effect of the pile depth Dis required to be found, bthe total length of the
floor, and b 1 the distance between the two mutually interfering piles.
The correction computed by equation (9.7) is additive or subtractive
depending upon whether the pile dis located upstream or downstream of
pileD.
In the case of the intermediate piling shallower than the end pile
(Dd) and b 1 ' 2 dthe mutual interference is negligible.
(ii) CORRECTION FOR FLOOR THICKNESS
The correction for floor thickness is interpolated linearly over the length
of the piling once the pressures at the key points are computed using
Figs 9.6 and 9.7. In equation (9.7) when computing the corrections to the
mutual interference with floor thicknessonly the net depths of the pilings
(i.e.D t,d t, where tis the floor thickness) instead of Danddare used.
(iii) CORRECTION FOR THE SLOPING FLOOR
The depths of the pilings to be used in the correction equation (equation
(9.7)) are always measured from the top level of the piling, for which the
correction is computed – see worked example 9.2.
The slope correction is only applicable to the key points of the pile
line fixed at the beginning or the end of the slope. Correction charts
following this approach were constructed by Khosla et al.(1954).
The (theoretical) percentage correction for sloping floors is negli-
gible for slopes less than 1:7; it is 2.3(%)4.5 for slopes 1:7 to 1:3, 6.5%
for slope 1:2 and 11.2% for 1:1; the actual correction is obtained by multi-
plying the appropriate value by b 2 /b 1 ,b 2 being the horizontal distance of
the sloping floor. This correction is additive for down slopes (following the
direction of seepage flow) and subtractive for rising slopes.
(b) Actual exit gradient (Ge)
On the basis of the potential theory of flow Khosla et al.(1954) suggested
the actual exit gradient given by (with one downstream end cut-off pile of
depthdwith the total base length of b)
GeHs/πd (9.8)where(1(1^2 )1/2),beingb/d.
For alluvial soils the critical (safe) exit gradient is around 1 in 1, and
with a safety factor of around 5.5 the permissible exit gradients are about
1:6 to 1:7 for fine sand, 1:5 to 1:6 for coarse sand and 1:4 to 1:5 for shingle.