Check out "Micropile Design and Construction Guidelines Implementation Manual" by Tom A. Armour, PE, Paul Groneck, PE, James Keeley, PE, and Sunil Sharma. It's an FHWA publication, available through them or (more easily) through ADSC-IAFD (Int'l Association of Foundation Drilling) for $50. This is currently "the" design manual for these systems.
What will you use the micropiles to support? There are issues with joint strength when subjected to flexure (i.e. lateral loading.) The effective EI is severely impaired by the threaded connections -
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"