source (a nice compact resource of trail info)
If surface water continually seeps slowly across a section of trail creating a perennial mudhole, a drainage lens that enables water to seep beneath the tread may be required. First excavate several inches of water-saturated soil in the trail bed, then backfill with a layer of large rocks. Add layers of progressively smaller rocks on top of the first layer, leaving large pore spaces between rocks at lower levels. Top this rock fill with soil or gravel to form the tread.
If saturated subsoil is extremely deep or unstable, first lay geotextile fabric on the ground, then add rock layers. Place additional geotextile fabric on top of the rocks and top with soil or gravel. Geo-textile fabric separates rock fill from the substrate, preventing soil from clogging pores between the rocks yet allowing water to percolate through the fabric and the fill material.