Can my builder build a concrete fence about 3 foot high over the top of a drain. He said the post and the boards aren't that heavy and only need a bit of soil underneath and the drain should stand that. i am very concerned as I stated to him that if the soil compacts with the weight of the posts it would break the drain. I cannot have wood because the banking is higher at one side than the other so needs to be a retaining fence, he also states it only needs going in about 16ins which to me for a retaining fence doesnt seem enough support. Can anyone help