Page 77 - Buried Pipe Design

External Loads 53

Soil Subsidence
Buried pipe is typically routed in competent soil and installed in a
compacted trench. These precautions provide reasonable assurance
that the soil will not deform, and therefore this effect is rarely included
in design. Where the potential for natural soil subsidence is real or,
more often, when subsidence has occurred, the buried line is analyzed
to assess its integrity. The soil movements are applied to the buried
pipe, either through soil springs or by directly deforming the pipe as a
beam following the soil contour. Stresses or strains are calculated. In
the case of a simple longitudinal pull of a straight buried pipe, as would
occur, e.g., at the interface between a buried pipe and a building pene-
tration as the building settles, the axial stress imposed on the pipe end
would be

2Ef

A
where E Young’s modulus of pipe, lb/in 2
building movement pull along pipe axis, in
A cross section of pipe wall, in 2
f pipe-soil longitudinal friction, lb/in

In certain cases, designers prefer to evaluate strains based on the
deformed shape. However, there is no consensus standard specifying
allowable strains for permanent deformation such as sustained from
soil subsidence.

Loads due to Temperature Rise
Buried pipelines are often operated at temperatures that do not signif-
icantly differ from the surrounding soil temperature. In these cases,
there will be little or no differential expansion and contraction between
the pipe and soil, and a thermal design analysis is not required. In cases
where the fluid is hot or cold, stresses are generated as the pipe expan-
sion is constrained by the surrounding soil. For long sections of
9
straight pipelines, the resulting longitudinal stress is
S L E (T 2 T 1 ) S h
where S L longitudinal compressive stress, lb/in 2
E modulus of elasticity of the pipe, lb/in 2
coefficient of thermal expansion, 1/°F
T 2 maximum operating temperature, °F
T 1 installation temperature, °F