Retaining wall with fooing cut into sandstone bedrock for sliding restraint.

[Prestressed Guy]

I am designing a 10' tall concrete restrained retaining wall. The soils report is allowing 400 psf for passive pressure but leaves me with a sliding failure.
The existing slope is a thin layer of soil and weathered / fractured sandstone, on top of competent sandstone bedrock. The footing of the retaining wall will embeded at least 18" - 24" below the surface of the competent bedrock. I intend to place the footings in a trench cut into the undisturbed sandstone bedrock by sawcutting the edge of the footing into the bedrock and then chipping out the rock within the footing edges. This condition seems to me like it should be more like full bearing pressure.

I have attached a prelim sketch of the wall section. The top of wall will be tied with reinforcement to the concrete slab at the retained height. The wall will be braced with erection bracing to support the top of wall during backfill and casting of the slab. The analysis is more like a basement wall that is restrained by the basement slab and the first floor diaphragm but the basement slab will be the sandstone bedrock and the floor diaphragm will be the reinforced slab on grade over the heal of the wall.

Here are the test pit log and wildcat dynamic cone log for the pit closest to my wall. It sure seems like being cut a couple of feet into that rock ought to keep my wall from sliding.

I am also attaching a prelim run on this wall. I modeled it with 100% fixity at the footing to minimize the top restraint needed. The total lateral reaction of the footing bearing on the edge of the bedrock socket is 3364 plf. Given a 12" deep footing this also comes out to 3364 psf on the rock.

 

[Prestressed Guy]

here is the design.

 

[Mjkkb2]

I would ask the geotechnical engineer about the passive pressure of the bed rock. You should be able to utilize a much higher value if you are embedding into bed rock.
Additionally, since you’re using the LRFD design,You are not required by code to provide a 1.5 Safety factor [it is already included with your 1.6 on the soil pressure].
The only other thing I can suggest doing is to thicken up the footing or provide a shear key.

 

[Prestressed Guy]

I did ask him and he increased the passive pressure from 200 psf to 400 psf. This rock is a very dense sandstone that makes up the cornices and cladding of most of the historical buildings in western Washington but it will only give slightly better lateral resistance that dirt. Yah, Right!
In the attached photo, everything white is made of the same stone I am embedded in and it has been doing a wonderful job for 126 years. I have designed the foundations of buildings that had deeper soil under a portion of the building while the rest was on a sandstone outcropping. I used 3"ø pin piles and they would hit full pipe capacity within 6" of encountering the rock.
The problem with the keyway is that the passive pressure is so low that the key still doesn't meet the sliding threshold. He has required at-rest pressures of 60-65psf depending on what backfill is used.
The geotech has his head pulled way back into his protective shell and is not willing to stick it out at all in spite of two pages of indemnification and hold harmless clauses. Nice gig.

 

[BAretired]

I have some concerns about the design:

1. Are you tying the grade slab to something solid or simply relying on friction between slab and grade to provide the required horizontal reaction to support the top of wall?
2. How will you prevent the rusting of dowels at the joint between slab and wall?
3. Placing vertical reinforcement in the wall as detailed will be difficult.
4. The drain to daylight is below the 1st Level and there are no weep holes through the wall.

Is there any advantage in placing fill up to the underside of grade slab behind the wall? It might be simpler to leave a hollow space behind the wall and reinforce the slab as needed to span across the gap. The slab could be keyed into the wall to protect the dowels from rust. There would be no soil pressure on the wall, so the Dayton braces and 3000# dead-men could be eliminated.




BA

 

[BAretired]

Haydenwse,

I had not read your comment when I posted above but that is one more reason to remove the backfill from behind the wall. You would not be relying on developing passive pressure.

