Installation Method vs. Short-Term and Long-Term Stress

Installation Method vs. Short-Term and Long-Term Stress
The viscoelastic nature of PE results in differences in the observed mechanical
properties as a function of time (and/or temperature). The apparent stress/strain
behavior of the material is time dependent under the influence of a sustained load.
This is referred to as "creep” properties. In this regard, we can distinguish between
"short-term” properties, such as those exhibited during a laboratory tensile test at
a strain (stretching) rate of two inches per minute, as compared with "long-term”
properties typical of conduit placement and sustained service loads.
Knowledge of the load-bearing capability of PE as a function of loading rate allows
one to select appropriate strength values to substitute into design equations. Loads
are applied to conduits both by the environment that they are placed into and by
the placement means under which they are installed; the chief difference being the
duration over which the load is applied. For example, a common means to install
multiple conduits is to directly plow them into the ground using either a railroad
plow or tractor-drawn plow. During this installation process, a certain amount of
bending and tensile stress is encountered over a rather short period of time (only
seconds to minutes). Whereas, after the plow cavity collapses about the conduit, the
ground continues to settle upon stones that may be pressing directly against
the conduit, thus setting up a long-term compressive load. For this application,
we see that we would require both long-term and short-term moduli to assess the
deflection resistance. Initially the conduit may offer resistance to ovalization, but in
time, the resin may yield under the sustained load, resulting in a reduced pathway
for the cable.
Numerous approaches to placing conduits have evolved over the years. Each
method presents its own unique set of challenges with respect to the potential for
conduit damage, or installation related predicaments. Perhaps one way to compare
the potential sensitivity to damage of the various methods is the following table.
Here the potential for damage is depicted by a numerical scale ranging from 0
to 5, where 5 is the most severe condition, resulting in yielding and permanent
deformation of the conduit; 4 is the potential for loads greater than 75% of yield
stress; 3 represents loads greater than 50%; 2 representing greater than 25%; 1 less
than 25%, and 0 representing no significant load at all. The shaded areas depict the
most severe condition.
Chapter 14
Duct and Conduit
482
TAbLE1
Relative Damage Sensitivity vs. Installation Method
Installation
Method
Short-Term Loading Long-Term Loading Recommended
SDR Range Tensile Bending Crushing Impact Crushing Tensile