Some places like Colorado School of Mines have tunnel/shaft graduate programs, but I don't know if anyone has a straight earth retention program. Try finding a school where one of the profs is prominent in the field and start there.
I have studied basic courses about retaining system during my grad school. looking for some prof or professional courses which offers training in live projects.
If you want training in live projects, try getting an internship with a design/build contractor or one of the consulting firms specializing in this type of work.