Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
(OP)
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.
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.





RE: Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
RE: Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
RE: Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
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
RE: Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
RE: Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
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.
RE: Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
RE: Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
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
RE: Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
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.
RE: Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
RE: Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
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.
RE: Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
RE: Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
RE: Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
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.
RE: Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
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.
RE: Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
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.
RE: Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
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
RE: Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
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?
RE: Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
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?
RE: Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
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
RE: Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
This was designed and built in the 1970's. They used a 2'-6" thick slurry wall and as they excavated down they installed soil ties (grouted and prestressed tendon anchors I believe). Once on the bottom, the installed footings, and constructed back upward, each level then creating a permanent lateral restraint on the walls.
Once completed they cut the tendons (or at least abandoned them as they were no longer needed.
If you can at least get a temporary permission to use tie-backs, then the neighbors wouldn't be damaged or disturbed by their presence.
RE: Slurry (Diaphragm) Wall Pilasters in Place of Tiebacks?
Thanks very much for the temp tie back idea. That could help.
maxwolf