Let's address these questions individually.
Sunlight contains ultra-violet rays that reduce the tensile properties of plastics with time. HDPE pipe installed in the ground is primarily in compression due to the annular profile design of the pipe. Slight tensile stresses in the pipe arising from trench anomalies and residual stresses from the manufacturing process are overcome by the large compressive stresses due to the soil overburden. Therefore, the net stress in the pipe is compressive. Additionally, once the pipe is backfilled it is protected from the effects of UV rays. The pipe's exposed ends are in areas of little or no stress and therefore a reduction in tensile strength due to UV does not affect the pipe's performance. AASHTO and ASTM specifications include requirements of coloration of HDPE pipes with carbon black to inhibit the effects of UV of the material. Carbon black has been proven to protect HDPE pipe for over three years with no change in tensile properties from UV effects. Therefore, when the pipe is stored prior to installation, the carbon black coloration inhibits the effects of UV rays and, once the pipe is installed, it is protected from UV by the backfill materials and exposed ends are located in an area of little or no stress. To illustrate the fact that carbon black protects HDPE from UV degradation, consider the insulation coating on overhead telephone and electric lines. These lines are insulated with HDPE that is continuously exposed. However, since the lines are under relatively low stress, the carbon black coloration protects the lines from UV degradation.
The effects of gasoline and whether the pipe burn will be addressed together since the concern with fuel spills is the potential for a catastrophe, which could damage the pipe. Gasoline does not adversely affect HDPE. The material does not soften or lose strength when exposed to gasoline; however the material will burn if exposed to an outside fuel source (gasoline). When examining the effects of a catastrophic event, one needs to consider the performance of other materials as well as the effects of the event itself. In the case of a fuel spill and a fire in a closed storm drain system, the fire would extinguish itself rather quickly due to an inadequate air supply. Studies have shown that even in an open culvert the fire extinguishes itself within the first few feet of the culvert. Repairs would be relatively simple and involve excavating the ends of the pipe and cutting off the damaged section. This could then be replaced with new pipe.
The Florida DOT conducted a study into the potential damage to HDPE pipes from fire and concluded the risk of a fire was minimal and headwalls would effectively eliminate damage. For a catastrophic event, many commonly used materials would be damaged and require repairs. For example, asphalt pavements would burn, metal pipes (especially asphalt coated) would be damaged and concrete pipes would be weakened due to cracks from expanding aggregate. Therefore, fire has the potential to damage a number of common construction materials. When evaluated from the risk potential of damage, HDPE pipe is no more susceptible to fire damage than other pipe materials.
