Building a retaining wall on a 30-35% slope
sandiegodiyer
| Posted in Construction Techniques on
I want to build a retaining wall on the hill behind my new house using those retaining wall block, the slope of the hill is around 30-35%, I was wondering how deep should the footing be? The wall would be around 5-7′ tall. is there a calculator online where I can do the math. If the answer is related the type of soil, I was wondering if I can use the soil test done by my neighbor a few houses up the street who has a similar slope for my calculation? I can’t imagine the soil would be that different if we are just a few hundred yards away from each other on the same hill. Thank you very much
Replies
https://www.sandiego.gov/sites/default/files/dsdib221.pdf
This might be of some help.
The details do depend on soil types, only you (or someone on site) can determine if a neighbor's soil analysis would apply to your location. soil types can be uniform over a good area, but there also can be transitions that can change soil structure and composition over a short area.
It does not seem to me that dry laid block is a good choice for your application.
Might be ok if you break up the wall into a stepping affair of two or more shorter walls.
I just finished renovating my 4’ tall, 90’ long wall that was professionally installed, but started to bow in several parts within 3-4years. I was too busy at the time and my wife and I agreed that hiring out would be best, but the contractor made several apparently common mistakes (and of course symptoms didn’t start showing until after the warranty). So i wanted to throw in some advice that you may already know, just in case it helps (even though I don’t know the footer depth for your particular situation).
- Geogrid is amazing. Structures the soil to nearly stand on its own independent of the wall. Layer horizontally every 2 feet of height, extending perpendicularly behind wall (into hill) at least as many feet as the wall is high. My contractor didn’t use it since my wall height didn’t mandate, but I think it would have prevented most of my issues.
- Make sure you get the drainage right. gravel/rock/high-draining aggregate for most of the wall height (aside from a layer of earth that can top it off; I left mine gravel as a low maintenance border this time) and going back 12-24 inches behind wall.
- Tamp or compact every few feet of backfill/earth added. My contractor skipped (or skimped on) this step and the ground all settled down about a foot below top of wall (and I couldn’t just add more dirt and re-sod because the bowing had started at that point).
- You probably already have, but watch several YT vid’s for best practices and common mistakes people/contractors make. Also, depending on how far that slope continues up the hill, you may want to have an engineer determine specs for the wall (and it could possibly be required in your area).
You need to understand the soil type, the soil slope beyond the retaining wall, the block type (including the angle the block references to the proceeding row), the foundation requirements, stone size, compaction schedule, geogrid sizing and placement, drainage, not to mention site access. Each manufacturer will have their own requirements. I recently installed a system from Allan Block (https://allanblock.com/retaining-walls/landscape-walls.aspx) and has so far held up well. They publish gravity wall tables/schedules for basic situations and separate tables for geogrid. Check with your building dept about the need for permits, especially as the wall size grows in height. a 5'-7' wall is quite tall for these sort of systems.
FixEmUpper is correct - check with your local building department to determine what they require in order to comply with applicable codes.
If you were in CT it would require that a professional engineer design the wall and provide drawings of it installation.
If you try to circumvent the permitting process required it may bite you in the butt when/if you go to sell the home.
How high is the hill?
What type of soil?
How deep would the soil be behind the wall?
35 degrees is approaching the angle of repose for many soils and after a heavy rain, it would be even less. No dry-stacked wall is going to slow it down much less stop it. That's a seriouse engineering job.
You are going to need an engineer to spec that thing, or you run the risk of it failing. Here, anything over 4' requires it.