# Need advice on large retaining wall



## alipep (Jul 7, 2020)

Hello! My name is Alice and I need your help

My husband and I are planning on taking a big project of installing a retaining wall. Pictures bellow!

Our house has a sloped yard. We had excavated (phase one) to install the retaining wall. The wall will be in a U-shape the smaller sides being 7 feet long on each side and the back of the wall will be 36. The height will be no more than 4 feet, most likely around 42 inches tall to avoid any problems with my town as 4 feet is the limit to have it engineered.

Our base trench is 24 wide x 18 deep, I plan on having about 12 inches deep of well compacted base and my first roll of concrete blocks barrier bellow grade. Behind the wall we will have a French drain system with perforated pipe and crush stone removing the water to day light far away from the wall and home.

Behind the wall should about 8 to 12 inches of crush stone between the back of the wall to the dirt.

I have clay soil that is always very saturated of water, because of the slope. It doesn't drain the water at all.

Key points:
We are in Worcester MA
Clay soil
Sloped yard
We will use concrete blocks, most likely from techo bloc
We believe that we should do ourselves if possible! 💪

My question are:
1- is my plan sufficient? Please point out any weak spots of my plan
2- is 12 inches deep of a base enough to estabilize the clay soil?
3- what material would be the best for my base, I got Recommendations of 3/4 crushed stone but I fell it will leave a lot of voids in between and the water, that never drains into soil, will just sit there.

Thank you so much for your input


----------



## corneliani (Apr 2, 2019)

You seem to be on the right track and are checking off all the important aspects. If you had a site plan with topography uploaded I could be of more assistance as I could see what the whole of the project looks like.. so far the only concern from your description is the footing depth (I'm from Atlanta, we don't worry about frost lines and hence can't comment on that) as well as that long 35' run of continuous wall. In long runs like that we either build a deadman every 8' or so (for timber walls) or we use geotextile grid (for concrete blocks). In your situation these may both be unnecessary (heck, from your description and attached pics it seems you're overbuilding as it is.. I'm not seeing the lateral load being an issue as yours is more of a landscape rather than structural wall) but if you have clay soil then you MUST be aware of the hydrostatic pressure that can build up behind that section no matter what. From your description you're going to be addressing this with the gravel bed and backfill .. make sure you wrap it in fabric to keep sediment out of it though.

To your questions:
1- your plan sounds sufficient but be very mindful of grade and water flow. Always create a water path, especially in clay soils.
2- a 12" base is enough, but it's not necessarily intended to "stabilize" the clay soil (clay is a great substrate to build on).. it's meant just as much to keep the water away from the structure & moving downgrade. 
3- The material choice for the base is a compromise between stability/compactability and permeability. Granite dust makes a great compactable base but not very good at moving water... so go with a mix of the two if you need to accomplish both tasks.

I'm impressed with your fearlessness in undertaking this project!! Best of luck.


----------



## alipep (Jul 7, 2020)

Thank you so much for your input, I thought about geogrid but that would make the project more than I could handle. I think I'll do the project as I stated and see what happens, worse comes to worst I'll have to hire out to take down the job and rebuilt. But I'm pretty confident about our plan. I like your suggestion of the granite dust for the base. We had a heavy rain 10 days ago and the water is still sitting there, it doesn't drain into the soil at all... so if I create a good solid base and install the french drain with the right pitch I should be able to get that water out of there easily, preventing the hydro pressure behind the wall.

The sloped yard around will be graded after the wall is built, I'll definitely follow your suggestion to direct water away... I thought about installing another trench way above the wall, like midway to the top to try catching some of the water that way too... do you have any thoughts?


----------



## corneliani (Apr 2, 2019)

Since pictures are worth a thousand words let's include some here for better visuals. I grabbed this from your specified block manufacturer... which brings me to another point I wanted to make: try & decide on that aspect since every system will have its own way of locking the blocks in. Some rely on a staggered system, others use locking keys that slide into built in keyholes. Look at their technical specs and it'll show you their recommended installation methods.



The 2nd point I'll make is that I feel like you're misunderstanding the role of the base foundation layer. Think of that layer not as just a stable foundation for your block but also as a "French drain" capillary system to allow for standing water to have a place to stay until it percolates into the ground. The slag I mentioned is so fine that it will not allow for that latter aspect, and that standing water will come to the surface. That was the point I was trying to make earlier .. create a place for water to flow, and the system that you described in your original post (which is what I included as a picture here) does just that: captures the water coming downhill and creates a point of least resistance (the gravel behind the wall) for it to enter but then it goes all the way down and into the footing if needed.i think you're headed in the right direction though, but wanted to clarify that point. IMO it's an important distinction to make.

Here's another similar design from the same manufacturer. You'll see the similarities and slight differences, from block to block.


----------



## alipep (Jul 7, 2020)

Thanks for going thru the trouble to further look into my project!

My local block supplier (camosse) provided me with the spec book which reinforced my original plan.

The thing with the base is, I know for a fact that my soil do not absorb water at all, so I was wondering if instead of using a 3/4 crush stone well compacted as a base I should use a more "solid" barrier so the water would actually be above the base behind the first block and therefore get into the French drain and be removed to day light away from the wall and house. I'm thinking the crush stone would allow the water sit in the base part first and it would only get to the French drain after collecting a lot raising all the way to the pipe, which, I'm thinking, could compromise my base in the long run... again, I have water sitting in the trench since 10 days ago because the soil doesn't absorbs well. What do you think? Does it make sense to try to keep the water near the pipe and make harder to penetrate the base?


----------



## corneliani (Apr 2, 2019)

No reason why you can't do it that way. Consider price as a determinant of material choice. Crusher run has similar compaction characteristics but cheaper since it's a byproduct of crushed concrete.

Last note: make sure you have a plate or jumping compactor on site. Loose/uncompacted soil can be a weak point for water intrusion.


----------



## alipep (Jul 7, 2020)

Great points! I'm planning on renting a compactor for the weekend to do the base and first few layers. Thanks again for all your input! It has been very helpful!


----------



## hsvtoolfool (Jul 23, 2018)

I would also research GeoGrid Fabric and learn how it's installed and stablizes the slope and wall. It think it's worthwhile insurance even on a 4 foot wall.


----------



## alipep (Jul 7, 2020)

The geogrid would be worthwhile, I agree... I'd need to get a person to excavate again but I agree it is the right direction


----------

