I am building several retaining walls in my sloping yard to level it off. The lowest wall is made of 6x6's to raise it 3 feet. I had to build retaining walls on the other two sides to contain the dirt from hitting the wooden fence on one side and on the other side to join the parallel wall. I have built trenches under the walls and filled with clear 3/4 inch gravel about 8 inches under the lowest wall and anywhere from 2 inches to 4 or 5 inches under the other walls which are built of 3x3's. I placed a 3 inch perforated PVC pipe behind the lower wall which is the biggest and covered it with gravel. I plan to add one 4 foot deadman in the wall(which is 14 feet long).
1) How much gravel do I have to put up the walls? I have placed over 15 inches wide behind the lowest 6x6 wall and so far I have it about one foot up the wall with some tapering to about 12 inches back from the wall but am running out of gravel. The other walls (which aren't going to be as deep and are running perpendicular to the fill) have narrower trenches and will be 30 inches deep at the bottom next to but not attached to the 6x6 wall. Your diagrams show gravel only about half way up the wall. Because it is made of wood, should the gravel go higher to minimize dirt touching the wall? Or can I just cover it with landscape fabric at this point and cover with dirt the next 18 inches?
2) the perpendicular walls (which close the square) are not attached to the parallel walls. Is this a problem? I stabilized the base 3x3's with 2 foot rebar every 4 feet and attached each layer with 10 inch spikes. Should I do more for a side wall than that? It is only taking force on the lower half which is deepest in added dirt.
3) I drilled holes for the spikes that make the spikes drop in in some places. Is that a problem? Should it be hard to pound them in?