Tall Retaining Wall with Short Length
Tall Retaining Wall with Short Length
(OP)
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?
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?





RE: Tall Retaining Wall with Short Length
RE: Tall Retaining Wall with Short Length
The footing could be enlarged to tale seismic forces if necessary, but would still be a lot smaller.
Mike McCann
MMC Engineering
RE: Tall Retaining Wall with Short Length
RE: Tall Retaining Wall with Short Length
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.
RE: Tall Retaining Wall with Short Length
RE: Tall Retaining Wall with Short Length
RE: Tall Retaining Wall with Short Length
f-d
¡papá gordo ain't no madre flaca!
RE: Tall Retaining Wall with Short Length
RE: Tall Retaining Wall with Short Length
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.
RE: Tall Retaining Wall with Short Length
RE: Tall Retaining Wall with Short Length
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.
RE: Tall Retaining Wall with Short Length
RE: Tall Retaining Wall with Short Length
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?