Improving municipal solid waste landfill liners using synthetic fibers and di-calcium phosphate

Abstract
Cracking of landfill’s clay liners after implementation reduces life expectancy and quality of the liners by increasing hydraulic conductivity due to preferential flow. The objective of this study was to produce a liner with less cracking potential, during and after implementation and more absorption capacity of contaminants. Furthermore, it was aimed to come up with simpler and more inexpensive liner compared to geosynthetic liners. For this purpose, three different levels of polypropylene fibers including 0.5, 0.75 and 1.0 of the weight of the soil were applied to a simple clay liner. To increase the contaminants absorption ability of liner, 0.2 percent Di-calcium phosphate was added. The clay behavior, its cracking and permeability of simple clay liner, liner with fiber and liner with fiber and DCP were examined in laboratory scale. The obtained results indicated that 0.75% of fiber can significantly reduce the surface and subsurface cracks. Although the use of 0.75% fiber has led to increase permeability compares to the simple liner at laboratory scale, but it was remained in the acceptable range of clay liners. The examined physical parameters also showed that use of the additives do not have any adverse effect on liner. This implys that the quantity of permeability, shear strength and the elimination of cracking in the presence of 0.75% fiber and 0.2% DCP are optimal for this purpose.
Materials & Methods
The collected soil sample were transferred to the laboratory, dried and prepared for testing. Based on the unified system of soil classification, the texture of samples was clay loam (CL) and had a plastic and liquid limit of 25 and 10, respectively. The specific gravity of this soil was measured at 2.65 g / cm3. The fiber used in this research was polypropylene (C3H6) with an approximate length of 2.5 cm. Di-calcium phosphate (DCP) with the chemical formula CaHPO4, 2H2O, was used as a phosphate additive. Three different types of samples were prepared and tested in this study. To prepare a simple liner sample, 500 mm of water was added to 3000 g of soil was sample, so that the percentage of moisture in the samples was 16.6%. Then the specimens were allowed to reach a uniform paste condition. This sample was transferred into the test cell and loaded for a period of time to prepare the permeability coefficient measurement. Fibrous samples were prepared similar to the simple one, except that before adding water to the soil, the required amount of fiber was randomly added to the soil and mixed well. Polypropylene fibers were used in three levels with 0.5, 0.75 and 1% by weight of soil. Third, in addition to the fibers, DCP was also applied. To prepare these samples, before adding water to the soil, di-calcium phosphate was added to the water and mixed with the mixer to form a gray-colored liquid. The resulting liquid was then added as sample water. The DCP value in all experiments was considered as 0.2% by weight of the soil sample. The prepared samples were then placed in a humidification room for 10 days in order to obtain the chemical equilibrium, and the effect of the additive on the soil behavior was stable. In order to evaluate the performance of improved dense clay liner with polypropylene fibers and di-calcium phosphate, permeability tests, direct cutting and determination of cracking value were performed on seven different samples each with two replicates. Some permeability tests were carried out based on ASTM-D2434 (2006) standard in a cylindrical cell made of Plexiglass with a diameter of 150 mm and a height of 130 mm. Direct cutting test was performed according to ASTM D3080 / D3080M-11 (2011) instructions.

Discussion of Results & Conclusions
The permeability of simple clay liner measured in 24 hours, was a value of about (0.8-1)×10-8 centimeters per second. The infiltration coefficient was increased by about four times in samples with 0.5% fiber compared to clay without fibers, and was obtained a value between (3-3.5)×10-8 cm/s. This process was also observed for the application of 0.75 and 0.1% fiber, and the permeability changes acquired in these two states between (3-4)×10-8 and (4-5)×10-8 cm / s. respectively. In other words, the hydraulic conductivity after application of these two quantities of fibers, on average, was about 3.5 and 4.5 times than permeability of simple clay, respectively. By adding 0.5% fiber in the presence of DCP, the permeability value was obtained to be about (2-3)×10-8 cm/s. Similar to the non-DCP samples, adding DCP simultaneously with fibers in the clay linear increase the conductivity by about three times than the simple clay sample. The permeability of the clay sample with 0.75% fiber plus DCP was 3 to 5 times larger than the simple clay sample. In samples with 0.1% fiber plus DCP, the hydraulic conductivity of the clay liner increased about 4 to 6 times than the simple one. Therefore, the fiber increases permeability, but the addition of DCP does not have much effect on penetration changes. The results of the direct shear test showed that the maximum shear strength in the case where DCP was present was not significantly different from that in which the fiber was used alone.
It seems that, by adding 0.5% of the fiber to the soil, the φ of sample increased, but in the following, with increasing fiber content, ie, 0.75 and 0.1% of the fibers, φ decreased. The reason for this soil behavior is the high volume of fibers compared to its light weight. At each step, increasing fiber, adhesion has also increased. In fact, the increase in fiber has contributed to the increased adhesion of clay linings.
The results indicated that the addition of fibers and DCP can increase the shear strength of the maximum soil. In addition, observing the effect of the fiber on the reduction of surface and inside cracking of clay liners is also a very important factor. Because these cracks have a significant adverse effect on the mechanical condition and the permeability of the lining. It also reduces the longevity and performance of the clay liners. The simple liner specimen, first had hairy cracks, but after passing the time and drying the surface, cracked deep and wide. The samples made in this study show a very small scale of clay liners, and in reality the dimensions of the liners are much larger. As a result, fractures and cracks will have a lot more size and depth. In samples with 0.75% fibers, the desiccation cracks completely faded and a smooth and uniform sample was obtained. From the integrity of this sample, it can be concluded that this fiber percentage has also been able to greatly eliminate deep cracks. 1% Polypropylene fiber has a lot of volume based on its light weight, which may cause improperly distribution. That's why it cannot completely absorb soil particles and completely prevent desiccation cracks.
The results observed in the fiber samples were also the same in the presence of DCP. That is, in clay sample with 0.5% fiber and in the presence of DCP, cracks behavioral had a similar effect as 0.5% of non-additive fibers. The analysis of the results obtained from the experiments shows that the hydraulic conductivity of the fibrous clay liners were lower than simple one, but the advantage of using the fibers is the reduction of the desiccation cracks, which can be a major factor in the significant increase in the hydraulic conductivity of the liners as a result of the preferential flow. In general, the elimination of contractions with 0.75% fibers was more pronounced than 0.5%, and the appropriate range for using fibers seems to be between 0.5% and 0.75%.