Page 125 - Buried Pipe Design

100 Chapter Three

load to increase. This caused a somewhat rapid increase in deflection for
all pipe samples during this period. A new stable or equilibrium deflec-
tion level was reached at about 400 days (9600 h). The water table con-
tinued to fluctuate on an annual basis for the 14-year test period. These
subsequent water table movements influenced deflection readings only
slightly since the initial saturation of the pipe zone.
Again, the soil around these pipes was a silty fine sand. For this soil,
over 92 percent standard Proctor density is necessary to insure a void
ratio less than the critical value. The installed densities were less than
92 percent, resulting in void ratios greater than critical. Thus, when the
water table rose into the pipe zone, soil consolidation took place and
caused pipe deflections to increase. This indicates that for pipe installa-
tion below the groundwater table, additional deflection control can be
obtained if the density is such that the void ratio is below the critical
value. The test site area was also subjected to small earthquake tremors
during the test period. Any effects are included in the deflection results.
The change in deflection, with respect to time, for this embankment
condition is greater than that measured in soil cell tests. This time-
dependent deflection is due to the increasing load that is taking place
in the embankment tests, whereas in the soil cell tests the load is
applied to soil just over the pipe and is held constant. The equilibrium
deflections, being approached by the curves in Fig. 3.15, are the same
deflections which would result if similar pipes were tested in the soil
cell at the same soil pressure, with the same initial soil density, and
with the addition of water.

Post evaluation of buried samples. Pipe samples excavated from the site
were examined visually, and no signs of cracking, crazing, or other poly-
mer damage were evident. Specific gravity, pipe stiffness, and wall
thickness measurements were taken for each sample and are given in
Table 3.8. Notably, the pipe stiffness for the foamed samples varied from
2
2
34 to 38 lb/in initially and ranged from 36 to 40 lb/in after 14-years.
The filled pipe samples varied from 45 to 50 lb/in initially and measured
2
2
44 to 49 lb/in after 14 years of buried service. These small variations
TABLE 3.8 Post Excavation Properties of Embankment Pipe Samples
Pipe *
Pipe sample Compound Thickness Specific stiffness, 60%
designation type average, in gravity lb/in 2 flattening
A Foamed 0.381 1.2 36.8 No cracking
B Filled 0.327 1.6 44.0 No cracking
C Foamed 0.417 1.2 40.5 No cracking
D Filled 0.331 1.6 49.0 No cracking
* Post evaluation pipe stiffness per ASTM D 2412.