Page 120 - Buried Pipe Design

Design of Gravity Flow Pipes 95

TABLE 3.5 Suggested Design Values for Constrained Soil Modulus M s
Stress Soil type and compaction condition, MPa
level,
kPa SW95 SW90 SW85 ML95 ML90 ML85 CL95 CL90 CL85
7 13.8 8.8 3.2 9.8 4.6 2.5 3.7 1.8 0.9
35 17.9 10.3 3.6 11.5 5.1 2.7 4.3 2.2 1.2
69 20.7 11.2 3.9 12.2 5.2 2.8 4.8 2.4 1.4
138 23.8 12.4 4.5 13.0 5.4 3.0 5.1 2.7 1.6
275 29.3 14.5 5.7 14.4 6.2 3.5 5.6 3.2 2.0
413 34.5 17.2 6.9 15.9 7.1 4.1 6.2 3.6 2.4
1 MPa 145 lb/in . 2

If both K and E are known, can be calculated as follows:
1 E

2 6K
Thus, the constrained modulus, M s can be calculated if the E and K
values for the hyperbolic model are known.
28
McGrath has suggested design values for M s , for use as a soil stiff-
ness parameter. These suggested values are proposed for use in the
Iowa formula for deflection of buried pipe and other design equations
that had adopted the use of E . The proposed values are secant moduli
and are listed in Table 3.5.

Deflection lag and creep
The length of time that a buried flexible pipe will continue to deflect
after the maximum imposed load is realized is limited. This time is a
function of soil density in the pipe zone. The higher the soil density at
the sides of the pipe, the shorter the time during which the pipe will
continue to deflect, and the total deflection in response to the load will
be less. Conversely, for lower soil densities, the creep time is longer,
and the resulting deflection due to creep is larger.
After the trench load reaches a maximum, the pipe-soil system con-
tinues to deflect only as long as the soil around the pipe is in the
process of densification. Once the embedment soil has reached the den-
sity required to support the load, the pipe will not continue to deflect.
The full load on any buried pipe is not reached immediately after
installation unless the final backfill is compacted to a high density. The
increase in load with time is the largest contribution to time-dependent
deflection. However, for a flexible pipe, the long-term load will not
exceed the prism load. Therefore, for design, the prism load should be
used, which effectively compensates for the time-dependent increase
in load with trench consolidation and the resulting time-dependent