Reinforcement of Pavement Subgrade using Granular Fill and a Geosynthetic Layer

International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2163
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Reinforcement of Pavement Subgrade using Granular
Fill and a Geosynthetic Layer
Johnny Oriokot*
Fibertex South Africa, South Africa
Johnny@geotextilesafrica.co.za
Edoardo Zannoni
Maccaferri Africa, South Africa
Edoardo.Zannoni@maccaferri.co.za
Denis Kalumba
Geotechnical Engineering, University of Cape Town, South Africa
Denis.Kalumba@uct.ac.za
Felix Okonta
Civil Engineering, University of Johannesburg, South Africa
fnokonta@uj.ac.za
Manuscript History
Number: IJIRAE/RS/Vol.04/Issue06/APAE10101
Received: 26, April 2017
Final Correction: 18, May 2017
Final Accepted: 28, May 2017
Published: June 2017
Citation: Oriokot*, J.; Zannoni, E.; Kalumba, D. & Okonta, F. (2017), 'Reinforcement of Pavement Subgrade using
Granular Fill and a Geosynthetic Layer', IJIRAE:: International Journal of Innovative Research in Advanced
Engineering Volume IV (Issue VI), 38-56.
Editor: Dr.A.Arul L.S, Chief Editor, IJIRAE, AM Publications, India
Copyright: ©2017 This is an open access article distributed under the terms of the Creative Commons Attribution License, Which
Permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
Abstract — Engineers are often faced with construction of pavement structures over soft and highly compressible
subgrade. Such conditions render the structures unable to withstand the required design loads and susceptible to
high settlements with associated excessive distress, leading to pavement damage. Because using imported quality fill
to improve the load-bearing capacity of the subgrade has limited benefits, an alternative construction approach is
needed to ensure the required strength of the soil structure is reached. The use of geosynthetics in soil to improve
bearing capacity and stability offers a better alternative to imported fill. This research was conducted to determine
the degree of improvement of the load-bearing capacity and reduction in settlement as a result of the geosynthetic
reinforcement of soft clay overlain by granular material. Compression tests on the unreinforced clay subgrade and
geosynthetic-reinforced two-layered soil composite were conducted at bench-scale using a Universal Compression
Machine at the University of Cape Town Geotechnical Engineering Laboratory. The geosynthetic products used to
reinforce the soil structure in this study included geogrids and geotextiles. The reinforcement layer was placed
alternatively within the layer of granular material and at the interface of the soil and a range of fill thicknesses and
depths of placement were tested to determine their influence on mobilized strength. The use of these composites could
potentially lead to reduced construction costs as a result of less aggregate material used, an extension in the design
life of the pavement structure and, consequently, a reduced necessity for maintenance work.
Keywords— Geosynthetics, Reinforcement, Subgrade, Pavement, Bearing Capacity
I. INTRODUCTION
The construction of pavement structures often occurs over soft and compressible subgrades that have low load-
bearing capacities and are susceptible to large settlements. This results in damage to the structures erected on
them [42].

