Jeff and Henrey: Great to read someone really interested in the why of trail building.Previous replys say it all so my two cents is only on helping you get more education on this matter.
My recommendation for getting started is studying the USFS Trail Construction and Maintenance Note book 2007 edition.
There are many good ones out there so have fun obtaining what may interest you the most.
The USFS manual can be obtained by ordering at this address.
http://www.fhwa.dot.gov/environment/rectrails/trailpub.htm and is free.
A word or two on Grade is important
Grade is designated as a %. 1 in 10 is as described by others is 10% 1 foot in elevation to a run of 10 feet or 10 feet in 100 is also 10%. I find it easier to picture a 10% grade as a 6 Degree angle. A 20 % is 20 feet in 100 or 12 degrees. That is about the limit for a reasonable hiking grade. !0% is the goal if possible. Continuing up tp 30% would be 30 feet elevation in 100 feet or 17 deg. trails in this grade and up will require structures to be built to protect the treadway from erosion.
A lot of trails espec. here in NE have much steeper grades thus maint. problems from poor lay outs.
Mostt trails in the west are well graded and close to the 10% that explains why they have so many switchbacks,both for sustainability and Equestrian use.
Trailwrights may be of some help if you are contemplating building trails in your town and have often worked in Mass. Recently helping out on the Mt. Tom Reservation.
I would recommend you to join us for a day or two and get some up close and personal experience. One of Our purposes is hiking trail maintenance edcation thrue hands on work shops. You can check us out at our web site at
www.Trailwrights.Org. It may be helpful to you.
CAUTION, you could get hooked and discover a lot of rewarding experiences.
Feel free to contact me by PM here on this forum anytime.
Hal