Investigation of physical, temperature and chemical effects on the short term and long term performance of high density polyethylene pipe

High-density polyethylene (HDPE) gravity drainage pipes are used in a number of applications, including: landfill leachate collection systems, mining heap leach pads and earth dam drainage galleries. For these kinds of applications, earth pressures acting on the pipe can be extremely large and the pipes can be exposed to aggressive chemicals, elevated temperatures and discontinuous backfill support. The structural response of the pipe is a complicated soil structure interaction problem. The performance of these pipes depends on the pipe and soil properties, as well as the backfill configuration. Design is further complicated by the thermo-viscoplastic nature of HDPE.; This thesis examines a number of different performance and durability issues related to the design of HDPE gravity drainage pipes. The influence of circular perforations on pipe response is examined through the use of three-dimensional finite element analysis. Guidance is provided in regards to the size, spacing and location of perforations. It was found that a stress concentration factor of 3.0 can be conservatively used for most perforated pipes, provide the holes are spaced at least 4 perforation diameters apart on centre. Better estimates of stress concentration factors derived from the finite element analysis are presented. The use of a unique backfill configuration in landfill where a gravel trench is surrounded by soft clay is examined through the use of large-scale laboratory testing and numerical analysis. Pipe deflections measured in a trench were 3.5 times larger than those from tests involving laterally extensive blanket backfill configurations. Issues of backfill modulus degradation, trench arching and sidewall material intrusion and how they lead to an increase in pipe deflections are examined. Large scale laboratory testing and numerical analysis is used to provide insight into the performance of HDPE gravity drainage pipes under extremely large earth pressures up to 3000 kPa. It was found that pipes deflected to extremely large amounts (in some cases up to a 25% o decrease in the vertical diameter), exhibited nonlinear behavior and exceeded recommended deflection limits without resulting short-term collapse. It was found that the short-term pipe-soil response could be modeled by using a linear elastic perfectly plastic soil model with a Mohr-Coulomb failure criterion and a viscoplastic constitutive model for HDPE. It was found that existing analytical and viscoplastic numerical models can be used to predict the short term elevated temperature response. It was found that elevated temperatures resulted in a reduction in HDPE modulus, and that the stress rate dependence of the material reduced at higher temperatures. In terms of the long-term response, it was found that under gravity loading conditions that the pipes experience both creep and stress relaxation. This means that stresses reduce with time, so it is conservative to use calculated short term stresses for design. Fifty year pipe deflections can be conservatively predicted using the industry recommended deflection multiplier of 1.5. Finally, the effect of synthetic landfill leachate on the durability of HDPE gravity drainage pipes is examined. Service life estimates were made using the experimental data. For pipes exposed to both air and leachate, the estimated service life ranges from 703 years at 5°C to 43 years at 50°C. It was found that landfill leachate accelerates the degradation of HDPE pipe materials, and that pipe thickness and pipe stresses also have an influence on the rate of degradation.