Relieving platforms

[InDepth]

Does anyone have any good references of how soil pressures are determined below a relieving platform? I have an existing vault where the owner wants to place 10 feet of soil on top of it. I am proposing a relieving platform with piles, but wanted to know how wide I should make the platform in order to prevent an increase in soil lateral load pressures on the existing vault structure. i.e. how do I prevent additional surcharge being imposed on the existing vault.

 

[InDepth]

Schematic Drawing.

 

[msquared48]

A few comments...

1. The top of the new structure would have to be closed to avoid additional loading to the top of the existing grease sump.

2. The additional lateral pressure should be uniform in nature, not varied.

3. The new structure will completely enclose the existing sump, so why not replace the existing, or just build a new one and bypass the original.

4. Where is the max watertable?

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

 

[ishvaaag]

If you close above the grease interceptor and add a compressible layer under your relieving platform you will have the grease interceptor entirely relieved from additional vertical stress, and a conventional slab to design. You can also relieve horizontal stresses the same way or at least get some degree by mere distnce and a complete wall support enclosure.

If you don't want to add compressible layers, in an elastic behaviour view, FEM is the answer; model the slab with the structure but not the fill, then add the fill (or equivalent load) and the changes are the increases in pressures. If constant section, even 2D models can be enough.

 

[InDepth]

thanks for the responses Mike and ishvaaag.

Probably my question is really the theory or methodology for calculating lateral pressure when you have a relieving slab (ishvaag, your comment hit on this). How far do I need to extend the slab beyond the existing structure edge in order to get zero additional lateral stresses (20 feet, 40 feet, 100 feet)?

1. The plan is to place a slab with compressible fill below the slab. The slab would prevent additional loading to the top of the existing structure.

2. I agree the additional pressure would be uniform ( but without a relieving slab). Is it possible for the additional pressure to be equal to zero if the slab is wide enough? or is there a method where the geotech calculates a lateral pressure based on a rotational movement of a soil block rotating and inducing lateral pressures beneath the relieving slab?

3. I may consider a secant or tangent pile wall to surround the existing structure and tie the tops with a slab (ring beam).

4. Water table is 30-40 feet below the existing structure.

 

[cntw1953]

Instead 2 T's, how about a H shape set-up. Means you bring the relief slab to just above the existing structure, thus forming a support system that is capable of distribute the load to pile walls. Leave a space in between the relief slab and the existing structure, and fill with compressible materials to absorb add'l pressure from deflection.

 

[ishvaaag]

Likely a 1H:2V line from the bottom of your grease interceptor can mark to where the relieving slab must be extended (if supported on deeper piled or walls) to forfeit most of the increase in lateral pressure from the surcharge. With the FEM model you can tweak this dimension.

 

[InDepth]

Thanks ishvaaag. I found out that they used an embankment load starting from the edge of the relieving slab; essentially not accounting for a 1H:2V cutoff line for surcharge. I agree that if I make the salb edge at 1H:2V then there wouldn't be substantial surcharge. I'm sure FEM would result in lower values.