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Earls Walls
J
NAMES K. MITCHELL AND JAMES G. COLLIN
Soil is our most abundant and least expensive construction material.
The range of soil types and possible states for any soil type are almost
limitless. When at a suitable density and moisture content, most soils
can be strong enough in compression and shear to be structurally useful.
On the other hand, like Portland cement concrete, soil is very weak in
tension; this limits its use for some applications, such as those requiring
slopes steeper than the internal friction angle of the soil, which is about
30 degrees in most cases. But also, as is the case for reinforced concrete,
the inclusion of reinforcement that is strong in tension yields a composite
material that combines the best features of both components
Figure 1 shows a pile of sand with the steepest slope that can be
maintained with the sand in its dry state. The same sand with sufficient
water added to give capillary stresses in the pores, i.e., fluid pressures
less than atmospheric, results in positive contact stresses between sand
particles, or an "apparent cohesion" that permits steep slopes to remain
stable, as shown in Figure 2, up to some particular height. Because
either wetting or drying of the damp sand will lead to collapse of the
slope, the use of capillary stresses can hardly be relied upon for long-
term stability.
On the other hand, the inclusion of reinforcements within the sand
can, as a result of stress transfers between the soil grains and the re-
inforcements, result in structures that are stable over long time periods.
Figure 3 shows the same dry sand as that in Figure 1, but in this case
strips of paper are incorporated as reinforcing elements. The paper used
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as the wall facing is needed to prevent running of the sand from the
region between reinforcements; however, it does not assume a major
structural or load-carrying function.
Over the past 15 years increasing use has been made of earth rein-
forced in a manner similar to that shown in Figure 3 to construct new
walls or to strengthen existing slopes. There are several reasons for this
in addition to the low cost and abundant supply of earth. Construction
is simple and rapid. Reinforcements and facing elements can be pre-
fabricated. Aesthetically pleasing structures are possible. Earth walls do
not require unyielding foundation support as is the case for most rein-
forced concrete walls. Keen competition among the developers of dif-
ferent reinforcement systems has led to rapid technological development
and continued cost reductions relative to both traditional types of rein-
forced concrete walls and other types of wall systems such as crib walls
and bin walls.
EVOLUTION OF EARTH WALLS
It has been known for several thousand years that tensile inclusions
in soil can provide reinforcement. Large religious towers, called zig-
gurats, were built by the Babylonians between 5,000 and 2,500 years
ago. These structures (see Figure 4) had walls faced with clay bricks in
an asphalt mortar with blocks of sun-dried mud behind. Layers of reed
matting were laid as horizontal reinforcing sheets in the mud. In some
ziggurats additional reinforcement was included in the form of ropes
about 50 millimeters (mm) in diameter placed perpendicular to the wall
and regularly spaced in the horizontal and vertical directions.
Reference is made in the Old Testament (Exodus 5:6-9) to the use
of straw-reinforced bricks by the ancient Egyptians. Many primitive
peoples used sticks and branches for reinforcement of mud dwellings.
The corduroy road was an early method for construction of roads across
very weak ground that was widely used in colonial America. Figure 5
shows a more modern example of a corduroy road. During the seven-
teenth and eighteenth centuries French settlers along the Bay of Fundy
used sticks for reinforcement of mud dikes.
The development of earth reinforcement for walls in its modern form
was pioneered by the French architect and inventor Henri Vidal. The
results of several years' study and experimentation led to Vidal's patent
in 1966 for Terre Armee. The first highway use of a Vidal reinforced
earth wall was near Nice, France (Figure 6~. A schematic diagram of
this type of wall is shown in Figure 7.
Vidal reinforced earth walls were first used in the United States in
1972 to provide support for California State Highway 39 along a steep
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EARTH WALLS
163
slope in the San Gabriel Mountains- north of Los Angeles (Figure 8~.
An earth wall was well suited for the site because of the overall instability
of the hillside and because of the ability of these walls to withstand
substantial deformations without failure.
CURRENT REINFORCEMENT SYSTEMS FOR EARTH WALLS
Over the past 10 years the Vidal reinforced earth wall has been the
most widely used. Galvanized steel reinforcing strips are connected to
pr'ecast concrete facing panels, which have largely replaced the metal
facings used in early walls. The standard precast concrete facing panel
is shown on the walls in Figure 9. For aesthetic reasons special finishes
or panel designs can be used, as shown in Figure 10.
A number of other reinforcing systems and materials applications have
been developed for earth walls as well. In the VSL Retained Earth
system, horizontally placed wire or bar mesh systems are used, as shown
in Figure 11. The Hilfiker welded wire wall (Figure 12) uses wire mesh
reinforcement and a facing consisting of wire mesh covered by shotcrete.
