[valmet10]

a lot depends on the type of soil /subsoil if it not draining then it must be clay , if you dig through clay when its too wet it will just smear the trench wall sealing them so when you back fill it wont allow water to get back to the drain /backfill , make sure any aggregate you use is clean no dust , will stop drainage pipe getting blocked ,also look at subsoiling across above drains helps break any pan you have as well as letting water to drainage pipes