Tall Retaining Wall with Short Length

[USGeotech]

I have a project in an alleyway between two existing buildings. The alleyway is about 7 feet wide and both buildings have basements that go down about 20 feet. At both ends of the alleyway, a retaining wall must be constructed. The walls will be about 20 feet tall but only about 7 feet long-the width of the alleyway.

The new walls must be designed as cast-in-place cantilever retaining walls. For the moment, I want to ignore the temporary earth support that will be needed to construct the walls and focus just on the permanent concrete retaining walls. The retaining walls must be constructed so that they are independent of the existing building foundations on either side. Backfill will be native well-graded sand and gravel with less than 5% passing the #200 sieve and the walls will be constructed well above the groundwater table and will not be affected by runoff since the sand and gravel is free-draining.

If I design the walls for the full height earth pressures assuming plane-strain conditions, then the design moments are very large resulting in very heavy reinforcement and very large footings. Clearly this is not a plane-strain condition and some arching between the two existing foundation walls will occur. But how much? Is there some known method to calculate the reduced earth pressure? Is this condition analogous to designing lagging between two soldier piles spaced 7 feet apart?

 

[BigH]

With, what I understand, 20 ft of fill placed between the two buidlings the design of the retaining wall is likely the lesser of the problems you face - or am I misunderstanding and the foundation bottom of the retaining wall is at the same level as the buildings' basements?

 

[msquared48]

If this a "U" shape structure with earth fill in the middle of the "U", why not just use concrete encased rebar ties between the walls rather than worry about the soil pressures due to a large footing. They use this construction a lot for offramps in the middle of the I-5 freeway here.

The footing could be enlarged to tale seismic forces if necessary, but would still be a lot smaller.

Mike McCann
MMC Engineering

 

[BigH]

Assuming that the 20 ft of fill is above the existing ground level between the buildings, my question was what will the 20 ft of extra fill do to the earth pressures on the existing buildings' basement walls? I realize there may be some arching - silo effect but still am wondering if the existing basement walls will take the added loads. - agree that your suggestion is reasonable so long as the building lengths are reasonable - a tie 200 ft long?

 

[InDepth]

Well I think you have some interesting issues here:
1) Yes, I think you could assume arching fir the earth pressure as long as the assumed point where the arches meet the wall are stiff and rigid...ie a perpendicular wall (new or existing basement walls.)
2) you could also jet grout a huge 20ft block in the soil and no need for temporary shoring...just a shotcrete facing. Ie a gravity block
3) Now, let's talk about the unbalanced load in both buildings you are creating by exvacating between the buildings (don't forget that Chinese failure when they created an unbalanced condition). You need to check if the floor diaphragm can take the soil load and can transfer this load to the shear walls. Then you need to check the new foundation pressures caused by the imbalance. You will have to do this for both buildings. There are a few tricks, but you ultimately have to follow the forces. If the diaphragm is inadequate you may have to either strengthen the diaphragm or add an internal shear wall to break the diaphragm....etc etc. Option 2: Provide struts between buildings to match the unbalance (hopefully floors align between the buildings).

Now don't forget that once you create an unbalanced condition you may be triggerring code requirements that change the building height and thus may also change earthquake or wind response of the global structure. Be very careful with trigger points and changing building response.

Just do a few calcs, follow the load path; verify possible existing building/diaphragm/wall/foundation response.

 

[InDepth]

By the way, you could consider a secant or tangent pile wall with a 4" shotcrete finish facing. Avoids adding temp shoring and gains space.

 

[hokie66]

Like BigH, I think the answer to your question depends on how the 20' tall retaining wall relates to the 20' deep basement.

 

[fattdad]

If you can't use the existing buildings' foundations what would be the basis to use the existing buildings' interface shear (arching) effects? I mean if the buildings were to be demolished, what would provide the resisting shear strength?

f-d

¡papá gordo ain’t no madre flaca!

 

[dcarr82775]

Look up 'Bin Pressures'. If I remember right it is based on grain silos, and will help reduce the pressures a little. I am assuming the 20' of soil in the alley already exists and for whatever reason excavations are needed at each end of the alley.

 

[USGeotech]

Thanks for the replies. The two existing industrial buildings both have basements and the footings of the new walls will be at about the same elevation as the existing footings. There will be no unbalanced earth pressure loads since.

Also the alleyway is the same length as both buildings - about 200' long. The two new "end" walls will be placed near the middle of the alley with the walls about 100 feet apart. The excavation will create a pit between the two buildings which extends down to the basement levels and be about 100' long x 7' wide. A couple of openings will cut into the existing foundation walls to provide access from the pit into the existing basements.

 

[InDepth]

Explain your reasoning how there won't be unbalanced soil loads if you remove 50% of the earth soil pressure from one side of both buildings.

 

[hokie66]

I think I can explain that, InDepth. The basement walls of the existing buildings are retaining now, but he is taking away the retained earth for 100' of the length. This could conceivably create global issues, depending on what happens at the far side of the basements, but locally, the existing walls should be fine.

If the new retaining walls have to be independent of the existing, I think that means that no load, arching or otherwise, can be placed on the existing. Cantilever walls or some type of tied back wall would be the options.

 

[InDepth]

Thanks hokie...that's exactly my point, the global issue...removing soil is fine locally, but he may run into trouble with his diaphragms and shear walls (globally) because of the soil load on the far side.

 

[TXStructural]

Aside from the issues created for the building foundations, you could probably buttress the back of the walls, rather than making them purely cantilever. This would let the footing width be governed by soil pressures rather than reinforcing development lengths and bending strength of the heel.

Out of curiosity, why can't the ends of the new walls be supported by the existing foundation walls (maybe a vertical corbel), so that the new wall acts in bending across the 7 ft length?