Tensar geogrids are high-strength plastic composite grids that can be
used in a variety of ways for containing, retaining, and reinforcing soil.
Since the grids can easily be interconnected, a variety of combinations
is possible. Large (l m x 1 m x 1 m) earth- or rock-filled baskets similar
to gabions can be stacked on top of each other to form walls or barriers.
The plastic grids can be used during reconstruction of failed earth slopes
to improve stability. Figure 13 shows an example of a brick-faced re-
taining wall using tenser geogrids.
Synthetic fabrics for geotechnical use, called- geotextiles, have been
used as earth reinforcement. They are particularly suited for relatively
low walls along remote or relatively lightly traveled roads. An example
is shown in Figure 14. A schematic diagram of the internal structure is
shown in Figure 15.
The "Kabil Stack-Sack Wall" shown schematically in Figure 16 is also
a simply constructed system that is adaptable to remote sites. The facing
consists of vertical reinforcing bars over which sacks full of premixed
but just wetted concrete are placed by dropping the sack over the top
of the bar. Wire mesh or chain link fencing is used for horizontal re-
inforcement, as shown.
Reinforcing systems are also used to strengthen existing ground in
order to improve slope stability or to enable slope steepening on ex-
cavations without internal bracing or active anchor systems. Soil nailing
(Figure 17) consists of the insertion, by driving or drilling, of reinforce-
ments into a natural or cut slope. Grouting around the reinforcements
is often used to assure good bond between the soil and the reinforce-
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meets. "Root piles," consisting of small-diameter, cast-in-place concrete
piles containing a single reinforcing bar down the center are also used.
Root piles are commonly installed at different inclinations, as shown in
Figure 18.
APPLICATIONS IN TRANSPORTATION SYSTEMS
Earth reinforcement systems of various types have become extensively
used in transportation projects. Probably the two greatest uses are for
retaining walls and bridge abutments, where they compete very favor-
ably, economically and aesthetically, with reinforced concrete. A bridge
abutment application is shown in Figure 19. They are equally useful for
rail networks. The need to rehabilitate and expand existing road and
rail systems in urban areas portends increased use in rights-of-way pre-
sently containing earth slopes that will have to be steepened or removed
in order to provide the needed space. Any situation requiring an ele-
vation change of more than a few feet is potentially suitable for use of
an earth wall.
Earth walls have also been used for waterfront structures, e.g., quay
walls. Here the facing elements play an additional role, namely, erosion
protection for the soil behind them.
DESIGN CONSIDERATIONS
Earth walls are subject to the same external design criteria as those
for a conventional retaining wall. That is, they must be stable against
sliding due to the lateral pressure of the soil retained by the wall, they
must resist overturning, and there must be safety against foundation
failure. Classical methods of soil mechanics have been used for the
analyses necessary for this part of the design, and they have been sat-
isfactory.
The internal design of the wall itself must ensure against (1) failure
of reinforcements in tension, (2) pullout of reinforcements, and (3) loss
of reinforcements by corrosion or other forms of deterioration.
To ensure against the first two failure modes requires knowledge of
soil-reinforcement interactions. These depend, in turn, upon soil type,
reinforcement type and geometry, and the stress state of the soil. The
effective friction coefficient between the soil and reinforcements controls
the stress transfer between the materials.
Many analytical, numerical, model, and full-scale field experiments
have been done to provide information on which the selection of needed
parameters for design can be based. Sufficient information has been
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EARTH WALLS
165
obtained from these studies and from experience with existing walls that
safe designs are possible for normal loading conditions.
The soil used in the reinforced zone of permanent reinforced earth.
walls is required to be cohesionless so that it will be freely draining,
there will be a reasonable frictional strength, and there will not be
problems owing to large creep deformations, as might be the case if a
clay were used.
For the Vidal type of reinforced earth wall the distribution of tensile
stresses along reinforcements has been found to be about as shown in
Figure 20. The locus of maximum tensile stresses as a function of depth
is also shown. This geometry can be used with soil property and rein-
forcement data and suitable factors of safety to select the length, cross-
sectional area, and vertical and horizontal spacings of reinforcements.
A similar methodology can be used for other reinforcing systems.
A generally accepted method for the seismic design of earth walls has
not yet appeared. High-magnitude earthquakes were considered in the
design of the walls at Valdez (Figure 9) and for other walls in areas of
known seismicity. The design methodology led to a somewhat increased
length of reinforcements and a higher density of reinforcements in the
upper part of the wall. To the best of our knowledge, there have been
no failures or significant distress of earth walls due to earthquakes.
