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Re: Soldier Beam Retaining Wall Design Practice

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Well, I'm the last person to be considered an authority on anything related to soil design values, but I would like to point out that now that we're required to consider earthquake forces in our design of retaining walls (at least in my area of California), I think in terms of 1/3 increase in earth pressure is more or less offset by 1/3 increase in allowable timber stresses.  Where does that leave us if we've already taken a 40% reduction in loads??

Ralph Hueston Kratz, S.E.
Richmond CA USA

In a message dated 11/10/08 2:12:01 PM, dfisher(--nospam--at)fpse.com writes:
Phil:
 
I typically use method #1.
 
 
David L. Fisher SE PE
 
 

 



From: Phil Doody [mailto:phil(--nospam--at)m-me.com]
Sent: Monday, November 10, 2008 4:01 PM
To: seaint(--nospam--at)seaint.org
Subject: Soldier Beam Retaining Wall Design Practice

 
Is there a consensus on the design of wood lagging for soldier beam retaining walls? Several design methods appear to be in use and each gives very different answers.
 
Basically, there appears to be at least three ways of designing wood lagging that I have encountered:
 
#1 Determine the uniformly applied load  on the lagging due to the soil pressure at a given depth. Select the lagging based on the required section modulus, S = (Moment)/(Allowable bending stress).
 
#2 Same process as above but instead apply only 60% of the theoretical uniform load.  The justification for the 0 .6 reduction factor is that soil movement causes the lagging to flex outward, and induces a redistribution of soil pressure away from the center of the lagging thus reducing the bending moment. This method is described in the Caltrans Trenching and Shoring Manual. 
 
#3 The third method of sizing lagging is based on FHWA recommendations contained in Federal highway Administration Report Number FHWA-RD-75-130.  A table in this report provides the minimum thickness lagging for various soil conditions, soldier pile spacings and excavation depths.  This method cannot be used if there are surcharges behind the wall.
 
The most conservative method is the first method and the one which I am most accustomed.  However, I would like to know if others use methods #1 and #2 and if anyone has observed lagging failures using these methods?
 
Thanks for your input, Phil Doody



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