Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?

[maxwolf]

Hi,

We have a site where the owner is considering a 5 level deep cellar (about 65' deep total) with the lower half of he excavation in clay. Some areas of the cellar would have the retaining walls going down full depth without any cellar slabs for lateral stability. All of the proposed retaining walls take loads from the proposed superstructure and are right up on most of the property lines where there are existing structures of about 6 stories height.

Even if the neighboring buildings have shallow cellars, so that tiebacks could pass below, tiebacks may not be permitted since variances cannot be counted on. One engineer noted that the tiebacks in clay might have to extend for up to 75ft into neighboring property. In lieu of tiebacks, has anyone seen pilasters (counterforts) done? (The only vaguely similar precedent I've seen is with the WTC site, where they are installing a pilastered slurry wall behind a portion of the original compromised wall.)

Thanks for any suggestions.

 

[civilperson]

Pilasters or counterforts can shink the 60' vertical span to a more manageable horizontal span, however, the pilasters need sufficient strength to collect the horizontal loads and transfer them down to the soil. A drilled pier could act as pilaster/foundation if deep enough to transfer horizontal loads in bending. The first approximation I would try 54"-60" diameter piles/piers/pilasters at 18' spacing going 24-30' below the lowest level. You need the horizontal subgrade modulus of the clay below the excavation for exact solution.

 

[hokie66]

This sounds like a case for top-down construction. Build the retaining walls as slurry walls, install temporary struts and partial floors as you progress down with the excavation.

 

[maxwolf]

Thanks, Civilperson. I ended up sketching a similar approach using a slurry wall counterfort that collected the horizontally spanning wall loads. I'll need to get a soils report to go much further.


Hokie66

The owner actually insists on top-down construction to make the schedule work, though I have my doubts as to how much time(money) it can really save here. One other issue another engineer brought up was that a slurry wall is not totally waterproof (though I thought it could be made so with sufficient admixtures, thickness and/or higher cement ratio/compressive strength). If a slurry wall can't be made totally waterproof, then I was told a secondary inner wall would be required. But then expensive program space it lost, which the owner will probably not accept.

Thanks again.
Max

 

[hokie66]

I don't know that you have any option here, considering the adjacent buildings. Time and cost will be whatever they are. Counterforts will take a lot more space than the slurry wall or a contiguous pile wall, even if you build an internal wall to allow for drainage between the walls. And I agree the wall will probably not be completely waterproof, but that is the nature of building so far below ground, whatever type wall you use. If you can't get to the back face of the wall to waterproof, it won't be completely waterproof.

 

[DRC1]

Pilasters transfer column loadds with only local increases in wall thickness. They do not provide lateral resistance. Counterforts will take up a considerable amount of interior room. In order to construct the conterforts, the exterior soil must be removed or independently supported. The only means of supprting the excavation with out going on adjacent property is to brace internally or provide a very stiff cantilever system by either a very deep slurry wall or a combined shheting section. This depth is beyond what either system can do economically. Bracing is feasible.
Further, no system (including Bracing) is sufficently rigid as to prevent unacceptable movements in the adjacent buildings.
I do not see any way to safely and economically supporting the buildings or the excavation with out going on the adjacent property. However, most areas require the abbuting owners either grant you an easement to to make necessary constructions to protect their property from your excavation, or make neceesary constructions themselves. In general your excavation can not be denied because an abbutter will not give you an easement. I would suggest a jet grout wall to act as earth retention and underpinning.
If you are below grade, you will have water no mater what. A properly constructed slurry wall is relatively impermiable and any seepage should be readily handled.

 

[PSlem]

There was a highway widening project in Atl where the adjacent property owner did not allow access. A tangent pile wall was installed and U shaped pipes cast in the piles whose curved shape mirrored the bending moment. Strands were run thropugh the pipes and heavily posttensioned. Wall height was about 35'.

 

[maxwolf]

DRC1

Thanks a lot for the response. The idea of extending a diaphragm wall down deep to get good cantilever action at the bottom makes good sense. I'll look into the jet grouting.



PSlem

Post-tensioning could definately cut down on the section depth. Thanks very much for the example.

Max

 

[JAE]

What size of building (in plan) is it? I wanted to post in here to use top-down but hokie beat me to it.

If your building isn't too large, then horizontal beams could be developed around the perimeter within the slurry wall (extra bands of reinforcing).

Basically what I'm suggesting are horizontal pilasters spanning from corner to corner instead of vertical pilasters. Vertical pilasters would still need some sort of lateral resistance and cantilevering off the ground would be tough via a slurry wall.

