Geosynthetic Refoirced Soil
Geosynthetic Refoirced Soil
Geosynthetic Refoirced Soil
Abstract: The paper begins with a historical review of reinforced soil technology, beginning with nature and the ancients, the development
of Terre Arme or Reinforced Earth by Vidal and Lee, and ending with the early uses of geotextiles for soil reinforcement in France, Sweden
(Wager and Broms), and the United States (U.S. Forest Service, Federal Highway Administration, J. R. Bell, T. A. Haliburton, B. R.
Christopher, and others). Then the advantages and basic behavior of geosynthetic reinforced soil (GRS) are described. An overview of
current design procedures, modified with the results of analytical research conducted at the University of Washington, leads to practical
suggestions for dealing with creep, pullout, and backfill drainage. Next the properties of reinforcement geosynthetics are discussed, and
important conclusions of research on soilgeosynthetic interaction at the University of Washington are presented. Although GRS is quite
a mature development, a few technical and several professional issues remain. The technical issues are relatively straightforward but the
professional issues are not: they are costly, potentially tragic, and threaten a wonderful technology and our profession. Some suggestions as to
what the profession can do about these issues are given. DOI: 10.1061/(ASCE)GT.1943-5606.0001674. 2017 American Society of Civil
Engineers.
Author keywords: Soil reinforcement; Geosynthetics; Geosynthetic reinforced soils; Design; Properties; Technical and professional issues.
Fig. 4. Results of pullout tests on a woven multifilament polyester geotextile; the text and Holtz (1973, 1977) give an explanation of these figures
(reprinted from Holtz 1973, 1977, courtesy of the Swedish Geotechnical Institute and the Association Amicale des Ingnieurs Anciens lves
de lE.N.P.C)
Of course it was very strong, and we soon learned that it was vir- walls. Reinforced Earth has three primary components (Fig. 5):
tually impossible for model walls to collapse, even under large sur- select granular backfill, galvanized steel strips for reinforcement,
charge loadings. The walls would greatly deform, but they did not and precast concrete facing panel elements (Vidal 1966; Schlosser
collapse, and this was true even with very short reinforcing layers and Vidal 1969). Worldwide patents were granted in 1966, and by
(L=H 0.4 and 0.3). Walls 1.2 m high could support a surcharge the early 1970s the technology was well established in France and
equivalent to about 5 m of fill without collapse. We also performed several other countries.
a few Taylor-Schneebeli pin-model wall tests similar to those The first Reinforced Earth project in the United States was
shown by Schlosser and Vidal (1969), although at that time we for the repair of a large landslide in Southern California in 1972
were unaware of the French tests. The results of all these tests on Highway SR39 in the Angeles National Forest north of Los
are described by Holtz and Broms (1977, 1978). Angeles. The site, a schematic of the repair, and the nearly com-
pleted wall are shown in Fig. 6. The slide and its remediation are
described by Chang (1974), Chang et al. (1974), and Walkinshaw
Terre Arme and Reinforced Earth
(1975).
Some years ago, French architect Henri Vidal invented a system he At about the same time, Professor Kenneth Lee at the University
called Terre Arme (Reinforced Earth) for construction of retaining of California, Los Angeles, began research on Reinforced Earth
The objective was to review existing test methods and develop new
tests as well as specifications for various uses of geotextiles for
highways; I was one of a small group of peer reviewers for this
project. The first phase of the project was summarized by Bell and
Hicks (1980) in an official FHWA report. Due to a disagreement
Fig. 5. Components of Terre Arme (Reinforced Earth): select backfill, between Professor Bell and the FHWA contract monitor, the final
galvanized steel strips for reinforcement, and precast concrete facing project report was never officially published; only a draft report
panels (courtesy of the Reinforced Earth Company) exists (Bell and Hicks 1982).
