Concrete Retaining Wall + surcharge

[Ben29]

I have a concrete retaining wall that is retaining approximately 15FT. See attached. An exterior building wall for a (1)-story wood-framed commercial building will be built on top of this retaining wall.
I designed this wall for at-rest pressure (62H per the geotech report). Here are my questions:

1) The geotech report says, "It is important that below-grade building walls that generally are designed for minimal displacements at the top of the wall should not be backfilled until the walls are adequately braced by permanent structural framing." So, obviously this isn't an option for me because we need to backfill the wall in order to place the slab on grade. I understand that you can shore the wall while it is being backfilled, however our office policy is to just design the wall for the at-rest pressures and figure that doing so will prevent us from having to shore the wall. Does anyone disagree with this logic?

2) I used a surcharge load of 250 PSF over the heel. To me, this 250 psf represents the backfilling and compacting. The actual first floor occupancy live load is 100 psf and we have a 4" slab on grade (so 50 PSF dead load). I'm not sure if it is appropriate to use 250 psf for backfilling/compacting. I think this is standard practice in our office. I would like to just use the 150 psf occupancy load for the surcharge load. Thoughts?

 

[PEinc]

That wall will probably cost more that the house. You show a conventional, cantilevered, concrete retaining wall with a very large heel (NTS? Dimensions? Shear key depth?). The concrete slab on grade should not be needed to support or brace the top of the wall. The wall should be capable of holding the backfill and the slab's DL and LL prior to framing building above the wall. Backfill for a 15' high wall will load the heel of the footing a lot more than 250 psf.

With respect to the Geotech Report mentioning that the wall shouldn't be backfilled until the wall is adequately braced, I would be surprised if the Geotech was expecting or envisioning the type of heavy duty cantilevered wall that you showed.

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[Scratch3r]

I had a situation where the contractor backfilled a lube pit prior to the floor going on over the pit. The contractor provided wood shoring that was completely outclassed. The wall bowed in a couple inches and the masonry overhung the exterior support wall. Looking back, I should have put the expected construction sequence on the plans and added an option to stiffen up the wall or add a tieback. You really can't design for every situation though.

 

[SteelPE]

What difference does it make if your wall deflects slightly during construction? Should you be using the active soils pressures here?

 

[SlideRuleEra]

1) I'm not sure if it is appropriate to use 250 psf for backfilling/compacting.
2) The actual first floor occupancy live load is 100 psf...
3) ...have a 4" slab on grade (so 50 PSF dead load).

1) IMHO, excessively high... in part because of the following:

2) The 100 psf live load will NOT be present during backfilling/compacting.

3) The 50 psf dead load from the slab will NOT be present during backfilling/compacting.

So, when the the backfill/compacting is being performed the surcharge allowance is 400 psf (250 + 100 + 50) ... and there is more:

If the design height of the wall is dimension "H" shown on the drawing, the top of the backfill is 4" below design wall height. (If design height of the wall is exactly 15', only 14'8' of backfill will have to be compacted... with force on the wall proportional to the square of the depth, every little bit counts.)

Also, most of the weight of the 4" slab is not really a surcharge, it is just "heavy" backfill which is added after compaction has been completed. The only surcharge portion of the slab weight is based on the difference of the unit weight of the concrete (150 pcf) minus the unit weight of compacted backfill (for discussion, say 110 pcf).

If any seismic (or similar environmental) loading was included in retaining wall design that can be part of the allowance for backfill/compaction. The chance of having a design earthquake while backfill/compaction work is taking place is extremely remote. This could reduce the need for a backfill/compaction allowance.

Other comments:

A 4" inch thick slab on grade cannot include rebar (which is specified) and meet rebar cover requirements. The slab is too "thin" for that.

I don't see a vapor barrier under the slab, assuming the space above the slab will be inside the building.

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[Settingsun]

Perhaps the geotech report's comment about below-grade walls was intended for basements, where the backfill is outside the building and the air void is inside. In this case, it would be common to design the wall as supported at the top by the floor slab so you obviously wouldn't want it backfilled while acting as a cantilever.

I echo SteelPE's question about whether movement can be tolerated in which case active pressure could be used for design. If movement can't be tolerated, you should check if there will be ongoing foundation settlement over time despite using at-rest pressures for design.

I treat compaction as a minimum lateral load for structural design only. If using light compaction equipment (recommended for cantilever walls), the minimum lateral load is generally 10~15 kPa. This becomes a design requirement that the builder must comply with. At some depth, the lateral pressure due to soil weight and live load surcharge will exceed the minimum compaction pressure so you design for K[sub]0[/sub]*(G+Q) from that point down.

Not considering compaction for stability is on the basis that some wall movement is tolerable (and possible, eg wall props would be designed for compaction pressure because they prevent the relieving movement). If the compaction pressure overloads the wall stability-wise, it will begin to slide/overturn and this movement relieves the compaction pressure back down to (approximately) active pressure. The movement then stops so ultimate stability failure doesn't occur.

 

[Ben29]

PEinc: So, does simply designing the wall for at-rest pressure ensure that the wall will not move at the top? I just tied the slab in hopes to provide a safety factor.

