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A Numerical Modelling Approach for Designing of
Capping Layer on Weak Subgrades with Geogrids
Shiran Jayakody
Queensland University of Technology
Chaminda Gallage
Queensland University of Technology
Research Article
Keywords: Capping Layer, Geogrid, Numerical Analysis, Pavement Design, Weak Subgrade
Posted Date: May 13th, 2024
DOI: https://doi.org/10.21203/rs.3.rs-4363172/v1
License: This work is licensed under a Creative Commons Attribution 4.0 International License.
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Additional Declarations: No competing interests reported.
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Abstract
A capping layer is a conventional mechanism used to stabilize weak subgrade, particularly in platform
design in the pavement industry, facilitating the operation of heavy machinery. The thickness of the
capping layer depends on the in-situ CBR strength and the target strength and stiffness properties of the
working platform. Geogrid reinforcement in subgrade stabilization has emerged as a sustainable solution,
reducing the demand for quarry aggregates by decreasing the thickness of the capping layer. However,
designing the capping layer on a weak subgrade when the strength is below 5% of California Bearing Ratio
(CBR) is dicult since conducting laboratory tests to investigate the properties of the weak subgrade
proves challenging due to its poor workability and inconsistent behaviors. Therefore, this study used
numerical analysis to assess the strength and stiffness with the parameters load bearing capacity and
strain modulus of stabilized subgrades with a capping layer and, with and without geogrid in the interface
of the subgrade and capping layer. The investigation focused on exploring different subgrade properties,
considering various CBR values below 5%, and altering the thickness of the granular capping layer. To
validate the developed numerical models, experimental data from large-scale model box tests were
employed. The obtained results were further analyzed to introduce an ecient and effective mechanism
for designing a capping layer thickness with and without geogrids, based on the subgrade strength
conditions. The design curves are ultimately applicable to optimizing capping layer design on the weak
subgrade of CBR below 5%.
1 Introduction
Subgrade stabilization is a signicant challenge in pavement construction, particularly in regions like
Australia where weak and expansive clay soils are prevalent [8, 19, 20]. Consequently, it is required to
focus on ground improvement techniques to reinforce the inherently weak subgrade. This is essential for
establishing a stable foundation as a temporary working platform at the outset of any project, facilitating
the operation of heavy construction machinery [9, 25]. The conventional approach to stabilizing weak
subgrades in platform design involves the use of a capping layer [26, 14]. Determining the optimal
thickness of the capping layer to achieve the desired strength while accounting for subgrade conditions
and material properties is paramount, as it contributes to minimizing project costs, reducing construction
time, and optimizing material usage [14, 27].
Geosynthetics presents a viable alternative for reinforcing the working platforms, avoiding the need for
further increasing the thickness of the granular layer [38]. Additionally, geosynthetics offer many
environmental and economic benets, particularly mitigating the demand for natural materials and
maintenance costs compared to alternative subgrade improvement methods [10, 11, 44]. The
reinforcement behavior of geosynthetics is complicated and depends upon various factors including the
type of geosynthetic, number of geosynthetic layers, type and gradation of the covering material, and the
positioning of the geosynthetic layers [40, 29, 5]. Numerous researchers have investigated the
reinforcement effect of geosynthetics on weak subgrades both in laboratory and eld settings [39], with
ndings consistently demonstrating signicant improvements in strength and stiffness properties [5, 24].
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Geogrid is a main type of geosynthetic which extensively employed for reinforcement purposes [35],
contributing to the enhancement of the modulus and bearing capacity of weak subgrades [2, 28]. The
laboratory characterization of geogrid reinforcement has been done by many researchers to incorporate
the geogrid material into the subgrade improvement with a granular layer [23, 21]. The studies have
revealed the benecial effects of the geogrid layer under loading, primarily through two main mechanisms;
lateral connement effect and vertical membrane effect [13, 36]. The lateral connement effect creates
the interface frictional interaction and interlocking between the granular layer and geogrid layer thereby,
reducing vertical stress on the subgrade by enhancing the stiffness in the granular layer [31, 46].
