Effect of fiber length on the consolidation parameters of coir fiber-reinforced soft clay

. Coir fibers are natural fibers with high tensile strength and pressure resistance, making them an alternative soil reinforcement material. However, the effect of fiber length on improving soil consolidation properties has yet to be largely explored. Therefore, this study investigated the effect of the length and content of coir fibers on soft soil mechanical behavior through a series of one-dimensional consolidation tests. Fibers of >10 mm to 30 mm and >30 mm to 50 mm lengths were mixed randomly into the soil with varying content, ranging from 0.00% to 1.25% of the total weight of the mixture. The relationship between the effective stress and the void ratio was explored, unveiling that the void ratio of soil specimens reinforced with coir fibers decreased more gradually than those unreinforced. An increase in the fiber content reduced the compression index (Cc) value and enhanced the coefficient of consolidation (Cv) value, with longer fibers producing more significant values due to more excellent interactions between fibers. Fibers of >30 mm-50 mm yielded optimum Cc and Cv values at the fiber contents of 0.50% to 0.75%.


Introduction
About 20 million hectares or more than 10% of soft soil deposits are formed across Indonesia.Soft soil consists of soft clay and peat soil.It is necessary to point out the soft soil since the constructed edifice might suffer harm before attaining its designated age of completion [1].
Various soil-reinforced methods have been developed to improve soil mechanical properties to meet specific technical requirements.One applicable method is mixing materials into the soil to enhance the bearing capacity and reduce the compressibility [2].Reinforcing the soil with coir fiber waste is also an alternative [3].It is one of the most widely investigated in reinforced soil compared to other natural fibers (i.e., banana fiber, sisal fiber, and jute fiber) [4].
Unfortunately, as Lone and Bawa mentioned, construction rarely employs coir fibers.Despite having a high economic value due to their abundance and low prices, coir fibers are only utilized as household handicrafts and fuel [5].Each coir fiber comprises 75% crude fibers and 25% cork, connecting one fiber to another.Coir fibers are shaped about 60-250 mm with 0.1-0.45mm diameter and have 15-327 MPa of tensile strength, elongating about 10-75% [6].
Coir fibers exhibit elevated levels of lignin and cellulose components [7].Significant amounts of lignin make coir fibers resistant to microbial deterioration and saline conditions.Conversely, the high cellulose concentration in coir contributes to its capacity to maintain tensile strength even when moist.Due to its high cellulose content, coir can retain its tensile strength when wet [2].It is supported by the tensile strength test of coir fibers by Widianti et al. [8], who explained that coir fibers have a high tensile strength value, so it is expected to increase the bearing capacity of soft soil.Compared to synthetic fibers, coir fibers are more elastic and have a higher coefficient of friction [9][10][11], making them strong and more durable than other natural fibers [12].
Kar and Pradhan [13] studied the compressibility of coir fiber-reinforced soil.The study revealed that soilmixed fibers declined the soil compression index (Cc).In contrast, the consolidation coefficient (Cv) value rose.The optimum fiber content to achieve maximum value was 0.8% of the dry weight of the soil [13].Lawer and Boadu conducted research using various coir fiber contents.The test results uncovered that the compression index dropped with increasing fiber content and reached the optimum value at the fiber content of 0.6% [14].Moreover, Khatri et al. proved that adding coir fibers to soil boosted the pre-consolidation pressure.The results of this study can be used for shortterm stability problems and temporary access road applications [15].Furthermore, Jeludin et al. compared additional coir fibers with varying lengths using a consolidation test.The results demonstrated increased compression index (Cc) values in coir fibers with differing lengths.A longer coir length caused uneven mixing due to low friction and overlapping, leading to lower strength [16].Subsequently, Jayasree et al. conducted consolidation, swelling pressure, and threedimensional (3D) shrinkage tests to analyze the effect of adding coir waste to volume change behavior.Mixing coir waste with expansive soil reduced the volume change behavior of expansive soil [17].
Besides, Phanikumar and Singla concluded that strengthening expansive soil with randomly oriented The swell-consolidation test results unveiled that the vertical potential for swelling and swelling pressure decreased at the fiber content of 0.25% but increased slightly when the fiber content rose to 0.30%.The swelling pressure dropped with increasing fiber length.The secondary consolidation characteristic of fiberreinforced specimens was better than unreinforced specimens.The decrease was due to the interlocking between soil particles and fibers and the resulting friction between soil and fibers [18].

Soil
This research employed clay soil obtained from Sentolo, Kulon Progo, Yogyakarta, investigated for its physical characteristics in a previous study [8].Table 1 displays the results.

