Arching effect of soil between vertical braces

[hairman]

How do you account for the arching effect of relatively stable (OSHA - Type B) soils between vertical braces in a shallow (less than 8' deep) braced cut with vertical braces spaced from 4' to 8' apart?

 

[GeoPaveTraffic]

Experience.

While leave the sides open and expose the works to falling objects from the side? Place a piece of sheet steel on teh back sid eof the vertical braces and you have no problem.

 

[hairman]

I am trying to determine the load that the brace must support. I am sure that the brace will not have to support the entire load between supports due to the arching effect of soil between braces, but I am not sure how to account for the arching effect.

 

[GeoPaveTraffic]

I'm not sure of your question. The braces will need to support the total load that the system is designed to support. If you are creating a shoring system to support the banks during an excavation, then the load on the system will be based on either Ka or Ko, depending on how long the excavation will be open and how much ground movement the surrounding area can tolorate.

The project geotechnical engineer should be able to design the system or provide the loads.

If you provide more information about the nature of the project, we may be able to help.

 

[hairman]

Geopave (and anyone else), thanks for your interest and help. The specific project is for an engineering justification for a shoring system that had been installed, functioned acceptably, and was cited by OSHA as not meeting the OSHA requirements found in 29CFR1926 Subpart P Appendix C.

The shoring system consisted of 2" by 6" maple heart vertical uprights with two ea 2" by 6" maple heart spreaders in a 7.5 foot deep trench. The bracing (sets) were spaced 4 feet apart along the tench.

The active earth pressure can be calculated using a number of methods including OSHA's. Cohesive soil has some ability to "arch" between supports, thus I am trying to develop some rational for applying a percentage of the total load between supports to each support. Any input would be appreciated.

 

[VAD]

There may be other articles that can lead you to an answer but if you have Tezaghi's "Theoretical Soil Mechanics" you can review arching and detremine lateral pressure which would be smaller than if you use an active coefficient of 1. Roughly you would probably be one third of the value computed. This would take care of the arching effect. However, you need to review the concept. Hope this helps.

You may also finf some info on this subject in other texts

 

[GeoPaveTraffic]

If it was a short term installation, then I would calulate the active pressure on each member assuming a 4 foot contributing area.

If you are trying to say that the arching means that you only have to consider a 2 or 3 foot contributing error, I think you are out of luck.

If OSHA's point is that sheeting should have been installed between braces, then your only argument is that the cohesion was enough to hold the surface, and again you are probabily out of luck.

 

[DRC1]

The braces will take all the load. Load will be derived from pressure envelope for 2 braces. Triangular ditribution probably not apprpriate. water table to be cosidered. OSHA has several pre approved plans incluudig skeleton sheeting, which sounds like your case. Construction details are very conservative and must be strictly followed. If te contractor deviates from published specs, he must (prior to OSHA visit) have a PE stamped drawing. Provding one after the fact will do little, even if design is correct. Ability of clay to span between vertical sheeting is simpy the applied lod vs availble strength. Braces will not see any arching effect.