Reinforced soil - a method that deserves improvement
Mensur Mulabdić, Dejan Mračkovski, Tomislav Glavaš
Faculty of Civil Engineering
University J.J. Strossmayer, Osijek, Croatia
e-mail: leta@zg.tel.hr
Key words: soil, reinforcement, pullout, test, geosynthetic
Abstract: Reinforced soil is analysed as a method of soil improvement, in regard of present
use in geotechnical and road engineering. Basic testing methods used for determination of
soil-geosynthetic interaction are compared (direct shear versus pullout) and analysed in
terms of mechanisms that determine interaction. The efficiency of geogrids compared to
geotextiles is discussed, and some basic presently used theoretical expressions for bearing of
ribs in geogrid are presented. It is demonstrated that gegrids are superior to geotextiles in
reinforcing soil. The need for better understanding of the interaction mechanism is stressed
and elaborated. Some aspects of current research at the Civil Engineering Faculty of
University in Osijek are presented.
1
Introduction
Geosynthetics are extensively used in civil engineering. Geotechnical engineering profited
most of different applications that comprise filtration, drainage, separation, reinforcement,
impermeability, erosion protection and water protection in tunnelling (see Fig. 1). Research
results and practical experience in using geosynthetics led to the rules for practical design
solutions in regard to application of different geosynthetic to geotechnical problems. Still,
research is needed to better clarify and model soil-geosinthetic interaction. This need is
further driven by the very intensive product development and demands for economical
solutions, not only in the sense of the cost reduction. Today engineers have to respect
demands for use of by products and "lower quality" materials trying to spare good materials
(ex. gravel). This growing demand is a constant in the policy of environmental protection.
Use of unfavourable materials is possible due to soil improvement brought by geosinthetics,
so that composite material (soil with geosynthetics) has better properties and more suitable
engineering behaviour than the soil itself.
Fig.1. Some examples of use of geosynthetics in geotechnical engineering
1
The question of durability of geosinthetics is well understood and many test methods are
developed for determination of long-term properties of geosynthetics. The special interest is
in determining decay of tensile strength, the crucial characteristic for soil reinforced structures
(Fig.2 for explanation of the relevant terms and requirements). Today it is possible to
determine geosynthetic durability in respect to structure life time, which is very important in
the sense of factor of safety in terms of time and reserve in functional property (CR ISO
13434:1998). Few years ago new technique called SIM (Stepped Isothermal Method) was
developed (GRI GS 10:2000). By using this test it is possible to detect creep effects and
tensile strength reduction for a very long period (hundred years) in a very short time (few
weeks). This is a tremendous improvement compared to classical testing of creep behaviour
under different temperatures and stresses, where only one time-log-cycle could be used for
extrapolation of laboratory results (that lasted a year or more) in order to have idea of
geosynthetic tensile strength in twenty or thirty years time, which is often shorter then is the
structure life time.
Fig.2. Schematic illustration of the factor of safety in regard to required geosynthetic property in time
(according to CR ISO 13434:1998)
2
Soil - geosynthetic interaction
It is well known that reinforcement in the soil improves soil properties. Also, we usually think
of that composite material as the one that gets higher shear strength due to interaction effect
with geosynthetic. On the laboratory specimen of reinforced soil exposed to (unconfined)
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compression friction developing on the contact area between soil and geosynthetic under
vertical stress acts in turn on the soil as an (increase in) confining stress, so that soil can
sustain higher load without failure. This can be described formally in terms of Mohr-Coulomb
criteria in the way that the reinforced soil has higher cohesion intercept and the same friction
angle compared to unreinforced soil.
The model for soil reinforcement in slope stability problems is presented on the Fig.3.
Reinforcement acts as a tensile element for sliding area (anchorage). The length of the
reinforcement outside failure zone must be long enough to absorb corresponding transferred
load, by friction developed between soil and geosynthetic. Explanation of the mechanism of
that interaction between soil and geosynthetic is of engineering interest. It can be shown that
this mechanism consists of three parts: friction between soil and geosynthetic, shear through
the soil (in-between ribs in case of geogrids) and passive resistance in front of ribs (for
geogrids). Following this explanation one can conclude that geotextiles are inferior to
geogrids in terms of the tensile force they can take in contact with soil, since friction angle
between geosynthetic and soil is always lower than soil friction angle. Many researchers
found that the majority of the resistance (almost 80%) to pullout from the soil of the grid
reinforcement comes from resistance of ribs perpendicular to direction of the force.
FAILURE
SURFACE
C
A
B
REINFORCEMENT
Fig.3. Reinforcement acts in the active zone under failure and outside of it
If the failure line in reinforced slope follows the plane of reinforcement (point A, Fig.3.) then
direct shear is responsible for resistance in that part of the failure surface. On the other hand,
if the failure line crosses reinforcement pullout resistance is responsible for transfer of load
into the soil (point B). At point C both bending and pullout occur. These two mechanisms
(direct shear and pullout resistance) are usually tested in laboratory. Pullout testing device is
shown in Fig.4. Dimensions are different in different countries.
