Soil Arching Effect 1

[pelelo]

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.

 

[PEinc]

There is no spacing between secant piles. They overlap one another.

http://www.PeirceEngineering.com

 

[msquared48]

If you are using concrete encased soldier pile, the arching effect is normally 2 to 3 times the pile diameter, depending on the recommendations of the Geotech.

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

 

[fattdad]

All I can say is for horizontally-loaded vertical piles, I'd use a spacing no closer than phi/10, where the spacing is from closest edge to closest edge.

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!

 

[pelelo]

Guys,

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.

 

[PEinc]

Just use a passive resistance width of 3b unless the soil is very soft. Most phi angles divided by 10 = about 3 feet. b equals the flange width of a driven soldier pile or the concreted (lean or structural) diameter of the drill hole for drilled-in soldier beams. Done this on several thousand soldier beam jobs, both permanent and temporary.

http://www.PeirceEngineering.com

 

[wmta]

Secant piles are not designed to have spacing between the piles, the arching between the pile is total dependent on the cohesion of the soil which my vary. The only reason i can imagine using secant pile is to limit deflection and potential ground lose where settlement is an issue.

I have use variations on secant walls for special conditions but the contractor's operation is critical for installation.

 

[pelelo]

PEInc, Thanks for your reply. If I have a soft to very soft clay or a loose to very loose sand, what would be the rule of thumb for the separation between drilled shafts?

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

 

[fattdad]

I don't quite think arching has anything to do with cohesion.

f-d

¡papá gordo ain’t no madre flaca!

 

[FixedEarth]

pelelo;

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://www.soilstructure.com/1dayshoringdesignclass.html

 

[BigHarvey]

First you must check the level of the water table. If your excavation is going under the water table, you can't do a soldier pile wall in silty sands without having the risk to see yhe material running into the excavation.
Tangent piles won't make it either in this case and you will have to turn to secant pile or diaphragm walls.

 

[fattdad]

On the question of my "units" related to my suggestion of phi/10: If you have a 14-in soldier pile and a friction angle of 30 degrees a spacing of less than 42 inches, i.e., (phi/10)*14 would affect the soil arching (not that anybody would place soldier piles that close in the first place). So the units are based on the pile width.

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!

 

[PEinc]

If you have soft to very soft clay or very loose sand, you probably should not use cantilevered soldier piles. There will be too much movemment. Steel sheet piling may be more appropriate.

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.

http://www.PeirceEngineering.com

 

[csd72]

You should definately use closed off supports for any loose granular material or for very poor clay. Otherwise you run the risk of it eroding between the piles every time it rains.

 

[apsix]

I agree, which is why I can't agree with those who appear to say that cohesion is not required.
It's true that the major stress in an arch is compression, but there will be minor transverse tensile stresses to be resisted.

 

[InDepth]

Look at the Caltrans Shoring and Trenching manual under the Soldier Pile Design section. They have equations based on phi. I would argue that these values are for temporary short term conditions. You have to analyze your arching stability for long term if you have to design a permanent structure.

 

[pelelo]

Thanks for your reply Indepth,

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

 

[PEinc]

Lagging is used between all soldier beams unless you use an alternate soil support method between soldier beams such as shotcrete or unless the soldier beams are installed as secant or tangent piles. Depth is not usually a significant consideration.

http://www.PeirceEngineering.com

 

[InDepth]

Pelelo, yes there are pictures out there showing no lagging between soldier piles. This condition is usually in "hard" self supporting materials...and the soldier piles are redundant safety measures. Ie the soil is almost rock.

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.