In the late 1970s, geosynthetics design and construction prac-
tice was not well developed, and specifications for geotextile ap-
plications were not generic or based on sound engineering research
sponsored by the National Science Foundation. Figs. 79 show
or experience. The FHWA realized that to improve practice, state
some of the small-scale model tests using metallic strip reinforce-
highway engineers would need formal training in geotextile
ment that Lee and his students performed. Fig. 7 is a top view of
engineering. The first FHWA geosynthetics courses were devel-
one of the tests that has failed, but the failure was restrained; a scarp
oped and taught by Professor T. Allen Haliburton of Oklahoma
is visible a few centimeters back of the wall face. Fig. 8 is a side
State University, who unfortunately passed away after only a few
view of that same model test; the wall has moved about 50 mm
courses were taught. A second contract was awarded to Dr. Barry
from the black line, which was the location of the wall face before
failure. Failure planes are clearly indicated by the bands of colored Christopher and me, and we updated Haliburtons draft course ma-
sand, and those failure planes are inclined at very close to the terials (Haliburton et al. 1981) into the Geotextile Engineering
Rankine theoretical failure plane angle for the sand. In the next Manual (Christopher and Holtz 1985). We began teaching 3-day
photograph, Fig. 9, the model has been allowed to collapse because training courses based on the manual and our own experience in
it was not restrained. Does this happen in the field? Yes, as shown practice, laboratory testing, and research. Since then, nearly 200
in Fig. 10, a failure that occurred on Interstate Highway 24, east of courses have been taught in most states (and several foreign coun-
Nashville, Tennessee. A description and analysis of the failure are tries), and it is fair to say that this educational effort by FHWA
reported by Lee et al. (1994). has significantly improved geosynthetics design and construction
The potential for corrosion of steel reinforcement was a practice and specifications for highway construction in the United
common concern in the early days of Reinforced Earth. Often at the States. Our Geotextile Engineering Manual was condensed into the
end of presentations about Reinforced Earth, speakers were asked, FHWA Geotextile Design and Construction Guidelines in 1989
Well, what about corrosion? And the answer always was: The (Christopher and Holtz 1989). The Guidelines were substantially
steel reinforcing strips are galvanized to take care of any potential revised and updated in 1996 (Holtz et al. 1996) and published as
for corrosion. Still, predicting corrosion of even galvanized steel in a textbook the following year (Holtz et al. 1997). Its latest version is
soils is often problematic, and we thought that plastics were virtu- Holtz et al. (2008).
ally indestructible. Thus we considered anything made of synthetic Finally, I would like to mention some early research and devel-
polymeric fibers was also likely to be virtually indestructible and opment work on geosynthetic reinforcement conducted at Purdue
not subject to corrosion. At least that was what we thought in the University, 19731988, work sponsored by the National Science
1970s. Foundation, Indiana Department of Highways, U.S. Air Force,
Netlon, and Gulf Canada, among others. Much of this research was
reported in ASCE conferences, the Second International Conference
Geotextile Reinforced Walls, 19711977 on Geotextiles, other geosynthetics conferences, journals, and major
It is interesting that in the home of Terre Arme, French engineers reports to sponsors. Specific subjects include pullout of GRS walls
were also building fairly large-scale model walls reinforced with (Salomone et al. 1979, 1980), reinforced embankments (Boutrup
geosynthetics. Fig. 11 shows two experimental walls reinforced and Holtz 1982, 1983; Humphrey and Holtz 1986a, b, 1987, 1989;
with a heavyweight (300 g=m2 ) nonwoven needle-punched poly- Humphrey et al. 1986), laboratory creep tests (Holtz et al. 1982),
ester geotextile that were constructed in 19711972 (Puig and and GRS for roads and airfields (Bourdeau et al. 1982; Holtz and
Blivet 1973; Puig et al. 1977). Besides the small dam spillway con- Harr 1983). There were also two state-of-the-art reports on geosyn-
structed in 1976 (Kern 1977), these two experimental walls were thetic reinforcement (Holtz 1978, 1982).
the only GRS walls constructed in France in the 1970s, apparently
due to the dominance of Terre Arme.
