Between the action of freezing and thawing of water in cracks nd the powerof growing roots boulders become rocks, then pebbles, sand then soil. Just look at how easily a root mass forms through a net pot. Drain pipes in a roots territory in nature don't stand a chance. All the need is one liittle crack, or inperfection to get a hold in or on and they rip and tear from there. On plastic pipe the start at the joints and work out from there.