I need to build a retainging wall ~6' tall, problem is im not very experienced and have some questions. Since this is my first big wall Im looking for a product that can go 6' tall without geogrid, keystones standard units can go 6' 8" in "ideal condiotions" my questions is how do i know if my soil is 34,30, or 26 sand/gravel or silty sand or clay? 2nd is how much do you backfill with gravel like if the wall is 6' tall does that mean I have to have 6' of gravel behind it all the way up? Because if it does my wall is like 70' long by 6' high. 70(long) X 6(HIGH) X 6(DEEP)=2520 / 27= 93 CUBIC YARDS OF GRAVEL IS THIS RIGHT? That sounds crazy thats what 9-10 dump trucks of gravel?? I will post some pictures this week. Thanks for all the help.