Question
Hi Sean,
We have a retaining wall that runs along our side yard (approx 50 ft). At the property/fence line above us we are experiencing erosion. I want to install a small wall/planter on the existing slope, about a 18"-24" from the property line, make it level with the property above us and use it as a planter. I will be using 4x4x16" cinder blocks. At the highest point, the wall will only be 18" tall. At the shortest, just 12". I do not plan on putting a footer in because I don't think the volume of soil behind it requires it. It will be secured with rebar and concrete.

My question is, what do I use for drainage? I have read numerous posts about gravel vs. drainpipe. If gravel is the choice, what kind and how much?

This wall is purely cosmetic. I just want to avoid problems in the future. I will be working around the existing larger plants on the hill.

Thanks for your help,
Cheryl

Answer
You would typically use gravel and drain pipe. Not one or the other.
The gravel goes behind the wall and the pipe at the bottom of the gravel. Water moving through the soil or on top hits the gravel and drops into the drain pipe. The qty of gravel depends on the length and ht of the wall. Generally you want about 4 to 6 inch layer of gravel behind the wall.
Take the length of the wall times the height times six inches. That will be your volume in cubic feet. The divide by 27cu ft to get the qty in cubic yards. Rock is sold in yards and tons. Ask the supplier to tell you how many yards are typical per ton.

Bags often sold in cu feet so you can figure that out also.

PS cinder blocks do not lock together well even when mortared unless you reinforce them with steel. I recommend a concrete wall unit made for this application like "keystone wall" or "anchor wall", or some similar unit masonry.

Also you will need a footer. It should be a gravel footer dug down to the depth of your frost line. The width should be 1.5 times the width of your wall block. You should compact this gravel with a vibratory plate compactor to be sure you have a solid level starting surface.


Good luck