Reinforcement durability is probably the area of greatest concern at
present. The rate of corrosion of metal reinforcements depends on many
factors, most of which cannot be controlled over the long term in the
ground, e.g., local chemical concentrations and stray electrical currents.
Galvanized steel has been extensively used. The by-products of zinc
corrosion cover the base metal and tend to seal affected areas. A cor-
rosion loss over the design life of the wall is taken into account in the
specification of metal reinforcement cross sections. Epoxy-coated steel
reinforcements now being developed offer the potential for high dura-
bility.
Nonmetallic reinforcing materials such as geotextiles, fiberglass, plas-
tics, and composites, while not susceptible to corrosion, may undergo
other chemical and/or biological forms of deterioration. Unfortunately,
many of the materials are new, and the effects of long-term burial and
exposure to the elements are not known. Hence, durability emerges as
an area of major concern, and further studies are likely.
CONSTRUCTION
Generally the construction of earth walls is relatively simple, rapid,
and straightforward. Equipment for placing and spreading the backfill
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soil and a roller for compacting it are required. Reinforcements can
usually be carried and placed by hand, as shown in Figure 21. If precast
facing panels are used, a small crane is required for handling them
(Figure 22~. Other facing types, e.g., geotextiles, do not require equip-
ment for installation.
A significant advantage of reinforced earth walls over reinforced con-
crete walls is that no formwork is required. On the other hand, con-
struction of a reinforced earth wall requires considerably greater space
behind the wall face, as the reinforcement lengths are usually at least
0.7 times the wall height, whereas the base width of a concrete wall is
only about 0.3 times the wall height.
COSTS
The costs for reinforced earth walls today are less, in constant dollars,
than they were 8 to 10 years ago. This is because the technology is
maturing and because there is competition among earth reinforcement
systems. In spite of the widely different technologies, a parallel is evident
with the electronics industry.
The total cost in any case will be composed of the costs of materials,
erection, backfill soil (if the onsite soil is unsuitable), and any special
aesthetic treatment of the facing that may be required. A reasonable
figure for the cost of materials and erection for walls in the height range
of 10 to 15 ft is about $15/ft2. For walls of 15 to 30 ft in height the cost
increases to $17 or $18/ft2.
In urban areas reinforced concrete is likely to be more economical
than reinforced earth for low walls up to 10 ft in height. The two types
are competitive for wall heights of 10 to 30 ft. and the earth wall is less
expensive for heights greater than 30 ft.
THE FUTURE
A number of reinforcing systems have been developed for construc-
tion of earth walls. Some are best suited for permanent high walls, some
are best for low walls, some have their best application in remote areas,
and others must withstand the rigors of high-traffic-volume urban areas.
As experience broadens and as uncertainties concerning the durability
question disappear, it can be anticipated that earth reinforcement will
see even greater use in surface transportation networks. The current
competition between the manufacturers of different reinforcing mate-
rials and systems can lead to more efficient designs.
The rapid development and importance of the field has been recog-
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EARTH WALLS
167
nized at the federal level. Work is in progress on National Cooperative
Highway Research Project 24-2, Reinforcement of Earth Slopes and
Embankments, administered by the National Research Council's Trans-
portation Research Board. An international team of experts has been
assembled to evaluate and compare the major systems and methods.
The results of this study, due in late 1984, will be in the form of a report
providing the user with the information required to make the analyses,
designs, and estimates needed for the rational choice of an earth wall
system.
BIBLIOGRAPHY
Bassett, R.H., and N.C. Last, "Reinforcing Earth Below Footings and Embankments,"
in Proceedings of the Symposium on Reinforced Earth, American Society of Civil En-
gineers, Pittsburgh, Pa., April 1978, pp. 202-231.
Binquet, J., and K.L. Lee, Bearing Capacity of Strip Footings on Reinforced Earth Slabs,
G.I. 38983, Report to the National Science Foundation, Washington, D.C., May 1975.
Elias, V., and D.P. McKittrick, "Special Uses of Reinforced Earth in the United States,"
in Proceedings of the International Conference on Soil Reinforcement: Reinforced Earth
and Other Techniques, Paris, Vol. 1, March 1979, pp. 255-259.
Goughnour, R.D., and J.A. Dimaggio, "Application of Reinforced Earth in Highways
Throughout the United States," in Proceedings of the International Conference on Soil
Reinforcement: Reinforced Earth and Other Techniques, Paris, Vol. 1, March 1979, pp.
301-313.
