There appears to be a bamboo-derived plastic out there with high tensile strength, but I don't think it'd work for this application, because it degrades within 50 days of being buried in soil. I wonder if being in gravel would delay that, due to the different microbes present?

It seems like there are usually biobased solutions out there, but they aren't as cost-effective or easy to procure as the forever plastics, so organizations don't bother with them. Perhaps some kind of incentive would get us moving in the right direction.