Eddie,
Trowelable grade mastic (or) roofing tar works well to seal voids between your anti seep collar and pipe. The heavy stuff not the runny stuff; available at most hardware stores and much cheaper than silicone or other sealants.
Theo asked a very valid question: Are you using a smooth or corrugated pvc pipe? Corrugated plastic pipe is far supperior when it comes to "water pipeing" along your principal spillway. Each corrugation acts as a miniture anti-seep collar.
Using concrete as a collar also works well if done properly.
There has been some discussion on hand tamping pipe versus heavy equipment tamping.
Hand tamping pipe is an acceptable method of compacting soil around the pipe if it is done properly. You will need to pay careful attention when hand tamping the bottom one-half of the pipe, as not to bridge or lift the pipe from its "bed" during the tamping process.
I honestly believe that hand tamping (the bottom half) of your pipe is superior to many other methods. Its hard work but you will have alot of control over quality of the job.
Just my thoughts,
Ed
