When using the Littlejohn method the root bearing capacity is provided by:
magnitude of grouting pressure
It follows from experimental measurements of micropiles that their bearing capacity also depends on the course of grouting and on the grouting pressure (grouting course often governs the micropile bearing capacity). The bearing capacity considerably increases with repeated grouting. Grouting pressures range from 0.1 to 3 Mpa, in some case they may reach up to 8 Mpa. The Littlejohn method gives the bearing capacity directly proportional to the grouting pressure.
LITTLEJOHN, G. S. y BRUCE, D. A. (1975).: "Rock Anchors - State of the Art. Part 1. Design". En Ground Engineering, Vol. 8, N° 4.