Types & Design of Temporary Bracing for Concrete Retaining Wall 1

[patrox777]

On a commercial project, I have a 12 foot tall and around 40 feet long, 12" thick, double matted steel concrete cast in place wall. There is a small 30" wide, 24" deep 'continuous' footing along it's length at the bottom to handle the 'vertical bearing' load.

Once the concrete wall has been poured, one side (the exterior' will recieve backfilling to 10 feet, while the 'inside' will remain at '0' feet and an interior slab will eventually be poured in place. The top of the 'wall' shall recieve precast double 'Tee's' to create a second floor slab.

The current engineered design is for 'in place' loading. Meaning AFTER the precast Tee's are secured to the top to counter the lateral load.

I have been contracted to design 'temporary bracing'for the wall to stablize it while the backfill is being placed and the precast double Tee's are installed on it's top.

Question 1: Besides the typical overturning loads on the wall (which I can assume shall resist nothing to be sure the braces hold)... of what significance is the pressure exerted while compacting the backfill? and are there any other considerations to think about?


Question 2: Any usual bracing for this situation? I was thinking along the lines of adjustable steel pole braces supported on the ground via 4 x 4 x 2feet thick spread footings and attached to the footings and wall with removeable expansion anchors. These braces we use all the time to support concrete tilt-up panels while the 'metal frame and roof structures' are being installed.

Thanks for the help,

Pat.

 

[SlideRuleEra]

The pressure on the wall during soil compaction can be very significant. Our geotechnical colleagues can probably give you a more detailed account, but unless you hear differently, I would assume the that the soil acts a hydrostatic fluid (weighing, say 120 cubic ft.) and that there is a surcharge on top of the soil, adjacent to the wall, equal to the weight of the compacting machinery + an allowance for the impact from the compacting machinery (say, also assumed equal to the machinery weight).

Be sure the concrete has cured adequately before backfilling is allowed.

In additon to braces near the top, put braces at an elevation below the level of the finished outside grade. Perhaps at the elevation of the centroid of the assumed construction soil loading.

Instead of making your expansion anchors removable, consider selecting a type that can be abandoned in place (with proper concrete cover & patching to ensure they don't cause problems later).

A good general purpose reference for this type work is the "California Falsework Manual". Free .pdf download here

http://www.dot.ca.gov/hq/esc/construction/Manuals/Falsework/Falsework.htm

Best Wishes

 

[DTGT2002]

Your plan seems to be in the ballpark.

I would hope you could find a solution such that you can locate only one brace per give location, with it's location based around directly resisting the force applied to the other side of the wall, while not being too eccentric as to exceed cracking stresses as the wall acts as a beam.

One consideration for the dead man footings will be whether or not the footings are dug into the ground or will all lateral resistance be provided by friction.

Good luck and let us know where you go with this.

That said, as you have limited experience with this, it would be fair to contact a contractor/engineer with wall shoring experience in your area.

Daniel

 

[DTGT2002]

One other thing, wind loading will be an additional concern.

 

[patrox777]

Thanks ya'll,

I'll contact some shoring folks. And yes, I'll be sure to note on the drawings that the wall concrete must achieve 2,500 psi strength (will check the original concrete spec'd for the wall) I'll locate one brace +/- 2/3's the way up the wall and estimate a machinery weigh then double it for additional loading than just hydrostatic ( there will be a french drain in place).

The 'footing' / deadman spread footing will be 4 x 4 x 2' embedded in compacted base material to resist slippage. Good call.

thank you guys.

 

[MikeE55]

In answer to part 1 of your question, assuming hydrostatic fluid pressure of 120 pcf soil will yield some very big loads. I think it is more appropriate to design for at-rest pressure and add some load for the surcharge due to the weight of the equipment.

 

[37ed]

I'm a little worried about the pipepole use.The design of the wall and footing really needed to be of a Battered wall design in order to stand on it's own for back filling anyway.
But if you must use the design keep the depth of your bracing bolts deep and big.

 

[SperlingPE]

Review the back fill procedures specified by the engineer of record. Backfilling should be done evenly and in very small lifts uniformly around the entire wall. This is usually done with a hand compactor (at least I hope so) within 2-3 feet of the wall so that desired uniform compaction can be achieved. Is there a vertical drainage course or drainage system specified for the wall? Review these details as well. There may be a drainage course in direct contact with the wall instead of soil. In this case only the top two feet will have soil in contact with the wall.

 

[TFL]

When we have done this in the past we have used tubular or wide flange braces back to an interior footing. Also as stated earlier, if you do not have a slab on grade at the bast to resist base shear you need to provide horizontal bracing at or near the base

 

[37ed]

WAS YOUR WALL FORMED WITH SIMONS OR SIMULAR FORMING SYSTEM?
IF SO REAPPLY YOUR PANELS TO THE POSITIVE SIDE OF THE WALL.
REAPPLY THE WALRES AND STRONG BACKS BUT DOUBLE THE LOCATIONS.PIN 100% AND APPLY NAILS SO THE PINS WILL NOT WIGGLE OUT DURING COMPACTION.TRY 4X4 KICKERS APPLYED MIDDLE AND TOP OF FORMS.TREATED OR YELLOW PINE IF POSSIBLE.APPLY EVERY OTHER PANEL.SECURE TO FORM WITH PINS AND NAILS,AND TO SLAB WITH WEDGE BOLTS.IF TO THE GROUND,SECURE WITH 36IN #6 REBAR.DIG A ONE FOOT HOLE AND PUT THE TURNBUCKEL IN THE HOLE AND PIN IT WITH THE 6 BAR .THIS WILL KEEP IT FROM "JUMPING OUT"

CONTACT SIMON ENGINEERS TO GET THERE OPINION FIRST.