Conversely, the vertical membrane effect mitigates the outward shear stress induced by the repetitive
loading, thereby, distributing stress more widely around the geogrid layer and reducing the stress on
subgrade [32].
Numerical analysis has emerged as a valuable tool for accurately simulating geogrid reinforcement in
subgrade stabilization, particularly in cases where laboratory and eld tests are impractical due to
constraints such as cost, time, and logistical reasons [43]. Recent advancements in numerical analysis
have provided insights into the complex behavior of geogrid reinforcement, particularly concerning its
interaction with granular layers and subgrade soils under both monotonic and cyclic loading conditions.
Abbas and Abd [1] and Das et al. [15] effectively simulated the inuence of geogrids on the bearing
capacity of sandy soil, highlighting the impact of load eccentricity relative to the slope crest on foundation
behavior and the density of the sand layer, respectively. Chowdhury and Patra [12] also conducted an
analysis on cohesionless soil materials, comparing settlement reduction in reinforced soil with geogrids
versus unreinforced soils. Kolay et al. [30] investigated the bearing capacity of cohesive soils overlaid with
thin sand layers and reinforced with geogrid material. Their study revealed a signicant improvement in
bearing capacity, with an increase of over 70%. The analysis of [3] also observed 50% of improvement of
bearing capacity with geogrids in sandy soil. However, numerical simulations have encountered
diculties, such as effectively capturing the interlocking effect, aperture structure (size and shape), and
interface friction particularly when granular layers are incorporated with geogrids [42, 13, 17]. Therefore,
geogrid reinforcement layer is assumed to have rough characteristics to prevent slippage at the interface
with granular materials, even when geogrid materials are represented by stretched elastic membranes
[16].
Previous studies have shown little research on strengthening the soft subgrade with a granular capping
layer and geogrid reinforcement. Determining the optimal thickness of the capping layer for working
platforms based on laboratory test results presents a signicant challenge, particularly for subgrades
weaker than 5.0% CBR. Strategically placing a capping layer becomes vital for stabilizing these weak
subgrades, ensuring durable construction platforms. The present study aims to examine the stabilization
of weak subgrades (CBR 5.0%) with a capping layer, both with and without geogrid reinforcement to
serve as a working platform. In this regard, the study presents an experimental validated 2-dimensional
(2D) nite element (FE) models of stabilized subgrades with varying thicknesses of capping layers, with
and without a geogrid at the interface between the subgrade and the capping layer. The properties of the
stabilized subgrades were analyzed based on two main parameters: bearing capacity and strain modulus,
≤
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considering variations in capping layer thickness and in-situ subgrade CBR. Furthermore, the results were
analyzed to develop design curves for determining the capping layer thickness on weak subgrades, with or
without geogrid (CBR≤5%), to achieve the desired strength.
2 Materials and Methods
This study aims to investigate the performance of extremely weak subgrade soils with California Bearing
Ratio (CBR) of less than 5.0%. Laboratory testing on such weak subgrade soil is challenging due to its
poor workability. Therefore, the nite element method (FEM) was employed as a solution tool for modeling
and analyzing the performance of weak subgrades with stabilization mechanisms. A granular capping
layer and a geogrid at the interface of the subgrade and capping layer were simulated to assess strength
and stiffness characteristics. The FEM models were initially validated using similar large-scale laboratory
test results to dene material properties and ensure the accuracy of the numerical analysis. Ultimately, the
results were used to develop design charts for determining the thickness of the capping layer with and
without geogrid, to achieve the target strength in platform designing on weak subgrade of CBR less than
5.0%.
