Lateral Earth Pressure on Lagging in Soldier Pile Wall Systems

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Lateral Earth Pressure on Lagging in Soldier Pile Wall Systems

Howard A. Perko, Ph.D, P.E., CTL|Thompson, Fort Collins, CO, USA hperko@ctlthompson.com
John J Boulden, SGM, Inc., Grand Junction, CO johnb@sgm-inc.com

Soldier pile and lagging is a conventional means of temporary excavation shoring. Timber lagging
design has traditionally been based upon the designer’s experience or empirical rules. One such method
is the Goldberg Zoino chart used by the Federal Highway Administration. Most of these methods
restrict the designer to a consistent soil profile, certain pile spacing, construction grade timber lagging,
and limited depth. The methods do not take into account surcharge loads or a variety of other factors
that could arise. A designer working outside of ordinary circumstances with unusual loads, varying soil
conditions, or alternate lagging materials has difficulty estimating lateral earth pressures.
In rigid earth retention systems, lateral earth pressure is generally assumed to be constant along the
length of the wall. In soldier pile and lagging systems, the lagging is often considerably less stiff than
the steel soldier piles. As the lagging deflects, the soil tends to bridge between the stiffer elements
resulting in a lower pressure on the lagging. Several previously published methods by others for
determining this reduced pressure are summarized and discussed. These methods typically consist of
using a portion of the active earth pressure in several different pressure distributions.
A simple theoretical model is presented for determination of lateral earth pressures on wood lagging.
The model is based on a three-dimensional “silo” shaped sliding wedge analysis. Results of the model
are compared with other published methods. The model compares well with these methods which
cover the normal spectrum of design situations. In addition, the model can be used to estimate lateral
earth pressures outside typical situations.

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