Wood Lagging
Wood Lagging
Wood Lagging
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Howard A Perko
Colorado State University
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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.
model
In fact, is within approximately +/- 5% of a
constant value of 1.4 over the range of most
granular soils. If =1.4 is substituted into Eqn.
(7) and Eqn. (8), one obtains the following very
[Fig. 6] Sample Results
simple equations for maximum pressure on
Figure 6-Sample Results lagging and the depth at which this pressure
The depth at which the maximum pressure on occurs and then remains constant.
the lagging occurs can be found by taking the
partial derivative of horizontal pressure (Eqn. 3) Dmax = 1.4l
(9)
with respect to depth and setting the result
equal to zero as shown below.
Pmax = K a ( w + 1.2lγ )
(10)
∂P 4 K a tan(ϕ ) If there is no surcharge, then the maximum
= K a γ 1 − D = 0
∂D l (4) pressure is even simpler.
Pmax = 1.2K a lγ
Solving for the silo height at which the (11)
maximum pressure occurs, Dmax, yields
l
Dmax =
4 K a tan(ϕ ) (5)
⎛ 2c ⎞ DISCUSSION
l⎜ γ − ⎟
l ⎠
Pmax = Ka ⎝ (12) The model can be used to determine required
2 tan � lagging thickness for many soil conditions and
which for cohesionless soils reduces to load cases outside those considered in the
Goldberg-Zoino chart. The lower portion of
K a lγ Table 1 shows the required lagging thickness
Pmax = (13) for the same three different soil conditions and
2 tan �
lagging spans of 1.52 to 3.05 m (5 to 10 ft) with
When Eqn. 13 is compared with the new model, a 9.58 kPa (200 psf) uniform surcharge at the
Eqn. 11, one observes that there are many ground surface. This is an important capability
similarities. In fact, both equations result in the when the soldier pile and lagging wall system
same pressure at an angle of internal friction borders roads, sidewalks, or other structures.
of approximately π/8 (22.5 deg). The trap door The model also allows for determination
analogy predicts much higher pressure (about of lateral earth pressures on other lagging
155%) at smaller angles of friction. The new materials. For comparison, the model was
model is not influenced as much by friction used to calculate required lagging thicknesses
angle but is becomes more conservative at for utility grade lumber. Utility grade lumber
higher angles of friction. is rated at only ¼ times the allowable bending
Using the model, timber lagging thicknesses strength of construction grade lumber
were calculated for three theoretical soils and according to NDS (2005). It is very apparent the
compared to the thicknesses recommended in effect that timber strength has on the required
the chart by Goldberg-Zoino and Associates. The lagging thickness. On average, utility grade
competent soil was assumed to have an internal lumber needs to be almost twice as thick as
angle of friction of 38 degrees and a unit weight construction grade lumber.
of 1,920 kg/m3 (120 pcf). The difficult soil was The model also could be used to size other
assumed to have an angle of internal friction of materials such as flexible steel decking
spanning between soldier piles. One caution strength of the brick wall was estimated based
is that the derivation of the model is based on on a “dry stack” approach using friction only.
the assumption that a lagging material that is Lateral earth pressures based on the model
considerably less rigid than the soldier piles. indicated that the brick wall would bridge
Rigid pre-cast concrete plank may be required between the soldier piles with sufficient factor
to carry full active earth pressures. of safety. The excavation was made without
Recently, the model was tested successfully on a excess movement or any damage to the nearby
project in New York City for an unusual shoring structures. The brick wall was approximately
condition. The model was used to estimate ten feet deep. The excavation extended to
the earth pressure on an existing 6-wyth brick a depth of approximately 5 m (16 ft) below
foundation wall that was braced with soldier existing grades. Wood lagging was used below
piles. A photograph of the wall system is shown the brick wall. A waler with cross braces was
in Fig. 8. The brick wall in the photograph positioned near the top of the soldier piles. The
existed directly adjacent a gas station with one- waler was removed after construction of the
story block structure, several buried tanks, and a structural mat foundation.
canopy. The developer did not have permission On another recent project, the Convention Center
to place shoring outside of the brick wall. The Hotel project in Denver, soil caving occurred
shoring contractor determined that there was during placement of timber lagging. The soil
too much risk of movement of the adjacent conditions consisted of fairly clean, poorly
structures to remove the brick wall. graded, coarse sand overlying claystone bedrock.
It was decided to attempt to use the brick wall The soil which flowed out from beneath the
as lagging between the soldier piles. Mortar in lagging formed a nearly perfect silo shape as
the brick wall was severely degraded such that shown in Fig. 9. This image gives credence to the
it was essentially held together by friction. The assumptions used in development of the model
soils were silty sands and high plastic silts with for sand soils. Although this caving occurred,
some ground water. The punching and flexural the soil successfully arched between the solder
[Fig. 9] Soil Silo Formed by Caving (Courtesy of Coggins and [Fig. 10] Separation Between Lagging and Cohesive Soils
Sons, Inc.) (Courtesy of Coggins and Sons, Inc.)
REFERENCES
1. Macnab, A. (2002) Earth Retention Systems
Handbook, McGraw-Hill, New York, pp. 319-
322
2. Goldberg, D.T., Jaworski, W.E. and Gordon,
M.D. (1976) “Lateral Support Systems and
Underpinning”, FHWA-RD-75-128, pp 118-
121
3. Hart, J. (2008) Coggins and Sons, Inc.,
Denver, CO (Personal Communication)
[54] DFI JOURNAL Vol. 2 No. 1 November 2008
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