Keywords

1 Introduction

Weak subgrades are a widespread challenge in a temporary access road or a permanent road constructed over a weak subgrade road construction. The paved or unpaved surface can be deteriorated due to deformation of the subgrade. The benefit of geosynthetics in unpaved roads constructed over a weak subgrade is known to give a reinforcing benefit to the roadway sections. Geogrids, geonets, and geotextiles assist in sharing the loads more effectively and raise the efficient bearing capacity of the subgrade. Geosynthetic materials have been commonly utilized as reinforcement/ stabilization in structures through boundless materials, such as slopes, roads, embankments, and retaining walls. Geosynthetic stabilization and reinforcement is a mechanical process. Geosynthetics located either on top of the subgrade or within the subgrade/base course layer work with the soil and granular material to make a reinforced section through separation, confinement, and/or reinforcement functions.

Geo-grids are widely accepted as reinforcement for enhancing engineering strength [1]. Reinforced soil technology is one of the mainly successful fields of civil engineering and has gained broad popularity because of its functional, constructional, and cost-effective benefits [2]. The geosynthetic reinforcement is mostly located between the sub-base and base layers at the interface between the subgrade and sub-base layers or within the base course layer of the flexible pavement [3, 4]. The placement location of reinforcement is the major factor influencing the bearing capacity of reinforced granular soil and maximum bearing capacity is examined when the depth of placement of reinforcement is lowered. Thus, geotextile assists in decreasing the vertical stress acting on the subgrade than in unreinforced pavements [5]. The use of geosynthetic reinforcement is effective in a weak subgrade with prominence for higher rut depths which can mobilize the ‘tension membrane effect’ of the geotextile [6]. The existence of the reinforcement layer raises lateral restraint or passive resistance of the fill material, raising the rigidity of the system and decreasing the vertical and lateral pavement deformation [7,8,9]. Reinforcement positioned high up in the granular layer hinders the lateral movement of the aggregate due to frictional interaction and interlocking between the fill material and the reinforcement which increases the apparent load-spreading capacity of the aggregate and decreases the required fill thickness [10, 11].

Geosynthetics have been in permanent use over the previous few decades as a reinforcing material in the cross-sections of pavements. Geogrids are mainly effective for reinforcement purposes and therefore, are mainly utilized in the design of road cross-sections. The primary objective behind the reinforcement is to decrease the structural section without changing the traffic capacity of the pavement or the durability of the pavement. An experimental test setup is needed to investigate the strength characteristics of both unreinforced and reinforced sections to investigate the behavior and to realize how and to what extent geosynthetics affects the engineering characteristics of subgrade soil. The function of geosynthetics in the design of a flexible pavement system has been calculated by performing laboratory tests on similar pavement sections, both in unreinforced and reinforced situations.

Many investigations in the field of geosynthetics application in pavement design have depicted that there is and decrease in base course thickness for specified structural capacity [12,13,14] and extension in the working life of pavement [15, 16, 17, 18] Performed tests on unpaved sections with varying base course thicknesses and depicted that a geotextiles-reinforced section with a 350 mm thick base layer performed the same as an unreinforced section with a 450 mm thick base layer. [19] Examined the construction of field-reinforced sections that contained a base course that was 50 mm thinner than that of unreinforced sections. Geogrid is extremely firm in tension, if compared to the subgrade material or aggregates, hence lateral stress is minimized in the reinforced base aggregate, resulting in less vertical deformation at the road surface. Due to shear interaction between the geogrid and material, the shear strength and thus the load distribution capacity of the utilized base course material is significantly improved [20].

This relation supports the decrease of reinforced aggregate layer thickness in comparison to the un-reinforced aggregate layers. There is a 40% reduction in the base course thickness after reinforcement in comparison to the unreinforced section for similar load-carrying capacity.

The main aim of this research study is to assess the performance of unpaved road sections reinforced with geosynthetics at different depths thereby also determining the optimal position of reinforcement. For this reason, extensive small-scale in-box static PLTs were conducted on several geosynthetic reinforced and unreinforced unpaved test sections. The improvement due to reinforcement has been assessed in terms of bearing capacity and base course reduction. The unpaved sections have been reinforced by applying one and a double layer of geosynthetics at different locations in the cross-section. Three types of geosynthetics (geogrids, geonets, and geotextiles) have been used in this study and their performance has been compared.

2 Material Used in the Study

2.1 Laboratory Model Tests

Figure 1 shows a typical cross-section of the testing set of a model device used in this study. The test box is rectangular-shaped, having inside dimensions of 1000 mm × 1000 mm and 800 mm in depth, and the walls of the tank have a thickness of 6 mm.

Fig. 1.
figure 1

Typical cross-section showing positions of reinforcements and loading configuration

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2.2 Model Footing

The model footing was prepared from a steel square plate with a dimension of 200 mm × 200 mm, and 25 mm thickness. The load is transmitted to the footing via a ball bearing which is located between the footing and the proving ring. A proving ring of capacity 100 kN was applied.

2.3 Test Material

Important physical properties of the clay soil are shown in Table 1and it was categorized as CH according to the Unified Soil Classification System (USCS), and A-7-5 according to the American Association of State Highway and Transportation\Officials (AASHTO) classification systems.

Table 1. Properties of clay soil
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The size of aggregates ranges between 10–20 mm along with 10% stone dust from total weight were utilized to make the base course compacted to a unit weight of 20.61 kN/m3. This particular blend of materials in the base course has been chosen to determine the lowest voids and optimum compaction filled with stone dust, briefly stated by [21].