Also in the early 1970s, U.S. Forest Service engineers in the Advantages and Basic Behavior of GRS
Pacific Northwest, working with Professor J. R. Bell of Oregon
State University, built several walls reinforced with lightweight There are a number of advantages of reinforcing soils, both with
nonwoven geotextiles for logging and fire protection roads in the steel and geosynthetics. First is cost, as shown in Fig. 13. When
Fig. 6. California Highway SR39 landslide repair (reprinted from Walkinshaw 1975, courtesy of FHWA): (a) the slide; (b) cross section of the
remediation showing the Reinforced Earth wall at the top of the slope; (c) the wall near completion
the height of the structure exceeds some 1012 m, there is virtually shown, for the same calculated factor of safety, there is a lower
no competition with conventional retaining walls. Other advantages probability of failure, i.e., greater reliability, for a reinforced steeper
include flexibility of reinforced soil structures. They are very toler- slope than an unreinforced flatter slope. One wonders why do we
ant of large settlements, and this means cheaper foundations on poor still design and construct unreinforced soil slopes?
soils. Construction is simple and rapid, and it is easy to change the The next few figures illustrate an important fact about GRS struc-
alignment and height of the structures. Because they are so flexible, tures. First, Fig. 14 illustrates that the stress exerted by the soils at
they can readily absorb seismic energy and therefore are safer than the face of the structure is very small, in fact it is zero! Now the
conventional structures in earthquake country. Attractive facing sys- thickness of the spacing of the reinforcing layers is the same as the
tems are possible, including green facing systems incorporating size of the particles, so in a way it is a bit artificial. But that is not
plantings with vines and shrubs. Such facing systems are of course the case in Figs. 15(a and b). These photographs illustrate that the
important because of sustainability and environmental concerns. facing system is only there to restrain the soil particles at the face
Steeper slopes are possible with reinforcing than are possible between each reinforcing layer.
without reinforcing. For example, compacted cohesive soils can Mr. Robert Barrett of Colorado has shown in Fig. 16 that it is
be constructed safely steeper than 2:1. Granular soil slopes can be possible to build a very impressive GRS structure with only bed-
steeper than the angle of repose. Finally, there is increased safety sheets (from Walmart). The slope angle of the face of the wall is
with reinforced slopes. As Cheng and Christopher (1991) have clearly much greater than angle of repose of the sandy backfill
Fig. 7. (Color) Top view of Lees model Reinforced Earth wall Fig. 9. (Color) Model wall allowed to collapse (image courtesy of
(image courtesy of Professor Kenneth Lee) Professor Kenneth Lee)
material near the bottom. The soils in the backfill have some silt
and capillary moisture to provide apparent cohesion sufficient to
support the backfill within the vertical spacing of the reinforcing
layers.
In conclusion, stress at the face of a GRS wall or steep slope is
very small and local, and therefore the facing is only necessary to
support the soils between reinforcement layers. A heavy structural
facing element is unnecessary, unless the reinforcing spacing is
large. On the other hand, Professor Fumio Tatsuoka (personal com-
munication, 2010) has pointed out that the heavy structural type
facing systems common in Japan for their GRS structures have
Fig. 11. Walls reinforced with a geotextile (reprinted from Puig
performed well in earthquakes. I am not sure a lighter and more
and Blivet 1973, courtesy of Bulletin des Liaison des Laboratoires
flexible facing would not perform just as well, but it is difficult
RoutiersPonts et Chausses)
to argue with strong empirical evidence.
Fig. 12. One of the first geotextile reinforced walls built by the U.S.
Forest Service in the Siskiyou National Forest, Oregon (image courtesy
of Bell et al. 1975)
Fig. 15. (Color) Two large-scale GRS structures [image (a) courtesy of
R. R. Berg; image (b) courtesy of T. M. Allen]
Design
First let us summarize the steps normally followed for the design of
both GRS walls and slopes (Holtz et al. 1997, 2008).
For GRS walls
1. Establish project scope, geometry, loading conditions
(e.g., permanent, transient, seismic), facing system, perfor-
mance requirements, and construction constraints, if any;
2. Determine foundation soil properties;
3. Determine backfill (reinforced and retained) soil properties;
4. Establish required safety factors or load and resistance factors,
as appropriate;
5. Determine preliminary wall dimensions [height(s), embed-
ment, reinforcement length];
6. Evaluate external stability (sliding, bearing capacity, overturn-
Fig. 14. (Color) Model of a GRS structure (image courtesy of B. R.
ing, overall or global slope stability, and seismic stability if
Christopher)
appropriate);
(1997) have one chapter on GRS slopes and another on GRS walls, the Mohr circle at failure cannot be above the Mohr failure
which is the traditional approach to the design of these structures. envelope.