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A further problem is that sites may have had the in situ soil disturbed either by previous excavations or the
dumping of weak material on them, leaving unstable sections on the site. These factors often render the strength
of the subgrade inadequate for construction. As such, these subgrades need engineering intervention before they
can safely support applied loads.
The problem of encountering soft subgrades consisting of clays and silts, the need to build infrastructure, like
pavement structures, over these sites, and the additional mandated environmental regulations that prevent
construction on alternative sites, have provided the impetus for the development of a number of ground
improvement techniques during the past 25 years [25]. The accumulation of the load as the construction process
progresses, including the continuous movement of construction machinery on site, and also the imposed loading
during the design life of the pavement structures, sometimes leads to migration of the fine grain particles into the
granular fill, and penetration of the large granular particles into the soft subgrade [33]. The effect is deterioration
in the structure of the soil layers, resulting in deformations and ultimately leading to failure of the structure. An
additional problem in the construction process is the limited availability of quality fill to stabilise the soft
subgrade. The feasibility of projects is affected by scheduling delays that occur when the required materials are
transported from distant quarries to the construction site, or when relatively expensive alternative ground
improvement methods are used [38].
When dealing with difficult sites for construction purposes, the conventional practice was limited either to
replacing the unsuitable soils, or bypassing them with costly deep foundations. The solution that involves the
addition of granular material, which acts as the fill layer, distributes the loads laterally, decreasing the stresses on
the soft subgrade. This method allows the support of greater loads, and reduces failure occurrences such as
settlements and pavement distress [24]. With higher strength properties than the in situ clay, the granular fill
bears most of the applied loads, distributing the load transferred to the in situ subgrade and improving the load-
bearing capacity of the pavement structure. Innovative ground improvement approaches are now used to solve
these unique soil-related problems, and often are considered to be the most economical means to improve an
undesirable site condition [12]. Construction with geosynthetics is one of the approaches that have been
incorporated in the design of pavement structures, with the aim of stabilising the soils to make them more
suitable for engineering applications [37]. The use of geosynthetics in geotechnical construction projects has
gained popularity over the past 30 years, supported by research conducted by several researchers [26], [35], [51],
[22], [19], [23], [27], [18], [3], & [36], and their use in large-scale civil construction projects has made the resulting
structures safer [24]. The inclusion of geosynthetics in soil layers has shown the benefits of enhancing the load-
bearing capacity of the soil, and reducing the settlement undergone by the structures [30]. The main objectives of
this research were to determine the degree of improvement of the load-bearing capacity of clay soil using
combined reinforcement of granular fill and geosynthetics, and to determine the degree of the reduction in
settlement. The research methods involved determining the optimum thickness of fill, determining the optimum
depth of placement of the geosynthetic layer, and identifying which of the geosynthetic products would provide
the greatest benefits.
II. MATERIALS AND EQUIPMENT
A. SOIL MATERIALS
I) GRANULAR MATERIAL
The granular material used in all reinforced experiments was sourced from Contermanskloof quarry in the
Western Cape, South Africa. The material consisted of grey, angular to sub-angular gravel, and had a medium
dense consistency as observed when excavated from the quarry. The sieve analysis conducted on the granular
material followed the testing standard presented in ASTM D6913, and the grading curve of the granular material
is presented in Figure 1. According to the USCS, the material is classified as well graded gravel, GW. The
percentage passing the 0.075 mm sieve was approximately 25%, and the in situ relative density of the material
was 0.94. The material had a California Bearing Ratio (CBR) of approximately 15% at 93% Mod. AASHTO and a
penetration rate ranging from 9 to 14 mm/blow were achieved from Dynamic Cone Penetrometer (DCP) tests
conducted on the material in situ. A plasticity index (PI) of approximately 11 was measured on the material. The
optimum moisture content of the granular material was 4.9% with a maximum dry density of 2240 kg/m³ MoD
AASHTO as provided by the supplier. According to materials classification methodology described by [28], which
is based on the Technical Recommendations for Highways No.14 [48] and South African Pavement Engineering
Manual [46], the material that was used in the tests was categorised as a G7. According to the AASHTO M145 [1]
and ASTM D3282 [8], the material was classified as an A-2-7.This granular material is used as selected fill in the
subgrade layer in pavement structures and road embankments according to [46]. As such it was used in this study
to represent the fill layer.

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Fig 1. Particle Size Distribution for G7 Granular Material
II) CLAY MATERIAL
The clay used to represent the subgrade material in all the experiments was kaolin. This clay was specifically
chosen because its uniform mechanical and chemical properties were ideal for experimentation and enhancement,
and it was easy to work with. Kaolin is a good representation of the soft soils in South Africa that pose a problem
on construction sites. The native kaolin ore is a lamellar aluminium phyllosilicate, and naturally occurring kaolin-
rich residual soils occur in the Eastern and Western Cape regions of South Africa. The kaolin clay was acquired
from Serina Trading, Cape Town, South Africa. The refining process for the clay included water washing,
hydrocyclone beneficiation and screening followed by mechanical-thermal drying. The clay had a specific gravity
of 2.6, moisture content of 4% and pH value ranging between 7 and 8. The mineralogical composition consisted of
kaolinite with trace mica and quartz. The kaolin clay was a grade HB powder, and had typical physical properties
and chemical analysis as presented in Tables I and II.
TABLE I - PHYSICAL PROPERTIES OF THE KAOLIN CLAY
Physical Properties Value
Abrasiveness (Einlehner tester) 64 g/m²
Mean particle size (D50) 1.1 micron
Residue ( > 45 micron) 1.5%
Reflectance 79% (off-white in colour)
pH value 7 – 8
Specific gravity of kaolin mineral 2.60
Mohs hardness 2.0 – 2.5
Moisture content 4%
Oil absorption (linseed oil) 45 mg/100g
TABLE II-CHEMICAL ANALYSIS OF THE KAOLIN CLAY
Chemical Analysis Value
SiO2 47.32%
Al2O3 36.52%
Fe2O3 0.56%
TiO2 0.82%
CaO 0.31%
MgO 0.18%
Na2O + K2O 0.92%
L.O.I 13.02%

