From: Chance, Acie <AChance(--nospam--at)lacsd.org>
Subject: RE: Soldier Beam Retaining Wall Design Practice
Date: Monday, November 10, 2008, 7:46 PM
The California Trench and Shoring Manual allows the lagging to be designed by the second method. If the lagging is used between soldier beams in a shoring design the 60% not to exceed 400 lbs./ft^2 is used. Surcharges are additive above the design loads previously mentioned. The soil will load the soldier piles at the ends of the lagging as the piles will be stiffer then the lagging (soil bridging). It has been a some time since I read the Shoring Design Manual but that is as I remember it and the way we have been doing it for the last ten years at least. We do not consider seismic for shoring. We do allow a 1.33 factor for short term loading.
I have never used soldier beam and lagging for a retaining wall which is permeate. Most lagging is wood or steel and not acceptable for long term loading on a permeate retaining wall which would be concrete or masonry. A permeate wall would be governed by the Building Code but Shoring is not governed by the Building Code to the best of my knowledge.
Acie P. Chance
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