Even Koerner (2012) does the same thing in effect; he has separate So why is the measured K h often so much less than K a ? There
sections on GRS slopes and GRS walls in his chapters on design are several possible reasons, and all are likely to contribute to this
with geotextiles and geogrids. So what is really going on? discrepancy:
(b)
Fig. 21. Two cases of a sand slope: (a) slope angle less than the angle
of repose; (b) slope angle slightly greater than the angle of repose
0 20 40 60 80 100 120
(a) Face Deflection (mm)
8.0
Strain (%)
Fig. 23. (Color) Selected results from Lee (2000) (reprinted from Lee 2000, with permission): (a) face deflections of Wall 1; (b) strains in the
reinforcement in Wall 3 with a 50-kPa surcharge
for example, by the flow nets in Fig. 25 from Terzaghi (1943) and RMC by Professor Richard Bathurst and his students, and on addi-
Taylor (1948). And we know what is good GRS drainage practice; tional work by T. M. Allen of WSDOT, the primary research spon-
one example is Fig. 26 from Sandri (2005), which addresses both sor. It is a purely empirical method based on many case histories of
surface and subsurface drainage. Of course, designs such as this are walls reinforced with both steel and geosynthetics. One of its great
expensive, but so is failure. We, the profession, have no excuses for advantages over current design practice is that it is independent of
not using good drainage practice, because we do know better. reinforcing materialit does not matter if the reinforcement is met-
allic or geosynthetics. The K-stiffness method enables a more ac-
curate estimate of reinforcement loads, and a step-by-step design
Other Approaches to GRS Analysis and Design procedure is available with a limit states design approach that is
Two other approaches to GRS analysis and design should be consistent with current design codes (i.e., LRFD). For details, see
mentioned; they are the composite material approach and the Allen et al. (2003, 2004); some recent refinements are described by
K-stiffness method. Both offer significant potential improvement Allen and Bathurst (2015).
to current design practice.
The composite material approach is based on the concept of
homogenization, which can greatly simplify numerical analyses of GRS Design Recommendations
GRS and other reinforced soil structures. Accurately modeling There is nothing wrong with traditional limiting equilibrium de-
the constitutive behavior of these materials is quite complex be- sign methods as long as it is recognized these methods are ultra
cause of significant differences in geometry and engineering prop- conservative. If you want to use them, then you should
erties of the soil and reinforcement as well as their high degree 1. Use the correct soil properties for overburden stress h and soil
of anisotropy. Early work on this was done at the University of friction angle 0 , preferably determined in plane strain and at
California, Davis, (Chang 1974; Romstad et al. 1976; Shen et al. the correct confining pressure. This is not as easy as it sounds.
1976), and more recently by Lee et al. (2007) at UW. First, there are not many plane strain devices available, and it is
The K-stiffness method was developed from some of Lees difficult to conduct triaxial and plane strain tests at low confin-
(2000) analytical results, experimental research conducted at the ing pressures. If you want to do advanced numerical modeling,
LEGEND
0.600
24-Aug-06 8:41
step 4120521
Flow Time 1.0200E+07
-4.698E+00 <x< 1.749E+01
-1.216E+01 <y< 1.004E+01
Gravel 0.200
Y-displacement contours
base layer
-2.00E-01
-1.75E-01
-1.50E-01
-1.25E-01
-1.00E-01 -0.200
-7.50E-02
Downloaded from ascelibrary.org by University of California, San Diego on 05/19/17. Copyright ASCE. For personal use only; all rights reserved.