Coir fibers
Coir fibers were primarily sourced from conventional markets and subsequently extracted from coconut husk waste, isolating the fibers for use (Fig. 1).The coir fibers were then sectioned into lengths ranging from > 10 mm to 30 mm and > 30 mm to 50 mm.They were thoroughly blended into the soil until uniformly distributed.Table 2 exhibits the tensile strength of coir fibers.

Mix design
This study utilized different proportions of coir fiber content, ranging from 0.00% to 1.25% of the total weight of the mixture.For each sample, coir fibers measuring >10 mm to 30 mm and >30 mm to 50 mm in length were added.The mixing was conducted under optimal moisture content and maximum dry density conditions.Table 3 portrays the mixture design.

Testing procedure
The one-dimensional consolidation test using an oedometer was carried out at the Soil Mechanic Laboratory of Universitas Muhammadiyah Yogyakarta based on ASTM D2435/D2435M-11 [19] (Fig. 2).Samples were prepared using a ring mold with a diameter of 60 mm and a height of 20 mm.Four different effective stresses of 34.7 kPa, 69.4 kPa, 138.8 kPa, and 277.6 kPa were applied during testing.

The relationship between the effective stress and the void ratio
The one-dimensional consolidation test results are presented in Fig. 3. Fig. 3 indicates that the specimens without coir fiber content experienced a significant drop in the void ratio curve as the effective stress escalated.In contrast, those reinforced with coir fibers exhibited a more gradual decline in the void ratio.

The effect of coir fibers on the void ratio (e)
The effect of coir fibers on the void ratio (e) was also investigated to identify the influence of coir fibers on clay soil.Fig. 4 displays the void ratio value for each effective stress and fiber content variation.
Fig. 4 exhibits that specimens without coir fiber content have a higher void ratio than specimens reinforced with coir fiber at each effective stress.Fig. 4 also shows that longer coir fibers result in a smaller void ratio at a specific effective stress and a certain fiber content.This reduction occurred due to the increased friction and adhesion between the soil and the longer fibers.Additionally, when the volume of coir fibers enlarged, the interactions between the fibers also increased, resulting in more significant compression of the fibers.

The effect of coir fibers on the coefficient of consolidation (Cv)
The effect of coir fibers on the coefficient of consolidation is exhibited in Fig. 5.
The coefficient of consolidation (Cv) values of the specimens were evaluated to determine the effect of the length and content of coir fibers on the soil compressibility.At an effective stress of 34.7 kPa, the specimen without coir fibers acquired a Cv value of 0.00097 cm 2 /s.Fibers of >10 mm-30 mm produced Cv values that increased as the fiber content rose.The specimen with 1.25% fiber content obtained a Cv value of 0.00271 cm 2 /s, representing a 180% increase from those without fibers.Fibers of >30-50 mm in length generated higher Cv values, with a maximum value reaching the fiber content of 0.50%.This rise in the consolidation coefficient could be attributed to coir fibers occupying the soil voids, slowing down the flow of water and reducing the time required for primary consolidation [20].However, the decrease in the Cv value might be due to the folded fiber strands during mixing, resulting in lower-strength soil samples [21].Additionally, the increased volume of coir fibers led to more inter-fiber interactions, yielding greater fiber compression dominance [13].

The effect of coir fibers on the compression index (Cc)
The effect of coir fibers on the compression index is illustrated in Fig. 6.
The compression index values of the specimens were evaluated to determine the effect of the length and content of coir fibers on soil compressibility.The specimens without coir fibers had a compression index value of 0.544, which declined with the rising length and content of coir fibers.Fibers of >10-30 mm in length generated Cc values that dropped as the fiber content rose.The specimen with 1.25% fiber content yielded the most significant decrease in the compression index value, 0.322, with a reduction of 69%.Soil mixed with fiber with a length of >30-50 mm produces a minimum Cc value at the fiber content of 0.75%, 0.235.The Cc value would rise over the increase in fiber content.More and longer fibers increase the value of Cc because more fiber interactions will make fiber compression more dominant [13].In addition, Jeludin et al. [17] disclosed that too much coir caused uneven mixing due to inadequate mixing.

Conclusions
The test results lead to the following conclusions.a.The relationship between the effective stress and the void ratio revealed that soil specimens reinforced with coir fibers experienced a more gradual decrease in the void ratio than those unreinforced.b.An increase in fiber content decreased the void ratio, reduced the compression index (Cc), and raised the coefficient of consolidation (Cv) value.c.Longer fibers generated a more significant decrease in void ratio, reduction in Cc values and increased Cv values.d.The 0.50% to 0.75% fiber contents produced optimum Cc and Cv values.

Fig. 1 .
Fig. 1.The typical coir fibers used in this study.

Fig. 6 .
Fig. 6.The effect of coir fiber on the compression index for various coir fiber lengths.

Table 2 .
Tensile strength of coir fibers.

Table 3 .
The mixture design applied in this study.