Residential SRW Design

[TRAK.Structural]

Posted this in the Structural forum, re-posting here for the Geotechnical take.

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? Are legit special inspections/geo confirmation during construction realistic for residential SRW work?
- 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?
- Any trusted resources out there for typical soil properties based on region (Charlotte NC area)?

 

[ABTSGeoScience]

As far as confirming in-situ bearing you could use a DCP (Dynamic Cone Penetrometer). That would give you a direct correlation with the Bearing Capacity. I don't think I would "design" a footing off the DCP results, but it is a good way to let you know if you are int the ballpark.

 

[geotechguy1]

I don't know about Charlotte NC - in Alberta these walls were usually design without any site investigation using what were considered conservative parameters in the region. Normally when the site has been stripped / excavated down to the base level you would do some kind of testing there, like NDMs, a hand-held bearing probe, or perhaps DCP (although very view use DCPs). Alot will depend on what the agency that has to approve permits / consents will accept.

In terms of notes / parameters, is it is a 'speciality type' block (like Allan Block, redi rock, etc) - if so the manufactures of those designer blocks can usually give you most of what you are asking for, although a PE needs to sign it off. Also there is usually a small consulting firm or two that specializes in churning these designs out.

In terms of software, I haven't used NCMA SRW. We used MSEW or a program called VESPA (

https://www.ctiware.com/vespa)

which has most of the common SRW blocks built into it.

 

[LOTE]

I responded to your post in the structural forum too. Have you considered having someone that specializes in SRW design that portion of the project for you? It might be cheaper than the time to teach it to yourself for one project.

You are typically dealing with sandy clay in Charlotte (28-degrees, 120 pcf is common).

The VESPA software has training videos, and it can layout and design the whole wall for you. NCMA SRW only does single section analysis.

 

[TRAK.Structural]

Lote - I did end up getting the Vespa software and going through the training videos, along with doing my own research and reading from NCMA and some of the block manufacturers. I feel pretty good about the wall design itself. I ran a few iterations with varying soil properties (28 deg and 120 pcf was one such run) to get an idea of how it affects the results.

I did let my client know that we need a Geotech onboard to complete the design. In the end my main concern is not necessarily what my original post was focused on; I think a Geotech is needed for this project to evaluate the global stability of the slope that this wall is going to be on. That's not something that I can reasonably or comfortably take on. And while they are engaged they can do some soil sampling to sharpen my design parameters.

Will see how it all shakes out; I think the cost for this extra consultant may be a sticking point but I think it's the right approach in my case.

 

[geotechguy1]

I'm not sure what your reason was for taking on the job originally - but it might be worth finding and making a relationship with a few small local companies. Normally there are small geotech companies that basically specialize in churning out SRW designs and global stability analysis and then doing the field inspections / testings. At least that was the case in Canada. To get a geotech to take responsibility for giving you the soil parameters (at least for an SRW wall) and testing will probably cost almost as much as just getting a little company that specializes in churning these designs out to do it.

 

[TRAK.Structural]

geotechguy - I don't disagree. I didn't expect the existing site conditions were going to give me reason for pause in terms of global slope stability when I decided to take the job. A lesson learned on my part for sure. Now just trying to do right by the client and help to make sure all the boxes are checked to provide a satisfactory design.

 

[TRAK.Structural]

Update on this one.

The client has told me he spoke to a geotechnical engineer who said slope stability wasn't an issue on his property. I don't have all the background but I don't get the sense that this engineer made a trip to the property, more of a general conversation over the phone type deal. I don't have any reason to doubt my client or the geotech but it's not my area of expertise so I feel most comfortable with some sort of official documentation that global stability is not an issue for the proposed wall.

Is a memo/letter stating such and sealed by the geotech a reasonable request? I'm considering finishing the wall design and noting on the drawings that global stability has not been evaluated and must be done either prior to or during the special inspections process during construction; but only if that is a realistic requirement that can be satisfied. Otherwise I may just have to bow out of this project.

Any thoughts?

 

[LOTE]

A good geotechnical engineer with knowledge of local soils could look at the topography of the site and determine whether global stability is necessary.

I was involved recently with a project where a geotechnical engineer did something similar (they did visit the site and perform a couple small tests, more to cover their basis after telling me the same think verbally). They provided a sealed letter with the test results attached and recommended soil parameters.

Are there slopes above/below the wall? Any groundwater anticipated? Any poor soils you arr aware of? If the answer is no to all of these, then I would agree with the geotech's assessment.

 

[TRAK.Structural]

See below a crude sketch of the existing/proposed section. There is some slope at the base of the proposed wall and a "ditch" that looks to stay dry most of the time but has steep eroded banks likely from flow during storm events. Those are what is giving me pause here. No knowledge of groundwater or poor soils but that's not from review of data, just a lack of data.

 

[LOTE]

I would want to run that in a global stability software with ground water at an estimates flood elevation with drawdown, even if the geotech doesn't think it necessary. Having more embedment or an undercut for that case may be necessary.

 

[TRAK.Structural]

LOTE - thanks for your input. I think I'm with you, need the global checks to be run.