-5.00E-02
-2.50E-02
0.00E+00
-0.600
Contour interval= 2.50E-02
Boundary plot
0 5E 0
Cable plot
-1.000
University of Washington
Seattle
-0.200 0.200 0.600 1.000 1.400
(*10^1)
(a)
0.00
0 2 4 6 8 10 12 14
-0.05
Set t lement ( m)
3.2 m (calculated)
-0.10
3.2 m (measured)
5.5 m (calculated)
-0.15
5.5 m (measured)
-0.20
-0.25
Distance from the facing (m )
(b)
Fig. 24. (Color) Settlement predictions by Saidin (2007) (reprinted from Saidin 2007, with permission): (a) FLAC settlement distribution;
(b) settlement prediction compared with field measurements
e.g., with FLAC, and you want the correct answer, you must use (2) fewer thicker layers of higher strength geosynthetics re-
the correct dilatancy angle. Again, this is not easy to determine quired for stability. The advantages and disadvantages of each
in plane strain and at the correct confining pressure. choice are shown in Table 2. I recommend the first choice
2. For the internal stability of steep GRS slopes, I think they can many thin layers of weaker reinforcing. It is less expensive, and
be designed as a very steep slope. As the slope angle increases, improved compaction and better face control are obtained. Note,
more or stronger reinforcing is required. Use some of the com- too, that thinner lifts provide improved quality assurance for the
puter programs mentioned in Computer Programs for GRS compaction process.
Design and pay attention to their caveats. Although I have not 4. Pullout of the geosynthetic reinforcement is not a problem
tried this, it is possible that conventional soil nailing or tieback based on our research in Sweden described in Experience in
design procedures could be used with appropriate adjustments Sweden, 19701975. If there is any reasonable amount of
for geometry and reinforcement properties. backfill on it, the geosynthetic will rupture before it pulls out
3. For spacing of the reinforcement, the designer has two choices: of the backfill. Sometimes, however, there is a problem with
use (1) many thin layers of lower strength geosynthetics; or large unexpected surcharges applied to the face of GRS walls.
Fig. 26. Good drainage design (reprinted from Sandri 2005, with permission)
As I have shown above, in the past 3040 years, GRS has devel-
oped from experimental, low-risk, and modest projects into a ma-
ture and rather common technology. However, there are a couple
of technical issues that remain:
1. We need a simple, what might be called a poor persons, plane
strain test device, preferably one that can also provide measure-
ments of volume changes during shear. This test would allow
designers to easily determine the plane strain friction and
dilation angles of their proposed backfill soils.
2. Although seismic design of GRS structures was not mentioned
in this paper, it is still an important issue in earthquake country.
Fig. 31. UCD stressstrain results on the Rainier Avenue sand at a Consequently, we need a seismic design procedure better than
confining pressure of 10 kPa; sand alone (NONE), two nonwoven geo- the old-fashioned Mononobe-Okabe pseudostatic analysis, even
textiles (NW1 and NW2), four woven geotextiles (PP1, PP2, PP3, and though we know GRS walls and slopes are much safer than
PET1), and stainless steel sheet (SS) (reprinted from Boyle 1995a, with conventional ones in earthquakes (e.g., White and Holtz 1996).
permission) Important progress was made by Anderson et al. (2008) using
the generalized limiting equilibrium approach, but important
limitations such as the flexibility of GRS structures and eva-
luation of deformations remain. My colleague at the Univer-
Southwest Research Institute. Also, Professors Robert Koerner sity of Washington, Professor Steven Kramer, suggests that
and Grace Hsuan of the Geosynthetics Research Institute at Drexel performance-based earthquake engineering (PBEE) is the ap-
University are experts on creep of geosynthetics (e.g., Koerner propriate approach, and because he wrote the book on geotech-
et al. 2006). nical earthquake engineering (Kramer 1996), I believe him.
Professor Jonathan Fannin (personal communication, 2007) has Besides these two technical issues, there are several professional
demonstrated that if one uses the very elegant British Standard issues that pose a serious threat to this wonderful technology. First
8006 (BSI 2010) to extrapolate 10,000-h creep data to 1,000,000 h there are simply too many failures! Koerner and Soong (1999,
(approximately 120 years, a common design life for public works), 2001) have studied failures of GRS structures, and they concluded
the allowable tensile strength determined is very close to the value that about 1% of all GRS structures have serious problems or have
obtained if the AASHTO (2002, 2014) default creep reduction failed. Most are due to
factor is used to reduce the T ult . Poor-quality backfills;
Finally, it is strongly recommended that fine-grained backfill Poor drainage or saturated backfills;
material should be avoided, especially for GRS walls and very Construction problems;
steep slopes. If this is not possible, then the interaction properties Inadequate global or external stability;
of the soil and geosynthetic should be determined using samples of Unexpected surcharges;
the proposed backfill. Also, the drainage designs recommended in Lack of proper inspection;
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