 

[DRC1]

If you have braces spanning corner to corner, there is a force at right angles to the brace that needs to be resolved eithier by a tie back or a raker.

 

[hokie66]

DRC1,

What JAE is saying is that for a relatively small building, the whole thing can be designed as a box, with each wall supported by the intersecting walls at the corners. That probably wouldn't work for underground carparks, as they would be too big, but the OP did say "cellars", so we don't know the size.

 

[DRC1]

Corner braces do not provide lateral restraint for walls and there have been failures (one rather spectacular one in DC) of this type of bracing. It must be supplemeted by cross lots, rakes or tiebacks.

 

[hokie66]

I take your word for it. Since I am always interested in why things fail, do you know in the DC case? I am thinking that the horizontally spanning walls would be problematic to achieve in a wall of this type because of the difficulty of making the reinforcement continuous.

 

[JAE]

"Corner braces do not provide lateral restraint for walls ..."

DRC1 - I suppose for a very large building footprint it might be un-feasible to use horizontal wall flexure to resist the lateral earth pressures.

But corner forces DO resist the lateral push of soil. It is simple statics that each wall, spanning horizontally, would be supported by the orthogonal walls which would be resisted by the opposing wall thrust. As hokie66 stated, we don't know the size of the building. A smaller footprint may be a condition where this idea can work.

If there have been failures in DC, as you say, I would wonder what the source of the failure was. It certainly wasn't the statics/analysis/concept. I would guess it was simply poorly engineered.

 

[DRC1]

Corner braces do transfer load, but if you break out the statics, there is a significant component nomal to the brace that is not resisted by the bracing. If the force is not resisted by rakers or tie backs, it might be resisted by friction agaist the wall elements or sufficent rigidty in the frame.
The DC case was a moderately deep excavation, soldier pile and lagging with corner braces. It was designed and built by a nationaly known firm that has a reputation for quality. The faiilure was due to the fact corner braces were used and the force normal to the brace was not accounted for in the design.

 

[hokie66]

DRC1,

Somehow I don't think we are talking about the same thing. JAE was not suggesting corner braces, he was suggesting that concrete walls continuous around the corner would brace each other. With a soldier pile and lagging situation, you would have to have braces in both directions at the corners, as the lagging cannot resist the force. That is a totally different situation from a continuous concrete wall as in a slurry wall. Having said that, I would still question how the horizontal reinforcement in a slurry wall could be made continuous.

 

[maxwolf]

All,

Thanks for the replies. Sorry for the delay over the weekend, but please see the attachmet for an idea of the dimensions and rough layout. The clear span of the cellar wall in one open area is 66ft hight.

I thought mutually orthogonal corners could provide lateral stifffness so I only sketched in counterforts and pilasters in the midspan areas to create essentially a vertical T-beam wall. Not too efficient though, since the slurry wall acting as the effective flange will be in tension, not compression from the inward lateral earth pressure. But with deep enough counterforts, I think it should work.

Max

 

[hokie66]

maxwolf,

I think your scheme has some problems:

1) You still have to do top down construction to brace the slurry walls. The walls, counterforts, pilasters, etc. won't be able to do the job as cantilevers.

2. The end 80' long wall would have to span horizontally, as the pilasters are not shown to be braced at top, thus may not do much good.

3. I don't have any experience with making a slurry wall span horizontally, but wonder how that can be done, that is how is the reinforcing placed under bentonite and made continuous?

 

[hokie66]

maxwolf,

We have probably all lost sight of the need to support the existing adjacent structures. Underpinning will be required if they are founded at high levels, but how would that interface with the other work?

 

[maxwolf]

Hokie66,

There's continuous lateral restraint at the top of all the slurry walls from the 1st floor slab, and from the trusses in one direction. So the counterforts would be laterally simply supported at the top (1st floor) and have a fixed end at the base. Sorry if the sketch wasn't clear.

Good question on the underpinning. I need more information from the neighboring site to the left. Not a problem at the top of the page since buildings are far from the prop. line. The additional pressure from the pressure bulbs of the neighboring building would have to be added to the calc for the counterforts. I need to make a conservative estimate for the added pressure and recalc it.

I'm not sure how to make slurry walls span horizontally. Horizontal rebar lapping is the issue I see when the cages are lowered into the slurry. Since the counterforted wall would act as a cont. multispan slab, horiz. rebar splices could be placed at zero moment transitions zones (from pos to neg moment). That could help reduce the need for cont. horizontal steel. If the horizontal spanning can't be done, I've got a rough calc that requires a vertically spanning diaphragm wall nearly 10ft thick. Shear governs.

You're comments are really appreciated.

maxwolf