 

[Ben29]

SlideRuleEra: I checked this wall for (2) scenarios:
1) checked for 250 psf surcharge over the heel (backfilling /compacting load)
2) checked the wall for 150 psf surcharge over the heel + weight of the building wall on the stem (occupancy load)

 

[SteelPE]

Tying the slab into the wall isn't a good idea as you need to design the slab to resist the loads from the wall, otherwise the slab will crack. If the wall is shored during backfilling/tying of the slab, the loads in the slab are going to be large.

You could design the wall for active pressures during backfilling and then design the surcharge using at rest pressures you just need to make sure the slab is capable of resisting those loads (which may not be small).

 

[Ben29]

steveh49: 10kPA = 210 psf. So, are we saying that we put a uniform load of 210 psf on the back of the wall and design for that? Or is the 210 psf a surcharge load whereby R(surcharge) acts horizontally at H/2 above the base and is defined as, R(surcharge) = ka * 210psf * H

 

[Ben29]

SteelPE: So is it true that the only way to ensure that the retaining wall doesnt move is to make sure it is restrained at the top via the slab on grade? In which case, the slab on grade will need to be appropriately designed for those loading conditions.

Simply designing the wall for at-rest pressures does not ensure that it will not move if it isn't retained at the top?

 

[SteelPE]

Everything moves under applied load. Remember the equation, force = stiffness x displacement. Even if the wall is restrained at the top it will still move slightly.

Designing for the at rest pressure will cause you to have a huge footing. Once the shoring is removed the slab will be loaded at everything will move slightly.

The only way I can see to eliminate movement if the wall is to introduce a prestressed element before the shoring is removed.

Also, I’m not entirely sure about this, but if you are using the slab to resist load from the wall, I believe the slab must extend beyond the failure surface of the soil behind the wall. Otherwise you system will not work.

 

[JoelTXCive]

I'm not sure if your drawing is to scale, but I think that shear key needs to be designed for flexure. It's gigantic.

I've had this happen on some of my projects. You add a shear key to overcome sliding.....but you start picking up active pressure on the retained side. Then you have to make the shear key longer to pick up some additional passive pressure. If the geotech tells you to disregard several feet of passive pressure; then it can get crazy fast.......Before you know it you've got a 5ft 'shear key'.

 

[JAE]

I used a surcharge load of 250 PSF over the heel. To me, this 250 psf represents the backfilling and compacting.

If your 250 psf is intended to account for backfill as well as top side surcharge, that isn't enough as others have hinted at here.
15 ft x 110 pcf = 1,650 psf of load on the heel from the backfill alone.

The soil bearing below the footing may be given to you as "NET" allowable pressure in which case any current soil on top of that footing bearing elevation can be added to your allowable net number.

So if currently you have 4 feet of fill existing over the footing bearing plane, and you are removing it - building the footing - then backfilling 15 ft - you would have a net add of 15-4 = 11 ft of new soil overburden - thus 11 x 110 pcf = 1,210 psf of heel surcharge. Add to that any live load above as well.

Also - the live load on the slab will impart additional lateral force on the wall, be it cantilever or not.

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[PEinc]

Ben29, steveh49 mentioned the conventional basement wall which your sketch does not resemble. Typical, residential, basement foundation walls usually have small, narrow footings - about 2' wide by 1' thick. These walls get their overturning resistance from the slab at the top of the wall. Usually, the slab provides a compressive reaction to keep the wall from falling into the basement. Your situation is the opposite; you want the slab to act in tension. If your slab is able to support the top of the wall, the foundation wall does not need to be designed as a cantilevered wall with a large heel.

I don't know where you and others are getting the 250 psf surcharge behind the wall. Yes, the weight of the backfill soil over the heel is a surcharge but it is much heavier than 250 psf, as JAE discussed. The soil over the heel helps prevent overturning and sliding.

If your slab at the top of the wall is to provide overturning resistance in tension, then, as mentioned above by SlideRuleEra, it will need to be thicker to fit the rebars. In addition, you need to consider the continuity of the slab to make sure the slab extends far enough behind the wall, without unreinforced slab joints, to provide sufficient frictional resistance or connection to the opposite foundation wall.

Designing for at rest earth pressure will give you a much stronger and heavier wall which will reduce wall movement. I believe that the overturning and sliding of the wall should be checked without the framed building load on top of the wall stem. Bearing pressure should be checked with the building load included.

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[Settingsun]

I believe that the 250 psf surcharge proposed by Ben29 is in addition to the weight of the backfill and is intended to account for the plant used to fill and compact the backfill. Ben, please correct if this is not the case.

However, I still suggest that compaction immediately behind the stem (for a few metres) be done by light equipment. Nonetheless, the compaction 'locks in' stresses in the soil until relieved by wall movement, even after removal of the compaction plant from the site. To account for this, calculate your 'traditional' horizontal pressure which is equal to vertical stress multiplied by horizontal pressure coefficient. Wherever this horizontal pressure is less than the compaction minimum pressure, increase to the minimum. This results in a constant pressure of 10~15 kPa near the top of the wall, then a linearly increasing pressure further down.

 

[JAE]

He stated in the original post that the 250 represented "the backfill and compacting". Two things - not one.
That's all I was going on...all we had.



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