Usage of geogrids in flexible pavement

Seminar report 2019-20 Usage of geogrids in flexible pavement
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1.1 INRODUCTION
The majority of roads in the world are ﬂexible pavements. At some point of time, this
type of pavement may suffer from different distress types such as rutting and fatigue cracking.
One of the major problems faced by the engineers in highway construction in plains and coastal
areas of India is the presence of soft/ loose soil at ground level. Roads constructed over this
loose soil demands higher thickness of granular materials resulting in the high cost of
construction. Alternately attempts of reducing the thickness of pavement layer to make an
economic construction will lead to early damage to the pavement which in turn will make the
road unserviceable within a short period after construction. This condition may be further
worsened if supplemented with poor drainage or lack of it. Some states of India is situated in a
region of high rainfall area suffers from poor drainage as well as weak sub grade condition. This
is one of the major causes of deplorable road condition in those states.
Looking at the poor road condition of some states of India use of geogrid is thought for
road construction to improve the performance of roads. Geogrid a geosynthetic manufactured
from polymers is selected for this purpose.Geogrids used within a pavement system performs
two of the primary functions of Geosynthetics: separation and reinforcements. Due to the large
aperture size associated with most commercial geogrid products, geogrids are typically not used
for achieving separation of dissimilar material. The ability of a geogrid to separate two materials
is a function of the gradations of the two materials and is generally outside the specifications for
typical pavement materials. However, geogrids can theoretically provide some measure of
separation, albeit limited. For this reason, separation is a secondary function of geogrids used in
pavements. The primary function of geogrids used pavements in reinforcement, in which the
geogrid mechanically improves the engineering properties of the pavement system. The
reinforcement mechanisms associated with geogrids.

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2.1 MATERIALS USED
2.1.1 Soil
The sub grade material is a silty sand. Table 1 provides a summary of the subgrade soil
properties. The soil is classiﬁed as SW-SM, which is a well-graded silty sand in accordance with
ASTM D2487 and in accordance with AASHTO M145. A set of laboratory unsoaked California
bearing ratio (CBR) tests (ASTM D1188) are performed on the soil at different geogrid
positions. When CBR increases the thickness of the pavement will be decreases and it results in
the lesser cost of construction.
Table 1: Engineering Properties of Soil
(Source:EJGE, Improvement of Flexible Pavement With Use of Geogrid)
2.1.2 Geogrids
A geogrid is geosynthetic material used to reinforce soils and similar materials. Geogrids
are commonly used to reinforce retaining wall as well as sub bases or subsoil’s below roads or
structures. Soils pull apart under tension. Compared to soil, geogrids are strong in tension.
Geogrids are commonly made of polymer materials, such as polyester, polyvinyl alcohol,
polyethylene or polypropylene. They may be woven or knitted from yarns, heat-welded from

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strips of material or produced by punching a regular pattern of holes in sheets of material, then
stretched into a grid.
Table 2: Tested index properties of the geogrid
(Source: ARPN Journal of Engineering and Applied Sciences)
Fig.1: Geogrids
(Source: https://geosynthetic.net)

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2.1.2.1 Types of geogrids
Based on the manufacturing process involved in geogrids it can be of
1. Extruded Geogrid
2. Woven Geogrid
3. Bonded Geogrid
Based on which direction the stretching is done during manufacture, geogrids are classified as,
1. Uniaxial geogrids
2. Biaxial Geogrids
3. Triaxial geogrids
1. Uniaxial Geogrids
These geogrids are formed by the stretching of ribs in the longitudinal direction. So, in
this case, the material possesses high tensile strength in the longitudinal direction than on the
transverse direction.
2. Biaxial Geogrids
Here during the punching of polymer sheets, the stretching is done in both directions. Hence the
function of tensile strength is equally given to both transverse and longitudinal direction.
Fig 2. Images of biaxial triaxial and uniaxial geogrid
(Source: https://happho.com)

