I have a small pond approx. 40' x60'. Just had a leaky wall shored up with 3 loads of clay. We upsized my overflow line to a 6" dual wall pipe (corrugated outside, smooth inside). Guess we did not pack it well enough and water is traveling along the bottom of the pipe. Since I have to re-dig it, I'd like to add some kind of anti-seep collar. Would concrete be sufficient? If so, any recommendations on dimensions and reg. concrete or hydraulic?

Snooter, I've used the flexible anti-seep collars from Agri Drain. All you do is cut out a hole, and slide the collar over the pipe, so they're very easy to install. If you go that way, be sure to cut the hole for the pipe to the exact dimensions provided with the paperwork.