Underground pipelines--Design and construction

Flexible thermoplastic p1pes under field and laboratory loading conditions have been
examined in the present study. The flexible pipes were tested under truck loading
application with shallow soil cover. The pipe-soil system response includes soil stresses
around and above the buried pipes, vertical pipe crown diametral strain, and
circumferential pipe wall strains. Modeling the pipe-soil system is made using plane
strain and thin ring assumptions. A thin ring model using Castigliano's theorem is
developed to analyze the behavior and response of a flexible pipe under well defined
loading conditions and simulate the behavior of the buried pipe under the live load
application. Laboratory work was carried out to study the pipe behavior and response
under two-point, three-point, and four-point loading configurations. The thin ring model
predictions show good agreement with classical solutions specially valid for two-point
and three-point loading configurations. Laboratory results were also in good agreement with the predictions. Laboratory results show that the maximum tensile strain for the
four-point loading test occurs at inner pipe crown region. Comprehensive efforts were
made to correlate the thin ring model predictions with the field test results; however, it
appears that the thin ring model cannot be used to simulate the effect of the live load
application. A major source of the differences between the predicted and measured
values is attributed to the applied load magnitude. A further investigation was carried out
to examine the applicability of the model to study the general pipe behavior. The
predicted hoop pipe wall strain profile was found to be similar to that of the reported
strain profile by Rogers under overall poor soil support condition. Comparison of soil
stress distribution shows that the 2D prediction approach provides nonconservative
results while the FE analysis agrees more favorably with the measured pressure data.
Overall, FE analysis shows that a linearly elastic isotropic model for the surrounding soil
and flexible pipes with a fully bonded pipe-soil interface provides a reasonable prediction
for soil pressures close to the buried pipes.

The primary goal of this study was to evaluate the service life of HDPE (High Density PolyEthylene) pipes. The following experimental tasks were carried out: (i) procurement of materials, and fabrication of test setups; (ii) creep evaluation: the performance of buried pipes (notched/unnotched), subjected to live loading, was studied in soil chambers for three levels of loading (service, 2/3 and 1/3 of service). The long-term behavior was accelerated with super-ambient temperatures; (iii) field monitoring: strains and diametral changes were measured for 10,000 hours. The analytical investigations were as follows: (i) extrapolation of the long-term performance at ambient temperature, based on the Bi-directional and the Arrhenius methods and (ii) 2-D Finite Element Analysis, using the software CANDE. The findings include: (i) the deflection threshold (7.5% vertical change of diameter) as the governing failure condition, (ii) similar life predictions, for Bi-directional and Arrhenius methods, with service lives of about 80 and 30 years at ambient temperature, for unnotched and notched specimens, respectively, subjected to maximum loading, and (iii) a reasonable agreement between analytical and experimental values.

Flexible plastic and metal pipes are increasingly being used for drainage and storm sewers. When flexible pipes are buried at shallow depths, the pipe behavior will not depend on the dead load pressure above the crown, but rather on the live load pressure (vehicle load). Field tests were designed to evaluate the performance of large diameter flexible pipes of 36 in. (915 mm.) and 48 in. (1050 mm.) under shallow burial depths subjected to the actual vehicle loading. The test pipes included high-density polyethylene (HDPE) pipes, polyvinyl chloride (PVC) pipes, steel pipes and aluminum pipes. AASHTO standard pipe installation procedures were followed and pipes subjected to vehicle loads simulating the effect of HS 20-44 trucks. Measurements of interior pipe-wall strains, soil pressures at different depths and pipe deformations were taken to determine the influence of surface vehicle loads. Results of field tests are compared with those based on theoretical analyses.