Introduction

In the course of urbanization, scarcity of land has led technology to shift its focus toward constructing over discarded land or landfills having very low bearing capacity and high compressibility. To enable construction over such lands, the soil needs to be stabilized and reinforced. Ground improvement techniques are an attempt to modify the existing land by enhancing its engineering properties. Many innovations have emerged to control the settlement of the subsoil as well as increase its bearing capacity so that these otherwise-abandoned lands can be potentially utilized. But the important objective lies in selecting the most suitable and economical method of improving the bearing capacity of the soil without sacrificing on the safety and reliability. One such way of improving the strength of the underlying soil is by using geosynthetics. Geosynthetics are polymeric products available in various forms (e.g., geocells geotextiles, geonets, geomembranes, geosynthetic clay liners, geogrids, geofoam and geocomposites). Geocells, also known as cellular confinement systems, are three-dimensional honeycombed cellular structures which form a confinement system when infilled with soil. These are flexible perforated planar polymeric strips welded together ultrasonically in series which act as a composite entity when infilled with soil. The cellular confinement reduces the lateral movement of the soil particles and forms a stiffened mattress that distributes overcoming loads over a wider area. The perforations present in the strips facilitate the drainage of fluids which enhances the performance of the cells. Geocells are widely used in construction for erosion control and soil stabilization. They are honeycomb-interconnected cells that completely encase the soil and provide confinement to prevent lateral spreading of infill material. The cellular pockets of the Geocells are infilled usually with sand, aggregates, cohesive soil or concrete depending upon the availability of the material. This flexibility of the geocell system to be able to use a large array of materials as the infill makes it more acceptable and convenient choice of ground improvement. Understanding the growing environmental concern, there is a vital demand to develop materials that are equally strong as the synthetic materials but also do not harm or pollute the environment. Coir fiber which is obtained from the husk of coconut is one of the strongest natural fibers. It has high tensile strength and retains much of its tensile strength when wet. Coir enables the production of various types of permeable fabric which when used in association with soil has the ability to separate, filter, reinforce, stabilize, protect or drain. Hence, they can serve as a better replacement for the commercially employed HDPE geocells. While the durability of coir geocells is a question, on the other hand, the polymer-based geocells are long-lasting.

Many researchers have conducted experimental and numerical studies on high-density polyethylene (HDPE) geocells. In recent years, few studies have been performed on geocells made from natural materials as well. Jute and coir are the most promising economically viable and vegetable fiber fabrics. Jute and coir fibers have higher water absorption capacity as well as a lower impact on the environment than manmade fiber [1]. Sudhakaran et al. [2] and Karthikeyan et al. [3] investigated the effect of soil–bottom ash mixtures reinforced with areca fiber and coir fiber, respectively. Latha et al. [4] investigated the effect of geosynthetics reinforcement form on the bearing capacity of square footings on sand. A biaxial geogrid and geonet were used for reinforcing the sand beds, and the study reported that geocell reinforcement is the most beneficial form of soil reinforcement system provided there is no rupture of the material. Rajagopal et al. [5] studied the influence of geocell confinement on the strength and stiffness behavior of granular soils. From triaxial compression test results on granular soil encased in single and multiple geocells, they concluded that the granular soil develops a large amount of apparent cohesive strength due to the confinement by the geocell. Hegde and Sitharam [6] conducted laboratory studies including plate load test on bamboo geocells and analytical studies to compare the values obtained by increasing the confining pressure and failure strain. Jones and Clarke [7] studied the residual strength of geosynthetic reinforcement subjected to accelerated creep testing and sited seismic events. Geosynthetic materials have a knack of developing creep, and this makes it necessary to study their creep behavior. Rajagopal et al. [5] studied the influence of geocell confinement on the strength and stiffness behavior of granular soils. A large number of triaxial compression tests were performed on granular soil encased in single and multiple geocells. In general, it was observed that the granular soil develops a large amount of apparent cohesive strength due to the confinement by the geocell. The magnitude of this cohesive strength was observed to be dependent on the properties of the geosynthetic used to fabricate the geocell.

