Temp wall bracing for early backfill

[Dzastr]

Rookie PE. Here is my situation:
I have a retaining wall that will be temporarily braced to allow for backfill prior to placement of the concrete deck that will go above the wall. Once the deck is placed it will act as a diaphragm and the bracing can be removed. I need to calculate the spacing of my pipe bracing.

Our soils report provides an Equivalent Fluid Pressure of 40, 60, and 350* psf/ft for active, at-rest, and passive conditions respectively. With the wall being 9' tall, my pipe bracing will connect to the wall at an h=7'. At a 1:1 slope that gives me a brace length of 9'11". At this length my pipe braces can withstand 6000lbs.

Is my calculation process below correct?

The force of the soil will be [40(psf/ft)*(9'^2)]/2 = 1,620 lbs/ft and it will be acting at an h=3' (1/3 of the way up from the bottom)
This gives me 3'*1620(lbs/ft) = 4,860 ft-lbs/ft
The horizontal force of my wall brace will be 6000lbs*COS(45) = 4242.6lbs and it will be acting at 7' yielding 29,700 ft-lbs.

(29,700 ft-lbs) / (4,860 ft-lbs) = 6' spacing

Follow-up question, how could I include a surcharge of say 300 psf?

Thanks in advance, please let me know if i can provide any clarification. Really appreciate this, trying to increase competency here.

 

[jayrod12]

conservatively you can assume your surcharge acts along the entire height of the wall and recalculate using the same method as above. That method appears to make sense.

 

[Splitrings]

If you are bracing your wall at the top you likely can't develop active wall pressure and should be designing for at rest pressures. I calculate a horizonatal brace force of 1050#/ft without a surcharge and 2800#/ft with. That is a huge surcharge, by the way.

 

[dcarr82775]

I have designed several of these, but they rarely get built as they cost too much. A few things to think about:

- a 300psf surcharge is overkill. 250 is typically used for a roadway with AASHTO loading. Surcharge could be less if traffic can be controlled.
- Is there going to be a crane of pump truck up there? The outrigger loads easily exceed 300 psf surcharge.
- With your brace up high on the wall you probably have something closer to at-rest pressures because the wall isn't free to rotate.
- What is the bottom of the brace reacting against? If a s.o.g. this is pretty simple, but if you need to provide some temporary footing it tends to get messy in a hurry.
- Take advantage of the fixity at the base of the wall to reduce your brace load up higher up.
- Can your wall span horizontally between the struts?

 

[manstrom]

See if you can treat the wall as cantilevered and then partially backfill it - 4' or so. This should eliminate the need for bracing and also help out the contractor.

IMO surcharges, full at-rest pressures and soil bearing capacities are very conservative for a temporary shoring detail.

 

[Dzastr]

Guys, cant thank you enough for your replies.

Since writing this I have pretty much moved to the idea that I should be using at rest condition. I The final condition of the wall will be at rest, and I do not want it to move prior to construction being completed.

The only surcharge loads that will be on this will be a wacker jumping jack compactor, maybe a walk behind rammax p33 24hhmr compactor, and probably a little bobcat/skidsteer on tracks not wheels. also, the weight of the 18" thick wet concrete when it is placed above. boom pump will be about 63 meters away on the street, no affect on the wall at all. Would you recommend 250 psf with that in mind?


I also found this in the soils report after posting last night:

Loads applied within a 1:1 projection from the surcharging
structure on the stem of the wall shall be considered as lateral surcharge. For
lateral surcharge conditions, we recommend utilizing a horizontal load equal to
50 percent of the vertical load, as a minimum​

I am interpreting that to mean that if i had a 300psf surcharge on the horizontal backfill at the top of the wall, i could use a 150psf (at a minimum) load over the back face of the wall.


We will be tying the bracing into an 18" thick mat slab which covers the entire structure floor, a 2' thickened section of this is the footing for the wall.

I am the concrete subcontractor on the project, I will be looking to the EOR to tell me if his wall can handle the brace spacing.

Also, we cant partially backfill because the purpose of the early backfill is to allow us to place the concrete slab that will be sitting on top of the wall. Need all the backfill in before we can do that.


Your comments are greatly appreciated

 

[RHTPE]

Dzastr - I did one of these a couple of months ago. Very similar circumstances. My tact was to compare the braced condition to the final as-designed condition. I assumed full-fixity between the wall and the mat slab which reduced the brace load substantially. The EoR insisted that the surcharge load be applied even though it really didn't exist during the duration that the bracing would be in place. My brace spacing ultimately came in at 36" c/c. No one mentioned the temporary stair tower that was in the way.

Don't forget to keep the brace connections to the wall out of the way of any yet-to-be-constructed beam connections. I kept the braces well below the elevation of the embed plates for the steel beams that support the ground level floor (those beams actually braced the wall in the as-designed condition).

I found the limiting factor to be the connection between the wall and the brace/mat as I could only utilize a single 5/8" (15mm) anchor.

Don't you just love the situations where the EoR requires the elevated floor to be complete before backfilling can proceed but then doesn't anticipate the ramifications on the construction sequence and schedule?


Ralph
Structures Consulting
Northeast USA