Keywords

1 Introduction

1.1 Background of the Present Study

In a country like India, construction of rural road networks plays a pivotal role in its socio-economic development. However, many of the existing rural roads are becoming structurally ineffective because of the rapid growth in traffic volume and axle loading. Another major problems faced by rural roads across the country are that they are built in poor subgrade with low California bearing ratio (CBR). Low CBR increases the construction cost due to large pavement thickness. IRC specifies a minimum CBR of 4% for rural roads. To achieve the specified CBR, subgrades are nowadays reinforced with geotextiles. Natural geotextiles like coir are good alternatives to geosynthetics due to its cost-efficient and eco-friendly nature. From the previous studies conducted by Vinod and Minu (2010), it has been found that the inclusion of coir geotextile as reinforcement in soil improves the California bearing ratio of lateritic soil. Sajikumar et al. (2014) studied the performance of coir geotextile reinforced using Benkelman beam deflection (BBD) test and concluded that the variation in deflections of reinforced roads is less compared to unreinforced roads. In the present study, two flexible pavements constructed by incorporating coir geotextile were considered and their deflection under traffic load was found out using ABAQUS. Also, a finite element model of laboratory CBR test for both coir geotextile-reinforced and geotextile-unreinforced section has been developed. Such a model could be effectively used to choose the type of coir geotextile suitable for different soils. In addition to this, pavement thickness reduction and overlay thickness reduction due to coir reinforcement are also found out for six pavement field data as per IRC specifications and catalogues.

1.2 Data Collection

Six roads reinforced with coir geotextiles were selected for the study. They are presented in Table 1. They are designated as road 1 to road 6.

Table 1 Road details
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The properties of coir geotextiles used for reinforcing the roads are presented in Table 2. The BBD values of the road after 6 years of construction are presented in Table 3, and the CBR values of the reinforced and unreinforced soil are presented in Table 4.

Table 2 Properties of coir geotextile (Sajikumar 2014)
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Table 3 BBD values (Sajikumar 2014)
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Table 4 CBR of subgrade
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1.3 Plan for Numerical Analysis

Numerical analysis was carried out for finding the deflection of road 1 and road 2 with two different coir geotextiles GT1 and GT2. For both the road sections, deflections were found out with and without incorporating coir geotextiles. Laboratory CBR prediction was conducted for worst condition, i.e. for clayey soil. All the analyses were done for with and without coir geotextile. Laboratory CBR prediction and deflection of reinforced section were analysed with two different coir geotextiles GT1 and GT2.

2 FE Modelling Methodology

2.1 Geometry Idealization

In this study, the road sections were idealized as a plain strain condition and hence modelled as a 2D deformable body. Soil and coir geotextile were idealized as a 3D deformable body. The road section model and laboratory CBR model used for the analysis are shown in Figs. 1 and 2, respectively

Fig. 1
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Model of reinforced road section

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Fig. 2
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Model of laboratory CBR model

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2.2 Material Behaviour

Subgrade soil has been considered as an elasto-plastic material. The plastic behaviour of soil is defined by Mohr–Coulomb model (friction angle and cohesion), and coir geotextile has been considered as a deformable material. The elastic modulus of GT1 and GT2 is 132 and 90 kPa, respectively. Poisson’s ratio of coir geotextile has been taken as 0.35. The subgrade soil used in the study is clayey soil.

2.3 Load and Boundary Condition

Traffic load for performance evaluation was simulated by giving moving load of corresponding cumulative standard axles (CSA). CSA were calculated as per IRC 37: 2001 load. The bottom portion of the road section is fully encastred, and vertical movement is allowed only on the sides. CSA calculated for each road from the traffic data collected are presented in Table 5.

Table 5 Cumulative standard axle load
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For laboratory CBR prediction, displacement of 5 mm is given over a circular area of 50 mm diameter which could simulate the actual laboratory condition of load applied through a plunger of 50 mm diameter. The bottom portion is fully encastred, and vertical movement is only allowed in the sides.

2.4 Meshing and Interaction

Road section has been modelled as a 2D plain strain condition with uniform mesh size. A 4-noded bilinear plain strain quadrilateral (CPE4R) was selected as the mesh element type. Coir geotextile also has the same element type and meshing. For CBR prediction, soil was modelled as a 3D object with finer mesh at loading surface and coarser mesh at far ends. An 8-noded linear brick element (C3D8R) has been used as the mesh element. Coir reinforcement was modelled as a 3D object with hexahedral shape and C3D8R element type. Interaction between coir geotextile and soil has been simulated by giving surface to surface contact with coefficient of friction. The meshed CBR model is shown in Fig. 3.

