Pressure from Adjacent Permanent Sheeting & Shoring

[KBVT]

I have a dilemma that I am hoping I can get some help with. I am currently designing perimeter cantiliver retaining walls for a below-ground precast garage directly adjacent to an existing garage. The geotechnical engineer has recommended a permanent sheeting & shoring system of secant pile walls with tiebacks between the existing garage and the new garage (the new garage extends 20' below the existing). The distance between the two garages is only 5', so the proposal is to place the retaining wall directly to the face of the secant wall.

My question is when that secant pile wall deflects 1/4" to 1/2", does the cantilevered retaining wall need to be designed for the bending associated with that lateral deflection? I am trying to convince the geotech to put some sort of semi-rigid barrier between the two walls that will crush under the force of lateral deflection of the secant wall, but will be stiff enough to not crush under the concrete fluid pressure during the pouring phase. He is insistent that the secant wall is taking the bending so the structural wall does not need to be designed for it and they do this method with bridge abutment construction all the time.

Another note, due to the precast system, there is ample gapping provided for lateral movement of the wall so compression forces are not induced in the double-tees.

 

[DRC1]

The secant wall should deflect during construction. If it is a premenant wall, double corrosion protected tie backs, epoxy rebar if required, then the secant wall will carry the load. If it is only a tempoary wall, the new cantilever wall must carry the retained soil. In Europe, they are often used as both temporary and permenant. Unless you leave about 3-4 foot of gap between the secant wall and the new cantilever wall, you will not be able to strip the back forms. In which case you should consider a one sided form.

 

[PEinc]

Is the existing garage on piles or caissons (drilled shafts)? If not, your wall needs to support the other garage. If you use tieback anchors, you will need an easement for going under the adjacent garage. Perhaps the adjacent garage needs to be underpinned in order to support the vertical lod from the adjacent garage. If so, the underpinning will still need tiebacks.

 

[KBVT]

Both garages are on the same property, and the secant pile wall is being designed as a permanent underpinning system. The geotechnical engineer is designing the secant pile wall to take the surcharge loads imposed by the existing garage.

My intention is to use a one-sided form and place my basement wall to the face of the secant pile wall. My concern is any movement of the secant pile wall which occurs after the placement of my adjacent structural basement wall will impose bending stresses. Even though the secant pile wall is being designed to take the load from the existing garage, won't having the walls placed side by side affect induce a lateral loading on the cantilever wall? I suggested using a rigid styrofoam layer fastened to the face of the secant pile wall (which should be finished with gunnite to leave a flat surface) that would crush under any long-term deflection of the secant pile wall but this idea has been rejected.

Any other thoughts?

 

[PEinc]

A secant pile wall is not "underpinning." It is a very stiff retaining wall, sometimes tied back. The secant pile wall is not "under" anything. Therefore, it is not underpinning. It is more like a concrete diaphragm (slurry) wall, but is less expensive.

Usually, a permanent tiedback wall should be separate from the new structure in front of it. This separation prevents just the movements you are mentioning. Possibly, a tiedback secant wall is stiff enough to eliminate the movement problem that has been seen with tiedback, soldier beam walls where the building foundation wall is attached to the tiedback wall.

Often, parking garages need to be open enough to allow ventilation. Perhaps, you can have an opening or space between the secant pile wall and the garage structure.

 

[PEinc]

You might want to call and talk to someone from Schnabel Foundation Company in their Washington, DC office. They can guide you in the right direction.

 

[DRC1]

I agree with PEinc that the garage should be underpinned. Although in theroy the wall can be designed to accomidate the anticipated loads by making it rigid, this does not always work. Actual deflections and ground movements are not well understood. This method can work for light structures that can tollerate differential deflection. This is not the case for a parking garage. The foundations should be adequately underpinned. Lateral loads may require a seperate system such as the secant wall depending on the situation.