Maluche, E., "Reinforced Earth Used as Supporting Structures in Hydraulic Engineer-
ing," in Proceedings of the International Conference on Soil Reinforcement: Reinforced
Earth and Other Techniques, Paris, Vol. 1, March 1979, pp. 335-339.
McKittrick, D.P., "Reinforced Earth: Application of Theory and Research to Practice,"
in Proceedings of the Symposium on Reinforcing and Stabilizing Techniques sponsored
by the New South Wales Institute of Technology, October 1978. (Available from the
Reinforced Earth Company, Rosslyn, Va.)
Mitchell, J.K., "Soil Improvement State-of-the-Art Report," in Proceedings of the Tenth
International Conference on Soil Mechanics and Foundation Engineering, Stockholm,
Vol. 4, June 1981, pp. 509-565.
Schlosser, F., Reinforced Earth, ISSN 0337-1565, Ministere de l'Equipement, Laboratoire
Central des Ponts et Chaussees, Paris, France, April 1976.
Vidal, H., "The Principle of Reinforced Earth," High w. Res. Rec., 282:1-16, 1969.
OCR for page 168
FIGURE 1 Dry sand with maximum stable slope.
TRANSPORTATION TECHNOLOGY
FIGURE 2 Moist sand can develop stable near-vertical slopes be-
cause of internal capillary water pressures.
k ~~ ~ ~~ it. ~ ~~ ~~ ~~ ~~ ~ ~~ ~ ~~ ~~ ~ ~ ~~.~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~ . ~ ~ _
: ~~ ~~:~:~'~
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169
FIGURE 3 Dry sand reinforced with strips of paper.
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FIGURE 4 (Top) Ziggurat of Ur in Mesopotamia, circa 2500 B.C. Reinforcement is
considered a major factor in its longevity. Photo Max Hirmer. (Bottom) Reconstructed
view of the Ziggurat. Photo John Freeman. Reprinted from M. E. L. Mallowan, Early
Mesopotamia and Iran, McGraw-Hill, New York, 1965, with permission.
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171
FIGURE 5 Corduroy road for crossing swampy areas.
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,. ~ . .
TRANSPORTATION TECHNOLOGY
F~GuRE 6 First highway use of modern reinforced earth wall in France between Nice
and the Italian border.
Dent
Thickness = t
' `~:
F~GuRE 7 Concept of the Vidal reinforced earth wall.
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173
F~GuRE 8 First reinforced earth wall constructed in the United States, along Highway
39 in the San Gabriel Mountains of Southern California.
F~GuRE 9 Reinforced earth walls used at Valdez, Alaska, in connection with the Alaska
Oil Pipeline Terminal.
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74
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FIGURE 10 Special facing panels for reinforced earth wall along Interstate Highway 70
through Vail Pass, Colorado.
a
i.~,..~.
FIGURE 11 VSL Retained Earth system.
in
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EARTH WALLS
~ ~ q Roadway
175
\ Li~ ' ~
Backing Mat Behind
9 Ga. 2" X 6", Typ.
FIGURE 12 Schematic diagram of Hilfiker welded wire wall.
comic d fit 11
Tensar Geogrid
reinforcement
-
Geogrids cemented
through brick faces to
act as ties.
/N
FIGURE 13 Tensar geogrids used to reinforce a brick retaining wall.
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FIGURE 14 Fabric-reinforced earth wall.
Typical overlap
12-36 in.
~ _ I
, _ ,
Paved surface
~ Fabric laver n
Wall height ( Fabric lever 2
<_ ~ Fabric layer 1
L Typically _'
~:;~
F~GuRE 15 Schematic diagram of fabric-reinforced earth wall.
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177
FIGURE 16 Schematic diagram of Kabil Stack-Sack Wall.
FIGURE 17 Soil nailing for improving cut slope stability.
. ~
r-,i
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~T~
"Reticulated Pall /~ ~' ~ ~ I
Radice" Structure
_ ~ surface
hi
Hi: ,; . ',,_
~ .
.
FIGURE 18 Root pile system for strengthening steep slope
FIGURE 19 Reinforced earth bridge abutment.
_ .
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EARTH WALLS
H/2
To
H
1' ~
H/2
~~ I
7777~73
7~/////
7777777
/////
~ / //~/
1. ~ ~ = 0.3 H 1
Resistant
Zone
///~
}it_
FIGURE 20 Stress distribution along reinforcement and locus of
maximum tensile stresses in a reinforced earth wall.
179
FIGURE 21 Reinforcement placement.
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FIGURE 22 Facing-panel installation.
Representative terms from entire chapter:
earth walls