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B. GEOSYNTHETIC REINFORCEMENT MATERIALS
Four different geo synthetics were used in this study: woven geogrids, extruded geogrids, woven geotextiles and
non-woven geotextiles.
TABLE III - MECHANICAL AND HYDRAULIC PROPERTIES OF MACTEX W1 8S WOVEN GEOTEXTILE
Material Property 8S
Tensile Strength (MD) EN ISO 10319 kN/m 83
Strain at max load EN ISO 10319 % 19
Tensile Strength (CD) EN ISO 10319 kN/m 81
Strain at max load (CD) EN ISO 10319 % 15
Static Puncture Resistance - CBR EN ISO 12236 kN 10.0
Dynamic Puncture Resistance – Cone Drop EN ISO 13433 mm 7.5
Permeability – normal to plane EN ISO 11058 m/sec 0.022
Opening Pore Size 90 EN ISO 12956 µm 120
TABLE IV - SUMMARY OF MATERIAL PROPERTIES FOR THE MACTEX H40.1 NON-WOVEN GEOTEXTILE
Material Property H40.1
Grab Tensile Strength ASTM D4632 kN 0.712
Grab Elongation Strength ASTM D4632 % 50
Trapezoidal Tear ASTM D4533 kN 0.267
Puncture (CBR) ASTM D6241 kN 1.824
Permittivity ASTM D4491 sec-1 1.3
Flow Rate ASTM D4491 l/m/m2 4482
Apparent Opening Size (AOS) ASTM D4751 mm 0.212
TABLE V - MATERIAL PROPERTIES OF THE MACGRID WG 8S WOVEN GEOGRID
Material Property 8S
Tensile Strength – MD (ISO 10319) kN/m 80.0
Tensile strength at 2% strain – MD (ISO 10319) kN/m 18.0
Tensile strength at 5% strain - MD (ISO 10319) kN/m 38.0
Strain at max strength - MD (ISO 10319) % 11
Tensile Strength – CMD (ISO 10319) kN/m 80.0
Tensile strength at 2% strain – CMD (ISO 10319) kN/m 18.0
Tensile strength at 5% strain - CMD (ISO 10319) kN/m 38.0
Strain at max strength - CMD (ISO 10319) % 11
TABLE VI - MATERIAL PROPERTIES OF THE MACGRID EG40 EXTRUDED GEOGRID
Mechanical Properties 40S
Minimum Average Tensile Strength kN/m 40.0
Tensile strength at 2% strain - Longitudinal kN/m 14.0
Tensile strength at 5% strain - Longitudinal kN/m 28.0
Typical strain at M.A.T.S – Longitudinal % 13
Minimum Average Tensile Strength kN/m 40.0
Tensile strength at 2% strain - Transverse kN/m 14.0
Tensile strength at 5% strain - Transverse kN/m 28.0
Typical strain at M.A.T.S – Transverse % 10
Typical junction strength efficiency % 95
The geogrids and woven geotextile are conventional reinforcing geosynthetics. The nonwoven geotextile is mainly
used for separation, but was evaluated in this study to assess whether meaningful reinforcement could also be
realised. The geosynthetic materials were sourced from Maccaferri Africa. The basic characteristics and
mechanical properties of the geosynthetics as provided by the supplier are presented in Tables III – VI.

