Geotechnical Engineering

DHARM

794 GEOTECHNICAL ENGINEERING

or x =

2

1/ 2

Q

γ′KKLpa′ −

L

N

M

M

O

Q

P

().P

...(Eq. 19.35)

Taking moments about S,
Mmax = Q(H + χ) – (Force due to pressure).χ/3
Taking the force due to pressure as being equal to Q,
Mmax = Q(H + χ) – Q(χ/3)
or Mmax = Q.H + (2/3)Q.χ ...(Eq. 19.36)
If the well rests on rock or on unyielding stratum, no rotation need be expected, and the
moment developed is transmitted to the foundation bed, which withstands it.

19.10.2 I.R.C. Method
Indian Roads Congress (I.R.C.) gives a procedure (IRC: 45-1970) for estimating the resistance
of the sand below the maximum scour level in connection with the lateral stability of a well
foundation. Their recommendations are based on extensive experimental investigations on
well foundation models carried out be several research workers. Elastic theory is permitted to
be used to determine the earth pressure at the side and soil reaction at the base, caused by
design loads. However, the ultimate soil resistance is to be computed for estimating the factor
of safety against shear failure.
The following assumptions are made in the elastic theory:
(i) The well behaves like a rigid body.
(ii) The coefficient of horizontal subgrade reaction increases linearly with depth.
(iii) Unit soil reaction increases linearly with the lateral deflection.
(iv) The well is acted upon by an external horizontal force and a moment at the scour
level.
Pressure Distribution on Sides
Figure 19.18 (a) shows a rigid well with its base at a depth D below the scour level. The well
may rotate about a point above the base, at the base, or below the base. If the centre of rotation
lies above the base, the latter moves towards the former, and hence, the frictional force at the
base acts in the direction of the horizontal forces, H. However, if the centre of rotation lies
below the base, it will be in the direction opposite to that of H.

In general, the frictional force F is given by

F = β.μ.W ...(Eq. 19.37)

where μ = coefficient of friction,

W = total load,

and β = a factor which lies between –1 and +1, depending upon the location of the centre of
rotation.
If the well rotates about a point C [Fig. 19.18 (b)], the lateral deflection at any depth z is
given by

ρH = (D – z)θ ...(Eq. 19.38)