2.1 Numerical Modeling
The PLAXIS 2D nite element tool was used for numerical modeling, assuming plain strain conditions. A
2D axis-symmetric modeling with 15-noded structural solid elements was used for modeling as shown in
Figure 1(a). Previous research has indicated that the consistency of the mesh size has some inuence on
the predicted results [6] thus, medium size mesh was used for both layers for more accurate results in all
simulations. The materials used in the numerical models were dened as homogeneous. The numerical
models comprised a gravel layer (capping layer) atop a subgrade layer (clay soil). The properties of the
capping layer materials remained constant for all applications, while the thickness varied as 100, 200, 300,
400 and, 500 mm. The subgrade maintained a constant thickness of 500 mm, but its properties were
altered to correspond to CBR strength of 0.5, 1.0, 1.5, 2.5, 3.5, 5.0%.
Table 1 presents the materials properties of capping layer and subgrade soils which were tested and
derived[45]. Capping layer materials represent Materials Type 2.1[33]high-quality base layer materials.
The hardening soil model with small-strain stiffness was employed for the granular capping layer
materials to account for the characteristics of very small-strain stiffness and non-linear dependency on
the strain amplitude of granular materials[7]. The subgrade soils used for validation of numerical models
represent CBR 1.5 and 2.5% clay soil since the laboratory tests were conducted on those subgrade soils.
The hardening soil model in Plaxis was applied for subgrade soil for simulating the behavior of soft
soil[22]. The properties of the subgrades for different CBR values (below 5%) were derived based on the
validated results of subgrades of CBR 1.5 and 2.5%. The geogrid reinforcement effect was achieved by
positioning a single layer of geogrid at the interface between the capping layer and subgrade. In PLAXIS
analyses, the axial stiffness of the geogrid was held constant at 1000 kN/m, with the strength reduction
factor Rint equal to 1.
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Monotonic load was applied on the surface in two load cycles. The rst load cycle was applied up to 550
kPa or 5 mm of vertical deformation, whichever comes rst, to represent the wheel pressure of an axle
load and [18], the second load cycle was applied till the failure of each model. The bearing capacity was
measured at 25 mm vertical deformation for consistent comparison of the strength and, the strain
modulus (EV2) was calculated as shown in Figure1(b) and equation (1) [18].
Table 1.Subgrade and Capping LayerMaterial Properties of Plaxis Analysis[33, 45]
Material
Type Material Properties
Type of
Model Unsaturated
unit weight
(kN/m3)
Void
ratio Specic
gravity E50(kPa) Eur(kPa C’
(kPa)
Φ’
(0)
Ψ
(0)
Gravel-
Capping
Layer
Small
strain
hardening
24.00 0.17 2.71 65e3 2e5 45 50 15
Subgrade-
Clay soil,
CBR 2.5%
Strain
hardening 16.50 1.3 2.60 4400 8800 50 2 0
Subgrade-
Clay soil,
CBR 1.5%
Strain
hardening 15.0 1.5 2.55 2700 5400 30 1 0
2.2 Model Validation
Certain assumptions were unavoidable in FEM, therefore, the material properties and other parameters of
the Plaxis 2D were conrmed by validating the numerical simulation results with three series of large-
scale laboratory test results conducted in a previous research study [45]. These tests have been
conducted in a specially designed steel box with internal dimensions of 1.0 m (length) x 1.0 m (width) x
1.2 m (height).
The congurations of the three tests series as follows:
Tests series 1; Two tests were conducted on subgrade soil with a thickness of 500 mm, representing
strength of 1.5% and 2.5% of CBR respectively.
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Test series 2; Three tests were conducted with granular capping layer thickness of 200 mm, 300 mm and
400mm on subgrade of 2.5% CBR.
Test series 3; Two tests were conducted with a 200 mm granular capping layer and a geogrid placed at the
interface of the subgrade and capping layer. Two different types of geogrids were used for these tests.
Additionally, two tests were conducted with a 300 mm granular capping layer and a geogrid placed at the
interface, using the same types of geogrids as those applied in the 200 mm capping layer tests.