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C. LABORATORY TEST METHOD
The main apparatus used in the research consisted of a bench scale steel “box” model (Figures 2 and 3). The
rectangular configuration of the box, 950 mm by 140 mm in plan and 500 mm deep, represented typical models
dimension used in literature. Metal bracings were fixed on either side of the equipment to prevent any lateral
deformation of the box by the applied loading which would have undermined the results of the investigation.
Fig. 2. Illustration of the internal dimensions of the steel “box” model
Fig. 3. Steel “box” model showing the metal bracing and trolley
III.METHODOLOGY
A. SOIL SPECIMEN
I) KAOLIN CLAY
The kaolin clay was mixed to give a CBR value less than 3%, to replicate a soft subgrade. At that CBR, a soil is
deemed to need stabilisation and reinforcement before construction can proceed [46]. To obtain the required CBR
value for the kaolin clay, correlation equations were used that related the CBR of the soil to its index properties.
Several researchers [49], [31], [29], [50], [43], [44], [21] & [47] have developed suitable correlations between CBR
values of compacted soils at natural moisture content and results of some simple field tests. The linear regression
model developed by [47] was used given that the laboratory and computed CBRs had the highest correlation. To
prepare the samples to the required specific moisture content of 30%, a known volume of water was mixed with a
measured amount of dry kaolin HB powder in an industrial mixer. This was determined by using the index
properties of the kaolin clay, the required CBR value of 3%, and a modified equation adopted from the linear
regression model developed by [47]. The prepared kaolin sample was stored in a plastic container, shown in
Figure 4, to allow for even moisture distribution throughout the sample. The container was kept in the
Geotechnical storeroom for 24 hours before the tests commenced and used within 72 hours, after which a new
batch was made for the next round of testing.

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Figure 4: Sample of mixed kaolin clay
II) GRANULAR MATERIAL
The granular material was mixed to the optimum moisture content by weighing out a known amount of the dry
granular material and mixing a measured volume of water into the material. The mixed sample was stored in
plastic containers in the Geotechnical laboratory storeroom prior to testing, as shown in Figure 5, to allow for
even distribution of the moisture throughout the sample.
Fig. 5. Sample of granular material
B. GEOSYNTHETIC MATERIAL
The geosynthetics were cut to specific rectangular sizes measuring 375 mm by 140 mm and 750 mm by 140 mm,
respectively, determined by the 140 mm-width of the loading box. The chosen measurements of 375 mm and 750
mm were selected to match the optimum width of the geosynthetics for all the tests of 5B, where B is the width of
the footing. The model footing widths of 75 mm and 150 mm were used. Samples of the geosynthetic products
tested are shown in Figures 6 and 7.

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Fig. 6. (top) Woven Geogrid (WGG) and (bottom) Extruded Geogrid (EGG)
Fig. 7. (top) Woven Geotextile (WGT) and (bottom) Non-woven Geotextile (NGT)
C. BEARING CAPACITY TEST PROCEDURE
The two soils were mechanically compacted and levelled in the loading box. A compaction effort equivalent to
1000kJ/m3 was subjected onto the soil layers, and was delivered by a 4.5 kg hammer dropped through a height of
300 mm for 25 blows [4] & [5]. The clay layer was prepared in all tests to a constant thickness of 250 mm, while
the granular material had varying thicknesses from 50 – 150 mm depending on the footing width. Two model
footings of widths 75 mm and 150 mm were used in the testing to measure the impact of footing width on the
depth of the fill, the depth of placement of the geosynthetic layer, and the bearing capacity of the composite soil.

