Permissible lateral movement of tied back caisson wall

[ajk1]

Is there a generally acceptable lateral movement of a caisson wall parallel to an existing reinforced concrete 10 storey condo building which is 3.5 m distance from the proposed new caisson wall to the nearest existing building footing founding elevation? The new building excavation is about 13 m deep. We want to specify the permissible maximum lateral movement of the new caisson wall. The movement would be measured at the uppermost level of tiebacks (this corresponds approximately with the bottom of the existing building spread footing which is 3.5 m away).

 

[MotorCity]

Displacement of any structure is a serviceability issue and is primarily governed by how much movement your structure can tolerate. I see 2 issues here: movement of the adjacent existing building and movement of your new building. For the existing building, what is the existing façade constructed of? A glass curtain wall or brick will most likely not be able to tolerate as much movement as say a precast wall. Also, if the existing building settles due to the adjacent excavation of your project, any rotation will be greatly magnified at the top of the 10 story building. For your building, is the caisson wall part of the permanent structure or is it just a temporary retaining structure? By caisson wall, I assume that it is a series of closely spaced caissons to give the appearance of a continuous wall.

 

[fattdad]

if the wall was designed for active earth pressures, you should expect one inch of rotation for every 10 ft of wall height. If the building is influenced by the movement, each story would then (potentially) also rotate one inch per story. Seems like that's be bad!

Such walls can also be designed for at-rest earth pressures (i.e., about 50 percent greater design loads. When you design the entire state of stress on at-rest earth pressures, then you'd limit all the typical Rankine earth pressure deflections. You'll still have some potential movement; however. Clearly, you could design an inadequate structure for the at-rest earth pressures - a structure that won't fail, just deflect under the at-rest earth pressures. It's a cat and mouse game. The structure will trump the applied load, so make sure you structure doesn't deflect under the at-rest soil pressure!

f-d

ípapß gordo ainÆt no madre flaca!

 

[ajk1]

I asked whether there was a number that we could specify as the maximum horizontal movement of the caisson wall at the uppermost tie back level (which corresponds to the bottom of existing footing). I think the answers that I have so far are that you do not have such a number, or a range. I don't care what happens to the new building...they have their own engineers. As I am working for the owner of the existing building to be assured that the new building shoring procedures will not adversely affect his structure. As I am a structural engineer (and not a geotechnical engineer) I have established from calculations what the maximum permissible movement of the existing building should be. Now I want to know what the maximum permissible movement of the caisson wall should be. The new permanent basement wall will be placed directly against the caisson shoring wall.

I suspect that I am posing a question that no one would want to answer...I am just wondering if there is any general number that is often used in such situations...for example 10 mm or 12 mm or 14 mm or ...?

 

[GeoPaveTraffic]

ajk1, you are missing the point of the other responses so I'll try to be as clear as possible.

There is NO one answer to the amount of permissible movement.

The amount of permissible movement of a wall that YOU are designing is dependent on what your wall is supporting. Therefore, as pointed out in the other posts, you BETTER CARE about both the existing building and the new building.



Mike Lambert

 

[fattdad]

interesting. . .

f-d

ípapß gordo ainÆt no madre flaca!

 

[KootK]

I see a lot of specs use 15 mm of movement as the trigger point at which work is suppposed to stop and a meeting of the minds take place to figure stuff out. That, coupled with a monitoring program checking in on the movement every two weeks or so. That's what "normal" looks like in my area.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[aeoliantexan]

The question needs site-specific analysis, but I would start with the rule of thumb that the settlement can be twice the lateral movement. MotorCity's comment worries me, though; it may be the tilt of the wall that does the damage even if the settlement magnitude would normally be acceptable. Underpinning should be considered.