16’ basement in bedrock

[Rosco22]

We are building a 16’ tall walkout basement, where the back and side walls are all bedrock, so the only pressure on the wall will be backfill. Currently, an engineer for the project has mentioned 20-30” thick walls and designing it as a retaining wall, but we were discussing the possibility of using angular backfill to lessen the forces on the wall. Maybe even using Geogrid in the angular backfil to reduce the lateral forces even more. We’re trying to get the wall down to 10-12” thick due to the design of the main floor.

Additionally, only the back and side walls will be poured concrete. The front/walkout portion will be stick framed 2’x8’ material. The floor trusses will sit within the basement, so the floor sheathing will connect directly to the sill plate. The back wall is 68’ long, with 6’ long by 8” wide buttresses at 16’ and another 20’ later, making the longest span 32’. Lastly, the entire back footer is on bedrock, and the majority of the side footers are also on bedrock. Only the front footer and a small portion of the side footers are in native soil.

Are we oversimplifying this by thinking the angular backfill will produce very little pressure on the wall, thus reducing the need for 20-30” thick walls with rebar grids on both sides of the wall? Any thoughts on wall thickness and rebar spacing? We’re trying to reduce cost, as the massive wall suggested in preliminary talks is cost prohibitive, forcing us to try and get more creative, but we obviously want to make sure it’s more than strong enough.

Thanks in advance for any thoughts or suggestions.

 

[SJBombero]

My thoughts:
The cost of a soils report with real soil pressure recommendations might pay for itself in cost savings.

If there is real pressure on the wall, a tied-back wall would definitely save thickness but at some additional cost. If the base is really bedrock then you shouldn't have to drill that far to get capacity out of a grouted rock anchor.

If the bedrock is self-supporting, geofoam would be an alternative backfill with little to no pressure. Also flowable fill.

 

[Rosco22]

Thanks for the reply! We have already excavated the site down to grade, and it is definitely all bedrock on the back and sides, and we are 100% on bedrock for 3/4 of the footers. It is being built on top of a cliff, and we brought in a hammer excavator to break the rock, as blasting wasn’t an option, and it took ages to get through everything. We also used the hammer to excavate the parking area above the site, and the entire hillside is nothing but incredibly solid rock.

We did discuss tieback anchors, but didn’t make it far enough in that discussion to work out any detailing. We also talked about pinning the footer with epoxied rebar versus hammering out where the footers go, but we may end up doing a combination of both.

I am personally unfamiliar with geofoam. Would that be in place of 3/4 angular backfill or would it accompany it? Also, if we went that route, is it possible to place a foot or so of backfill material from the site on top to naturalize everything?

I guess the major question is if there’s a structurally sound way to build this at a 10-12in wall, and if there’s a way to calculate the load that the angular backfill would produce on the wall.

Thanks again for the reply. Definitely curious to learn more about geofoam.

 

[le99]

Yes, it is possible, but a few things to consider:

1. Ensure there is no harmful pre-existing slip plane(s) in the rock.

2. Provide proper drainage between the rock and the concrete walls, or pumping, to alleviate hydrostatic pressure. Keep in mind that any effort can fail.

I won't consider granular backfill if the rock is intact and sound.

 

[HouseBoy]

Seems like angular backfill and/or grids would be helpful BUT..... I thought the soil had to move slightly in order for the grids to take effect. Might be OK but I'd wonder about whether or not deflection is something to worry about.

Geofoam is used in highway fill sites when weight savings is a factor (I think) .
It have used it in single family residence projects. It is helpful when backfilling access is difficult and/or for weight savings. It's basically 4 ft x 4 ft x 8 ft foam blocks that can be cut into any size you need. It's not cheap though (last time I priced it, they were $600 per block custom cut and delivered). Not a bad way to go if you need it but I would think you can limit the pressure with good backfill controls and if it's easy enough to get regular backfill into place, that's probably god enough. Drainage is still important too!

Regarding the buttress walls - I'd want to be sure the footing width (and thickness) is appropriate for the action of the buttresses.
Personally, I would not want less than 12" wall thickness and with buttresses, the wall will have horizontal bending that is positive and negative (so... prob need two layers of steel in some places anyway).
Also, the anchorage of the top of wall to the floor framing might not be a good place to take up a bunch of horizontal force. The wood framed side walls may not be very stiff (plus it is sometimes hard to get the load out to them). Again, compatibility of deflections seems like an important consideration for a 68 ft long wall.

The buttress idea seems good to me but you prob need "buttresses" at the ends also.

 

[Rosco22]

Le99- There’s definitely no slip planes. It is layered sandstone, but in our area the layers are basically flat and fairly thick. It is 100% self sustaining.

Also, I can’t imagine the hydrostatic pressure would amount to much. There is a very small amount of water that seeps from one spot in the rock, but with angular backfill and a good French drain I would think the water would never be able to even challenge the cold joint, much less build up and put pressure on the wall.

Did you mean to say that you would consider angular backfill? Or that you won’t?

