Area of passive resistance behind support post 1

[Trevski]

I'm trying to design some small scale retaining structures, timber or metal posts with boards/lagging between them.

As far as I can see the only restoring moment is from the passive resistance of the soil behind the support posts. This is only a narrow strip of soil compared to the supported soil.

Can you call upon a greater area of soil than that directly behind the post? I would imagine there is a wedge of soil providing passive resistance.

Thanks for any clues

 

[fattdad]

You can multiply the passive pressure acting on a soldier pile by the factor Cp. In granular soils, it's approximated as phi/10. Don't forget to sum your moments and horizontal forces to see just how much of the passive pressure you can actually use.

f-d

¡papá gordo ain’t no madre flaca!

 

[Trevski]

Thanks for this f-d. Is there a similar factor that can be applied in cohesive soils?

 

[BigH]

I"ve seen 1.5 x diam of post to be the "area" to apply the post's passive pressure (see Golder and Seychuk's paper in Pan Am Conf on Toronto Subway - 196something.

 

[oldestguy]

I've use twice the passive resistance, since there is shear on the sides of the post. This is a common usage for piles at bridge abutments.

 

[Trevski]

Thanks for this, I will use 1.5 to be conservative.

 

[PEinc]

Many engineers use 3x the width, but not more than the soldier beam or post spacing.

 

[msquared48]

2 to 3X is what I have seen and used, depending on the geotech and the soils.

Mike McCann
MMC Engineering

 

[InDepth]

I agree with msquared48. 2 to 3x the pile diameter if it is cast in competent concrete depending on soil type (if very loose, may only get 1x). If the post is driven, then multiply by 1.5 (thus 3x and 4.5x) due to increased compaction around a driven pile. See Caltrans and NAVFAC. In caltrans, you can calculate an effective area based on phi (arching factor).

It sounds like you have small retaining walls, so I'm assuming you have are installing the posts into top soil...so 1 to 1.5x dia is probably a good assumption.

For the posts, I would recommend steel. What you will find is that the wood will creep over time due to the load (increased deflection), and rot will usually form at the ground intersection. Use pressure treated wood for the lagging and don't forget to treat the cut edges. For even increased resistance, drill borax rods in the ends.

 

[hokie66]

Whatever passive resistance you decide on, by all means lean the post back toward the retained earth. Stops embarassment when the wall inevitably rotates. Leaning toward the bank is good, leaning away is bad.

 

[PEinc]

InDepth,

You said, "2 to 3x the pile diameter if it is cast in competent concrete depending on soil type (if very loose, may only get 1x). If the post is driven, then multiply by 1.5 (thus 3x and 4.5x) due to increased compaction around a driven pile."

It seems to me, if the pile is encased in competent(?) concrete, then it can't also be driven for "increased compaction" and the already increased width shouldn't be again multiplied by 1.5.

I know some people talk about multiplying the width (or diameter) by as mush as 4.5, but I would never use more than 3x.

 

[RWF7437]

Three references you may find useful are:

Pole Building Design by Donald Patterson, 1957. and

Archon Engineering's WinPost ( about $40 )available at Sales at archonengineering.com

and Chart for embedment of Posts, Outdoor Advertising Association of America ( circa 1960 ) which has been reprinted in many structural engineering handbooks.

 

[InDepth]

PEInc,

Maybe a clarification,

let's use examples

If a 24" diameter pile is drilled, a Wx inserted, and then cast in competent concrete - use between 4-0" to 6-0" effective width (2 x pile dia to 3 x pile dia)

If a 24" diameter pile is drilled, a W14x53 inserted, and then cast in uncompetent concrete - then use 2 x bf to 3 x bf = 16" to 24" effective width

If a HP10x57 is driven into a soil (no concrete), the effective width is 3 x bf to 4.5 x bf = 30.7" to 46"

Some shoring softwares use this procedure. The Caltrans manual has a partial discussion on this.

 

[born2drill]

InDepth,

What is "uncompetent concrete?" Even if you fill a drilled hole with lean mix or flowable fill, I would still count on that entire section to provide passive resistance. Weak concrete is still going to be stronger than the soil it replaced. Maybe if you backfilled the hole with sand I'd agree, but that's bad practice and shouldn't be done anyway.

I've always been told to use 2x diameter for a drilled in beam or pipe section, 3x bf for a driven HP beam. Assume passive pressure acts on the front of the beam. Draw a 45 from the back flange of an HP to the front, you get roughly 3 x bf. Draw a 45 from the widest part of a circular shaft to the front, you get 2 x diameter. It always made sense to me. By that logic I also can't justify using anything over 3x.