Drainage Tile and Retaining Wall

nabz

Hello,

I'm currently in the process of digging out an area for a retaining wall. My backyard is such that the water from the home behind me runs down towards my home and then passes between our home.

Since my retaining wall that I'm building is over top of the area that most of the drainage occurs, I've cut out the trench for the retaining wall and in the middle of (underneath where all the soil will go) I'm going to add a drainage tile. The wall itself will probably be somewhere like 2 to 3 feet high -- stepping up every few feet towards the fence at the back of my home -- since the slope is downwards towards my home.

In order to resolve the drainage issue and keep my retaining wall from separating/falling, I'm building in a trench for a drainage tile. I'm unsure of the size I will need (diameter of the drainage pipe) and whether or not I will need landscaping fabric.

I know I can get the drainage pipe witha sleeve on it, put it in the trench and bury 5-6 inches of drainage rocks/stones over top of it, but will I still need the landscaping fabric over top of the rocks to keep the soil from getting into the rocks. OR is that just overkill and does the sleeve on the drainage tile do the same job as the landscaping fabric. If I do need landscaping fabric any ideas on the width I'll need?

The length of the pipe that I will need to run will need to be about 18feet or so...

i apologize if this is a lengthy question but any ideas would be greatly appreciated. I don't often do these kind of jobs so am kind of clueless (but learning).

Thanks in advance!

Thalweg

Nabz,

There are a lot of potential complicating factors and questions here. First of all I'm not sure I'm visualizing exactly what you're doing. It sounds like you're almost building your retaining wall on top of your drainage ditch (tile), or at least very near it. The surface around your drainage tile is most certainly going to move over the years. So I hope your wall is flexible or has a very good foundation.

The size of your drainage pipe would be determined by your expected runoff, which will be determined by expected precipitation, slope, soils, and surface cover. Unless there is some reason to do otherwise, I usually design for the 2-year, 24-hour rain event. However, this might be over thinking the issue. Unless you've got a big contributing area (the area upstream of your site), or get very heavy rains, the 4-inch drainage pipe from the BORG is probably more than adequate. If you're expecting big flows, you might need something bigger.

I personally don't care for the sleeved pipe. But it's value, and landscape fabrics value, are dependent on your soil type and vegetative cover. If you get some good grass growing on the site, and have a good gravel over the drainage tile, you shouldn't be moving a lot of sediment into the pipe. Just make sure your rock is clean. The riskiest time is right after you lay the pipe when there is a lot of available loose sediment. The sleeves tend to plug up somewhat during the first flush and slow the whole system down forever. Just make sure that your pipe has a good place to drain and enough slope to keep things flowing, and any sediment that gets into the pipe should flush out OK. I might reconsider this though if you've got really silty soils. Silt is really hard to keep from moving. Some of the landscaping guys might disagree with not using the landscape fabric. I generally focus on water flow. I'm not really good at growing things.

I will often put a layer of compost, or compost/peat moss mix on top of the rock. It usually has a pretty high infiltration rate and a lot of nutrients to help get grass growing quickly. However if you live somewhere with soils that drain well and have high organic content, that may not be necessary.

I hope that helps!

nabz

Thanks Thalweg...this gives me some ideas on what I need to do.

I think I will use the 4" pipe and make sure I use the the clean drainage rocks.

Thanks again!

ccarlisle

Yup, 4" performated drain pipe is what we use up here too; geotextile fabric next to the soil, surrounding the pipe and the clean 3/4" stone gravel. Just make sure the drain pipe has somewhere to go...