The subgrade properties in test series 2 and 3 represented a strength of 2.5% CBR. The capping layer
materials were consistent across all tests, with properties based on experimental results from previous
studies as shown in Table 1.
Figures 2, 3 and 4 illustrate the comparison between laboratory measured and FEM predicted vertical
deformation vs stress at the surface under monotonic loading. Figure 2 presents results of clay subgrade
and Figure 3 presents the stabilized subgrade with capping layer thickness 200, 300 and 400 mm.
According to the graphs, UBC and EV2 values agreed with those model box test results and numerical
simulations.
Figure 4 (a) and (b) present the validation of the geogrids reinforced models for 200 mm and 300 mm
capping layers thickness on subgrade with 2.5% of CBR strength. The best-tting curves align with the
UBC and strain modulus; however, they do not exhibit the anticipated nonlinear behavior. This discrepancy
arises from the challenge of accurately simulating the nonlinear behavior of granular materials in Plaxis
2D. The comparison of extracted strain modulus values shown in Figure 5, considering both with and
without geogrid effects across different capping layer thicknesses, demonstrates good agreement. This
conrms the validation of the developed models in predicting the impact of capping layer thickness and
geogrid use on the stabilization of weak subgrade soil.
There were slight differences observed in the numerical analysis and laboratory test results may be due to
the following reasons: (1) in-large scale model box tests, it was dicult to accurately control the
consistency of the material properties such as moisture homogenizations, uniform compaction etc., (2)
The measured results were based on realistic three-dimensional contact load while the numerical analysis
was 2D simulation. Overall, the stress vs vertical deformation responses predicted by numerical
simulation are fundamentally consistent with measured results from laboratory tests. Therefore, the
developed and validated FE models of stabilized subgrades with a capping layer were employed for
further analysis under different conditions of subgrade and capping layer thicknesses, both with and
without geogrid reinforcement effect.
3 Results and Discussion
A series of numerical analyses were conducted to determine the effects of geogrids with a granular
capping layer on the stabilization of weak subgrade soil. Total vertical settlements and bearing capacity
were observed as strength indicators, and strain modulus (EV2) values were calculated as the key
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performance indicator for monotonic loading on the subgrade models. The modulus results were further
analyzed to develop design curves for determination of capping layer thickness, with or without a geogrid,
on soft subgrades with low strength below CBR 5.0%
3.1 Strength Properties of Weak Subgrade
Recent research outcomes have highlighted the signicance of accurately evaluating the strength
properties of the subgrade when a new mechanism introduces for designing the capping layers [37]
particularly for low strength subgrades. Investigating the strength characteristics of a weak subgrade,
when the CBR is below 5.0%, presents challenges due to poor workability of materials. Hence, validated
Plaxis nite element models corresponding to CBR values of 1.5% and 2.5% were utilized to determine the
properties of subgrades with different strengths conditions below 5.0% of CBR in the material model
during Plaxis analysis. Subsequently, Plaxis models were developed to represent weak subgrade CBR
values of 0.5, 1.0, 1.5, 2.5, 3.5, and 5.0%, and were analyzed to determine their strength and stiffness
properties. Figure 6 shows the comparison of numerical analysis of low and high subgrade representing
CBR 0.5, 1.0, 3.5 and 5.0%. The numerical results illustrate high vertical deformation, approximately
150mm when CBR is below 1.0%, gradually reducing with increasing CBR up to 5.0%. In contrast, a higher
concentration of stress from the loading plate can be observed, with values around 405 kPa in the
subgrade with a CBR of 5.0%. The stress bulb propagation extends to a large depth, with stresses at
around 25 kPa at a depth of approximately 500mm, as observed at the lower limit of the bulb. However, a
reduction in stress propagation is observed with decreasing CBR, being lowest in the subgrade with a CBR
of 0.5%, with stress only around 40 kPa below the loading plate. The extracted values from the numerical
analysis, including ultimate bearing capacity and strain modulus values of all subgrades, were plotted in
Figure 7 alongside the results obtained from laboratory tests. The laboratory test results of CBR values of
1.5% and 2.5% show close values, conrming the validity of the numerical models. These models were
employed to derive the material properties of weak subgrades for the analysis of strength properties of
stabilized subgrades with a capping layer, both with and without a geogrid.
