We have an existing (poor) retaining wall that is about 2 feet high. 30 inches in front of it, we’re adding a new retaining wall, mainly to establish some plant beds along fence line. The land in front of the new wall is level. What drainage precautions do we need to take for this wall?
If you are building your retaining wall out of the retaining wall bricks, all you need to do is to fill a space, between dirt and brick (approximately 6 inches wide), with gravel, that will allow the water to filter through the gravel and come out in between your retaining wall bricks on the other site. If you build it out of solid concrete wall, you do need to install some drainage pipe, fill the trench with gravel around the pipe, and make water exits in your wall for that pipe. So when water builds up behind your wall, it will go through gravel into your drainage pipe and exit the wall in the openings that you will make for it.
Add 6 inches gravel between dirt and brick. Make sure to add landscape fabric!
If the water coming from brick is not welcome, add drain.
Yes.
But the complexity of the drainage is dependent on the materials used, primarily.
C27s should be able to do this easy.
I’m unsure of your soil conditions in California. We install hardscapes in Iowa, but here are a few tips. When building your retaining wall, you’ll need to trench out an area of 24” to 30”. Fill the trench with ¾ to 1” clean or clear angular stone (not pea gravel and stone with fines); different parts of the country call it different names. You’ll need at least 4” of that base stone before leveling out the wall on it, with the bottom layer of block being ¾ covered or more. On the bottom of the 2nd layer place the drainage tile, make sure it runs downhill, and it has some sort of sock or fabric wrapping. Your backfill of rock, the same rock as the base, needs to be at least 18” in width. This will reduce the hydrostatic pressure from building behind the wall. It’s always a good option to use filter fabric to separate the angular stone from the subgrade, thus keeping the angular stone from becoming plugged with dirt and not allowing it to drain. The top 6” of the wall can be top soil. Keep in mind when planting, especially large shrubs or trees, the wall surcharge zone. The surcharge zone is calculated by multiplying the walls height by 2 and that will give you the distance need to plant large objects. I’ve seen many walls heaving because of tree or shrub roots.
For any retaining wall constructed the drainage flow must be accounted for. It’s almost impossible to tell you what your best drainage solution might be. Soil conditions, grades and current entry and exit points of the ground/storm water runoff all play a factor into the size and type of drain. Remember, retaining walls create what are effectively dams that will prohibit the flow of water. If you have soils with clay components, i.e. Bentonyte, your proper flow drainage of storm/melt water is likely the most important component of your landscape design and ijstallation. Clay does not absorb water. What that means in a nutshell is that any water that is standing anywhere in that yard is going to turn into a low spot and unaddressed, will create a sink spot that will only get bigger and bigger, causing a plethora of issues…All bad.
Failure to provide drainage for any retaining wall at any grade will result in the wall at minimum, washing out if not eventually failing all together…Guaranteed.
A large part of the answer is determined by soil conditions & the material used to construct the wall. At minimum, in addition to your 6" gravel drainage zone you will want to have a sleeved, perforated 3" accordion tube running around the outside perimeter of the wall to suck up any residual water By whicking it via the nylon sleeve and the perforated pipe to make sure goes round the outside edges.