Ok RM......just for the sake of discussion....
If PVC pipe is not impermeable to water.. how do houses use it for water piping under pressures of up to and above 60psig for many years and you never see water weeping, short of pipe condensate on the exterior of the pipe. In the ground there is only atmospheric pressure on the outside and the pressure on the inside could vary slightly depending tempature, but would be close enough to call equal... so 14.7 psi avg outside vs. 14.7 inside.
What was the avg humidity or dew point level when the people in question sealed the pipe? If atmospheric moisture is sealed in, then the interior of the pipe becomes a microenvironment, like a terrarium, moisture can not get out. It will condense and vaporize with tempature changes like during seasonal changes, even daily changes, and it doesn't take alot of moisture to screw up metal, the larger the pipe and the higher the humidity, the more moisture gets sealed inside.
Easy fix.... Pull a vacuum on the pipe, like an A/C system to remove the moisture, negative.... or a below atmospheric pressure causes water vapor to boil and become easily removalble, you remove the air, you remove the moisture. Moisture in an A/C system equals death, acids and all kinds of nasty for the system, which is why when you open a system, a humid day will take longer to evacuate, provided one doesn't use a low pressure dry gas like nitrogen to keep the atmospheric moisture forced out.. Or one could pull a negative pressure and and then charge the pipe with a dry inert gas, again like nitrogen, just a pound or two above atmoospheric to combat any issues w/permeability and exterior pressure.