Answer: Wish I had an absolute answer for you. Bougainvillea roots tend to grow down and out, usually staying in the top 12" of soil, but some can go a further down. If they reach the pipe, they'll probably grow in a different direction, but they could grow around it. It is possible that they could exert enough pressure when growing around it that it could be squeezed shut after a period of years. The only way to know is to dig around the pipe and check its status, which could compromise the health of the bougainvillea. I wish there were some other way to check it out!
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