is to say, it should not be tensile. It is the general prac-

tice not to give any relief due to skin friction while calculating the maximum pressure at the

base in clays, but to consider it for calculating the minimum pressure, so that the worst condi-

tions are taken into account in either case.

Maximum Moment in Steining

The maximum moment in the steining occurs at the point of zero shear. Referring to Fig. 19.15,

the depth χ to the point of zero shear, S, is such that the applied force and force due to the

mobilised earth pressure balance each other. With a factor of safety η, Kp′ = Kp/η.

[1/2γ′(Kp′ – Ka).χ^2 .L] = Q