excavation setback 8

[pelelo]

Engineers,

I am involved in a project which consists on a 10 ft excavation to place 3 ft diameter pipe.

Anyone knows if there is an approach of computing the setback so the overexcavated soils do not become a surcharge for the excavation (excavated walls)?. I am not familiar with this.

thanks.

 

[oldestguy]

There probably are local rules for minimum excavation side slopes. Unless sheeted bracing are used, those local rules had better be followed. The placement of excavated material usually is OK from top of cut if these slope rules are followed.

 

[Lomarandil]

As a quick answer, look into the OSHA sloping and benching standards for trenches. A refined analysis or shoring may be able to improve that.

----
just call me Lo.

 

[fattdad]

do a Culman analysis?

Make sure it's at least 14 ft away (5 ft plus 10 cos 30). That'd be the active wedge plus 5 ft (phi of 30).

Not sure what else we can say?

Well, you can look at the OSHA stuff too.

f-d

ípapß gordo ainÆt no madre flaca!

 

[PEinc]

fattdad gave a good distance estimate for a sheeted cut. However, if the excavation is not sheeted and you have an OSHA Type C soil, the minimum excavated slope is 1.5H:1V plus the 5' setback for the spoils pile. 10' at 1.5:1 = 15' + 5' = 20' to spoils pile.

http://www.PeirceEngineering.com

 

[Mad Mike]

The science and legislation has been nailed above by Fattdad and PEinc, but consider also the practical side.

Consider how impractical it might become to stockpile the soil 15 to 20 feet away from your trench- as opposed to simply excavating and dumping (repeat), it would now be required to excavate, manoeuver, dump, manoeuver, excavate (repeat)...

Have a good look at your soil profile and if possible, recommend stockpiling the excavated material at a distance within easy boom reach of the excavator being used on the job. If the sidewalls are strong as concrete, it would be redundant to have a contractor spend half his time and effort avoiding a slight surcharge- I'm referring to un-shored trenches...of course where the surcharge significantly affects lateral support design, fall back to the science as above.

All the best,
Mike

 

[pelelo]

Thanks a lot engineers.

After reading some OSHA manuals, OSHA recommends a minimum setback of at least 2 feet from the edge of the excavation.

 

[EireChch]

A 2 foot setback has to be wrong....

 

[Okiryu]

2' is too small but this is what is indicated in the USACE EM 385 Safety Manual - Excavations Section, page 609 of this PDF link:

https://www.publications.usace.army.mil/Portals/76/Publications/EngineerManuals/EM_385-1-1.pdf

c. Excavated material shall be placed at least 2 ft (0.6 m) from the edge of an
excavation or shall be retained by devices that are sufficient to prevent the materials from
falling into the excavation. In any case, material shall be placed at a distance to prevent
excessive loading on the face of the excavation.

Anyway, we recommend a minimum of 1.5 m setback from the unsupported excavation edge.

 

[EireChch]

Okiryu, 1.5m in general or for this specific case of a 10ft excavation.

To me it has to be based, as f-d indicated, on the active wedge. Excavation height and soil friction angle determine the set back distance.

ps. commiserations re the defeat to SA. I was over for the Japan v Ireland game where Japan beat us. We lost to the better team on the night. Japan is a beautiful country, full of the nicest people!!

 

[Mad Mike]

The minimum 2' setback presumably applies to stable sidewalls only, the intention being to prevent material spilling over into the trench. I wouldn't consider using this minimum if any rounded boulders are present.

Where the sidewalls are potentially unstable, the setback has to take account of the material friction angle and/or be considered as a surcharge in the shoring design.

Good luck, and sorry South Africa tore into you this past weekend- I must confess, I wasn't thinking of you as I cheered them on.
Mike

 

[Okiryu]

I use 1.5 m in general. Excavations deeper than 1.5 m needs some kind or shoring or slope back.

And yes, everybody is watching de rugby World Cup here! I am not a rugby guy but I watched the last game with SA... sad about that lost...!