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The geosynthetic layer was placed at varying depths in relation to the footing width, ranging from within the
granular layer to the interface of the two soils. Plate loading tests were conducted using a large capacity 100kN
Zwick Universal Compression and Tensile Testing Machine. The strength of the geosynthetic-reinforced two-
layered soil was determined by applying a compressive load to the soil surface. Figures 8 and 9 show the depth of
placement (D) of the geosynthetic within the top layer and at the interface of the two soils for the varying
thicknesses (Z) of the granular material; of which the depth of placement, D, and the thickness of fill, Z, are both
related to the width of the footing (B).
Fig. 8. Geosynthetic layer placed within granular fill material
Fig. 9. Geosynthetic layer placed at the interface of the two soils
The detailed procedure for the bearing capacity tests is as described below:
1. The clay was placed in the loading box in layers of 125 mm up to a thickness of 250 mm, which was kept
constant throughout all the tests.
2. The soil was compacted and levelled for each of the layers to allow for preparation at maximum density at a
compaction effort equivalent to 1000 kJ/m3.
3. The granular material was then placed on top of the clay soil at varying depths of 50 – 150 mm. When the
depths were greater than 100 mm, the fill was placed in 50 – 100 mm increments. Each layer was compacted
and levelled as described in step 2.
4. The geosynthetic layer was placed at the varying depths of 50 – 150 mm, and centrally located below the
model footing to allow for symmetrical reinforcement.
5. Once the sample was prepared, it was wheeled into position to be loaded into the Universal Compression
Machine.
6. The model footing was placed centrally on top of the granular layer, and levelled.
7. Using the machine interface, the settings that were necessary for each test, and those to be recorded, were
applied. The settings included the dimensions of the footing and the loading rate, which was set at 1.2 mm/min,
replicating undrained conditions [7].
8. The test began and ran until a vertical displacement of 30 mm was attained, at which point failure in a
pavement structure would have occurred [11]. From [46] it is stated that a rutting level of 20 mm is considered
terminal for the road structure; as such running the tests to 30 mm was sufficient.
9. The process was repeated for each of the 76 tests conducted, for the unreinforced soil and for the geo
synthetic-reinforced composite.

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Test conditions and procedures were similar throughout the tests programme; however, different granular fill
thicknesses, depths of placement, and geosynthetic types were used. The testing schedule for the woven geotextile
is shown in Table 7, which was similar for all the geosynthetic products.
TABLE VII -
TABLE OF THE TESTING SCHEDULE FOR THE WOVEN GEOTEXTILE (WGT) USING THE 75 MM AND 150 MM MODEL FOOTINGS.
Model Footing
Width (B)
Thickness
of granular
material (Z)
Thickness
to Width
Ratio (Z/B)
Depth of
placement
(D)
Depth to
Width Ratio
(D/B)
Test No.
75 50 0.67 50 0.67 WGT/B75/Z50/D50
75 1 50 0.67 WGT/B75/Z75/D50
75 1 WGT/B75/Z75/D75
112.5 1.5 50 0.67 WGT/B75/Z112.5/D50
75 1 WGT/B75/Z112.5/D75
112.5 1.5 WGT/B75/Z112.5/D112.5
150 50 0.33 50 0.33 WGT/B150/Z50/D50
75 0.5 50 0.33 WGT/B150/Z750/D50
75 0.5 WGT/B150/Z750/D75
112.5 0.75 50 0.33 WGT/B150/Z112.5/D50
75 0.5 WGT/B150/Z112.5/D75
112.5 0.75 WGT/B150/Z112.5/D112.5
150 1 50 0.33 WGT/B150/Z150/D50
75 0.5 WGT/B150/Z150/D175
112.5 0.75 WGT/B150/Z150/D112.5
150 1 WGT/B150/Z150/D150
IV. RESULTS
The applied loads against the vertical displacements were recorded for all of the experiments using the Universal
Compression and Tensile Machine, and graphs of the mobilised bearing pressure against settlement were
generated from the data obtained. The results for the unreinforced kaolin clay, used as control, were compared
with results for the geosynthetic-reinforced composite to determine the degree of improvement. Table 8 shows
the different symbols used in the graphs representing the results obtained. The results of the study, shown in
Figures 10 – 16, revealed that the inclusion of the granular fill over the soft subgrade, without any geosynthetic
reinforcement, led to an increase in the load mobilised at a given displacement on the layered soil structure.
However, further increases in the thickness of the fill, beyond the optimum, did not result in an improvement in
the bearing capacity of the two-layered soil structure. The incorporation of geosynthetics in the two-layered soil
led to a further increase in the applied load of the composite irrespective of the position of placement. As the
thickness of the granular fill was increased with geosynthetic reinforcement included in the soil structure, there
was a subsequent increase in the applied load for any given displacement, which could be directly related to an
increase in the bearing capacity of the soil. However, as the thickness was increased beyond a certain limit, there
was a reduction in the improvement in strength of the composite. The results for the woven geotextile at a
constant depth of placement are presented in Figure 10a.
TABLE VIII - TABLE OF SYMBOLS USED TO REPRESENT THE RESULTS OBTAINED.
Symbol Denotation
UR Unreinforced
GR Gravel-Reinforced (No Geosynthetic)
B Plate Width
Z Fill Thickness
D Depth of Placement of Geosynthetic Layer
X Clay Thickness
WGT Woven Geotextile
NGT Non-woven Geotextile
EGG Extruded Geogrid
WGG Woven Geogrid

