Page 107 - Buried Pipe Design

Design of Gravity Flow Pipes 83

From Table 2.2,

H
C c 1.5 0.07 16.73 ≈ 16.7
B c
2
2
W c C c B c 16.7(120)(1125) 3131 lb/ft
At the transition width W c ≈ W d
SF
W 3-edge 3131
LF
SF
W D load 2505
LF

Minimum Required Strength for SF 1.5
Bedding class Three-edge, lb/ft D load, (lb/ft)/ft
B 2472 1977
C 3131 2505
D 4270 3416

Flexible Pipe Analysis
Installation design
Three parameters are most essential in the design or the analysis of
any flexible conduit installation:
1. Load (depth of burial)
2. Soil stiffness in pipe zone
3. Pipe stiffness.

The design load on the pipe is easily calculated using the prism
load theory, as discussed in Chap. 2. 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 approaches the prism load.
Thus, if this load is used in design, the deflection lag factor should be
taken as unity.
The soil stiffness is usually expressed in terms of E (effective soil
modulus, lb/in ). Effective soil modulus E is dimensionally the load
2
per square inch. 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.
For flexible pipes, pipe stiffness rather than crush strength is usu-
ally the controlling pipe material property. Pipe stiffness may be
expressed in terms of various parameters as follows: