Our current septic task access is via a 18" corregated pipe with a concrete cover on top of it. The cover is already about 15" below grade. We are redoing the grading in this area and will be adding an additional 15" to this area, making the future total length to grade 2.5 feet. I would like to keep this low profile and cost effective. Here's the best idea I have come up with. Use a coupler like so:

http://m.lowes.com/pd/ADS-18-in-Dia-Corrugated-Split-Coupler-Fitting/3515574

I will use this to attach a 2.5 foot section of additional 18" corregated pipe then purchase a cover, such as the one seen in the link below:

http://www.septicsolutions.net/store/TufTite.htm

Would I need some kind of sealant with using the coupling to prevent excess water from entering the tank? I also can't seem to  find anywhere that sells corrgated pipe by the foot. Lowes has the pipe I need but only sells in 20 foot length at $300 each. Ouch! I know they sell one piece risers but it doesn't appear I could find them in the length desired. Any thoughts would be great appreciated!

If it were me, I would just replace the total length instead of using two pieces.  I bought my pipe and cover from a local septic company.  It is plastic, and they sold it to me by the foot

What county are you in?

Know any farmers putting in drain tile?

The 'centers' of these coils could be the pipe you seek. They are generally 6 to 8 feet long...
Finding a local dealer/installer of farm drain tile could be another route to getting your pipe.

Berks. Lehigh is less than 10 minutes from me. That's probably a wise idea to do.

NJD, I have never even noticed that. I will keep my eyes open.