Earthen Pit Wall Sloping

[ksheridan]

Looking for regulatory restrictions regarding the maximum sloping of earthen pit walls.

Industry: Oil/Gas/Geothermal
Application: Drilling "reserve" pit, or Geothermal flow "test" pit

Concerned about functional AND safety requirements/regulations.

 

[msquared48]

Depends on the material and the natural angle of repose which will vary.

You need a geotech involved.

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

 

[ksheridan]

@MSQUARED48

Assuming soil classification A or B (per 29CFR1929), can the walls be shear/near vertical? Do all walls have to be sloped?

 

[msquared48]

Professionally, I would defer to a geotech ion that, and there are many in this forum.

However, I would be concerned with erosion if the wall is exposed to the elements. Do you have any bank stabalization measures available to prevent erosion?

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

 

[ksheridan]

@MSQUARED48

It is a lined pit, depth ~20 feet, ~130 x 130 ft dimensions.

It is in the cut portion of a wellpad.

 

[PEinc]

You need to provide more information. What is a lined pit? A 20' deep excavation may be open cut if you have enough layback area. However, in the US, an open cut excavation over 20' deep needs to be designed by a professional engineer. If you need to install an excavation support systen instead of a sloped open cut, there are many ways to do it depending on site conditions and the site logistics plan.

http://www.PeirceEngineering.com

 

[ksheridan]

@PEInc

The pit is lined to prevent brine loss to the surrounding landscape during drilling or flow testing of new geothermal wells.

Typical liner is 30 mil PVC.

Please review the pictures, as they may better tell the story. The pits are "spec'd" by a Civ Eng, but I'm not sure all requirements are being taken into account.

The shear walls of the pit seem to be in opposition of OSHA excavation regulations, and may pose a safety hazard should livestock or personnel accidentally enter the pit.

http://files.engineering.com/getfil...0c-b0ef-d2e6114e8f21&file=IMG_0599_-_Copy.jpg

http://files.engineering.com/getfil...4b-98d1-1eb0b051f2fe&file=IMG_0584_-_Copy.jpg

 

[PEinc]

Unless this 20' deep pit is in competent bedrock, it needs to be shored. The only other way you could make this excavation is for some engineer to do a slope stability analysis showing that the 20' high vertical cut is stable. That is unlikely to happen. Therefore, you are left with installing a sheeting system or a soil nail wall.

The fact that the pit is lined is irrelevant. The pit must stand safely open in order to install the liner.

http://www.PeirceEngineering.com

 

[PEinc]

One other option is to safely slope the sides of the pit and then cover them with the liner.

http://www.PeirceEngineering.com

 

[TGLG]

If you are redesigning/rebuilding this thing, you may also want to put some thought into the grading of the area surrounding the pit. If surface runoff flows toward or past the edge of the pit, it might cause erosion which could cause the side of the pit to collapse.

 

[BigH]

Agree that in order to design a safe slope one needs to know the material type that the cut is in, the groundwater level and the like. Open cuts are best if you have the room and you can barricade about the pit for safety purposes.
Unless I am mistaken, the 20 ft pit with 130'x130' size is not considered an "excavation" in the sense of OSHA - I thought (though I may be fuzzy on this as I am relying on memory) that this applies when the base of the pit would be less than 15 ft wide. I'll if I am shown to be wrong.
Still, it should be designed for either temporary or permanent conditions whichever applies.

 

[ksheridan]

@BigH

If you are correct on the comment of 'excavation', would I still have the expectations that the earthen pit should have:

1. Engineered retention/shoring for vertical walls

OR

2. Sloped walls chosen with respect to the soil type/classification

?

 

[BigH]

That's basically what I said in the last sentence. I was just trying to point out my understanding of what is required by OSHA on the width of the excavation. But in the end, a proper open cut slope angle that is conducive to the stability of the slope wall and the lining aspects or a designed retaining structure is needed.

 

[dicksewerrat]

Osha doesn't care what you call it. The wall can fall on a worker, therefore it MUST be designed by a PE. And the worker has to get to the edge to get out.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com