DHARMCAISSONS AND WELL FOUNDATIONS 789
If the width of the pier is greater than the size of the dredge hole, it is assumed that the
weight of a cone of concrete having an apex angle of 60° is carried by the slab, and the remain-
ing load is transmitted to the steining.
The well cap should be provided with a minimum reinforcement of 0.8 kN/m^3.*19.10 LATERAL STABILITY OF WELL FOUNDATIONS
It is generally assumed that the well tends to rotate about the base due to lateral forces. This
is opposed by the resisting moment offered by the soil around the well and at the bottom.
The minimum grip length of a well foundation has been evaluated earlier from hydrau-
lic consideration. If the total resisting moment from both the sources of soil around the well
and that at the base exceeds the overturning moment caused by the external loads, the well is
considered to be safe with respect to lateral stability and deformation considerations. Other-
wise, the grip length obtained by hydraulic considerations is increased such that this criterion
is satisfied with a reasonable margin of safety. No separate analysis is made for lateral defor-
mation in current practice.
For an analysis of lateral stability of a well foundation, one must have an idea about (i)
the position of the axis of rotation of the well, (ii) the pattern of mobilisation of lateral earth
pressure, and (iii) the loads coming on to the well foundation. The third aspect is a relatively
easy one, while the first and second are interrelated and are more complex in nature. There is
difference of opinion among academicians, researchers, and engineers regarding the position
of the axis of rotation of the well subjected to lateral forces.
Many hypotheses have been propounded with regard to the mobilisation of earth pres-
sure against a well foundation subjected to lateral loads, at the point of incipient failure. Most
of these assume the soil involved to be cohesionless and a few have been extended to cohesive
soils as well.
The following are some of these approaches:- Pender‘s Hypothesis (Pender, 1947)
- Banerjee and Gangopadhyay’s Analysis (Banerjee and Gangopadhyay, 1960)
- Satish Varma’s approach (Satish Varma, 1966)
- Balwant Rao and Muthuswamy’s analysis (Balwant Rao and Muthuswamy, 1963)
- Murthy and Kapur’s analysis (Murthy and Kapur, 1969)
- Chowdhury’s analysis (Chowdhury, 1967)
- Sankaran and Muthukrishnaiah’s Hypothesis (Sankaran and Muthukrishnaiah,
1969) - Terzaghi’s method (Terzaghi, 1943)
- I.R.C. Method (I.R.C., 1972)
- Lazard’s Hypothesis (Lazard, 1957)
Methods 3 and 4 may be adapted for cohesive soils also, while method 10 is applicable
only for cohesive soils. All the other methods are applicable only for cohesionless soils.
Most of these approaches involve two-dimensional analysis of an essentially three-
dimensional problem; each has its own merits as well as demerits.