Geotechnical Engineering

DHARM

786 GEOTECHNICAL ENGINEERING

where q = Pressure at the base

=

Total weight

Area of Plug

r = radius of the imaginary inverted arch. For granular soils, the hoop tension is relieved by

active earth pressure around the curb. The net hoop tension is given by

T′ =

DqD

r

(^11) ppb
2
44 .( )−+^12
R
S
|
T|
U
V
|
W|
...(Eq. 19.19)
where p 1 = (1/2) Ka γ′ D^2 and p 2 = (1/2) Ka γ′ (D – b)^2 , b being the height of the curb, and
D = depth of the curb below scour level.
At the junction of the curb and steining, a moment M develops due to H, and is given by
Mo = H. b/2 ...(Eq. 19.20)
Suitable reinforcement is provided at the inner corner to take care of this moment and
is anchored into the steining.
IRC recommends a minimum reinforcement of 720 N/m^3 in a well curb. The inner slope
of the curb should not be more than 30° for ordinary soil and 45° for cohesionless soil.
(3) Concrete Seal or Bottom Plug
The concrete seal or bottom plug has to be designed for an upward pressure equal to the pore
pressure at the bottom minus the pressure due to self-weight. It is usually designed as a thick
plate as already mentioned in sub-section 19.2.5 under caissons.
Based on the theory of elasticity (theory of plates), the thickness of the bottom plug is
obtained from the following equation:
For Circular Wells:
for simply supported conditions,
t^2 =
33
8
W
c
()+ν
πσ ...(Eq. 19.21)
where t = thickness of the bottom plug,
σc = allowable flexural stress for concrete,
ν = Poisson’s ratio for concrete (taken as 0.15),
W = Total uniformly distributed load on the plug,
( =
π
4
Dq^2
i where q = uniform pressure acting on the plug)
Substituting for W and v, this is expected to lead to Eq. 19.4 for Circular Caissons.
For Rectangular Wells:
for simply supported conditions,
t^2 =
3
41
qB^2
i
σαc()+1.61
...(Eq. 19.22)
where Bi = width or shorter dimension of the well,
α =
B
L
i L
i
, i being the longer dimension of the well.