Soil Arching Effect
Soil Arching Effect
(OP)
Hello engineers,
I'm working on an excavation project and my team is thinking to use Secant piles as a possible solution.
Since the arching in soils will take effect, I would like to know what will be the separation between piles. I believe this depends of how big or little is the arching effect, but I have no idea how to determine the distance between piles.
FYI, the soil in this project is silty sand (SM), Nave is about 25 blows/ft.
Please let me know how can I come up with an adequate separation based on the arching effects of this SM?
Thanks in advance.
I'm working on an excavation project and my team is thinking to use Secant piles as a possible solution.
Since the arching in soils will take effect, I would like to know what will be the separation between piles. I believe this depends of how big or little is the arching effect, but I have no idea how to determine the distance between piles.
FYI, the soil in this project is silty sand (SM), Nave is about 25 blows/ft.
Please let me know how can I come up with an adequate separation based on the arching effects of this SM?
Thanks in advance.





RE: Soil Arching Effect
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RE: Soil Arching Effect
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Soil Arching Effect
I really don't know that much about secant piles, however and don't feel like googling to sound real smart.
f-d
¡papá gordo ain't no madre flaca!
RE: Soil Arching Effect
Thanks a lot for your feedbacks.
I meant soldier piles nstead of secant piles, sorry about that.
Fatdad, if I use phi/10, that distance is in meters or in feet?.
Btw, can u please give the reference from where u got that info?, or is it just a rule of thumb?
Msqueared, my piles are not cased, thanks for ur information though.
RE: Soil Arching Effect
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RE: Soil Arching Effect
I have use variations on secant walls for special conditions but the contractor's operation is critical for installation.
RE: Soil Arching Effect
Another question, Will soldier piles work on loose to very loose sand?. Wouldn't some soil in between the piles fall off the surface?.
Thanks again
RE: Soil Arching Effect
f-d
¡papá gordo ain't no madre flaca!
RE: Soil Arching Effect
Arching is assumed to take the shape of a half a circle. There are 3 items with regards to soil arching. The arching length which is the soldier beam spacing minus one drilled hole diameter. So 8 ft spacing and 2 ft diameter hole will yield arching length of 6 ft.
The arching depth (diameter of the half circle) is simply half the arching length or 3 ft in our example. Then there is the soil pressure on the lagging boards. As fattdad alluded to it is not dependent on cohesion. It is the equivalent active earth pressure multiplied by the arching length. so EFP of 40 psf/ft and arching length of 6 ft will give a soil lagging pressure of 240 psf. See this link for a shoring class next month: http:/
RE: Soil Arching Effect
Tangent piles won't make it either in this case and you will have to turn to secant pile or diaphragm walls.
RE: Soil Arching Effect
If the piles are closer than this, you really get into the plane strain sort of soil mechanic.
I realize that you are getting different answers that lead you to (about) the same answer, but that's the way I see the problem.
f-d
¡papá gordo ain't no madre flaca!
RE: Soil Arching Effect
Spacing of soldier beams depends on the soil and the installation method. Usually, driven soldier beams are spaced at about 8 feet. Maybe less if you have loose soils that may be hard to lag. Maybe a little more if the soil is more dense or stiff. Drilled soldier beams that are completely backfilled with lean concrete or flowable concrete fill are often spaced at up to 10 feet on center.
Designing soldier beams in loose sand is no problem. The problem is knowing how to and being able to properly install lagging in loose material. Search the forums for threads about installing lagging. There are a number of them, I believe.
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RE: Soil Arching Effect
RE: Soil Arching Effect
It's true that the major stress in an arch is compression, but there will be minor transverse tensile stresses to be resisted.
RE: Soil Arching Effect
RE: Soil Arching Effect
I got the Caltrans Shoring and trenching manual. When using soldier piles, I didn't know there is a depth "D" in which we need to use laggings.
I think I have seen somewhere soldier piles with no laggings at all.
Would like to know what are the conditions of using laggings at some depth D.
Thanks again
RE: Soil Arching Effect
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RE: Soil Arching Effect
On 99% of jobs you will need lagging from the top to the bottom. I believe Caltrans specifies a maxium 400 psf lagging load because of arching effects (Temporary). Lagging with proper backfill behind the boards is required to minimize settlement/movement of the soil behind the shoring system.
As far as your question...you may have an excavation that penetrates different soil layers or even rock. Obviously you would lag in front of the soil and stop lagging in rock.
In general you should consult your geotechnical engineer about the lagging design pressures, especially if there are surcharge loads adjacent to the excavation.