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2.1.2.2 Functions and working of geogrids
The purpose of geogrid is to retain or obtain the aggregates collectively. By using this
method of interlacing the aggregates facilitate in an earthwork that is settled mechanically. The
apertures in geogrid allow interlacing the aggregates or the soil arranged over them. It offers the
following functional mechanisms while being used for pavement construction:-
1. Tension Membrane Effect
This mechanism depends on the perception of vertical stress distribution. This vertical
stress originates from the deformed shape of the membrane. This mechanism was treated as the
primary mechanism in earlier stage. But in due course it is proven that the lateral restraining
mechanism is the most important factor that should be considered.
Fig 3. Tension Membrane Effect
(Source: https://theconstructor.org)
2. Enhancement of Bearing Strength
As soon as Geogrid is set up in pavement, the lateral movement of the aggregate is
decreased significantly. It leads to reduction of stresses; that if prevails would have shifted to the
sub grade. The Geogrid layer contains adequate frictional resistance that counters sub grade
lateral movement. Thus, this mechanism makes the bearing strength of the layer greater. Cutback
of external stresses signifies that inward stresses are developed and due to this the bearing
strength is raised.

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Fig4. Mechanism for Improved Bearing Capacity
3. Lateral Preventing Capacity
The stresses generated with the wheel loadings directing to the pavement lead to the
lateral movement of the aggregates. Because of this the constancy of the entire pavement
arrangement is impacted. The Geogrid functions a restraint against this lateral movement.
Fig5: Lateral Preventing Capacity
2.1.2.3 Uses of geogrid in pavement
The three primary uses of a geogrid in a pavement system are to –
1. To serve as a construction aid over soft subgrades.
2. To improve or extend the pavements projected service life.
3. To reduce the structural cross section for a given service life.

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4. To study the different aspects, regarding changes in properties of soil.
2.1.2.3 Ecological benefit of geosynthetics
Summarizing, geogrid reinforced constructions are increasingly used as safe, ecological
and economical solutions. Compared to conventional solutions with concrete they often have the
benefit of lower costs and less environmental impact. This is impressively documented in a
broadly based study of the European Association of Geosynthetic product Manufacturers
(EAGM), regarding a total of four case studies comparing a conventional construction method
with one or more alternatives using geosynthetics. The study was conducted by the ETH (Swiss
Federal Institute of Technology) in Zurich and ESU-services Ltd.. Evaluation criteria in this
context included the cumulative energy consumption, the global-warming potential, the
photochemical formation of ozone (better known as summer smog), the fine-dust potential, the
acidification with nitrogen and Sulphur oxides, eutrophication, i.e. nutrient enrichment of
watercourses with fertilizer, and the land and water requirements. The study took account of the
entire life cycle from the extraction of raw materials, the manufacture of the product, the
transport to and installation on site as well as dismantling and disposal.
2.1.2.4 Applications of geogrid in pavement
The Geogrid construction in pavement construction has following features:
1. Improvement of sub grade
The sub grade, which is the most important load bearing strata, is made solid and strong by the
geogrids. The problem of soft sub grade can be solved by this method.
2. Reinforcement of pavement base
The thickness of base if increased would increase the stiffness of base. But increasing thickness
enormously is not economical. The reinforcement to a given base layer would give adequate
stiffening that helps in reduction of thickness and time of construction. This also helps in
increasing the life of the pavement.

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Fig:6. Effect of geogrid position on maximum deflection
(Source:ARPN Journal of Engineering and Applied Sciences)
3.1 EXPERIMENTSAND REULTS
3.1.1 CBR Test
Design methods that determine the required layer thickness of the stone aggregates with
the reinforcement in the subgrade are usually based on the CBR of the soil subgrade.Although
the CBR test is only valid for uniform materials, it can show the qualitative beneﬁt of geogrid
reinforcement to the material resistance under the same conditions of test and hence can be used
for comparing the results. Therefore, CBR tests were conducted on selected soils unreinforced
and reinforced with geogrid at various positions. It was cut in the form of a circular disc of
diameter slightly less than that of the specimen to avoid separation in the specimen by the
reinforcing layer. The dry weight required to ﬁll the mould was calculated based upon MDD and
the volume of the mould. The water corresponding to OMC was added and mixed thoroughly.
The experimental study involved performing a series of laboratory CBR tests on the
unreinforced and geogrid reinforced natural gravel specimens. One layer of geogrid placed on
top of layer three of the sample height and the CBR tests were carried out. All tests were
performed inside a modified proctor mould at soaked and unsoaked condition according to
ASTM-D 1883 .The mould was a rigid metal cylinder with an inside diameter of 152 mm and a
height of 178 mm. A manual loading machine equipped with a movable base that travelled at a
uniform rate of 1.27 mm/min and a calibrated load indicating device were used to force the
penetration piston with a diameter of 50 mm into the specimen.The test specimens were

