DHARMCAISSONS AND WELL FOUNDATIONS 779The area of cross-section of a well foundation is controlled not only by the allowable
bearing pressure but also by the size requirements of the pier or other superstructure. The
desired area of cross-section is suitably adjusted to the shape chosen, the detailed discussion of
which has been given in the previous section.19.9.1Grip Length
A well foundation should be sunk below the maximum scour depth such that there is adequate
lateral stability. The depth of the bottom of the well below the maximum scour level is known
as the ‘Grip Length’. Thus, one of the important factors governing the depth of a well founda-
tion is the criterion of the minimum grip length necessary to provide adequate lateral stabil-
ity, besides the other factor of placing the bottom on a stratum with sufficient bearing power.
For the first criterion, it is necessary to obtain the depth of scour from hydraulic
consideration.
The scour depth can be ascertained by one of the following approaches:
(a) Actual sounding at or near the proposed site immediately after a flood, at any rate
before there is any time for silting up appreciably.
(b) Theoretical methods taking into account the characteristics of flow like the direc-
tion, depth, and velocity, and those of the river bed material.
In case the first approach of taking soundings is not feasible, the second approach may
be used and the normal depth of scour may be calculated by Lacey’s formula:
d = 0.473(Q/f)1/3 ...(Eq. 19.10)
where d = normal scour depth, measured below high flood level (m),
Q = design discharge (m^3 /s),
and f = Lacey’s silt factor.
The silt factor may be calculated from the equationf = 1.76 dm ...(Eq. 19.11)
where dm = mean size of the particle (mm).
The regime width of the waterway, w, can be computed as
w = 4.75Q1/2 ...(Eq. 19.12)
If the actual waterway, L, is less than the regime width, the actual scour depth, d′, is
given by
d′ = d(w/L)0.61 ...(Eq. 19.13)
Recommended values of Lacey’s silt factor, f, for particular particle sizes in the ranges
of coarse silt to boulders are given in the Indian Standard Code of Practice:
IS:3955-1967-“Indian Standard Code of Practice for Design and Construction of Well
Foundations”.