Page 86 - Buried Pipe Design

62 Chapter Two

The pipe is 8.75 ft in diameter, so the water does not even have to be to the
top of the pipe. Thus, if a water table can rise in the embedment, the impor-
tance of densifying the embedment soil, including soil under the haunches, is
evident. If the soil does not liquefy, the soil gives support to the pipe and pre-
vents buckling. Then the buckling equation is
P cr h 1.15 P b E′

2 E ⎛ ⎞ 3
t
P ⎜ ⎟
b 1 − ν 2 ⎝ ⎠
D
where P cr h
E′ soil modulus
E pipe modulus of elasticity
pipe Poisson’s ratio

Soil bearing
An empty pipe below the water table may rise through the soil by the
means of penetration if the soil’s bearing capacity is too low. This may
be more critical than the soil wedge for resisting flotation.

Example Problem 2.11 In the previous example, suppose that the soil is so
2
poor that the bearing capacity is only 300 lb/ft . Soil resistance is W s
2
(105/12 tank diameter)(300 lb/ft ) 2.625 kips/ft, where
W w 3.752 kips/ft buoyant uplift force per unit length of tank
W s 2.625 kips/ft effective bearing capacity
W p 0.580 kip/ft weight of steel pipe

W W w W s W p
0.543 kip/ft and is a net upward force
The pipe will rise through the soil by penetration because of a low bear-
ing capacity. To prevent this, a better soil with higher bearing capacity must
be used, or the pipe must never be allowed to be empty.

Internal vacuum
For a pipe with an internal vacuum, treat the vacuum as a positive
external pressure and add it to any acting external water pressure
before making the buckling analysis. The performance limit for inter-
nal vacuum and/or external soil pressure only is ring inversion.
Embedment usually prevents total collapse. Critical vacuum p is sen-
sitive to the radius of curvature. Ring deflection reduces critical vacuum.
Because vertical radius of curvature r y is greater than r, ring stiffness
3
EI/r y is less than EI/r and the vacuum at collapse is less for a deflected
3
ring than for a circular ring.