2.4 Reinforcement

Three types of geosynthetics (as shown in Fig. 2 during placement) with varying tensile strengths have been utilized in this study. The properties of these geosynthetics are given in Table 2.

Fig. 2.
figure 2

Geosynthetics creep testing machine, a) Nonwoven geotextile; b) Biaxial geogrid

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The tensile strength of the geotextile and geogrid, obtained by the wide-width strip method [1] with a creep testing machine was illustrated in Fig. 2.

Table 2. Properties of Geosynthetics
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3 Model Constructions and Testing Procedure

The typical cross-section of the testing setup is illustrated in Fig. 1. The test setup includes a 250 mm aggregate base layer overlying subgrade clay of thickness 450 mm contained in a rectangular tank. The subgrade layer is set in three layers compacted to attain a unit weight of 13.80 kN/m3 which is 90% of its maximum dry unit weight as determined by the Modified Proctor test. The gravel aggregates were located on top of the subgrade layer and blended with stone dust were located in 250 mm thickness compacted to attain a maximum dry unit weight. The load was used on the top manually by a mechanical jack in slight increases till getting a failure. The settlement of reinforced and unreinforced clay soil was measured utilizing two dial gauges located on the different sides of the footing. The small-scale in-box static plate load test during the placement of reinforcement in the laboratory is shown in Fig. 3.

Fig. 3.
figure 3

The test set up under progress for plate load test a) Biaxial geogrid; b)Geonet; c) Geotextile

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4 Results and Discussion

Results obtained from different laboratory tests by plate load to observe the load intensity versus settlement curves in both the unreinforced and reinforced conditions as illustrated in Figs. 4, 5, and 6 at different depths. The higher load intensities were recorded for a total settlement of 12.5 mm in all the test conditions. The position of geosynthetic reinforcement within the subgrade layer is one essential factor in the strength of unpaved sections. The following reinforcement configurations were selected to study this effect: placing geosynthetic at the base/subgrade interface, placing geosynthetic at 0.2 H, 0.4 H, 0.6 H, and 0.8 H of the subgrade layer, and placing double reinforcing layer at the upper (0.2 H and 0.4 H) and lower at (0.6 H and 0.8H) of the subgrade layer thickness. The effectiveness of reinforcement for the specified type of clay soil has been determined in the top one-third of the subgrade layer. [16] Examined that utilizing a geogrid at the top of the third layer in a soil sample by varying the plasticity index causes a significant rise in the CBR value compared with unreinforced soil in both soaked and unsoaked conditions. [22] Investigated the same trend while examining the result of geogrid reinforcement on the ultimate bearing capacity of sand.

It is examined from the results that there is a substantial increase in the load-carrying capacity of reinforced unpaved sections as compared to unreinforced conditions. The BCR value at the 12.5 mm settlement range for biaxial geogrid is 1.21–1.53, for geonet, it is 1.10–1.42 and from 1.05–1.51 for geotextile by moving the location of single layer reinforcement.

Fig. 4.
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Load intensity versus settlement curve for single biaxial geogrid reinforcement

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Fig. 5.
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Load intensity versus settlement curve for single nonwoven geotextile reinforcement

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Fig. 6.
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Load intensity versus settlement curve for single geonet reinforcement

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Figure 7 depicts load intensity versus settlement curves for the tests performed in both the unreinforced and double-reinforced conditions at varying depths. As predicted, the BCR rises as the number of reinforcement layers increases. The BCR is defined as the ratio of the bearing capacity of reinforced unpaved sections to that of the unreinforced unpaved section. Among the two-layer geosynthetics, biaxial geogrid performed better than geonets and geotextiles. [11] Explained that bearing capacity rises with an increase in the number of geogrid layers from 33.33% with a single layer to 44.44% with a double layer, though double-layer reinforcement may become too expensive in road construction.

Fig. 7.
figure 7

Load intensity versus settlement curve for double-layer geosynthetic reinforcement

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4.1 Ultimate Bearing Capacity of Reinforced Unpaved Sections

The experiment part of this study is considered to be a two-layer system. Since the performance of the granular aggregates base layer is much bigger than that of the underlying subgrade soil layer, a punching shear failure will exist in the granular aggregates layer followed by a general shear failure in the underlying soil layer. This kind of failure mode was primarily investigated by [13] for stronger soil overlying weaker soil. The cohesion and friction angle of the aggregate (base course) and the soft clay layer soil (subgrade) have been obtained from the experimental test as aggregate: Ct = 8.82 kPa (due to dust particles), Фt = 42°, and subgrade: Cb = 18 kPa, Фb = 19°. The adhesion, Ca, the punching shear coefficient, Ks, and the mobilized friction angle, \(\delta\), can be obtained by the graph given by Meyerhof and Hanna as 7.23 kPa, 5.2, and 30°, respectively.

5 Conclusions

The following major conclusions are drawn from the results presented from the in-box static plate load experimental tests carried out on several geosynthetic reinforced and unreinforced unpaved test sections.

  1. 1.

    For a single layer of reinforcement, the upper one-third position gave the maximum enhancements under static loading conditions.

  2. 2.

    Laying the geosynthetic reinforcement in double locations yielded the largest enhancement. However, double-layer reinforcement may become uneconomical in road construction.

  3. 3.

    The highest BCR value of about 1.53, with bi-axial geogrid, about 1.51 with geotextile, and about 1.42 with geonet were investigated when reinforced within the top one-third of the subgrade layer.

  4. 4.

    From the test result, the bearing Capacity of unpaved sections due to single reinforcements attained a rate of about 34.60% with bi-axial geogrid, about 29.81% with geonet, and about 32.50% with nonwoven geotextile.