Look for "Chance Anchors" on the web. They may be able to help you over the phone or get a copy of their design manual. Better yet, they may have a licensed contractor in your area that may be able to help with a specific problem.
Generally, the actual in-place capacity is determined in the field as they drill. The anchor manufacturer/installer has various guides that determine the capacity based on the torque.
Usually, you simply specify a required capacity, and the installer will provide - and certify that it meets the capacity. For small projects, a level of conservatism and prudence will do. For larger projects, and in cases of less redundancy or critical elements, you may want to specify some load tests to verify their numbers.
, the Deep Foundations Institute website. Search there paper archive. I believe they have a committee for Helical Piles, you could even email them, they are good about responding.
