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MSE Wall Global Stability Under Existing Homes and Wall Questions

MSE Wall Global Stability Under Existing Homes and Wall Questions

MSE Wall Global Stability Under Existing Homes and Wall Questions

(OP)
There is a new development proposed adjacent to my home. The developer's engineers designed a 37ft tall 3 tier concrete block retaining wall that is approx. 25.5 ft from existing homes. The global stability failure line crosses the property boundary and is under the existing occupied homes. If the wall were to have a global failure the existing homes below the wall could collapse. Has anyone seen a situation like this before? Are there regulations that exist to prevent a wall of this size being built this close to occupied homes? I'm also concerned that any type of wall failure would impact the residents below the wall. I've attached a picture.

RE: MSE Wall Global Stability Under Existing Homes and Wall Questions

Just because the critical fail surface crosses the property line, it does not mean there is a problem. What is the minimum safety factor? I would be more concerned about having someone with MSE wall design experience peer review the design and then making sure the wall is built properly with inspection done by a qualified inspector. It is nice that the designer performed the global stability analysis. Too often, MSE walls are designed only for their internal stability, sliding, and overturning. Too often global stability is ignored.

www.PeirceEngineering.com

RE: MSE Wall Global Stability Under Existing Homes and Wall Questions

A section showing the original ground surface would be nice.
Also, any drill hole data and info on the site geology
Surely, something this size has a decent study behind it.

RE: MSE Wall Global Stability Under Existing Homes and Wall Questions

(OP)
The factor of safety for global stability is greater than 1.3 and was required by the County. The engineer says it is 1.31 for this wall design. I've attached the wall cross sections and the geotech borings. I agree about the construction of the wall and building it correctly. That is one of my biggest concerns. I've been an construction inspector acting as QA working with the QC and contractor to make sure the project was built correctly.

RE: MSE Wall Global Stability Under Existing Homes and Wall Questions

I agree with PEinc, if its designed properly and to code and constructed properly then I no see no issue.

RE: MSE Wall Global Stability Under Existing Homes and Wall Questions

I would ask about the service life of the wall and what happens when the wall needs to be repaired or replaced. MSE walls have improved over the years, but we are currently working on several walls that only lasted 10-20 years before they became a problem. These are spots where the walls cannot be demolished without impacting an adjacent structure. We’re using soil nails to repair a 30’ tall MSE. It’s costing the owner ten times the cost of the original construction. I doubt the owner of the residence will be able to pay for that. Are they going to demolish their house to replace the wall? What happens??

RE: MSE Wall Global Stability Under Existing Homes and Wall Questions

I just realized you're in Colorado. To expand on PEinc’s comments, We have designed hundreds of MSE walls in Colorado. I can tell you that I’m never impressed by the “inspections” done on MSE walls out there. They usually get a technician to do some part-time compaction testing, that’s it. A wall of this magnitude (size and impacts of poor performance or failure) needs full-time inspection with someone checking grid lengths, installation techniques, etc. along with compaction testing. I hope you can sell full time inspections to the County. Something needs to change in CO regarding MSE wall inspections.

RE: MSE Wall Global Stability Under Existing Homes and Wall Questions

Quote:

we are currently working on several walls that only lasted 10-20 years before they became a problem.

What kind of problems? We do MSE walls regularly, and we expect a 75 year service life. We don't allow steel reinforcements in some types of reinforced backfill, but the geogrids we spec are supposed to be good for at least 75 years.

RE: MSE Wall Global Stability Under Existing Homes and Wall Questions

BridgeSmith - We still design MSE walls, but I'm not convinced of the 75-year service life on all MSE walls—specifically dry cast blocks.

We've seen walls designed, constructed, and/or blocks manufactured incorrectly. Usual suspects of issues we have seen in person are outside corners pulling apart, causing significant cracking and dislodging of blocks, fence posts installed too close to small block MSE walls causing rotation of the upper portion of the wall at each fence post location, blocks that have 100% disintegrated back into aggregate after 10-15 years. We've also seen issues when the contractor and their designer decide to tie walls together, that should be separate, where they butt together.

There is one designer in the area that we've had to get pulled into their new and old projects after they messed up. They have been "designing these walls for decades" and it's their bread and butter (aka block manufacturers go to him), but I can't figure out why. It must be cheap to use them, and no one has probably actually sued him enough. Most owners we work for are not that litigious.

RE: MSE Wall Global Stability Under Existing Homes and Wall Questions

emj_highway_engineer, I would be more concerned about the stability of the ~9' to 10' high, steeply sloped, red hatch, 28o, c = 0, loose soil placed at the property line, in front of the base of the wall. What keeps that from failing?????

www.PeirceEngineering.com

RE: MSE Wall Global Stability Under Existing Homes and Wall Questions

Likely the design involves some sort of back drain system to keep the mse portion of backfill from becoming saturated. If so, the selection of the long-term phreatic surface is likely dependent on the performance of the drainage system. What happens if the system does not work as envisioned? This means long-term monitoring/maintenance is critical to the wall performance. Who's responsible, what costs are expected and what mechanism is set up to make sure this occurs.
IS there any redundancy incorporated in the design to address these likely impacts?

RE: MSE Wall Global Stability Under Existing Homes and Wall Questions

I would concur with PEinc and add: I see no provision for drainage anywhere on the cross section nor a retaining wall for the Red/Pink patch in front of the MSE wall (defined as "loose" soil)

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