3.2 Strength of Stabilized Subgrade
Validated models (Figure 3 and 4) were employed to estimate the strength properties: bearing capacity
and vertical deformation of the stabilized subgrade with a capping layer and, both with and without a
geogrid at the interface of the capping layer and subgrade soil. The subgrade properties were varied to
compromise the strength as per CBR range of 0.5% to 5.0% and the capping layer thickness was varied as
100, 200, 300, 400 and 500 mm for a single type of material.
Numerical analysis revealed a signicant improvement in the stabilized subgrades when a geogrid is
placed at the interface of the subgrade and capping layer. Figure 8 represents an example of numerical
analysis comparing low and high CBR values of the subgrade (CBR 1.0% and 3.5%, respectively),
illustrating the vertical deformations of the stabilized subgrade proles under the same loading conditions
(550 kPa) and with a consistent capping layer thickness of 200mm, both with and without geogrids. Figure
8(a) demonstrates that the reduction in vertical deformation with the inclusion of a geogrid is
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approximately 26.6% for a 200mm granular capping layer when the subgrade CBR is 1.0%. Similar effects
are observed with an increase in subgrade CBR, as shown in Figure 8(b), which exhibits approximately a
23% reduction in surface vertical deformation when the subgrade CBR is 3.5%. However, the effect of the
geogrid decreases with an increase in subgrade strength (CBR) as well as with capping layer thickness.
The numerical analysis further reveals that the reduction in vertical deformation ranges from
approximately 34% to 9.0% when the capping layer thickness increases from 100mm to 500mm for the
stabilized subgrade with the lowest CBR of 0.5%. For the stabilized subgrade with a CBR of 5.0%, the
reduction ranges from approximately 14% to 2.5% over the same increase in capping layer thickness from
100mm to 500mm.
The bearing capacity was analyzed as the typical strength parameter in numerical analysis, and the load
bearing capacity values of the analyzed models were extracted at a vertical deformation of 25mm for
standard consistency in comparison, as shown in Figure 9 [41]. The results show that in-situ subgrade
with a CBR less than 1.0% can be improved above the respective required minimum strength for
constructions, which is 5.0% of CBR, with a 100mm thick capping layer and a geogrid at the interface.
However, it requires a 200mm granular capping layer without a geogrid. Considerable enhancement of the
bearing capacity (over 1.0 MPa) can be predicted when the capping layer thickness is above 400mm for
weak subgrades with a CBR less than 1.0%. However, when the subgrade CBR is 1.0%, a signicant
improvement in the bearing capacity, exceeding 2.0 MPa, was achieved with a greater capping layer
thickness of 500mm and a geogrid at the interface. Further analysis revealed a diminishing effect of the
capping layer thickness on the bearing capacity of weak subgrades as the layer thickness increased, as
illustrated in Figure 10. The highest percentage increase in bearing capacity, 115%, was observed in
subgrade CBR 0.5% with a 100mm capping layer, but the magnitude was lower due to the weakest
subgrade, and the effect of the geogrid gradually decreased to 80% and 62% with 200mm and 300mm
capping layers, respectively. In contrast, it signicantly dropped for the same subgrade strength to
approximately 20% with the increase in 400mm and 500mm capping layers, reecting the reduction of the
geogrid reinforcement effect with increases in capping layer thickness on soft subgrade. This trend is
commonly observed when the subgrade strength increases from 0.5 to 5.0%. The percentage increase in
bearing capacity was reduced from 115% to 25% when the subgrade strength varied from 0.5% to 5.0% for
a constant capping layer thickness of 100mm. With the increase in capping layer thickness, the effect of
geogrid reinforcement further reduced on the subgrade with high strength of CBR 5.0%, as it was observed
to be around 2-4% for 400-500mm capping layer thickness. In summary, Figure 10 illustrates that geogrid
reinforcement is highly effective when the in-situ subgrade CBR is less than 2.5% with 100-300mm
capping layer thickness, as it shows over a 30% increase in bearing capacity. This agrees with the
phenomenon that the inuence of geogrid reinforcement decreases with the distance of the loading from
the geogrid. Therefore, it is desirable to stabilize weak subgrades with 200-300mm capping layer
thickness, although it depends on the requirement of the target strength.
