Foundation Wall 40 Feet Below Grade 4

[JrStructuralEng]

Please excuse my metric units (<--Canadian)

I am designing a foundation wall for a condo that is approximately 12m below grade at the base. The condo foundation is stepped so the lowest portion of the wall is simply supported and has a clear span of ~2.5m. Silty/Clay soil and the geotech determined equivilent fluid pressure distribution to be 15.7 kN/m3. There is no option of using a lighter free draining granular backfill material since this portion of the foundation wall is right on the property line.

Ok, so here is the questions/problems.

Question 1. Should I be using a dead load or live load safety factor? i.e. 1.25 or 1.5. I am assuming this is considered dead load.

Question 2. I am getting very very large shear from the lateral soil pressure. ~310kN per meter of wall at the base. Using the shear strength of concrete alone (30MPa) I would need a 26" thick wall! My question is does the reinforcing mats on the inside and outside face of wall help with shear in this plane? It seems like shear reinforcing should be perpendicular to the wall. I have never put shear reinforcing in a wall, or needed it, so I am open for ideas. I am hoping the mats help, although logically I don't see why they would.

Please help! any comments welcome. (By the way there is no option to use buttresses in this case)

 

[JrStructuralEng]

gwynn - I will crack out the text books and try to remember general method

hokie - your meaning, it would only make sense to have thickening at the slab supports, just like you would thicken an area at a column for a floor loading with a drop panel.

DRC1 - I have talked to the geotech about this. He states it is an equivilent fluid pressure taking into account both hydrostatic and soil loading. If it was drained the report says it would be around ~60lbs/ft^3. The values he gave us are very common loading conditions for this area. I agree we should use a drainage panel. Thanks for the idea. ***What do you mean by vertical rebar will, and does take shear?*** I thought shear reinforcing in a slab or wall was completely different.

 

[miecz]

Agree with AggieYanks suggestion to calculate your shear at some distance above the base.

Your next option is to increase your concrete strength. I'm guessing that you're using a 20 MPa mix. With a 35 MPa mix, you should be able to get this wall down to 18 inches, without shear reinforcing.

Sounds as though your design water table is at ground level? If that's the case, it seem like overkill to factor your wall pressures by 1.5. Also,you may want to check buoyancy before you reduce your wall thickness.

 

[OldPaperMaker]

JrStructuralEngr,
I'm still curious about what is retaining the 31.5' of soil that is located above the 8.5'of "simple span" retaining wall at the parking level. Are the 4 floors of condos a reinforced concrete building that is designed to retain the soil on the right side of your cross section? (on your previous sketch)

 

[JrStructuralEng]

The building is set into a hill. The wall restraining the rest of the soil (~30ft)is concrete, and has full height buttresses on the exterior that put a big bending moment into the parkade roof. Keep in mind that the section I sketched is worst case, the grade is dramatically reduced as you move towards the front of building.

Only the parkade floor and main floor are concrete. The other floors are wood.

If your still confused perhaps I can send a sketch.

 

[OldPaperMaker]

JrStructuralEng,
Thanks. I think that now I get what's going on.

 

[hokie66]

Three more things from me:

1) Disagree that all your problems with shear are solved by considering the shear at d from the support. True, that is where the diagonal tension controls in the design models we use, and is where you should design your shear reinforcement in the form of stirrups. But you still have the issue of direct sliding shear at the bottom and top joints, which is where I suggested relying on bearing of the wall against the slab rather than keys or shear friction. At the bottom, you probably have a floor slab which can take the load, and at the top, the slab can be deepened in the vicinity of the wall to incorporate a bearing lug.

2) You probably know this, but the global horizontal load on your building will be large, and the design of the overall building has to take this load.

3) The "equivalent hydrostatic soil pressure" which the geotech has given you includes a component of water and a component of soil. That is what I discussed before in recommending that you use the water as dead load and the buoyant soil pressure as live load. After all, the density of water is well known.

 

[JrStructuralEng]

I appreciate the post hokie. Appriciated. By bearing lug, you mean in order to prevent punching shear I am assuming.

 

[hokie66]

No, I am not talking about punching shear, which is the type of shear which occurs around a column or a concentrated load. I am just referring to the horizontal reaction of the wall at the two concrete floors. My suggestion was to take the load in bearing by keying in the full depth of the wall top and bottom. In other words, the top of the lower slab would be say 75 mm above the bottom of the wall, and the bottom of the upper slab would be say 75 mm below the top of the wall, not necessarily for the entire slab, but for a decent width away from the wall. A continuous drop panel if you like, but not continuous over the wall.

Looking again at your problem, since you are built into a hill, surely therely there will be the opportunity to drain the space above the main slab to the ends and around the sides, thus reducing the hydrostatic load. The drainage would have to penetrate the buttresses, and the drainage system would have to be built so that it can never clog, but that should be achievable, and I would think the savings in the structure would pay for a blue ribbon drainage system. As it is, the hydrostatic pressure has a big impact on the basement wall, the buttressed wall, the main slab, the parking level slab (hydrostatic uplift), and the columns which support the buttressed wall.

Rereading your posts, I now see that the upper floors are not concrete. Do you have enough dead load to resist the hydrostatic uplift?

 

[JrStructuralEng]

The entire structure will be supported by large cast-in-place concrete piles on concrete walls/grade beams with a minimum of 6" of void form between piles. Does hydrostatic uplift still need to be considered?

 

[calculor1]

shouldn't there be a Ka or Ko factor?

 

[miecz]

With the magnitude of pressure that the geotech has you designing your wall for, hydrostatic uplift is a real possibility. (That's why I suggested you check buoyancy earlier in the thread.) Ask your geotech if you need to design your base mat for hydrostatic uplift. As someone else mentioned, with these driving pressures, you also need to check your overall resistance to sliding.

 

[hokie66]

For your original 15.7 kN/m3 "equivalent fluid pressure", there would have to be a large component of hydrostatic pressure. If the hydrostatic pressure acts on the wall, it acts on the slab as well. The void would simply fill up with water, and the uplift would have to be resisted by the mass of the structure combined with the tensile capacity of the piles, and the ground slab would have to be designed to resist this loading. I think you need to query the geotech in more depth to determine realistic design pressures. Built into a hillside, as I discussed previously, it is hard to see how you would get this much pressure at the bottom.