BA

 

[Prestressed Guy]

Thanks for the response BA. I will try to add info by number.
1. The bedrock slopes upward the heel side of the wall at 2:12 and is at top of slab elevation at the far side of the slab. the wall returns at both ends with the slab tied in to the wall as in the section shown. the slab will be held in place by both soil friction outside of the slip zone and by diaphragm action to the side wall step down with the rock on cut-in steps. The slab extends about 30' back from the wall.
2. I have not addressed rust yet. I am still trying to get a workable design. I could provide a keyway and/or water-stop. I could also use epoxy or galvanized rebar.
3. I am not sure I see your concurn about placing the vertical rebar. can you elaborate. The way I see it, place the horizontal and vertical rebar were needed and tie to form ties to possition.
4. The slope on the toe side continues at 2:12 for 60' before dropping to 4:12. Day light isn't a problem. ;>)
For your last point, how would i place fill up to the underside of the slab without a retaining wall. In order to get a void behind the wall, wouldn't that just require a 2nd retaining wall? The only way that I could see that working would be to file the back side of the wall with geofoam but that would be much more expensive than a simple retaining wall.

 

[Mjkkb2]

I also don't see how it is feasible to keep a void unless you lay back the slope of the backfill (BA -is that what you mean?) I would recommend thickening up the slab up top for some distance away from the wall and reinforcing it as simple span slab. You likely will not get the required compaction that close to your wall (as the heavy equipment will not be used). The simple span (rebar) will provide you some spanning over any void that may develop as a result. We do it all the time.
This will also help you with the tensional reaction from the assumed wall brace point.

I also concur regarding protecting the rebar, if it rusts out you may have a quite real failure potential.

And lastly, if it were me I would just design the wall as cantilever with a big footing. I know it's tall and will need a monster footing but bracing by slab on grade is a bit risky....

 

[Prestressed Guy]

I am looking at both options for restrained and cantilevered but neither solves the problem of sliding restraint. The primary issue is how to resist the footing lateral reaction for sliding.

 

[BAretired]

1. If the grade slab rests on deep fill, future settlements can be expected. Perhaps the grade slab should be structural. If the fill settles, there won't be much friction between slab and fill.
2. Rust is not a major concern.
3. I would extend the #5@1'-6" Verts. down to the top of footing for ease of placement and provide nominal dowels to tie them.
4. The drain location, as you have pointed out, is not a problem.

I do not know the length of retaining wall and do not have a cross section of the existing grade. Creating a void behind the wall would reduce or eliminate lateral pressure on the wall but would necessitate a structural slab at the 2nd Level adjacent to the retaining wall.

In order to get a void behind the wall, wouldn't that just require a 2nd retaining wall?

I was thinking of placing fill at its angle of repose below the slab as near as possible to the wall, so no second retaining wall, but you would need to retain soil at each end of the void.

BA

 

[Prestressed Guy]

Thanks for the responses BA and Mj.

Does anyone have any thoughts on the sliding restraint due to bearing on the rock face of the footing trench? This will be fairly deeply into the sandstone and well below the typical 6" of weathered / fractured surface. It seems to me that this is a case of bearing capacity not passive pressure. Passive pressure is a yielding condition and this is unyielding homogenous bedrock.

Here is a wider section and a plan of the slab. This will be the parking lot for a 3-story apartment which must be built prior to the excavation and construction of the building due to lot constraints. Heavy equipement can access the site from the road below but they need the wall in to create the building pad. During construction, the lot will be used for parking only due to limitations of the driveway. all delieveries of material will be from below.

 

[Mjkkb2]

I don;t have enough expertise to comment on the rock/soil stuff. Hopefully a geotech will see it and help.
I remember on one project having a dead men footing that was supporting a 55 deg (+/-) brace that I had issues with passive pressure/sliding. I asked my geotechnical engineer if it shouldn't be more like bearing considered the load applied is more vert than horizontal (I was also going to build my footing at a right angle to the brace). I didn't get 2000 PSF (soil bearing) but I got something in between the passive and the 2000. I do believe it is for the geotechnical guy to answer and give you some guidance....