Many researchers tried to define mathematical expression for the pullout resistance coming
from the ribs perpendicular to force direction. Jewell (1990.) states that maximal value of the
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pullout force corresponds to full friction in the soil at the same vertical stress. He uses socalled bond coefficient to characterise interaction between soil and reinforcement:
Pp  2 LrWr n ' f b tan 
(1)
where fb is bond coefficient, as a measure of apparent friction at 'n. When all the force in
pullout would be considered as friction on contact soil-geosynthetic the part fb tan would be
friction coefficient. This is valid if the pullout resistance is understood as if the cloud of the
soil is pulled out together with the reinforcement, and friction on his surface is characterise by
tan.
Fig. 4. Pullout testing equipment - basic assembly (according to Farrag at all, 1993.)
After arranging for the geometry and contact areas it can be written:
 tan  
 B   '  1
   b   n 
f b   s 
 S   b '  2 tan 
 tan  
4
(2)
where s is coefficient determining part of the contact surface available for the friction, and
b is coefficient determining part of the ribs surface available for bearing (see Fig.5).
Pullout resistance generated from bearing capacity of the ribs is dominant, and very often
pure friction for geogrids could be neglected.
Jewell (1990.) comments results presented by Palmeira & Milligan (1989.) as:





model of bearing capacity at ribs changes from general one to puncture one
(see Fig. 6) when B/D50 becomes greater then 7.5
as B/D50 goes over 10 failure deformation increases
S/B ratio influences fb value (bond coefficient)
fb value can not be over one
ribs behind the first one have lower bearing capacity which determines
degree of interference (ratio between bearing of all n ribs and n x bearing of
the first rib)
Fig. 5. Notation for the geogrid elements important in pullout
Bearing capacity of the rib can be expressed according to Jewell (1990.) as:
 b '  c' N c   n ' N q
(3)
Jewell (1984.) gives the expression for the case of general failure in granular soils in terms of
the following ratio (claimed to be lower value):
5
b'
  ( 2  )tg
Nq 
 tg (  )  e
n'
4 2
(4)
This value is for round ribs, and for square section ribs one should multiply this value by 1.2.
Upper value is defined for the puncture failure (characteristic for deep foundation):
Nq 
b'
 '
 tg 2 (  )  etg '
n'
4 2
(5)
Many measured values fall in-between these two curves.
Improvement of the theory can be expected only if assumptions in equation (1) are changed
(fb is not limited to one) or in change of expression for ’b / ’n (Jewell, 1990.).
This expression (1) is analogue to the direct shear expression, with only difference in
coefficient: bond coefficient for pullout versus coefficient of friction between soil and
geosynthetic in direct shear. Some authors also express pullout force in this way. Authors of
this article think that expression (1) is influenced by tradition coming from geotextiles (for
which only friction exists) and lack of better description of soil-geosynthetic interaction.
However, this expression enables direct comparison of the pullout mechanism for geotextiles
and geogrids: ratio of bond coefficient to friction coefficient in soil (tan) should be much
higher than ratio of friction coefficient for soil to geosynthetic (tan) to coefficient in soil
(tan). This is indication of the mechanism of interaction between soil and geosynthetic.
Indeed, literature review shows that first ratio (for geogrids) is well above one (1.2 - 1.9) and
second one (for geotextiles) is about 0.8. These numbers are influenced by the type of the
geosynthetic and type of soil, as well as by other factors reflecting geometry of the geogrid
compared to grain size distribution (Palmeira and Milligan, 1989.) and shape and hardness of
the soil particles for noncohesive soils.
Some of the authors modelled rib resistance similar to shallow foundation failure, see Fig. 6.
These models describe relatively well the results of the laboratory tests. They are developed
for the inextensive geogrids.
general failure
puncture
failure
modified
puncture
failure
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Fig.6. Models for description of resistance to pullout from ribs perpendicular to force direction (from
Bergado at all, 1994)
3
Soil improvement
It is widely accepted that interlocking effect is extremely important for the pullout resistance
for granular soils. However, there is no technique for direct measurement of it neither
parameters representing that effect in the expressions for pullout force. Jewell (1990.) stated
that improvement in the theory could be achieved only by change in the expression (1) what
can be accomplished if the value of the angle of friction in the soil can be changed. Author
think that angle of friction is different at different distances from the reinforcement plane up,
and that special testing techniques are needed to better describe this phenomena. On the other
hand, there is no good theory to predict deformations of the reinforced soil, which is
necessary for the ultimate state of serviceability, according to Eurocode 7.
4
New research
According to most design rules for calculation of necessary reinforcement in the slope
stability problems (embankments, cuts etc.) the total force to be supplied by the reinforcement
is determined for equilibrium with sufficient safety for the cross section of the reinforced soil.
Based on this force and available (factored) tensile strength of the chosen reinforcement it is
possible to determine number of reinforcing layers required in the cross section. Some codes
use smaller distances among reinforcement at the toe than at the crest. Two wedge method is
very much used in calculating those forces (used in British code). This artificial decision
regarding vertical distances of the reinforcement takes no account of the real interaction of the
soil and reinforcement.
