Page 130 - Buried Pipe Design

Design of Gravity Flow Pipes 105

P A P

L
L
PA
L
E
σ

L L
Figure 3.19 Concept for predicting settlement of soil by means of
stress-strain compression data from field or laboratory.

This demonstrates that a flexible pipe is deflected down about as
much as the sidefill settles. The vertical soil strain in the fill depends
upon the soil compressibility and the nominal load (Fig. 3.19). Curves
shown in Fig. 3.20 relate soil strain to the soil pressure.
The use of soil strain to predict pipe deflection then becomes a simple
exercise. The ratio of pipe deflection to soil strain can be determined from
Fig. 3.18. This value will usually be unity for most flexible pipe installa-
tions. The load on the pipe is calculated using the prism (embankment)
load theory, and the soil strain can be determined from Fig. 3.20.

80% Standard Density
Vertical Soil Pressure P, lb/ft 2 Compacted 90% Standard Density Tangent Note: Probable error is about half the
Slope
E t
distance between adjacent lines

Soil

70% Standard Density
Void Ratio
Zone of Critical
Secant Slope at A
W c
E s
∈ s Loose Soil

Vertical Strain ∈ s , %
Figure 3.20 Plot of vertical stress-strain data for typical trench backfill (except clay)
from actual tests.