Page 128 - Buried Pipe Design

Design of Gravity Flow Pipes 103

Percent Deflection

Time (Hours)
Figure 3.17 Steel and PVC pipes with the same pipe stiffness
2
(F/ y 46 lb/in ) and installed in the same manner with 85
percent standard Proctor density in silty sand soil.
Watkins’s soil-strain theory
A number of variations of Spangler and Watkins’s modified Iowa formula
have been proposed. All can be represented in simple terms as follows:

load
Deflection (3.7)
pipe stiffness (constant) (soil stiffness)
Upon analyzing data from many tests, Watkins wrote the Iowa for-
mula in terms of dimensionless ratios as follows:
y PR s
(3.8)
D E s AR s B
2
where P vertical nominal pressure at the top of pipe level, lb/in , and
R s stiffness ratio. (This is the ratio of soil stiffness E s to pipe-ring stiffness
EI/D . This quantity includes all the properties of materials, soil as well as
3
pipe.) Since for a solid wall pipe of constant cross-section I t /12, then
3
E s D 3
R s 12
Et 3
where E s slope of stress-strain curve for soil at load in question in
a one-dimensional consolidation test
P/
vertical soil strain
A, B empirical constants which include such terms as D L and
K of the Iowa formula