surcharge from adjacent building to retaining wall

[nozar afshar]

I am excavating 1.2m deep right next t a 7 story building. Depth is 1.2m below that building's foundation which is strip foorting. The footing is located right on the edge of the excavation. I have calculated a surcharge of 10 ton/sq-m for that footing. From the typical formulas, i come up with a very high lateral forces. Could somebody tell me what the lateral pressure would be on my retaining wall and what type of retaining wall would be appropriate. Angle of friction is 31 degree and C= 1 ton/sq-m. Some colleagues tell me that the lateral pressure i am computing from the typical formulas is not realistic because the depth is only 1.2m . I appreciate if someone with experience on this subject helps me out.

 

[Ron]

My concern would be for the existing footing that you will be undermining!

You need to consider a means of protection for that footing before you damage a building. The retaining wall is probably the least of your worries!

 

[structSU10]

So if I understand this correctly you are permanently undermining the existing building? You should look into underpinning the foundation rather than supporting the lateral pressure directly.

 

[nozar afshar]

Thanks Ron. In fact the existing footing is my main concern. That is why i am trying to design a retaining wall to support the soil underneath the footing before starting to excavate in that area. My question is 1)What would be the forces on that wall? For example is the Boussinesq equation the right theory considering the depth is only 1.2m ? 2)What would be the suitable structure to do the job? A wall, steel truss or etc?

 

[Agent666]

The problem is to build your wall you need to potentially undermine the existing building (even if only a short period), the very thing you are trying not to do by adding the wall. This is a risk you need to address with your design.

You need to underpin the building somehow, I've seen it done by excavating under the footing over a short localised length and backfilling with concrete. Alternatively a secent/palisade wall with closely spaced piles could be used. A normal retaining wall with strip footing is unlikely to be stiff enough and or control long term settlements. Would you be excavating below the water table for the new structure? You also didn't say how far away your wall is going to be from the footings?

Talk to your geotech engineer about the best approach for the soils present. If you are putting in piles in a continuous wall, they can also handle the analysis using a program like Wallap or similar which is far more realistic than you using hand methods.

Also be aware that the existing building may rely on the soil you are removing to transfer the lateral loads to the ground (you are removing the passive pressure load path by removal of the soil beside the footings).

 

[Agent666]

Additionally I'm imagining the line load and point loads from the building columns are huge for a 7 storey structure. This means huge lateral loads on your wall (which you would want to be 'stiff' to limit settlements that occur due to wall movement). What is the magnitude of these loads to give us an idea?

 

[oldestguy]

Your thought of using a retaining wall to hold back lateral loads from a footing is wishful thinking. Even if you can build it without damaging the adjacent building, the new wall will be massive and damned expensive. Look to underpin as the way to go.