Page 163 - Buried Pipe Design

Design of Gravity Flow Pipes 137

Analytical Methods for Predicting
Performance of Buried Flexible Pipes
Introduction
There are various methods for predicting the structural behavior of
flexible conduits. Included here is an in-depth analysis of the various
methods—pointing out strengths and weaknesses with emphasis on
large-diameter profile-wall HDPE pipes. Comparisons of test data
with predictions from the various theoretical methods are made.
Methods discussed include (1) full-scale testing, (2) semiempirical
equations (such as the Iowa formula), (3) closed-form analytical
(such as the Burns and Richard elastic solution), (4) finite element
methods, and (5) model testing (with dimensional analysis).
The Burns and Richard solution and the Iowa formula are both lin-
ear elastic theories. Both assume the soil and the pipe structure to be
linear elastic materials. The assumption that the soil is elastic can
lead to large errors. The Burns and Richard solution allows for a non-
linear soil modulus correction to account for overburden pressure.
With the same soil modulus or the same modulus correction, these
two methods are shown to produce almost identical results. For large
diameter PE pipes, the Burns and Richard method, although still in
error, offers some advantages over the Iowa Formula. It produces
results such as strain, horizontal deflection, and thrust that are not
directly available from the Iowa Formula. The presently used soil
modulus correction in the Burns and Richard solution is shown to be
incorrect.

Flexible pipe design and analysis
Installation design. Traditionally there were three parameters that
were considered most essential in the design or the analysis of any
flexible conduit installation. A fourth needs to be added to the list:
1. Load (depth of burial)
2. Soil stiffness in pipe zone
3. Pipe stiffness
4. For profile-walled pipe, the profile itself

Load. The design load on a flexible pipe is easily calculated using
the prism load theory. This load is simply the product of the soil unit
weight and the height of cover. Research has shown that the long-term
load on a flexible pipe can approach the prism load. 32,46 This load is
conservative. Thus, if this load is used in design, the deflection lag