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From the graph it could be observed that, as the thickness of the aggregate fill was increased keeping the
geotextile at a constant depth of placement of 50 mm, there was a subsequent increase in load-bearing capacity.
Analysing the results at settlement of 30 mm, the unreinforced subgrade had an approximate mobilised bearing
pressure of 85 kPa. With the inclusion of the woven geotextile at 50 mm depth and the aggregate thickness of 50
mm, the load bearing pressure improved to approximately 150 kPa, which is equivalent to a 75% improvement.
When the aggregate thickness was increased to 75 mm at the same depth of placement, the load-bearing capacity
was approximately 210 kPa, which is equivalent to an improvement of 145%. However, as the aggregate thickness
was increased to 112.5 mm, the load-bearing capacity only improved to 180 kPa, which is only equivalent to an
improvement of 110%. This indicates an optimum thickness of aggregate of 75 mm, which is equivalent to 1B, B
representing the width of the footing, as shown in Figure 10b.
The results for the woven geotextile at the interface is presented in Figure 11a. From these graphs it can be
observed that with an initial increase in the thickness of the aggregate, there was an increase in the load-bearing
pressure of the composite from 85 kPa for the unreinforced subgrade to 150 kPa for 50 mm aggregate thickness
and depth of placement of the geotextile, and to 200 kPa for 75 mm aggregate thickness and depth of placement.
However, as the thickness of the fill was increased beyond 75 mm to 112.5 mm, there was a reduction in the
improvement to 160 kPa. These increases in the load-bearing capacity are equivalent to approximate
improvements of 75%, 135% and 85% respectively.
Fig. 10. (a) Applied bearing pressure against settlement and (b) Mobilised bearing pressure against varying fill
thicknesses for the woven geotextile placed at a constant depth of 50 mm using the 75 mm footing.
This indicated that an optimum thickness was achieved at a thickness equivalent to 1.0B, B represents the footing
width, as shown in Figure 11b.

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Fig. 11. (a) Applied bearing pressure against settlement, and (b) Mobilised bearing pressure against varying fill
thicknesses for the woven geotextile at the interface using the 75 mm footing.
A similar trend is observed with the extruded geogrid at the interface, as shown in Figure 12a, where there was an
initial increase in the load-bearing capacity with increasing aggregate thickness, and improvements ranging from
75% – 135%. The optimum thickness was also obtained at 1.0B, as shown in Figure 12b.
As the depth of placement of the reinforcement layer was increased, at constant granular thickness, there was an
observed reduction in the strength of the composite structure. However, when different geosynthetic products
were used there was also an observed change in the strength. The woven geogrid produced results that indicated
an increasing load-bearing capacity up to an optimum fill thickness; with further increases in fill thickness a
reduction in improvement was observed, as shown in Figure 13a.

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For the unreinforced subgrade, a load-bearing capacity of 85 kPa was reached. When the subgrade was only
reinforced with the aggregate, the load-bearing capacity increased to 135 kPa, equivalent to an improvement of
65%.
Fig. 12. (a) Applied bearing pressure against settlement, and (b) Mobilised bearing pressure against fill thickness
for the extruded geogrid at the interface for 75 mm footing using the 75 mm footing.
When a woven geogrid was added at a depth of 50 mm for the same fill thickness of 75 mm, there was an increase
in the load-bearing capacity of 160 kPa, equivalent to an improvement of 85%. However, a further increase in the
depth of placement to 75 mm only led to an increase in the load-bearing capacity to 145 kPa, which is equivalent
to an improvement of 70%. The optimum depth of placement for the woven geogrid was obtained as 0.67B, as
shown in Figure 13b.
Fig. 13. (a) Applied bearing capacity against settlement, and (b) Mobilised bearing capacity against depth of
placement for the woven geogrid using 75 mm footing.

