Page 254 - Buried Pipe Design

228 Chapter Four

a data acquisition system and passed to a computer where they were
analyzed for cross-sectional shape and bending strain. Bending strains
were determined by comparing the initial shape profile with the
loaded profile. The pipe and assembly were buried in a soil load cell
and the vertical load was applied by hydraulic cylinders. Water was
pumped through the hose to the bladder to pressurize the pipe.
Test procedure. Before each test, the soil moisture content was
brought to an acceptable range (7 to 10 percent). Soil was then com-
pacted in layers, and the soil density was determined for each layer.
Initially, when the pipe was placed in the cell with no overburden,
2
the pipe was pressurized to 25 lb/in . Then a set (four) of pipe profiles
was taken. The pressure in the pipe was maintained until the cell was
loaded with soil, the loading plate was placed on the soil, and the
hydraulic loading cylinders were placed. Before any hydraulic pressure
2
was applied to the cylinders, the 25 lb/in pressure was removed from
the pipe and another set of profiles was taken.
The process of loading the cell was accomplished in four steps. For
each test, the desired surface load was converted to the appropriate
pressure required by the hydraulic cylinders to produce that load. This
pressure was then divided into approximately four equal steps. After
the pressure reached the value at each step, the readings from the pro-
filometer were monitored and that pressure was maintained until the
profilometer reading quit changing for approximately 1 minute. This
process took up to 5 minute depending on the step size and relative
density of the soil.
When the appropriate soil load was achieved and the profilometer
readings had stabilized, another set of profiles was taken. At this
point, internal pressure was applied to the pipe using pressurized
2
water. The internal pressure was added in 25 lb/in increments from 0
2
to 125 lb/in . After each increment, a set of profiles was taken after the
profilometer had stabilized and a constant internal pressure was
achieved. At the completion of the test, the surface and internal loads
were removed, and the cell was unloaded.
The majority of the tests were run using silty sand. The optimum
moisture content for this soil was 10 percent. All the poor haunch tests
were run using this soil. To obtain a poor haunch, special care was
taken not to compact the soil near the pipe from the base of the pipe to
approximately one-third of the distance to the pipe spring line.

Finite element modeling of rerounding. The ability for the FEA code
to handle unloading and reloading of soil was added. This allows for
a soil element to be in either tension or compression and to respond
correctly when the load is reversed. This characteristic is needed to
model soil as it is unloaded when the internal pressure begins to