Structure over MSE Wall
Structure over MSE Wall
(OP)
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.
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.





RE: Structure over MSE Wall
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.
RE: Structure over MSE Wall
RE: Structure over MSE Wall
RE: Structure over MSE Wall
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
RE: Structure over MSE Wall
RE: Structure over MSE Wall
RE: Structure over MSE Wall