Structure over MSE Wall

[moe333]

A proposed project has a 2 tier MSE wall with a 2 story building located several feet from the back of the wall facing. The total height of the MSE wall is about 30 feet (15 feet per tier), so the geogrid length will probably be about 12-15 feet long. The backfill in the reinforced zone will likely be a silty to clayey sand, so draiange will not be great. The soil in the retained zone may be similar but probably higher clay/silt content. The building will be either wood frame or CMU.

If the building if founded on shallow foundations, I think it would move laterally along with the wall, so I am thinking the portion of the building located over the reinforced zone should be founded on piers drilled below the bottom layer of geogrid in the upper tier of the wall. The piers would be drilled through the geogrid, but I think this may not affect the geogrid significantly. I would recommend that the piers be set back as far as possible from the back face of the wall.

The portion of the building beyond the geogrid would be on shallow foundations in the compacted fill. The piers would be founded in the compacted fill such that there shouldn't be significant differential settlement across the building.

I am trying to evaluate what effect the yielding of the MSE wall may have on the piers, and the lateral stability of the piers considering the project site is in seismic zone 4 in Southern California. I think the majority of the (static) yielding of the wall may occur during wall construction, or shortly thereafter, so that this may not be an issue. However, during an earthquake, there will probably be some movement. I will analyze the lateral capacity using LPile, but am not sure how to model the free face of the wall.

Any previous experience with a similar project, or suggestions/comments would be appreciated.

 

[rochplayer]

I have used MSE walls to support shallow foundations for bridges in high seismic areas (Alaska and Oregon). Bearing pressure was limited to 4000 psf and "inextensible" MSE wall reinforcing was used (e.g. steel). I have also designed MSE walls with inclusions (e.g. piling), but usually these are built first and the reinforcing is placed around the piles.

I have concerns about drilling through the geogrid - you are cutting the effective length of the grid and you may not have enough reinforcement left. The MSE walls need to be designed internally to account for the piers, not simply build the wall and drill through the grid. Talk to a wall vendor such as Keystone/Tensar (for geosynthetic reinforcement) or RECO or Foster (for steel reinforcement) and see what they recommend. My general recommendation is don't damage the reinforcing!

Also, you are setting up your foundation for an inherint differential condition if one side of your building is supported on spreads and the other on piers. They distribute stress differently into the soil, so you need to be very tight on your analysis.

Also, if lateral movement is a concern, in my opinion you may be making the situation worse with your above approach. The piers you are suggesting will be drilled through the reinforced zone and as a result would be fairly integral with it. They may be "dragged" within the MSE wall mass if it deflects laterally during a seismic event. A shallow footing on top of the wall, above the reinforced zone may actually experience less movement that the pier supported foundation. I have done no analysis, but that is my initial review.

Can you not get better backfill? FHWA and the wall vendors I have experience with recommend no greater than 15% fines in the reinforced zone backfill soil.

Do you have an outside reviewer within your firm you could discuss this approach more with? There are some big risks in the approach you are taking. I also recommend talking with the engineers at some of the wall vendors, especially if you are proposing to do the internal wall analysis yourself.

 

[GeoPaveTraffic]

I agree with the last part of what rochplayer has to say; the approach being taken is has a lot of risk and not only during a siezmic event. Placing a building in the active zone of any retaining wall is risky, placing it in the the active zone of a flexible wall is very risky. If I did it at all, an open question, I would send the client/owner a very strongly worded letter about the risks and ask them to acknollege the letter. No client is worth going out of business over.

 

[MSEdesigns]

I will like to add to what rochplayer said that the length of the grids will need be a lot longer to comply with global stability issues. You are talking about very poor soils for an mse structure. Too much fines in your soil is an invitation to lateral displacement. A rough estimate using the empirical curve of the FHWA and assuming L/H ratio of 1.0 will give you about 0.35 ft of lateral displacement assuming a surcharge of 420 psf.

 

[moe333]

Thanks for the input. I am concerned about damaging the geogrid, but the pier spacing and diameter turns out to be a very small portion of the area of the geogrid that the strength decrease could be overcome by a small overdesign in the geogrid.

The differential settlement should not be significant since both the piers and shallow foundations will be founded in well compacted granular fill that is not very compressible.

I would think that founding the building on piers would result in less lateral movement than on shallow foundations since the top of the wall will move more than bottom.

I will look at the FHWA reference for lateral displacement, and may consider inextensible reinforcement. I am interested to know if anyone has looked at lateral displacement of piers that are in proximity to a retaining wall and ways to model this with LPile.

Thanks

 

[Boonie]

As stated previously, unless the distance between the the two terraces is 2H (for the lower wall) the the system should be analyzed as a single structure. The global stability of the system will drastically increase the lengths of the geogrid in the lower wall. Similar structures are designed and constructed with silty clays and clayey silts in the southeast regularly. The key to utilizing these "poor" soils is to be conservative when preparing the sub-grades and providing adequate drainage measures to protect the reinforced zone (i.e. blanket and chimney drains). When addressing the impact of piers/piles in the reinforced zone, you can reduce the coverage percentage of the geogrid to account for the loss due to installation damage.

 

[MSEMan]

To put your question into perspective MSE walls are usually designed for a "permanent horizontal deflection" of 50mm or so in seismic conditions. If you model this deflection at the interface of the base of the MSE wall and the pile support you may find that you have a problem with brittle failure of the pile.

 

[attihc]

Suggest that the structure be located outside the influence of the MSE wall, and be supported on shallow foundations bearing on the retained compacted fill. The long-term behavior of MSE walls is uncertain. There are potential risks.