i`m a builder, if i were to dig foundations i would use a device to determine where underground drains were located, dig an oblong shaped trench slightly behind where blockage in drain is, making sure sides of trench were straight and neat not narrowing as you get deeper, as you get closer to pipe containing blockage, be more carefull untill you reveal pipe, if you are behind blockage you can then pull it out from behind. hope this is of some help.
bang on mate well put