Page 167 - Buried Pipe Design

Design of Gravity Flow Pipes 141

was not present and provided for nonsymmetry about that axis in
his semiempirical method.
2. As the solution is used by some, with the overburden soil modulus,
results will be nonconservative for flexible pipe installations. Whereas
this correction may work for rigid pipe, it should not be used for flexi-
ble pipe. The solution itself, without the overburden-dependent soil
modulus, does not require the pipe to be rigid. However, it is for the
small-displacement theory which does require small displacements.

Finite-element methods. 19,23 A more complete discussion of the finite
element analysis (FEA) technique is discussed later in this chapter.
The FEA method has been shown to be successful in predicting the
behavior of buried flexible pipes. In particular, recent research at Utah
44
State University has shown that the FEA is the most successful method
in the prediction of the behavior of large-diameter HDPE pipes.
However, the user must be forewarned that the FEA results are only as
good as the ability to model the behavior of soil-structure interaction.

Comparison of results. In the following figures, height of cover is
calculated by dividing the vertical soil pressure by an assumed unit
weight of soil. For these figures, a unit weight of 19.1 kN/m (120 lb/ft )
3
3
has been used. Figure 3.32 shows a comparison of the various ana-
lytical methods with test data for 48-in-diameter HDPE pipe. The

50
160
140
40 120
Height of Cover (Meters) 30 V - Burns & Richard 100 Height of Cover (Feet)

80
20
H - Burns & Richard
60
V - Iowa Formula
H - FEA Analysis
V - Test Data
10 V - FEA Analysis 40
H - Test Data
20
0 0
–1 0 1 2 3 4 5 6
Deflection (Percent)
Figure 3.32 Comparison of test results with various analytical methods for a
48-in-diameter HDPE pipe buried in silty-sand soil compacted to 97 percent
standard Proctor density.