Vinod et al. [8] studied the effectiveness of horizontally placed braided coir rope reinforcement for the strength improvement and settlement reduction in loose sand through laboratory plate load tests on model footings. The model test results indicated that up to about a sixfold improvement in strength and about ninety percent reduction in settlement (vertical displacement) can be achieved through the use of the proposed reinforcing method. Chen et al. [9] found that the confinement effect of geocells improves vastly the shear strength of granular soil. Triaxial tests were conducted on geocell-reinforced sand samples of different sizes. The influencing factors examined include the shape (circular, rectangular and hexagonal cross sections), size and number of cells. It was also found that circular cells induced the highest apparent cohesion among cells of all shapes, while the apparent cohesion induced by hexagonal cell was the lowest. The study by Pokharel et al. [10] investigated the factors influencing the stiffness and bearing capacity of single-cell–geocell-reinforced bases which included the shape, type, embedment, height of geocells and quality of infill materials The results showed that the geocell placed in a circular shape had a higher stiffness and bearing capacity of the reinforced base than that placed in an elliptical shape. Also, the geocell that has a higher elastic modulus had a higher stiffness and bearing capacity of the reinforced base. Moghaddas Tafreshi et al. [11] stated that the use of granulated rubber layers along with geocells is shown to reduce the plastic deformations and to increase the resilient displacements compared to the comparable non-rubber construction. By optimal use of geocells and granulated rubber, deformations can be reduced by 60–70% compared with the unreinforced case, while stresses in the foundation soil are spread much more effective. Mandal [1] recommended the use of jute for ground improvement system and concluded that natural fibers have very good technical properties, strength extensibility flexibility and durability. Currently, jute and coir are the most promising economically viable and vegetable fiber fabrics. Jute and coir fibers gave higher water absorption capacity as well as the lower impact on the environment than manmade fiber does. Hejazi et al. [12] have attempted to classify the fibers and revive the idea of randomly placing geocells which will increase the strength. They divided fibers based on their origin as natural (sisal, coconut, bamboo, cane, barley straw, jute, palm fibers) and synthetic (polypropylene, polyester, polyethylene, polyvinyl, glass, steel, nylon fibers), based on their ultimate tensile strength and specific gravity. Kumar and Kolathayar [13] reviewed the potential of tire waste in soil reinforcement and its application in Geocells as infill materials. Kolathayar [14] presented the overview of the vibration isolation of foundation using HDPE and natural geocells and stated that there are very few studies conducted on natural geocells. This paper studies the performance of coir geocell with different infill materials compared to that of the commercially available HDPE geocells, as a soil reinforcing material.

Materials

The materials used for testing are shown in Fig. 1.

Fig. 1
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Materials used in the testing

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Sandy soil bed The soil used in the testing program was river sand passing through 4.75-mm sieve with specific gravity Gs of 2.65. Table 1 presents the properties of sand used for the testing, as soil bed and infill material. According to USCS soil classification system, the sand was classified to be SP, poorly graded sand. The angle of internal friction of sand obtained through the direct shear test was 46.36°.

Table 1 The properties of the sand used as an infill material as well as for the soil bed
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Coir geocells Coir geocells were hand-stitched from a coir mat that had Aratory ribbed yarn. The geocells were fabricated with a pocket size of 125 mm × 105 mm and a cell depth of 75 mm. The properties of coir mat and coir geocells are presented in Tables 2 and 3, respectively.

Table 2 Properties of coir mat
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Table 3 Properties of coir geocell
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HDPE geocells Commercially available HDPE geocells of pocket size 250 × 210 mm were bought and sized down to the pocket size of 125 mm × 105 mm. It is necessary to scale down the geocells in order to fit the model and footing size as well as to make sure that the footing was placed upon the geocell network to replicate field condition. The properties of HDPE geocells are shown in Table 4.

