I need to build a retaining wall approximately 100 linear ft with a maximum height of 12', average height about 8'. I'd like to build it out of natual sandstone because I can get the stone for free. I plan to build this myself. The excavation is complete. I've got a trench dug about 18" in depth and apprx 4' out from where the top of the wall will be - further at maximum height. I've put a drainline in the bottom of the trench and covered it with 6" of washed gravel with weed guard on top of the gravel. This leaves apprx 12" of trench still below grade. My plan was to put very large stones in the bottom and just build my way up layer-by-layer, back filling, and compacting the soil behind the wall every 16" to 2'. I hadn't planned on putting mortar between the stones but I could. Am I missing anything? Is mortar required or not? Any other suggestions?

The wall you described will NOT work. 12’ of earth is extremely tough to hold back. A retaining wall of this height should be designed by an engineer.

That being said, in my vast experience with retaining walls the footing for a wall this height would have to be concrete greater than 6’ wide and 16” deep with heavy steel reinforcement that extends up into a concrete wall at least 12” thick (or greater) with 2 steel reinforcing mats embedded. Then you could place your stones in front of the concrete for looks if desired. This description I have given is NOT to be used as an engineered design but is based on the many reinforcing walls I have installed that were designed by an engineer. The exact dimensions will have to be done by an engineer licensed in your state that is familiar with your geological conditions.

In summary if you build a wall 12’ high with only sandstone it will not hold. The only other way I know of to help hold this wall would be to use mortar or Portland cement to help hold the stones together AND incorporate geological grid into the wall and hillside in flat horizontal compacted layers (much wider than the 4’ you now have). Bottom line, it will not last unless you seek professional assistance in the overall design. Spend more now in the planning stage to save yourself money in the long run.

Moderator: Construction Systems, Foundations, and Masonry & Stone

For more information about me, my qualifications, and/or home inspections please visit my website at:
www.consultationdirect.com