Page 198 - Buried Pipe Design

172 Chapter Three

Fig. 3.53 with data in Fig. 3.51 shows that compaction simulation did
improve the correlation between strains predicted by FEA and test
results. The general shape, maxima, and magnitudes all compare very
well.
Additional comparisons that were made with this condition included
soft elements in the shoulder areas of the pipe. Because soil placement
techniques do not allow compaction directly above the pipe, a completely
homogeneous compaction is not obtained in an actual installation.
For a flexible pipe, the soil will be of a lesser density at the shoul-
ders and crown of the pipe. One noticeable result with soft-crown
analyses is that generally the pipe strain at the 135° position of the
pipe (see Fig. 3.53) increased. This is due to the lowered stiffness of the
soil in the shoulders, which allows for more bending deformation in
the pipe. Compaction simulation for the soft-crown condition did
decrease the bending strains and ring deflections because the soil
would respond in the rebound range initially, thus inhibiting deforma-
tion at the low-pressure ranges. Because compaction simulation did
not include adding loads directly over the pipe, a soft-crown condition
was actually created with the homogeneous case. This is because the
soil at the crown was uncompacted and did not respond on the stiffer
rebound modulus at the lower-pressure ranges as did the surrounding
soil elements that had received the compaction loads directly.

Poor haunch installation at 90 percent relative compaction. Figures 3.54
and 3.55 show the results for the poor haunch installation with a silty
sand soil. A poor haunch condition, as used here, occurs where soil is
placed in the haunch areas but is not compacted. The physical pipe
data are as follows:

Curve
Parameter A B
Stiffness, lb/in 2 10 10
Thickness, in 0.285 0.300
Surface pressure, lb/in 2 35.5 30.0
Vertical deflection, percent 3.14 2.21
Horizontal deflection, percent 1.30 1.09

Figure 3.54 shows the load-deflection response, and Fig. 3.55 shows
the pipe strain around the pipe for a surface pressure of 35.5 lb/in .
2
Again, the initial steepness of the load-deflection curve, the relative
magnitudes between the vertical and horizontal deflections, and the
shape and magnitude of the strain plots should be noted. The bending
strains are higher than before at the 30° to 45° positions of the pipe