# Use of Lignin, Waste Tire Rubber, and Waste Glass for Soil Stabilization
## 5 Conclusions  
The complex interaction between soil and additives such as lignin, glass powder, and rubber tires was investigated using principles of material and soil mechanics. The aim of this research is to determine the most suitable combination of hybrid wastes to improve the subgrade of road pavements. The results of this experimental program will provide important information for the design of road pavements where the subgrade is improved with hybrid wastes such as lignocellulosic, glass powder, and rubber tires. In addition, the most effective mixtures of hybrid wastes are proposed to achieve the most subgrade improvements. The results of the laboratory tests allow engineers to determine the most important parameters for the design of road pavements that include subgrade stabilization with hybrid wastes. The most important observations resulting from the test program are the following:  
* The homogeneous distribution of grains in the stabilized clay soil has caused the microstructure of the soil to change positively so that the void content and optimum water content of the soil have decreased while the maximum dry unit weight has increased. * Since lignin is water-repellent due to its structure, it is thought that the water content of lignin-added samples is lower. Waste tire rubber-added samples absorbed water more than lignin-added samples. It was found that glass powder had a positive influence on the unit weight values of all samples tested. * Increasing the amount of waste glass powder and lignin additives as well as the curing time had a positive effect on strength, which is consistent with several research papers [24; 71; 72; 73; 74]. Figure 10: Resilient modulus versus bulk stress relationships at a confining pressure of 41.4 kPa. Regardless of the amount of additives, an increase in the curing time increased the strength of the samples at all mixing ratios. Based on these results, it is assumed that waste vehicle tires have a positive effect on the load-bearing capacity of the control sample due to their steel fiber content, as also reported in various research articles [65; 66; 67]. The soaked CBR value of the samples with lignin admixture is more pronounced compared to the samples with rubber tires. The results show that the most effective mixture for the combination of waste glass powder and rubber tires contains 20% glass powder and 3% rubber tires, based on the dry weight of the soil. For the combination of waste glass powder and lignin, the optimum mixture consists of 15% glass powder and 15% lignin, based on the dry weight of the soil. These results provide valuable insights into the sustainable use of waste materials for soil stabilization in road construction projects. This study focuses on the stabilization of a single soil type with waste materials. This limits the generalizability of the results of this study to the potential to improve different soil types with waste additives. In addition, the prepared mixtures can be conditioned for longer curing times, and the changes in their strength can be observed. Future studies can focus on the potential of using the wastes used in this study as additives for soils with different Atterberg limits, particle size distributions, and shear strength. In this way, the results can be evaluated with a statistical approach and a general conclusion can be drawn about the use of waste as a stabilizing material. Further research is needed to investigate the rubber tire crumb in terms of physical properties and angular and rough particles on mechanical properties. In addition, to ensure the adequate CBR of the subgrade, the appropriate thickness of the layer to be subjected to soil improvement needs to be further determined. Moisture content is a critical factor affecting the resilient modulus, i.e., increasing the moisture content beyond the optimum moisture content will result in a decrease in the resilient modulus due to a decrease in effective stress. Conceptualization, S.G., C.G., B.E.K., P.J. and B.Z. ; methodology, S.G., C.G., B.E.K., P.J. and K.G.