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Soil-Cement Frequently Asked Questions
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FAQs > Determining thickness
of soil-cement for upstream slope protection
Q: How is the thickness of soil-cement for upstream
slope protection of embankments determined?
A: Typical thickness design of soil-cement
slope protection depends on the severity of the intended application
and on the method of construction. Soil-cement can be placed for
slope protection in horizontal lifts adjacent to the slope (stair-step)
or parallel to the slope (plating). PCA guidelines for the stair-step
and plating methods of construction are summarized below.
Stair-Step Construction Method
 Soil-cement
for embankment slopes exposed to moderate-to-severe wave action
or high velocity lateral flows should be constructed using stair-step
method of construction. To determine the minimum thickness required,
designers should consider the density of the proposed soil-cement
and all possible loading conditions, including but not limited to
uplift due to wave action and hydrostatic pressures that may develop
during rapid drawdown. The thickness would then be determined to
insure that the soil-cement weight is heavy enough to resist all
upward forces. In determining the soil-cement thickness, designers
should also include a safety factor in the form of an additional
sacrificial amount of soil-cement to account for wear or loss of
surface materials, which can occur especially if the lift edges
are unformed or not compacted to a high density or the structure
is exposed to severe freeze-thaw action. Based on the performance
of soil-cement bank protection projects over the past 50 years,
the majority of any surface wear and material loss will occur within
the first couple years of service.
On many projects, the required thickness based on structural analysis
would be less then the thickness dictated by the embankment slope,
lift thickness, and construction equipment. Stair-step construction
on most projects has been accomplished by equipment approximately
8 feet wide placing soil-cement lifts 8 to 9 feet (2.4 - 2.7 m)
wide, 6 to 12 inches (150 - 300 mm) thick. For slopes, ranging from
2H:1V to 4H:1V, the resulting thickness of soil-cement measured
perpendicular to the slope would be between 2 and 2.5 feet (0.6
- 0.8 m). (Figure 1). This thickness compares favorably to a typical
rock riprap with bedding design. If desired, the overall thickness
can be reduced by placing narrower widths. On some projects, conveyor
belt or other placement methods were used to reduce the lift width
to 6-ft. (1.8 m) or less.
Plating Construction Method
 Plating
method of construction can be used on slopes 3H:1V or flatter on
projects not exposed to severe applications. Although this method
has been used on a few projects where the slopes were as steep as
2.5H:1V, it is generally not recommended for this steep a slope
because of the difficulty in placement and compaction. The steeper
slope requires the use of lighter compaction equipment or additional
cable supported equipment working from the crest to maintain stability
of the vibratory roller.
Similar to the structural requirements for stair-step construction,
the required thickness for plating method is typically determined
based on structural analysis considering all possible loading conditions
and accounting for possible loss of surface materials during the
desired service life of the structure. The analysis should include
but not be limited to hydrostatic pressure uplift due to rapid drawdown
and wave action.
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| Figure 1. Relationship of slope, facing thickness,
layer thickness, and Horizontal layer width. |
For detailed information on soil-cement design and construction
methods, refer to PCA publication EB203,
Soil-Cement Guide for Water Resources Applications.
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