Compressible Fill For Seismic Separation 1

[pgyr]

A building is to inset into a hillside to a depth of about 18 feet. Setbacks do not a allow a sloped excavation so a soil nail wall is being constructed for shoring. Our contractor has suggested to the owner that constructing the soil nail wall as a permanent part of the structural system will reduce cost by presenting zero lateral earth pressure load on the structural wall. The structural is proposing wall construction of reinforced masonry blocks offset from the soil nail wall by a void space of 8 inches.
The problem is the void space needs to be filled, either fully or at the top, to create a surface for topsoil and vegetation and to create positive drainage away from the building. The project is in a seismic area, so seismic loads cannot be transmitted through the fill to the wall.
My experience with pea gravel fills leads me to be believe that during an earthquake, shaking would consolidate the gravels and transmit loads from SN wall to structure. Is there a compressible foam product out there that would work? Something else?
Thanks

 

[BridgeSmith]

Check out Terraflex elasticized geofoam. An 8" thickness may not give you the displacement capability that you'll need, but it's something to look into. It's only elastic down to about 90% of its original thickness, after that the deformation starts to become permanent.

 

[pgyr]

HotRod10
Thanks much. The soil nail wall could be setback to form a a wider gap for the top 3 feet or so to accommodate a greater thickness of terraflex foam.

 

[BridgeSmith]

It just depends on how much movement you'll need to accommodate. It also depends on whether you want it to respond elastically or are ok with replacing it if it experiences the expected deformation during the design seismic event. There are alot of unknowns when it comes to whether the building will actually move as much as what you design for. For the bridge where we used it, we expect permanent compression set for the design seismic event. In the unlikely scenario that it actually experiences enough deformation for that to occur, we would replace it. For us, it was all about how much pressure from the foam that the structure could handle, not about the survivability of the compressible inclusion.

 

[MotorCity]

Even if the void is filled with a compressible material, it will still transmit load to the building. A compressible material needs two resisting surfaces to compress (i.e. the soil nail wall and the building).

The only way to have zero lateral pressure on the building is to keep the void space. Maybe enlarge the void such that it becomes a useable outdoor space?

 

[pgyr]

MotorCity,
No room for grading, leaving a void is not practical for drainage and pedestrian traffic reasons. The terraflex foam offers a low lateral pressure option, not a zero lateral pressure option.

 

[PEinc]

Anchored walls should be kept separate and independent from adjacent buildings. If not, one or both structures could crack because the building and the wall will move differently. Perhaps you can build some type of horizontal plate or cantilevered slab that comes off the building and overlaps the top of the wall? This horizontal slab or plate could support a required amount of topsoil and act as a slip joint when the wall and building move differently. Bridge expansion dams do this same thing. This PDF shows a slip joint closure plate for a 40' high permanent soil nail wall for a parking garage in North Jersey. Doing this for a retaining wall with variable height would require more thought.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1562867172/tips/Anchored_wall-building_slip_joint_svvtm6.pdf[/url]

http://www.PeirceEngineering.com

 

[pgyr]

PEInc,
We thought about using a detail as you propose, but the wall height varies, top of wall is sloped so it becomes complex and expensive to construct. The compressible foam seems to be a pretty good option, however, I'm waiting to hear back from the structural engineer if they agree.
THANKS!

 

[PEinc]

I don't see how compressible foam will work, as MotorCity said above. Summation of forces; equal and opposite reactions. Compressible material compresses from equal pressure on opposite faces. Build a sloped slide plate or slab.

http://www.PeirceEngineering.com

 

[pgyr]

PEInc,
Sorry to be unclear:
With the proper thickness,the stress/strain curve for the foam results in very low load transfer at anticipated differential movement during shaking.
I am waiting for the structural engineer to analyze anticipated seismic deflection then the load generated compressing foam at that strain, then the ability of structure wall to handle that load. Foam thickness to be increased if necessary dependent on results.

 

[PEinc]

If you lay a compressible material on the floor and then stand on the material, does the floor see any less load than your weight? If you weigh 200 lbs. and stand on a bathroom scale, doesn't the floor still see your 200 lbs. of added weight?

http://www.PeirceEngineering.com

 

[pgyr]

PEinc,
The physics of this situation are not an analog to person standing on a foam floor.
The foam behaves elastically, seismic deflection can be calculated generating a resisting force, kx where k is the spring constant, x is the deflection.

 

[MotorCity]

The driving force (seismic) must equal the resisting force (the wall), regardless of how much the foam compresses.

If you remove the wall, does the foam still compress?

Draw the free body diagram.

 

[PEinc]

My "foam" and your "spring" are both elastic materials. The load in the spring or foam will be applied to the building if they are touching the building.

http://www.PeirceEngineering.com

 

[pgyr]

The force=KX. Draw the freebody diagram, spring, fixed on one side, deflected on the other resulting in...F=KX. As long as the deflection in total between the building wall and the soil nail wall is in the range of strains where the foam performs elastically, K is the load. More than if it were an air void, much much less than a full rigid fill.

 

[PEinc]

I agree about F = kx. However, the force on the wall will also vary with x. You can't compress a spring from one end (the wall) unless it is restrained at its opposite end (the building). So, you will need to know x and k. k is known when you buy the material; knowing x (wall movement) is more difficult.

http://www.PeirceEngineering.com

 

[BridgeSmith]

The foam acts as a seismic isolator, similar to elastomeric bearings used for the same purpose. It reduces the forces transmitted to the soil nail wall and the building wall by the movement of the building. As pgyr said, more pressure than an air void, but less than soil or granular backfill, which is essentially rigid. As the foam begins to compress during the seismic event, the frame of the building and the foam will resist and limit the displacement. Even with an air gap, the building wall will only displace some limited distance; with the foam in place, the displacement will be further limited.