Table 4 Properties of HDPE geocells
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Seashells Seashells used for the testing program were collected from coastal regions of Kanyakumari district in the state of Tamil Nadu, India. Seashells were washed to remove organic matter and dust and then sun-dried before being used in the tests. They were then mechanically crushed, and the seashells passing through 12.5-mm sieve and retaining on 4.75-mm sieve were used as a substitute infill material.

Experimental program

Sieve analysis and direct shear tests were performed to determine the properties of the sand. Direct shear tests were conducted on different proportions of crushed seashell and sand mixture in order to find the optimum mix ratio. To find the settlement of the sandy soil bed under different reinforcement conditions, plate load tests were carried out. A standard model plate load test was conducted to evaluate the performance of different geocells and infill materials in improving the bearing capacity of the soil. A steel tank of span 700 mm × 700 mm and a depth of 700 mm was prefabricated for conducting the load test. The load application system consists of single acting hydraulic jack along with a manually operated pump. The load as measured through the load cell attached between the footing and the hydraulic jack. A steel rigid surface plate of 100 mm × 100 mm in plan and 12 mm thick was used as footing which was placed on the soil at the center of the tank. Two linear dial gauges of least count 0.01 and full range 50 mm were attached to the inner surface of the tank, and their tips were placed on the surface of the plate above 10 mm inwards from the edge of the plate to measure the downward displacement of the footings.

The steel tank was filled with 100 kg of sieved sand using sand pluviation technique. The relative density of 54% was maintained throughout for all model experiments. To find the height of fall for obtaining required relative density, many trials were performed and a calibration chart was prepared based on the height of fall and their respective relative densities. The HDPE geocell was placed on the foundation bed, and the cell pockets were filled with infill materials. Figures 2 and 3, respectively, show HDPE and coir geocells filled with seashell as infill material. The loading plate was placed at the center of the tank, and loading cell was attached between the hydraulic jack and footing. The load was applied using a manually operated hydraulic jack. The settlement of the plate was measured by dial gauges for every 20-kg-load increment. The test was terminated after the failure of the footing where it could not take the further load.

Fig. 2
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HDPE geocell network infilled with crushed seashells

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Fig. 3
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Coir geocell network infilled with crushed seashells

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The load tests were carried out for the following:

  1. 1.

    Unreinforced soil bed

  2. 2.

    Coir geocell-reinforced sand bed with sand as infill material

  3. 3.

    Coir geocell-reinforced sand bed with crushed seashells as infill material

  4. 4.

    Coir geocell-reinforced sand bed with crushed seashells and sand in a proportion of 20% and 80%, respectively.

  5. 5.

    HDPE–geocell-reinforced sand bed with sand as infill material.

  6. 6.

    HDPE–geocell-reinforced sand bed with crushed seashells as infill material

  7. 7.

    HDPE–geocell-reinforced sand bed with crushed seashells and sand in a proportion of 20% and 80%, respectively.

Results and discussions

Direct shear tests were conducted on different proportions of sand and crushed seashells in order to obtain the optimum mix ratio of the same (Table 5). The highest angle of friction of 30.5° was achieved when 20% of crushed seashells and 80% of sand were used.

Table 5 Direct shear test results for different mix proportions of sand and crushed seashells
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The plate load tests were independently performed as mentioned in the procedure. The values of the settlement corresponding to different values of the load were noted, and load–settlement behavior was plotted for different scenarios as shown in Figs. 4, 5 and 6.

Fig. 4
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Bearing pressure–settlement behavior of unreinforced soil bed and geocell-reinforced soil bed with sand as infill material

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Fig. 5
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Bearing pressure–settlement behavior of sand bed reinforced with coir geocell with different infill materials

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Fig. 6
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Bearing pressure–settlement behavior of sand bed reinforced with HDPE geocells with different infill materials

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When sand was used as infill material (Fig. 4), the settlement noticed for HDPE–geocells-reinforced sand bed at 840 kPa was around 40 mm, while the same settlement of 40 mm was observed for unreinforced sand at a lower bearing pressure of 105 kPa. When the soil was reinforced with coir geocells, it showed a good improvement in the bearing capacity. A settlement of 40 mm was observed at a higher bearing capacity of 1780 kPa. Thus, it was observed that the settlement is very high when it is unreinforced when compared to the settlement in the reinforced sand. HDPE material is more brittle comparatively, and hence, it shows a sudden increase in settlement values after a certain point. This leads to the conclusion that natural fiber made geocells perform better than the commercially available HDPE geocells. This coir geocells further promote the growth of plants, while the HDPE geocells may serve as a hindrance to plant roots.

