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.

 

[JAE]

In my city, there was a deep excavation done for a governmental building (75 feet deep with a 12 story building on top).

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.

 

[maxwolf]

JAE,

Thanks very much for the temp tie back idea. That could help.

maxwolf