The Lizzi method is currently the most popular method used. The root bearing capacity is provided by:
average limit skin friction
coefficient reflecting the influence of borehole
Coefficient J reflects the influence of the borehole diameter - it ranges from 1.0 for hole up to 100 mm and 0.8 for the hole from 200 mm.
Average limit skin friction of the micropile root can be found in the literature. The help contains three tables with reference values of limit skin friction. The first one is created by the program authors using various literature sources, the second one contains values of τm according to DIN 4128, and the third one includes values published in by Klein and Mišov (Inženýrské stavby, 1984). The third table contains measured values of skin friction of anchor roots for various soils, root diameters, number if groutings, etc. - using this table yields rather realistic results.
Lizzi, F. (1982). "The pali radice (root piles)". Symposium on soil and rockimprovement techniques including geotextiles, reinforced earth and modern pilingmethods, Bangkok, D-3.