Residential Segmental Retaining Wall

[TRAK.Structural]

I've done a fair amount of retaining wall design in my career but hardly any in the residential sector. I am considering doing a project that will require a segmental retaining wall that will have about 9 feet of unbalanced soil. The system will be the typical dry-stacked blocks with geo-grid back into the soil. Looking for any guidance on the points below:

- Geotech investigation isn't likely an option here, if using a low presumptive bearing pressure how does this get confirmed during construction?
- Does anyone have any general notes (for a structural note sheet) related to SRW's that they are willing to share?
- I have the NCMA SRW software but haven't found much useful training info on it; anyone know of a good training resource for this or know of any pitfalls to using this program?

 

[JoelTXCive]

Check out this link to an old thread......

https://www.eng-tips.com/viewthread.cfm?qid=487698

 

[jerseyshore]

I use Enercalc/ Retainpro for these designs generally, but Allan Block does have free design software that is decent.

Some manufacturers have some general notes you can use as well.

First figure out the brand of block they want to use and then what brand of geogrid.

Since you are new to these designs, many manufacturers also have typical pre-engineered designs in their catalogs that you can compare with your final design to make sure you are in the ballpark.

 

[milkshakelake]

If you're using a conservatively low bearing pressure, I still recommend doing geotechnical investigation (like dynamic cone penetrometer test) to verify bearing pressure. But in a pinch, you can use a pocket penetrometer. They're like $50 or so, and it's better than just looking at the prepared subgrade and saying, "Looks fine to me, I guess." I don't think they would hold up in court if the wall collapses, but it's really better than nothing.

I think you might be better off posting this in the geotechnical forum. I've gotten good responses about "unconventional" retaining walls (i.e. not the one you find in every textbook; you know which one I'm talking about).

 

[XR250]

They're like $50 or so, and it's better than just looking at the prepared subgrade and saying, "Looks fine to me, I guess." I don't think they would hold up in court if the wall collapses, but it's really better than nothing.

I would argue it is worse than nothing.
Either call out a presumptive capacity of get a Geotech.
Why take the risk of buying the wall by using a device that I have only seen mobile home installers use.

 

[Jrit]

One note to add, segmental walls aren't designed for a bearing pressure. The wall design will give you the applied bearing pressure. The width of the retained zone typically yields a bearing capacity with a large FOS (and much larger than what is typically seen for a strip or column footing). Settlement for poor soils are the typical problem. We state the testing firm should confirm the foundation soils are adequate for the applied bearing pressure, which in our area would be done with hand auger borings and dynamic cone penetrometer testing. Larger walls are a different animal.

 

[TRAK.Structural]

Jrit - Is it common to have testing/special inspections for a residential retaining wall like this (more than just a county inspector)?? I can note that they are required on my sheets, but I'm not convinced that there is a real shot of that kind of construction verification happening for this type of project?

Since you are on the geotechnical side, I can control (or at least specify) the type of soil that will be placed in the reinforced zone; but do you have any suggested resources for typical properties of the existing retained soil behind the reinforced zone? This project is located in the Charlotte NC area.

 

[milkshakelake]

@XR250 I disagree that it's worse than nothing. OP indicated there's no geotechnical, so presumably they're taking all the risk on this. Not something I'd do. But if they're taking the risk, having some small measure of testing to check a presumed conservative bearing capacity is better than nothing. I have seen inspectors use the tool to do quick localized checks, though those projects did have real testing.

I would add that if this project fell into my lap, and there's no budget for soil testing, I'd personally walk away. Putting money where my mouth is, I'm doing that on a project right now.

 

[LOTE]

TRAK.Structural - I went from doing almost solely DOT MSE/SRW walls to a lot more residential over the last couple years. Here are a few things I have learned about smaller residential walls:
- Regardless of what you specify for the reinforced fill zone, there is a good chance the contractor will just install 12" of crushed stone followed by onsite fill, even if it doesn't meet your specified reinforced fill. When checking designs, I always do a "what if" check to see if I am still comfortable with the design if onsite fill gets replaced with my specified reinforced fill.
- I specify that 3rd party should test the foundation and fill soil, and inspect the leveling pad, fill placement, and geogrid placement. And geogrid should extend past the face to confirm the spacing (can be cut after construction). I would say only about half of small residential projects does this go as specified.
- There is a chance you will never hear anything after you send off drawings, then a while later get a call from the contractor asking for a sealed form that the wall was built to plan (after they had no inspections or testing, maybe a couple field photos). Be prepared for how you will respond to this.
- For shorter walls, I start with the USDA web soil survey to get a general idea of soil layers, groundwater depth, etc. then have the contractor verify this prior to design, then specify testing/inspection during construction.

Here are a few general pointer on design:
- SRW grid lengths are typically controlled by sliding or pullout. You can lengthen the gris to ~0.8*H before there is minimal reduction in applied bearing pressure because you are approaching the weight of the soil itself. I always show the applied bearing pressure at each change in grid length that the foundation must meet.
- I tried to use the NCMA software and hated it. I also tried Allan Block's software and found it glitches too often for me to feel comfortable with the results, and only applies to one manufacturer. I like to use Vespa2 by CTi Software because you can design the whole wall in one go, then export the full wall elevation view and cross sections into CAD. It has all the common blocks and grid testing preloaded, and you can export to ReSSA or Gslope for global analysis. I have also worked closely with their team on a client specific program and they know their stuff. Anything non-standard I check in MSEW+. Vespa2 has some training videos, and I have one I created for my team that I can share.

For general notes, Allan Block and Keystone both have a lot of great resources. Also look at some of the DOT's APL drawings (FDOT APL section 548, for instance).

 

[jerseyshore]

There is a chance you will never hear anything after you send off drawings, then a while later get a call from the contractor asking for a sealed form that the wall was built to plan (after they had no inspections or testing, maybe a couple field photos). Be prepared for how you will respond to this.

I don't know if I've ever seen a more accurate statement before on this forum. Depending on the type of work you do and type of contractors who are doing the install, this situation could happen at an alarming rate. Around here most MSE walls are installed by landscapers and they take out permits maybe 50% of the time. And when you design 2 or 3 walls for a landscaper, they build the 4th one on their own and send you 4 blurry photos asking for a letter.

 

[TRAK.Structural]

LOTE - thanks for the thorough response and the references!!

 

[geotechguy1]

>And when you design 2 or 3 walls for a landscaper, they build the 4th one on their own and send you 4 blurry photos asking for a letter.

Unfortunately there is always a few P.E's down on their luck career wise or nearing / past retirement willing to churn out these designs for 2-3k and sign a letter for $500. If this wasn't the case or the regulatory bodies were strong the landscaping industry would probably come to be controlled by larger firms or smaller firms owned by P.Es / P.ENgs. In the current setup landscaping companies pop in and out of existence day flies, make some money, and stop existing after a summer or two before being replaced by a new one operated by the same guys. All long-term liability for crap construction is left with homeowners and engineers while most of the profit goes to the dirtbags that run landscaping companies.

 

[gte447f]

LOTE's advice is pretty spot on for the residential SRW market. Note that LOTE said to specify "that 3rd party should test the foundation and fill soil, and inspect the leveling pad, fill placement, and geogrid placement," even though this is likely not going to happen. If there is a problem with the construction, you have covered your bases, and the contractor is the one who has not met the standard of care, because he/she has failed to construct the wall in conformance with your plans.