Reinforcing of subbase (granular) layers in road construction is a matter of intensive study in
recent years. The reduced thickness of these layers when reinforced compared to
nonrfeinforced is obtained by design diagrams based on CBR values. The general problem in
prediction of long-term road behaviour in terms of deformation is not solved, although many
numerical models are developed but not yet calibrated enough for real situations. Use of
resilient modulus for this purpose should improve calculation of expected deformations. Here
comes reinforcement, when present, since it changes (improves) soil characteristics, but yet in
unknown way. Higher shear stiffness means lower deformations. This phenomena remains to
be further investigated.
Some researchers report that it is better to use stronger reinforcement at larger distances than
less strong reinforcement at smaller distances, due to interference of reinforcement when put
close to each other.
In Europe recently started new research project in the form of COST 348 action
("Reinforcement of Pavements with Steel Meshes and Geosynthetics") dealing with
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reinforced soil) with the task to analyse present state of practice and research and offer proper
methods for analysis of the reinforced soil.
Mulabdić at all (1997.) showed that there is change in soil rigidity going from reinforcement
level upwards, based on measurement in situ, using SASW technique (Spectral Analysis of
Surface Waves). In the research project currently running at the Civil Engineering Faculty in
Osijek (principal investigator is the first author of this article) the working hypothesis is that
both, internal friction angle and soil stiffness change with distance from reinforcement grid
level for granular materials, due to interlocking, as Fig. 7. suggests. The nature of this change
depends on physical, mechanical and geometrical properties of the soil and grid. The project
will test in details those relationships in the very big pullout test, specially instrumented for
this purpose. Also, influence of technology of installation of geogrids will be tested, in regard
to creation of the interlocking effects.
vertical distance from the reinforcement
dds=?
ds
max
0
G, 
Fig. 7. Expected change of G and  in granular soil around grid (ds=direct shear)
Direct shear in soil will be performed as well in order to assure parameters for modelling of
the composite material behaviour. This might offer the possibility to select proper geogrids
for different soil types.
Having measured stiffness and angle of internal friction in the soil it will be possible to
calculate in a better way deformations of the composite material, using advanced soil
constitutive models. Many researchers agree that pullout test is suitable for investigation of
the soil-reinforcement interaction. Palmeira and Milligan (1989.) showed that some tests had
higher bond coefficient than soil internal friction angle would give.
This implies that internal friction angle is different, and that statement by Jewell (1990.) in eq.
(1) might be questioned: fb might be greater than one, if proper angle of soil friction is used
(see Fig.7). Most of the research in pullout was done on sand and fine material, and are not
directly applicable to coarser materials. Real challenge will be to test gravels in the pullout
test since interlocking effects are significant and those materials are very often used in road
and railway construction, where better knowledge is very much welcomed. There is urgent
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need also in detecting form of the failure surface around grid, what could help in better
numerical modelling and choosing proper vertical distance among reinforcement.
5
Conclusion
The paper points out the significance of use of reinforced soils in today's geotechnical
practice. Efficiency in terms of construction time, economy and technical advantages of the
reinforced soil advocate more frequent us of this technology. This is especially important in
cases when we have to preserve natural resources of gravel and use weaker material for
embankment construction.
Some of the aspects related to calculation of reinforcement and detection of soilreinforcement mechanism are discussed. It is stressed that more sound understanding of these
mechanisms is needed. Pullout test proved as a best available commercial and research tool
for studying such phenomena, when properly instrumented and conducted.
In regard of the newer research projects some leading ideas from the current research
undertaken at the Civil Engineering Faculty University of Osijek were presented. The big
pullout device instrumented for measurement of the reinforced soil properties will enable new
insight into the interaction mechanism of the soil and reinforcement. This will, in turn, help in
more rational design of the reinforced soil structures.
References
1
CEN REPORT CR ISO 13434:1998 – Guidelines on durability of geotextiles and
geotextiles-related products
2
GRI TEST METHOD GS10:2000 – Standard Test Method for Accelerated Tensile
Creep and Creep-Rupture of Geosynthetics Material Based on Time-Temperature
Superposition Using the Stepped Isothermal Method, Geosynthetic Research Institute
3
Jewell, R.A. (1990): “Reinforcement bond capacity”, Geotechnique 40, No. 3, pp.
513- 518.
Farag, K., Acar, B. Y. (1993): “Pull-Out Resistance of Geogrid Reinforcement”
Geotextiles and Geomembranes, Volume 12, pp. 133-159.
4
5
7
Palmeira, E.M., Milligan, G.W.E, (1989): “Scale and other factors affecting the
results of pull-out tests of grids burried in sand”, Geotechnique 39, No. 3, pp. 511524.
Bergado, D.T. and Chai, J. (1994): “Pull-out Force/Displacement Relationship of
Extensible Grid Reinforcements.” Geotextiles and Geomembranes, Vol. 13, No. 5, pp.
295-316.
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8
Mulabdić, M., Szavits-Nossan, A. & Kovačević, M.S (1999). Model testing of
geosynthetics in reinforced soil, XII ECSMGE , Amsterdam , F.B.J.Barendts at all
(editors), Proceedings of the XII ECSMGE, Vol 2, pp 817-822, Balkema, Rotterdam,
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