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compacted in accordance with the procedures given in ASTM-D 1557 using modified effort .The
soil at the optimum water content (OMC) were placed in 5 layers within CBR mould. Each of the
layers were compacted by 56 blows of a 44.5 N rammer dropped from a distance of 457 mm.
The soil samples are compacted in CBR mould at its optimum water content and then the CBR
tests were carried out under soaked and unsoaked conditions. The pavement layers were
simulated by the experimental setup for a CBR test. The reinforced and unreinforced soil
samples were tested for CBR using the CBR testing machine.
Fig-7 Inclusion of Geogrid at various Positions
(Source: IJESRT, usage of geogrids in flexible pavement design)
(a) (b)

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(c) (d)
Fig8 : load vs. penetration graph of soil with various conditions(a) Without geogrid
(b) With geogrid @ H/4 from bottom (c) With geogrid @H/2 from bottom (d)With geogrid
@ 3H/4 from bottom
Values of CBR at various positions of geogrids:
Description at 2.5 mm At 5mm
Without geogrid 1.67 1.36
With geogrid @ H/4 distance from the bottom 1.80 1.29
With geogrid @ H/2 distance from the bottom 2.50 2.74
With geogrid @ 3H/4 distance from the bottom 3.91 1.80

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Table -3 CBR Value Variations with Geogrid Application in Soil Sample
From the results of the CBR tests on soil reinforced with and without geogrid, It is clear
that there is a considerable amount of increase in the CBR value of a soil with the geogrid
reinforcement. The amount of increase depends upon both the type of soil and geogrid stiffness.
It is clear that soil with geogrid @ 3H/4 from bottom have high CBR value, which was of higher
stiffness indicating that the stiffness of the grid also has considerable effect on the bearing
capacity of the reinforced soil.
Fig 9: CBR Contrast with geogrid Application

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3.1.1.1 Optimum Position of the Grid
The results of CBR and triaxial tests indicate that for maximum benefit, the geogrid
should be placed at 3H/4 from bottom The tests were conducted at four different positions of the
grid and, as shown in Tables 3.The maximum value of all strength parameters was obtained in
the case when the grid was at 3H/4 depth from bottom of the specimen height.Putting the grid
at100%depth (below the specimen) is as good as having no reinforcement in the sample. The
exact position of the grid was obtained by plotting the improvement in the CBR value of the soil
with depth of the grid. Such plots for CBR value of the soils reinforced with and without grids
are shown in Figure8. All these curves indicated that the maximum CBR of a soil is obtained
when the grid is placed at 3H/4 from bottom, theoretically if the soil is assumed to be isotropic
and if the stress difference due to gravity is neglected, then, the same benefit should occur in
triaxial tests when the grid is placed at any depth from the top of the specimen. However, the
present study shows more benefit of the reinforcement when the grid was placed at at 3H/4 from
bottom.
4.1 DESIGN OF PAVEMENT
4.1.1Pavement design methods
The evolution of road design can be captured by the image in Fig. 1. Road design from
the Roman era through to the 1930’s was based on engineering judgment or past experience.
The idea of providing a harder and stiffer surface material with less stiff materials providing
support was conceived by the Romans and forms the basis of design today. This philosophy
forms the basis for our current need to protect the subgrade and ensure serviceability of the
pavement structure layers. In the 1930’s following the Great Depression and a proliferation in
new technologies, the cover based design approach was developed, but still required a significant
amount of engineering judgment. It was only in the 1960’s that the $20million (1960’s) AASHO
road test was performed and led the way for the launch of the AASHTO series of design
methods. With these new more sophisticated cover based design methods came a greater
number of design inputs. In the mid 1970’s the South African’s released an innovative linear

