Geotechnical Engineering

DHARM

800 GEOTECHNICAL ENGINEERING

Taking the centre of rotation to be at a height of 0.2 D above the base, it can be shown
that

D 1 = D/3 from similar triangles.

Taking moments about O,

Ms′ = 0.096D^2 [γD(Kp – Ka)]

or Ms′ = 0.1γD^3 (Kp – Ka) nearly.

If submerged unit weight is taken,
Ms′ = 0.1γ′D^3 (Kp – Ka)
For a Well of length L,
Ms = 0.1γ′D^3 (Kp – Ka). L ...(Eq. 19.56)
Resisting Moment from front and back faces
The frictional forces on the faces act in the vertical direction and produce a resisting
moment Mf, given by

Mf =

11

3

. γδ′−DK K (^2) ()sinpa
For a rectangular well of length L,
Mf =
11
3
L. γδ′− (^2) ()sin(/). 2
NM
O
QP
DK Kp a B L
or Mf = 0.183 γ′(Kp – Ka)LBD^2 sin δ ...(Eq. 19.57)
For Circular Wells,
Mf = 0.11 γ′ (Kp – Ka)B^2 D^2 sin δ ...(Eq. 19.58)
with a shape factor of 0.6.
Total Resisting Moment
Mr = Mb + Ms + Mf ...(Eq. 19.59)
IRC: 45–1970 recommends a reduction factor of 0.7 in the total resisting moment of the soil.
Thus Mr = 0.7(Mb + Ms + Mf) ...(Eq. 19.60)
The applied moment, M, should be less than Mr.
Thus M ≤ 0.7(Mb + Ms + Mf)
In this is not satisfied, the grip length has to be increased and the computations re-
peated, until this is satisfied. In the computation of applied moments, the moments due to tilt
and shift of the well, if any, should also be considered.
Check for Maximum Pressure
The maximum average pressure should not exceed half the ultimate bearing capacity
W/A ≤ qu/2
where qu = Ultimate bearing capacity of the soil.
19.10.3 Lazard’s Hypothesis
Lazard (1957) analysed the stability of foundation for transmission towers in c-φ soils, the
depth of foundation being 1 to 3 m and the diameter (or width) being 0.55 to 1.00 m. The
foundation is laid by first drilling a hole in the soil and filling the hole with concrete. Thus this