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Slope Stabilization
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Geotechnical engineers have long recognized the utility of lightweight fill to reduce
mass and associated gravitational driving forces. Lightweight materials that have been used
in embankment construction include chipped bark, sawdust, dried peat, fly ash, slag, cinders,
cellular concrete, lightweight aggregates, expanded polystyrene, shredded tires, and sea
shells (Holtz and Schuster, 1996).
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EPS geofoam is up to 50 times less massive than other lightweight fills (see table). To
achieve a net reduction in driving mass of 100,000 kg within an embankment having an in-place
soil density of 2100 kg/m3 would require placing about 90 m3 of
lightweight fill of 1000 kg/m3 density or about 50 m3 of EPS geofoam.
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Material
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Density for calculation (kg/m3)
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Bark1
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1000-1100
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Sawdust1
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1000
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Cellular concrete waste1
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1000
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Light expanded clay (Leca)1
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800-1000
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Waste bricks of Leca1
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1000
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Tires2
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700-950 (in-place)
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Pumice3
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1225
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Expanded polystyrene geofoam4
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15-120
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1 Rygg, N., Sorlie, A., (1981). "Plastic Foam in Road
Embankments." Soil Mechanics Conference, Stockholm.
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2 Humphrey, D.N., Dunn, P.A., and Merfeld, P.S. (2000). "Tire Shreds
Save Money for Maine." TR News, No. 206, January-February, 2000.
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3 Holtz, R.D. and Schuster, R.L., (1996). Stabilization of Soil Slopes. In:
Landslides Investigation and Mitigation. Transportation Research Board Special Report 247.
National Academy Press, Washington, D.C., p. 429-473.
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4 Dependent on drainage conditions and nominal geofoam density.
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Advantages to using geofoam for slope stabilization may include:
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Maximizing available right-of-way
Reduced construction schedule and traffic impact
Relatively clean construction adjacent to waterways
Reduced labor and future maintenance
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To improve a slope safety factor against failure, different approaches may be considered. A
conceptual scheme of slope stabilization with geofoam is shown below. The balance of driving
and resisting segments within a potential failure mass may require adjustment. A potential
sliding block can be divided into an upper and a lower masses. The upper mass, contributing
more to driving or instability, is referred to as a driving block. The lower mass,
contributing more to resistance or stability, is referred to as a resisting block. These may
include ground water lowering, removal of material mostly from the upper or driving block, or
increasing the size or influence of the lower or resisting block by providing a berm. Except
for ground water lowering, the overall effect of either decreasing the driving block or
increasing the resisting block is to change the geometry of the slope by decreasing the
average iclination.
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The density of geofoam is 50 to 100 times lower than soils. Acceptable improvement in safety
factor can be achieved by soil excavation and replacement with geofoam in the driving block.
Such improvement in stability occurs without requiring a change in the final slope geometry.
The excavation back slope would be sloped to be self supporting. Free draining material would
be placed as transition between the natural soil and the geofoam along the back slope and as
leveling course along the base. The final geofoam block configuration balances the amount of
geofoam required (cost) with other design and construction factors such as ease and quickness
of construction, traffic and environmental impacts, and construction space limitations.
Provision of a concrete slab cover may not be necessary.
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Installing EPS geofoam into an embankment requires that existing soil fill material be
removed. Sloped excavations that roughly parallel the shape of the geofoam fill and the
existing failure surface facilitate construction. Sloped excavations may not be possible in
some cases due to construction right-of-way limitations, requirements related to traffic,
poor soil, or a combination of these factors.
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Where inclined excavations are not an option, a temporary steel sheet-pile wall may be
installed to permit vertical excavation adjacent to the geofoam fill, as shown below. In both
sketches, the concrete slab and pavement are optional.
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