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A similar optimum depth of placement was observed for the woven geotextile in Figure 14a and the non-woven
geotextile in Figure 15a, even with an increased thickness of aggregate tested. The optimum depth of 0.67B is
shown in Figures 14b and 15b.
placement for the woven geotextile for a constant granular thickness of 112.5 mm using the 75 mm footing.
The extruded geogrid had slightly different results, showing a continuously increasing load-bearing capacity as
the placement depth increased – see Figure 16a. The load-bearing pressures obtained ranged from 130 kPa – 200
kPa, which is equivalent to an improvement of 50% - 135%. The resultant optimum depth of placement was
obtained as 1.0B as shown in Figure 16b.

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placement for the non-woven geotextile for a constant granular thickness of 112.5 mm using the 75 mm footing.
placement for the extruded geogrid for a constant granular thickness of 75 mm using the 75 mm footing.
From all the results obtained and the trends seen, a greater increase in the load-bearing capacity of the reinforced
soil composite was evident when the geosynthetic was placed within the fill layer as opposed to at the interface.
This increase could be attributed to the mobilisation of the added reinforcement provided by the geosynthetic
layer at a higher level, which would lead to the spread of the applied load through the higher-strength fill material,
thus providing more support to the structure. As a result, the stress transferred to the weaker clay subgrade is
reduced, causing less destabilisation of the structure and allowing for greater loads to be exerted on the soil. This
corresponds to an increase in the load-bearing capacity of the soil.

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V. DISCUSSION OF RESULTS
The benefits of reinforcement provided by the geosynthetics are associated with the mechanisms of soil-
reinforcement interaction that include the lateral restraint of soil particles and the tension membrane action.
These mechanisms lead to the alteration of failure surfaces with a resultant improvement in the load-bearing
capacity and added benefit of reduction in settlements, which have been presented by [26], [11], [20], [39] & [32].
The lateral restraint of particles is attained mainly through the interlocking of the soil particles in the geosynthetic
apertures, an action that was greater in geogrids as they possessed apertures making it possible for the
interlocking to occur. The lateral restraint of soil particles is shown in Figure 17.
Fig. 17. Lateral restraint of soil particles due to geosynthetic layer.
However, when the friction between the geosynthetic layer and the soil was relatively high, there was an
increased interaction between the geosynthetic and the soil, allowing for the transfer of the lateral stresses from
the soil to the geosynthetic layer. This transfer occurred in the non-woven geotextile, and added to the
reinforcement benefits.
Unbounded granular soil fabric has no tensile capacity and cemented or stabilized soils have limited tensile
strength. When geosynthetics are used as reinforcement elements in soil, the most important feature in the
provision of this reinforcement is the bulk and interfacial interaction between the soil and the geosynthetic. This
is attributed to the necessary transfer of the stress in the soil to the geosynthetic material. The purpose of this is to
inhibit the development of tensile strains in the soil, and also to support the tensile stresses that the soil cannot
withstand [32]. The tensile stress supported by the geosynthetic improves the mechanical properties of the soil by
reducing the shear stress that develops and allows a greater shear resistance. As such, the shear strength of
reinforced soil relies on the mobilised shear resistance in the soil and the mobilised tensile stress in the
reinforcement.
There are many factors that could have an effect on the soil-geosynthetic interaction, such as the material
properties of the soil, the construction process, and the mechanical properties of the reinforcement. The
mechanisms of interaction that are critical in reinforced systems are:
• Skin friction along the reinforcement (interfacial),
• Soil-soil friction (inter granular), and
• Passive thrust (induced) on the bearing members of the reinforcement.
The skin friction is the resistance that is mobilized between the soil and the surface of the geosynthetic material as
shown in Figure 18(a). In geotextiles this is the only mechanism that is developed. However in geogrids, there is
also the development of soil-soil friction as the granules protrude through the apertures of the reinforcement. In
addition, there is also the passive thrust that the granules exert on the bearing members (ribs and junctions) of
the geogrids, as shown in Figure 18(b) [32].
The tensioned membrane action is easily mobilised in geotextiles. However, woven geogrids are also able to
activate this action as they are also flexible. In addition, it is usually only beneficial if deformations occur in the soil
structure before the tensioned membrane action can be activated. As such, the depth of placement of the
reinforcement is critical to allow for this action to take place. From Figure 19 it can be seen that when there is no
geosynthetic reinforcement, the stresses in the granular base and subgrade are equal, which shows that the
applied load is transferred through to the softer subgrade layer below. However, on inclusion of the reinforcement
there is a reduction in the stress in the subgrade as there is effective vertical support by the geosynthetic due to
the transfer of the vertical stresses to the geosynthetics in the form of lateral stresses (tensile strain).