The settlement spotted for sand bed reinforced with coir geocells infilled with composition of 20% seashells and 80% sand at 2020 kPa was around 35 mm, while the same settlement of 35 mm was observed for sand bed reinforced with coir geocells infilled completely with crushed seashells, at a lower bearing pressure of 1540 kPa (Fig. 5). It was observed that the settlement of the sand bed is very high when the coir geocells are infilled with sand alone. It exhibits a settlement of 35 mm at a further lower bearing pressure of 1480 kPa.

Similar was the case when the coir geocells were replaced with HDPE cells. The composition of 20% seashells and 80% sand proved to show less settlement (Fig. 6). When only crushed seashells or sand were filled in the geocell pockets, the settlement was higher. Figure 7 presents the comparison of bearing capacities of coir geocell and HDPE–geocell-reinforced soil with different infill materials. The bearing capacity of coir geocell-reinforced sand was obtained as two times higher than that of HDPE geocells. The preference of reinforcing soil with more economical and environment-friendly coir geocell is practical as well as sustainable. From experimental study, it can be inferred that settlement versus load plot for coir geocell was obtained linear and hence, it is an ideal substitute material for HDPE. Substantial results were obtained while using seashells as infill material in coir and HDPE geocells. Maximum bearing capacity was achieved for a combination of 20% seashell and 80% sand infilled in coir geocell.

Fig. 7
figure 7

Comparison of bearing capacities of coir geocell and HDPE–geocell-reinforced soil with different infill materials

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Conclusion

This paper demonstrated the suitability of coir geocells as an alternative to HDPE geocells through a series of model laboratory plate load tests. The paper also evaluated the performance of sand and seashell as infill materials in HDPE and coir geocell pockets along with sand. Major findings from the study are listed below.

  • The bearing capacity of coir geocell-reinforced sand was obtained two times higher than that of HDPE–geocells-reinforced sand bed.

  • The preference of reinforcing soil with more economical and environment-friendly coir geocell is practicable as well as sustainable.

  • The bearing pressure–settlement plot for coir geocell was linear, and it can be inferred that coir is an ideal substitute material for HDPE.

  • Substantial results were obtained while using seashells as infill material in coir and HDPE material. Maximum bearing capacity was achieved for a combination of 20% seashell and 80% sand infilled in coir geocell.

  • Reinforcing the soil with abundantly available, eco-friendly and cost-efficient coir fiber woven into geocells and seashells suitably mixed in the backfill turns out to be a superior approach to surge the bearing capacity of the weak subsoil and thereby provides access for establishing construction over the same.

  • The employment of coir material will most likely develop a business arena for the farmers, and the utilization of seashell waste will relegate its harmful impact of being a threat to the environment.

  • Reinforcing the soil with abundantly available, eco-friendly and cost-efficient coir fiber woven into geocells and seashells suitably mixed in the backfill turns out to be a superior approach to surge the bearing capacity of the weak subsoil.

  • The employment of coir material will most likely develop a business arena for the farmers, and the utilization of seashell waste will relegate its harmful impact of being a threat to the environment.

  • The results observed are subjected to the scale effect as the tank size and footing size were small compared to real field scenario. However, the general trend and basic mechanisms could be similar.

  • Coir is a natural degradable material, and hence, durability of these geocells for long-term applications is a challenge. Proper treatment needs to be done to prevent decay of coir fibers, thereby increasing the durability.