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Pressure Diagrams (Lagging)

Pressure Diagrams (Lagging)

(OP)
I am designing a permanent 11-ft tall soldier pile (8'-0" o/c) and lagging system. The owner wants a timber lagging option in addition to precast. Normally, timber lagging is not really designed, but selected based on historical performance. However, this particular wall will be retaining an abandoned pumping building and associated water tower. Our geotech has provide a surcharge of 300 lbs/sq.ft to take care of those structures. We have designed the precast lagging, which was easy to do using un-reduced lagging pressures without increasing costs significantly; However, we want to try to sharpen the pencil a bit for the timber lagging.

My question is what would you recommend for pressure distribution on this lagging? I ran some numbers with the sliding wedge method which is supposed to be constant beyond 1.4 times the lagging span (approx. equal to 11-ft in my case), but those values seem too high for such a relatively short wall. Something tells me that the aspect ratio of my pile spacing/retained height is such that the sliding wedge is not the way to go. Comparably, when I checked using a uniform 50% reduced pressure (Earth Retention Systems Handbook, Macnab, 2002), the pressure at 11-ft was quite less than the sliding wedge. Also, for the uniform reduced pressure diagram does the surcharge get reduced by the same 50%?

Phi = 30
Saturated Unit weight = 120 pcf

Any thoughts would be helpful. Thank you.

"It is imperative Cunth doesn't get his hands on those codes."

RE: Pressure Diagrams (Lagging)

A couple questions for you:

8' spacing with a 11' tall wall, curious as to what size your soldier beams are? Are they drilled? Usually I need a pretty big beam for that tall of cantilevered wall in order to limit deflection or is this an anchored system?

The wood lagging would be temporary, correct? You might be ask the geotech if any cohesion could be used for the short term condition.

The Caltran Trenching and shoring manual addresses this (see here click accept at the bottom to download document). Basically they say to use 0.6* the pressure to account for arching. They go on to say that studies show a maximum of 400 psf can be used. This is all without surcharge. Surcharge would be directly added. They then reference the FHWA manual for minimum thickness based on soil type.

EIT
www.HowToEngineer.com

RE: Pressure Diagrams (Lagging)

(OP)
HP 12x84 drilled in 24" dia. holes

1-3/4" deflection is not a concern as those two structures described can handle the movement, as they have been abandoned (forever). The owner has determined that a wall is cheaper than demo of the structures.

Wood lagging is a permanent alternative to precast - owner will decide based on bid results.

I don't quite understand why their lagging earth pressure varies with depth indefinitely, regardless of the 60% reduction afforded or not. Seems super conservative. I am aware of the FHWA tables - thanks. Either way, 60% of Ka is obviously in the ballpark with what a mentioned above. Still not sold on 1.0 Ka being applied to the surcharge separately, as you mention.

"It is imperative Cunth doesn't get his hands on those codes."

RE: Pressure Diagrams (Lagging)

This is an interesting topic. I often struggle with lateral earth pressure assumptions for permanent cantilever soldier pile walls using precast concrete lagging panels. As I understand it, the sliding wedge method and other similar models that allow a reduction in lateral earth pressure on lagging to account for soil arching, are all based on the assumption that the lagging material is considerably less rigid than the soldier piles. Does anybody know of a method for calculating earth pressures on much stiffer lagging materials, such as concrete? I also understand that it may be aggressive to use these soil arching models when the temporary wood lagging is place behind the back flange of the piles (back-lag), which is most often how the walls I design are constructed. For these reasons, I typically will calculate lateral earth pressures on concrete lagging panels using a traditional Rankine approach (p=Gamma*H*Ka), with the pressure increasing with depth, and never reaching a depth of maximum pressure.

RE: Pressure Diagrams (Lagging)

(OP)
Uncivil,

That is precisely why I didn't reduce the pressures for our concrete lagging option. Frankly, I believe that it is a non-issue when concrete is the lagging material of choice for a permanent wall. It is very easy to change reinforcement size or depth in the section, without much increase in cost, to meet the un-reduced lagging pressure. On the other hand, timber lagging is only regularly sawn in certain cross sectional sizes in certain species of wood. You can't incrementally increase the timber lagging capacity in the same subtle manner that you can with the concrete.

"It is imperative Cunth doesn't get his hands on those codes."

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