Page 164 - Buried Pipe Design

138 Chapter Three

factor should be taken as unity. A design procedure that calls for a load
that is less than the prism load should also incorporate an appropriate
lag factor in the procedure.

Soil stiffness. The soil stiffness is usually expressed in terms of the
parameter E′, where E′ is a soil modulus term and is, dimensionally,
2
the load per unit area (normally MPa or lb/in ). The soil modulus E′ is
a function of soil properties such as soil density, soil type, and moisture
content. Experience has shown that soil density is the most important
parameter influencing soil stiffness. 35,44,56 As discussed early in this
chapter, the secant modulus from a constrained soil test may be used
in place of E′ with some acceptable error.
Pipe stiffness. The most commonly used terminology is pipe stiffness
(F/ y). For a given pipe product, this term is readily determined in the
laboratory by a parallel-plate loading test

F EI
Pipe stiffness 6.7 (3.32)
y r 3

Profile of the pipe wall. When a pipe deflects under load, bending strains
are induced in the pipe wall. These strains vary through the pipe wall.
Profile-wall pipes are designed and manufactured to minimize the use
of material by increasing the section modulus of the pipe wall. The con-
cept of a profile-wall pipe is not new since corrugated steel pipe is truly
a profile-wall pipe and has been available for many years. Some of the
newer plastic pipe products are of this type. That is, the plastic is
placed primarily at the inside and outside walls or in ribs for greater
pipe stiffness. Many of these products have been shown to perform with
the profile section acting as a unit as designed. For adequate safety, for
any such product, the design should include sufficient plastic between
the inner and outer walls and/or between the ribs to carry shear and to
ensure that the profile section indeed acts as a unit. Also, the cross-
sectional area per unit length and the individual wall component thick-
ness should be sufficient to resist localized buckling.

Long-term properties of plastic. In a finite element program, an incre-
mental analysis with a decreasing pipe modulus shows that using
the so-called long-term modulus has little influence on the overall
behavior. Thus, inclusion of the viscoelastic properties of the pipe in
the analysis is not justified. Error caused by imprecision in the soil
terms totally masks any benefit gained by a viscoelastic analysis.
As previously stated in this book, in both the trench and the
embankment, it takes substantial time for the full load to reach the
pipe, and changes in deflections with time are due to increasing loads