Retaining Pressure of Gravel Between Two Masonry Walls 4

[Deker]

I've got an architect that would like to build a freestanding site wall using two wythes of breeze block separated by a 2" cavity that will be filled with river rock. The rock will be sandwiched between wire mesh so that it can be seen through the breeze block without falling out of the cavity. See attached sketch.

If I'm basing my design on NCMA TEK 03-16A, I'm limited to a service load of 50 psf on the wall. Assuming these walls are resisting an at-rest retaining pressure of (0.5)(100 pcf)(5') = 250 plf / ft imposed by the rock doesn't even put me in the ballpark of considering this to be feasible, especially considering that I'll need to include the effects of wind and seismic load on the wall.

But the fact that the walls are only retaining a 2" layer of rock has to result in some reduction, right? I understand that fluids exert equal pressure in all directions, but obviously rock is not a fluid and I'm not sure the EFP method is valid in this case. I've seen some threads that cover variations of this issue, namely narrow backfill between a wall and rock face, and I've also looked briefly into applying silo retaining pressure methods, but I'm curious how others would quantify the retaining pressure in this situation.

 

[GeorgeTheCivilEngineer]

Consider it as a freestanding wall, not a retaining wall. You will need to tie the leaves (wythes) together and the containment force will be resolved as an axial force in those ties.

I'd want to check a normal wall tie is sufficient (I suspect not, at least at the base) so may need some fancier tie design.

The narrow retainment issue isn't one I've thought about for years. I agree yes - retaining walls are resisting the triangle of soil, if that triangle is smaller then the resultant lateral forces will be smaller. Same effect having a steeper angle of repose in the backfill material has. I will go have a look at some of those threads.

 

[Settingsun]

Probably wouldn't have considered it, to be honest. Since you've raised it, I'd have applied the method posted here, even if it was meant for larger spaces.

https://www.eng-tips.com/viewthread.cfm?qid=498131

Look at the vertical friction on both sides. That's a key difference to the water case. The stones would be levitating at some depth if your full lateral pressure actually developed, so not loading what's below. Makes me think the limiting value at some depth from the linked thread has a kernel of truth.

 

[HTURKAK]

I would consider two options ;

- Calculate the lateral pressure of rock fill applying silo retaining pressure methods,

- If possible , provide tie wires btw wire meshes and neglect the pressure ..


My opinion...










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[JStructsteel]

Ignore the rock, design for the wind or seismic. Thats going to control. Tie the wythes together.

 

[EdStainless]

All that I think about is the mess when the mesh rusts and rock spills out everywhere.
Unless of course this is in an arid location.

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P.E. Metallurgy, consulting work welcomed

 

[Deker]

Thanks, everyone. Using the method steveh49 posted in the thread he referenced results in a max pressure of 9 psf, while Janssen's silo pressure methods suggested by HTURKAK are highly dependent on the angle of friction between the rock and retaining surface. The pressure predicted here ranges from 250 psf for δ=0 deg to 23 psf for δ=20 deg. Any ideas on how to estimate a reasonable friction angle for design?

I like the idea of tying the wythes together, but I was concerned that the ties would interfere with installation of the mesh. I suppose the mesh can be installed as the wall is built and stop / start at the tie locations. I don't have much experience with joint reinforcement. How can I evaluate the embedment depth required in the bed joint to ensure that I can develop the force in the ties?

EdStainless - I imagine the architect will specify a stainless mesh, but the site has an average annual rainfall of less than 5".

 

[JLNJ]

Maybe you can draw an analogy with the hydrostatic pressure seen in tall formwork. Gravel has to exert less pressure than fluid concrete.

 

[phamENG]

What about, instead of using mesh, you make the fill a Gabion with the wire mesh? That way the wire contains everything and the walls are for show?

 

[Deker]

JLNJ & phamENG - Thanks for your suggestions. The gabion idea is interesting. I'll pitch it to the architect and contact some manufacturers to see how slender they can make them.