Drainfield design varies by areas of
the country but here are some of the types you may see.
They should give you an idea of how the drainfield is supposed to work.
A 4" pipe with holes only on the bottom half of the pipe is placed in the center of a large size gravel bed, above the gravel a piece of red rosin paper keeps the soil from washing into the gravel. As waste water flows from the septic tank it flows out through the holes into the gravel and then seeps into the soil.
This style of drainfield pipe is a little diffrent. It has an 8" pipe with holes only on the bottom and surrounding this pipe is a large amount of recycled foam blocks. The whole thing is wrapped by a polly mesh. This system works well and can save money because it does not require gravel.
Searching google from this site should bring you a response more targeted to the septic system.