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elastic mechanistic empirical design method, which again increased the required inputs for
design. However, with this increased level of design sophistication, came the need for improved
methods of field and laboratory testing as well as techniques of material characterization.
Since the 1990’s there has been a proliferation of design methods that vary between
purely empirical methods to very sophisticated finite element - non linear - mechanistic
empirical (M-E) design methods (e.g. MEPDG). It is now possible to design using information
as detailed as the asphalt mix recipe for example. This continual development of design methods
has been driven by road asset owners’ and designers’ need to more accurately predict
performance, better accommodate new material types and account for rapidly changing traffic
patterns. However, it should be noted that there are many road asset owners that adopt less
sophisticated empirical design methods with great success and are extremely successful in
managing their roads. Therefore, it should be noted that a successful design approach can
therefore not solely rely on sophisticated design tools alone, but needs to be combined with a
thorough understanding of the performance of local materials and local environmental
conditions.
To fully capture the benefit of a geogrid in a design method, the linear elastic M-E design
approach best accommodates and can capture the unique contribution of the geogrid that
influences the construction, the application of a single load and the accumulation of deformation.
The M-E design approach is also better suited to including a geogrid’s benefits, because the
required inputs force the user to better define their local materials and provide a means by which
the geogrid contribution can be validated post construction, during and after the structural design
life. The next section defines the M-E design method in greater detail and addresses how the
geogrid benefit can be captured within both the mechanistic and empirical components of M-E
design.

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Fig10: Evolution of Design Methods
(Source:Tensar International Corporation, Atlanta)
4.1.2CBR method recommended by IRC (Indian road congress):
In this method, the chart contains several curves (A, B, C, D, E, F, and G) which
represents the different levels of traffic intensities. Based on this we will find out the layers
thicknesses.
Data required for design:
a. CBR value of soil subgrade
b. CBR value of sub base course
c. CBR value of base course
d. Traffic intensity

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Fig:11 Depth of construction Vs CBR graph
4.1.3 Flexible Pavement DesignProcedure:
4.1.3.1 Calculation of total thickness (T):
In this step, firstly for the given value of traffic intensity select appropriate curve from
classification table which is shown in the below chart. Now, from the given CBR value of
subgrade soil read the total thickness (T) with respect to selected curve.
4.1.3.2 Calculation of sub base course thickness (tsb)

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By using the above chart, for give CBR value of sub base course material and for traffic
intensity value read the thickness of pavement which is above the soil sub base. It is denoted as
(Tsb). Which is highlighted by circle in the below fig. but here we have to find tsb.
Therefore, thickness of sub base course tsb =T – Tsb
Fig:12 Calculation-of-pavement-sub-base-course-thickness
(Source:https://theconstructor.org)
4.1.3.3 Calculation of base course thickness (tb)
Repeat the above procedure again, from the CBR value of base course and from traffic
intensity value read the value of thickness of pavement which is above the base course (ts). From
this we can find out the value of tb. tb = Tsb – ts
CBR Value without geogrid:
CBR: 1.67 %, N: 47.45 msa ≈ 50 msa i.e., not fit for laying a road directly on the Subgrade soil;
which needs Stabilization to it.
CBR Value with geogrid at 3h/4 from bottom: CBR: 3.91 %, N: 47.45 msa ≈ 50 msa i.e.,
thickness of GSB: 300 mm, G. Base:250, DBM: 115 mm, BC/SDBC:40mm

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Fig-13: Pavement Design Catalogues
Thickness is calculated with the help of MSA and CBR value and it is concluded that the
geogrid at 3h/4 from bottom sample have more properties than others.

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5.1 CONCLUSION
The positive effects of geogrid reinforced subgrade courses can economically and
ecologically be utilized to reduce aggregate thickness. And it can also increase the life of the
pavement and can also decrease the overall cost of the pavement construction with an increased
lifetime. Some of the advantages of this study was,
1. thickness of Pavement is reduced by using geogrids,it also result in the reduction of the
cost of road construction.
2. Pavement thickness is designed based on CBR as per IRC:37-2012.
3. Load carrying capacity of the road or Strength of road is increaded by the used of
geogrid.
4. Usage of geogrids results in the incease in Service Life of Road.

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REFERENCES
[1] Barksdale R. D., S.F. Brown and F. Chan. (1989). “Aggregate base reinforcement of
surfaced pavement”. Geotextiles and Geomembranes , Vol:8,page no:165-189.
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[13] Perkins S.W. (1999). “Mechanical response of geosynthetic reinforced flexible
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[18] Venkatappa Rao G., Gupta K.K. and Singh P.B. (1989) “Laboratory studies on
Geotextiles as Reinforcement in Road Pavement”, Geotextile Proceedings of the
International Workshop on Geotextile,Banglore
[19] Wathugala G. W., B. Huang and S. Pal. (1996). “Numerical simulation of geogrid
reinforced flexible pavements”. Transportation Research Record 1534, Transportation
Research Board, National Research Council, Washington, D.C., USA. Page no:58-65.

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