_________________________________________________________________________________________________
Fig. 18. Soil-geogrid interaction mechanisms: (a) shear between soil and plane surfaces and (b) soil bearing on
reinforcement surfaces [32].
Fig. 19. Illustration of the tensioned membrane action of geosynthetic reinforcement [13].
During the construction phase deformation occurs as the soil is compacted, delivering the highest pressure the
soil will ever take, thus developing the tensioned membrane effect in the geosynthetics and lateral confinement of
the soil particles. The increase in load-bearing capacity with increasing settlement is due to the mobilisation of
more of the reinforcement mechanisms of the geosynthetic that include lateral restraint and tensioned membrane
effect, that are particularly activated with high settlements. At low vertical displacements, the aggregate particles
primarily provide support to the footing as the load is applied. At higher vertical displacements, the aggregate
particles are destabilised to a greater degree, being laterally distributed with an increase in outward shear stress.
This shear stress is transferred to the geosynthetic membrane that has high tensile strength, thus resisting the
lateral distribution of the aggregate particles, keeping the composite soil structure stable, and leading to an
increase in the load-bearing capacity. The geosynthetic product with the greatest improvement in the bearing
pressure was the Extruded Geogrid (EGG), and the layer geometry that gave the best increase in the load capacity
was when the geosynthetic layer was at a depth of placement of 0.67B and the fill thickness was 1.0B, B represents
the footing width.
VI. CONCLUSIONS
The study investigated the reinforcement benefits of the inclusion of a geosynthetic layer in a two-layered soil
representing pavement structures. The necessity of the reinforcement was due to the occurrence of soft subgrade
soils on sites that had relatively low load-bearing capacities and were susceptible to high settlements. This would
lead to failure of the pavement structures during the construction phase and through their design life.

_________________________________________________________________________________________________
Loading tests were conducted on the unreinforced soft kaolin clay, clay reinforced only by granular material, and
on geosynthetic-reinforced two-layered soil composite, using a Zwick Universal Compression and Tension
machine. The types of geosynthetic products used in the experiments included: extruded geogrids, woven
geogrids, woven geotextiles, and non-woven geotextiles.
The following conclusions can be made:
1. The study demonstrated that the inclusion of a geosynthetic layer in a two-layered soil, either within the fill or
at the interface, had the benefit of improving the soil structure by increasing the load-bearing capacity and
subsequently reducing the settlement.
2. The improvement in load-bearing capacity due to the addition of only a layer of granular fill ranged from 20% -
50%, whereas on inclusion of a geosynthetic layer either within the fill layer or at the interface of the soils,
there was an overall improvement that ranged from 35% - 160%.
3. The overall reduction in settlement due to geosynthetic inclusion, at a pressure of 85 kPa, ranged from 35% to
60% and this differed with different products. The least reduction in settlement was observed in the woven
geotextiles, while the most reduction was observed in the extruded geogrids.
4. It was observed that, when the geosynthetic was placed within the fill layer, the reinforcement from the
geosynthetic was mobilised at shallower depths than when it was placed at the interface of the two soils. This
led to greater improvements in the load-bearing capacity for the former configuration than the latter. The
optimum depth of placement obtained was 0.67B and the optimum range of fill thickness obtained was 1.0B –
1.5B. These were both dependent on the geosynthetic product used as they each had different reinforcement
benefits.
5. Geogrids generally provided better reinforcement improvements than geotextiles.
6. The reduction in base/sub-base thickness, possible due to geosynthetic reinforcement, was dependent on the
initial unreinforced thickness, and ranged from 25% - 67%.
ACKNOWLEDGEMENTS
The authors would like to express their sincere appreciation to Maccaferri Africa for providing the geosynthetic
material used in the research, and the Geotechnical Engineering Research Group of the University of Cape Town for
the support.
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