3.3 Stiffness of Stabilized Subgrades
Evaluation of subgrade stabilization using a capping layer typically involves considering ultimate bearing
capacity and modulus values. However, recent research emphasizes the importance of analyzing the
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modulus of stabilized subgrades to ensure consistent and precise assessment[34]. The strain modulus
(EV2) was evaluated to analyze the effect of capping layer thickness and geogrid reinforcement on the
stiffness properties of the stabilized subgrades. The results of EV2 values concerning different capping
layer thicknesses on various subgrade CBR levels (ranging from 0.5 to 5.0%) are depicted in Figure 11.
Continuous lines represent EV2 values with the effect of geogrid reinforcement, while dashed lines
represent EV2 values with only the capping layer. In the comparison of geogrid reinforcement, the most
signicant impact was observed for a 100mm capping layer thickness, exhibiting a strain modulus
improvement ranging from 1.2 to 1.8 times within the subgrade CBR range of 0.5 to 5.0%. This effect
gradually diminishes with increasing capping layer thickness, with improvements ranging from 1.15 to
1.55 for 200mm, 1.05 to 1.23 for 300mm, and minimal effect for 400mm and 500mm thicknesses,
resulting in approximately a 1.04 times improvement when the subgrade strength varies from 0.5 to 5.0%.
The effect of geogrid reinforcement becomes minimal with increasing capping layer thickness when the
in-situ subgrade CBR increases to 5.0%. The effect is signicant for 100mm and 200mm capping layer
thickness within the subgrade CBR range of 0.5 to 5.0%. However, the maximum improvement of the
subgrade with a CBR of 0.5% is less than 35 MPa, even with geogrid reinforcement and a 500mm capping
layer. This improvement is not considered signicant, as suggested by Adorjányi [4], who recommends
enhancing the subgrade modulus to at least 40 MPa with a capping layer to ensure a safer working
platform. Furthermore, it was observed that a 100mm capping layer thickness did not achieve stiffness
beyond 40 MPa, even with geogrid reinforcement for subgrades with a high CBR of 5.0%. Therefore, the
ndings recommend improving weak subgrades with a CBR below 1.0% using chemical methods such as
lime and/or y ash mixing to enhance the CBR to at least above 1.0% to achieve the ultimate effect of
geogrid reinforcement with a granular capping layer.
3.4 Optimal Capping Layer Thickness for Weak Subgrade
The numerically evaluated EV2 values of stabilized subgrades were further analyzed to develop design-
curves to estimate the required thickness of granular capping layer to achieve the desired strength of
platform designing on weak subgrade. A correlation was developed for different CBR strength of
subgrades between two parameters, subgrade improvement ratio (SIR) and cover ratio which are dened
in equations (2) and (3) respectively.
