Retaining wall bracing

[Bengineer76]

Hi, I'm a rookie structural engineer, and I'd really like some feedback on my shoring design.

There's a 19' high stone retaining wall next to a building that someone planted a tree on top of. The roots are now pushing through the wall and causing a 40' section to begin to topple. To prevent this, I'm designing a frame to brace the wall against the building while it's removed/repaired.

My structure consists of W-shape columns up against the wall every 5 feet (9 in total) With two W-beams connecting the columns horizontally (welded). There are 3 sets of 2 compression members at 45 degree angles (also W shaped) that connect the columns to the facade.

See the attached PDF for pictures.

I'm currently designing the members

I calculated the vertical effective pressure triangle for the soil behind the wall (120 PCF and friction angle = 30 degrees) and found that the resultant occurred 6'4" from the base of the wall at a magnitude of 7.2 kips/LF.

I assume I should treat the horizontal braces as compression members and the diagonal braces as tension members. The columns would actually be more like loaded beams with one fixed end, and the horizontal W beams would also be treated like a beam with opposing point loads creating bending moments. Is this how I should approach the design process, or am I not on the right track?

Also, what is the best way to resolve the lateral earth pressure?

Should I weld a plate to the spot where the braces are joined to cut down on punching shear? I'm not sure how much load the brick building facade can withstand.

Any help or advice would be greatly appreciated

 

[larsacious]

I don't believe your approach will work. I believe this is a Mechanically Stabilized Earth (MSE) wall. The stone facing is non-structural and used for aesthetics. You are detailing to brace a wall through non-structural stone facing at concentrated points. The way the MSE wall works is that it is a large soil mass reinforced with a wire or geofabric and it resists the lateral force by behaving as a large rectangle. I would expect that if you attempted to brace this wall as shown, then the wall would potentially fail above and below the horizontal braces.

Also, your bracing system appears to be statically unstable where the diagonal brace meets the horizontal beam at the top of your sketch.

 

[dcarr82775]

I am not sure I understand what your brace is doing. Is this a temporary brace to support the retaining wall until can be repaired? If some portion of the retaining wall is to remain why no make use of it's strength top reduce the loads in your bracing.

 

[IceNine]

That seems like an awful lot of lateral load to put into an existing building.

 

[hokie66]

Bracing a retaining wall against a building wall could be a recipe for disaster. Brick has little out of plane bending strength, so does not provide a rigid support for braces.

 

[shobroco]

I think you'd better do a bit of exploration behind the wall to see what holds it up & why it is really failing. You can't fix it without excavating behind it anyway.

 

[Bengineer76]

Thank you all for the suggestions.

@Larsacious & dcarr82775: The bracing is to support the wall while it is being repaired. There are 9 piles with concrete footings that support the wall, and those are braces up against the building with 6 braces in 3 locations.

@IceNine & hokie66: I agree with you, I'm seriously considering starting from scratch (my boss gave me the concept, so I don't know how many liberties I can take with the design).

@shobroco: I'm fairly sure the wall has anchors, but they've rusted out and failed in several places, so I've assumed a worst case scenario. As far as I can tell, the reason this particular section is failing is because of a very large tree (which has recently been cut down) right up against the edge of the wall. The tree roots are poking through the stone in several places

Anyway, I'm going to do a complete redesign, and I'll post that once I'm done. I appreciate all the advice!

 

[msquared48]

After you have the tree removed and contact a geotech for a soils report, what about solving the problem with soil nailing or tiebacks?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[versaengine]

Hi.
I doubt the tree is the cause of failure.

I advise do NOT brace with existing building.

I didn't see pictures, are there actual photos?

What is above the wall. There is a new method of stabilization, a variation on soil nailing where a track-hoe fires a large nail on a pattern, I would consider this.

I dont think you will be able to repair wall as proposed. Cant you take it down and rebuild properly?

In all cases make sure a geotech provides appropriate design lateral pressure and considers global stability.

Could you build a new wall in front of it? Maybe soldier piles in drilled shafts and tie backs above and precast facing elements with a clean drainable backfill between old and new walls.

Good luck