Embankments Toe Drains

I have gone through few rather difficult references for drain/filter design but still uncertain about what a practical method of toe drain design for an old embankment made from clay would be. Do designers usually apply Terzaghi (or USBR) filter design criteria for the toe drain design or do they simply sandwich the drain pipe within gravel materials surrounded by fine sands. Thank you for shedding some lights on this. your feedback.