SIR =EV2 (Stabilized subgrade)/EV2 (subgrade) (2)
Cover ratio = Thickness of granular capping layer / Diameter of loading area (3)
(in this study, diameter of loading area = 200mm)
Figure 12 presents six developed curves representing different strengths of subgrades CBR from 0.5 to
5.0%. These curves determine the necessary capping layer thickness to achieve the desired improvement
level (EV2) of the subgrade, provided the CBR of the existing subgrade is known. Two curves in each gure
represent the subgrade stabilisation with and without geogrid reinforcement. For instance, when it
requires to increase the EV2 by 10 times without geogrid reinforcement (SIR=10) for a subgrade with a
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CBR of 1.0% (corresponding cover ratio=3.1), the calculated capping layer thickness would be 620mm (3.1
x 200mm). When it with a geogrid reinforcement the “cover ratio is 2.5” thereby, the required capping layer
thickness is 500mm (2.5 x 200mm).
The developed design graphs illustrate the signicant impact of capping layer thickness on subgrade
strength, particularly when the CBR is lower. For instance, the EV2 of a subgrade with 0.5% CBR can
increase approximately 20 times with a 700mm capping layer and a geogrid in the interface, whereas it
only exhibits an increase of about 3 times for subgrades with a strength of 5.0% CBR. This observation
suggests that the improvement ratio is less sensitive to variations in layer thickness when the in-situ CBR
strength of the subgrade is higher. Furthermore, the graphs demonstrate the high eciency of geogrid
reinforcement in the middle range of subgrade CBR from 1.0 to 3.5%; conversely, they show that the two
curves (with and without geogrid reinforcement) are merging for low subgrade CBR below 1.0% and high
CBR of 5.0%.
The developed curves are applicable for effectively designing the capping layers on weak subgrades when
the CBR is below 5.0%. However, the design is specic to applying high quality base course materials with
similar properties to material Type 2.1 as per MRTS05 [33] specications. In contrast, the mechanism is
applicable to develop similar charts for different types of capping layer materials.
4 Conclusions
The numerical analysis conducted in this study assessed the strength and stiffness of stabilized
subgrades by examining parameters; load-bearing capacity and strain modulus, both with and without a
geogrid at the interface of the subgrade and capping layer. The investigation aimed to explore various
subgrade properties, including different CBR values below 5%, and varying the thickness of the granular
capping layer. Based on this investigation, the following conclusions were drawn:
The numerical analysis revealed a greater effect on geogrid reinforcement when both the subgrade
CBR and capping layer thickness are lower. It observed a 34% reduction of vertical deformation when
a geogrid was introduced with a 100mm capping layer on a subgrade CBR of 0.5% and it was only
14% for subgrade CBR of 5.0%. Conversely, for the high thickness of the capping layer, 500mm the
reduction of vertical deformation was revealed 9.0% for subgrade CBR 0.5 and only 2.5% observed for
subgrade with 5.0% of CBR strength.
The study found that assessing stiffness in stabilized subgrades is more effective than examining
strength. For example, subgrades with CBR below 1.0% can meet construction strength requirements
of 5.0% CBR with a 100mm capping layer and geogrid but need a 300mm layer without a geogrid. Yet,
for stiffness, a 300mm capping layer with geogrid achieves the stiffness of a 5.0% CBR subgrade
(22.5MPa). Thus, for both strength and stiffness, a 300mm capping layer with a geogrid is optimal; if
not, over 400mm without a geogrid is needed.
The inuence of geogrid reinforcement on load-bearing capacity diminishes as the distance between
the surface loading and the geogrid increases, and this effect is further diminished with higher
subgrade strength. This was particularly noticeable with increasing thicknesses of the capping layer,
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where the effectiveness of geogrid reinforcement on a high-strength subgrade with 5.0% CBR
decreases. It was observed that the bearing capacity improvement of around 2-4% for capping layer
thicknesses ranging from 400mm to 500mm.
The increase in stiffness with geogrid reinforcement also showed minimal effect with the increase of
capping layer thicknesses to 400mm and 500mm, resulting in an approximate 1.04 times
improvement for subgrade CBR of 5.0%. Conversely, the effect on stiffness gradually increases with
decreasing capping layer thickness, reaching a maximum average value of 1.5 times increase in
stiffness with a geogrid and capping layer thickness of 100mm.