Fig. 3
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Meshed CBR model

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3 Data Analysis

From the available field data of BBD values, the reduction in overlay thickness due to the inclusion of coir geotextile has been calculated as per IRC 81:1997.The overlay thickness depends on two major factors, namely stability of the existing flexible pavement and anticipated traffic loading in terms of CSA load repetition during the desired design life of the overlay. In addition to this, the reduction in pavement thickness has also been evaluated from the CBR data of six roads as per IRC SP 72: 2015.

3.1 Overlay Thickness Reduction

Overlay thickness is generally provided for continuous maintenance of roads. Additional overlay thickness is determined using overlay thickness design curves of IRC 81: 1997. For this curve, two parameters are needed for evaluation which are BBD values in mm and CSA. As per IRC 81: 1997, there is no need of any upgradation work when the deflection is below 0.45 mm for 100 million standard axle loads.

3.2 Pavement Thickness Reduction

As per IRC SP 72: 2015, pavement thickness for low volume rural roads is calculated based on CSA and CBR of the subgrade.

4 Results and Discussion

4.1 Numerical Study for CBR Prediction

Numerical simulation of laboratory CBR has been done for both reinforced and unreinforced soil. Reinforcement is placed at the centre of the section. The deformed shape of the soil subgrade is shown in Fig. 4.

Fig. 4
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Deformed shape of unreinforced soil

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CBR prediction of reinforced soil has been done by using two different grade coir geotextiles, i.e. GT1 and GT2. The load versus penetration curve for GT1 coir geotextile-reinforced subgrade soil is shown in Fig. 5. The reaction force developed in GT1-reinforced clayey soil is shown in Fig. 6.

Fig. 5
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Load versus penetration curve of GT1-reinforced and GT1-unreinforced section

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Fig. 6
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Reaction force developed in GT1-reinforced section

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The reaction force developed in GT2-reinforced clayey soil is shown in Fig. 7. The load versus penetration curve for GT2 coir geotextile-reinforced subgrade soil is shown in Fig. 8 (Table 6).

Fig. 7
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Reaction force developed in GT2-reinforced soil section

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Fig. 8
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Load versus penetration curve for GT2-reinforced and GT2-unreinforced section

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Table 6 Results obtained from analysis
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4.2 Numerical Study for Deflection

The displacement is considered as a response of applying traffic loads. The magnitude of the displacement beneath the centre of the load at the end of loading is taken as the maximum deflection experienced by the road section. Displacement contours on deformed shape for unreinforced road Sects. 1 and 2 are shown in Figs. 9 and 10, respectively.

Fig. 9
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Displacement contour of unreinforced road 1

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Fig. 10
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Displacement contour of unreinforced road 2

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The same road section is reinforced with two different geotextiles GT1 and GT2. Displacement contours of the reinforced sections are shown in Figs. 11 and 12, respectively.

Fig. 11
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Displacement contour of GT1 reinforced road1

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Fig. 12
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Displacement contour of GT2 reinforced road1

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The maximum deflection obtained from the numerical analysis for both road 1 and road 2 is as shown in Table 7.

Table 7 Maximum deflection obtained from the analysis
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From the BBD values and CSA loads, the overlay thickness is calculated for all six roads with the help of overlay thickness design curves as per IRC 81: 1997. The allowable limit of deflection having no need of any improvement works in the pavement as per IRC 81: 1997 is 0.45 mm. Roads 1, 2, 3 and 4 have the deflection values less than the allowable limit of deflection for both unreinforced and reinforced case. So, there is no need of upgradation of pavement. The deflection values for road 5 and 6 are not within the allowable limits. So, the upgradation is required for road 5 and 6. The calculated overlay thickness for road 5 and 6 is shown in Table 8. Overlay thickness required for coir geotextile-reinforced pavement is much less than that of unreinforced section.

Table 8 Overlay thickness
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4.3 Reduction in Pavement Thickness

The pavement thickness of roads both with and without coir geotextiles calculated as per IRC: SP 72: 2015 is presented in Table 9.

Table 9 Pavement thickness
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The average reduction in pavement thickness is about 20% was observed in coir geotextile-reinforced roads.

5 Conclusion

The pavement thickness reduction of six roads due to modified CBR value and a numerical simulation was also made to predict the modified CBR values of coir geotextile-reinforced soil, and the following are the conclusions drawn from the study.

  • From the numerical simulation, it is found that GT1 coir geotextile improves the CBR better than with GT2, which is in accordance with the actual case as reported by various researches.

  • Pavement thickness reduction was evaluated to be about 20% as per IRC design charts due to coir geotextile reinforcement.

From the study on the structural and numerical performance of coir geotextile-reinforced roads, the following conclusions can be drawn from the study.

  • The defection result shows that there is considerable reduction of deflection due to the coir reinforcement which reflects the rigidity and increased load bearing capacity of pavement.

  • The percentage reduction of overlay thickness of reinforced road is about 60–80 percentage of unreinforced section.