House Boy - The way we use geogrid when building block or stone retaining walls is it essentially makes the entire 4ft deep area of face stones and gravel a solid unit. That keeps the layer above that and so forth and so on from wanting to put outward pressure on the face stones. So, and this is just an assumption, I would think that by layering geogrid in the angular backfill, it should reduce or negate the lateral pressure on the wall.

I did some research on geofoam, seems like it could be useful if needed, but costly. Would definitely like to find a solution where we use gravel, since it is cheap and easily accessible. Maybe a combination of both? 10ft of gravel to mimic a normal basement and the geofoam blocks for the next 4ft, then 2 ft of regular backfill to naturalize the area surrounding the house?

We will be pouring the sidewalls as well as the back walls, so essentially a buttress on both ends and then two towards the center. The longest section of wall between buttresses is 32ft. The only walls that will be stick framed are the front walls, as they will have no backfill on them at all.

We were already leaning towards #5 or #6 on 10-12in centers on the compression and tension sides of the back wall. Maybe it would be appropriate to only use a double grid in the areas surrounding the buttresses?


Is there a way to calculate to lateral pressure of the angular backfill against the wall if we considered the bedrock completely irrelevant? Assuming one side is a immovable concrete wall, and then 6ft away was our concrete wall, if we filled that void with angular backfill, could we determine the pressure that would place on the wall by itself?

 

[EireChch]

Rosco22 - you need to get a geotech or geologist(familiar with local rock mechanics). They should map the rock face to determine the discontinuities. I dont think you are qualified to say that there are no discontinuities.

A days mapping and a report is likely 2-3k dollars (a complete guess from non US engineer maybe others could interject). As Bombero said, you could save that easily if its good news. If its not then at least youve done your due diligence.

It could well be that the rock is as sound as you say but its something you really need to confirm. Dont try cut corners on this as it will be very expensive to fix if you have a bulging wall.

If it is sound and self supporting then there could be no lateral pressures at all!!! Your wall could be 8inch thick.

Also, lee99 raises a good point re drainage. It doesnt matter if there is a one inch gap or a 100ft gap filled with water behind the wall. The hydrostatic pressure is the same!

Also, also, do a search on previous threads. Some great advice on how to calculate the earth pressures.

thread255-232396
thread255-421968

 

[XR250]

What would he fill the gap between the bedrock and the wall face with that provides no lateral pressure (other than foam)?

 

[le99]

Rosco22,

No, I would not backfill for which could build up pressure against the wall. You may consider filling the excessive excavation with lean concrete.

 

[WinelandV]

XR250,

If we're talking soil only (not free-standing water), the lateral pressure is derived from the wedge of soil behind the wall pushing against it. A 2" wide column of gravel 20' high isn't going to deliver 1200 psf laterally at the base of the wall because there's not the wedge of material driving the load.

Water will, though, because fluids are essentially magic. ;-)

But that's why EireChch is cautioning about having good drainage.

Please note that is a "v" (as in Violin) not a "y".

 

[XR250]

OK. So at what thickness does the lateral pressure of gravel become significant? In practice, it will likely be at least 24" wide to get the formwork in there and to waterproof.

 

[EireChch]

XR250 - If there is a perforated drain pipe at the base to collect seepage and appropriate waterproofing to back of the wall, does it need to be filled with anything at all? this is considering that its confirmed that the rock is self supporting.

To be honest, I have not constructed a retaining a wall with rock directly behind so I have never actually had to detail this. But everything that I have seen does show some backfill, whether it be sand, flowable fill or lean concrete, insulation etc. What would be the risks with not filling the void?

Re the question, of calculating earth pressures if the void was backfilled with soil, check out Geotechnical Engineering by Handy and Spanger. Page 552 has an example of your exact situation.

 

[le99]

@EireChch,

Think about constructing tunnels deep in the rock strata, the voids were filled by pressure grouting.

 

[EireChch]

Thanks Lee, yes I understand that for a major infrastructure project. But for a residential house.. what is the risk of a void if it can be kept dry (well not flooded at least) and there is no risk of rack face collapse?

 

[XR250]

XR250 - If there is a perforated drain pipe at the base to collect seepage and appropriate waterproofing to back of the wall, does it need to be filled with anything at all? this is considering that its confirmed that the rock is self supporting.

The gap would have to be large enough for a person to waterproof the wall. I cant imagine the local AHJ allowing the trench to stay open.

 

[JStructsteel]

Can you use lean concrete or CLSM as backfill, and lessen the pressure?

 

[SwinnyGG]

One word, gents: shotcrete.

68' long 16' deep walkout basement on the edge of a cliff? Tony Stark can probably afford to shotcrete this and be done with it.

 

[structSU10]

What about using the rationale from a grain silo? There are formulas / experiments for tall silos and the reduced lateral pressure for skinny silos.

One reference:

https://library.csbe-scgab.ca/docs/journal/27/27_1_43_raw.pdf

 

[EireChch]

I agree that I would likely recommend that it be backfilled with something like flowable fill or granular backfill if it was my project too. Why would I do that, likely because it appears to be that thats the way its done...

You said yourself that AHJ wouldnt allow it be be open, why, because thats not what they normally do.

If its dry and able to stand, I cant think of a good reason why it should be backfilled.