The developed curves are employed for precisely designing capping layers on weak subgrades when
the CBR is below 5.0%. The graphs illustrate the improvement ratio is highly sensitive when the in-situ
CBR strength of the subgrade is lower. For instance, the EV2 of a subgrade with 0.5% CBR can
increase approximately 20 times with a 700mm capping layer and a geogrid in the interface, however,
it only exhibits an increase of about 3 times for subgrades with a strength of 5.0% CBR.
Declarations
All authors certify that they have no aliations with or involvement in any organization or entity with any
nancial interest or non-nancial interest in the subject matter or materials discussed in this manuscript.
Author Contribution
Authors Contribution:Shiran Jayakody: Conceptualization, Methodology, Software, Investigation,
Validation, Visualization, Data Curation, Writing - Original Draft. Chaminda Gallage: Supervision,
Conceptualization, Methodology, Project administration, Funding acquisition, Resources and Editing.
Please address all correspondence concerning this manuscript to me at Email:
s.jayakodyarachchige@qut.edu.auContact details of co-authors:Chaminda Gallage:
chaminda.gallage@qut.edu.auThank you for your consideration of this manuscript. Yours Sincerely,Shiran
Jayakody
Acknowledgment
This study is an integral part of the research project (Project No IH18.06.1) sponsored by SPARC Hub at
the Department of Civil Engineering, Monash University funded by the Australian Research Council (ARC)
Industrial Transformation Research Hub (ITRH) Scheme (Project ID: IH180100010). The nancial and in-
kind support from the Department of Transport and Main Roads (Queensland), Logan City Council, Global
Synthetic (Australia), Polyfabrics (Australia), and the Queensland University of Technology is gratefully
acknowledged. The nancial support from ARC is highly acknowledged.
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Figures
Figure 1
(a) Plaxis 2D model (200mm capping layer with 500mm subgrade) (b) Parameters of Strain Modulus, EV2
calculation.
Figure 2
Page 16/23
Stress vs vertical deformation curves subgrade CBR 2.5% for validation
Figure 3
Subgrade CBR 2.5% with capping layer 200mm, 300mm and 400mm for validation.
* CL = Capping Layer, GG = Geogrid, MB = Model Box, NA = Numerical Analysis, SG = Subgrade
Figure 4
Subgrade CBR 2.5% with geogrid reinforcement and (a) capping layer 200mm, (b) capping layer 300mm
for validation.
* CL = Capping Layer, GG = Geogrid, MB = Model Box, NA = Numerical Analysis, SG = Subgrade
Page 17/23
Figure 5
Strain modulus (EV2) values of validated numerical models and laboratory test results.
Page 18/23
Figure 6
Vertical deformation (left side) and stress distribution (right side) at the failure of the subgrade soil with
varying strengths represented by CBR 0.5, 1.0, 3.5, 5.0%.
Page 19/23
Figure 7
Correlation of Ultimate Bearing Capacity and strain modulus with the CBR of Weak Subgrade Soil
respectively.
Page 20/23
Figure 8
Comparison of total vertical deformation: (left) without geogrid and (right) with geogrid, at a surface
stress of 550 kPa and subgrade conditions of CBR (a)1.0% and (b) 3.5%, with a 200mm capping layer.
Page 21/23
Figure 9
Inuence of capping layer thickness and geogrid on Ultimate Bearing Capacity at 25mm vertical
settlement for various subgrade strengths, ranging from CBR 0.5% to 5.0%.
Page 22/23
Figure 10
Effect of geogrid on the bearing capacity for different capping layer thickness and different subgrade CBR
0.5 to 5.0%.
Figure 11
Strain modulus (EV2) with geogrid and capping layer on subgrade varying across different in-situ CBR
ranges from 0.5% to 5.0%.
Page 23/23
Figure 12
Design curves to predict the capping layer thickness on soft subgrade soil with and without geogrids.
NG – No geogrid, WG – With geogrid reinforcement