 

[BAretired]

Does anyone have any thoughts on the sliding restraint due to bearing on the rock face of the footing trench? This will be fairly deeply into the sandstone and well below the typical 6" of weathered / fractured surface. It seems to me that this is a case of bearing capacity not passive pressure. Passive pressure is a yielding condition and this is unyielding homogenous bedrock.

I tend to agree with you on this point, but it would be nice to have agreement from your geotech. Perhaps you should get a second opinion from someone more familiar with the local sandstone.

Here is a wider section and a plan of the slab. This will be the parking lot for a 3-story apartment which must be built prior to the excavation and construction of the building due to lot constraints. Heavy equipment can access the site from the road below but they need the wall in to create the building pad. During construction, the lot will be used for parking only due to limitations of the driveway. all deliveries of material will be from below.

I was expecting bedrock to be a little lower than you are showing in the vicinity of Gridline H. As it is, there will be considerable excavation into the bedrock to accommodate your foundations. This could be a significant cost item.

Backfilling against the wall to minimize slab settlement would add considerable lateral pressure to the wall. In lieu of adding fill, I would investigate the cost of providing a structural slab over the entire parking area, leaving the existing grade as it is.



BA

 

[WARose]

[blue] (Haydenwse) [/blue]

Does anyone have any thoughts on the sliding restraint due to bearing on the rock face of the footing trench? This will be fairly deeply into the sandstone and well below the typical 6" of weathered / fractured surface. It seems to me that this is a case of bearing capacity not passive pressure. Passive pressure is a yielding condition and this is unyielding homogenous bedrock.

If the Geotech green lights it (and I would make sure he/she understands the situation).....I don't see why it would be an issue. It would almost certainly engage fairly quickly.

By the way, this brace you have.....is that permanent or just during construction?

 

[WARose]

[blue] (Haydenwse) [/blue]

I am also attaching a prelim run on this wall. I modeled it with 100% fixity at the footing to minimize the top restraint needed.

Looking at this.....I don't think I'd do that for the final/issued design. (Unless the bearing pressure on the bottom checks out.) To me it's not embedded deep enough in the bedrock for that. With piles socketed into bedrock....I normally embed a minimum of 2*Diameter before I'd call it "fixed".

I'd call it "pinned" for the analysis. (Assuming that brace is permanent.)

 

[Prestressed Guy]

BA, the rock excavation is what it is. This contractor has a lot of experience in this rock and the equipment to break it. A good hydraulic hoe-ram on an excavator any you break it up like concrete.

WARose. I think you have misunderstood my statement of fixity. I have analyzed the wall to have 100% fixity to the footing. The footing is then analyzed as a cantilever retaining wall footing with restraint at the top which reduces the overturning at the bottom, the stresses in the wall stem and the restraint needed at the top. I do this often on basement walls. I analyze the wall for how high it can be backfilled as a cantilever wall and then specify that the wall cannot be backfilled above that elevation until the floor diaphragm is in place. This allows them to partially backfill the wall to reduce fall hazard.

 

[WARose]

I think you have misunderstood my statement of fixity. I have analyzed the wall to have 100% fixity to the footing. The footing is then analyzed as a cantilever retaining wall footing with restraint at the top which reduces the overturning at the bottom, the stresses in the wall stem and the restraint needed at the top. I do this often on basement walls. I analyze the wall for how high it can be backfilled as a cantilever wall and then specify that the wall cannot be backfilled above that elevation until the floor diaphragm is in place. This allows them to partially backfill the wall to reduce fall hazard.

I agree the wall is fixed to the footing.....however unless the bearing pressures (and stability) check out prior to the brace being put in place (at whatever elevation you fill it to at that stage).....I'd be careful treating it that way.

Even after the brace is in place, I'd be careful about how "fixed" I called that (for the final design): you could potentially (treating it as completely fixed) attract a lot of force to the bottom and (ergo) underestimate how much load that brace sees. (Or misestimate the forces elsewhere.)

I'm not sure what what loads will be in place when (during the various stages of this)....but I just wanted to alert you to that.