# Application of Lignin for Slope Bioengineering: Effect on Soil Improvement and Plant Growth
## 4 Results and Discussion  
### Compressive and Shear Strength Characteristics  
Pocket penetrometer tests. Figures 1 and 2 show the results from a series of PP tests conducted over 40 days on each soil treated with different lignin concentrations. These results are compared with the data obtained for the untreated soil (only water) to estimate the positive contribution of lignin treatment. As can be seen in Figure 1, the strength of Soil 1 treated with lignin increased over time; however, so did the strength of the untreated soil. The latter can be attributed to the soil surface drying, which was observed during testing. However, compared to the untreated soil, the soil mixed with lignin solutions consistently produced greater values of soil strength. The difference in strength (almost twofold) was the most pronounced when Method 1 was used. According to [PERSON] et al. [3], an increase in the strength of lignin-treated soil can occur because lignin fills the pore space between soil particles and adds a biopolymer coating to the particles. A similar tendency was observed for Soil 2 (Figure 2), where the soil specimen mixed with 3% lignin solution exhibited a much greater value (1.5-2 times compared to the untreated soils) of strength over time. Figure 1: Unconfined compressive strength of Soil 1 treated with (**a**) 3% lignin solution and (**b**) 1% lignin solution. Figure 2: Unconfined compressive strength of Soil 2 treated with (**a**) 3% lignin solution and (**b**) 1% lignin solution. The comparison between the lignin treatment methods indicates that the mixing of soil with lignin (Method 1) was associated with greater increases in soil strength. It can also be observed from Figures 1 and 2 that, for both Soils 1 and 2, the strength of soil treated with lignin solutions by Method 2 was similar to the strength of untreated soil. The data given in Figures 1 and 2 suggest that the addition of lignin could increase the strength of both high (Soil 1) and low (Soil 2) plasticity soil, especially when Method 1 was used. Interestingly, for both soils, the noticeable increase in the strength of lignin-treated soils began to occur after 7-10 days from the beginning of the experiment. This finding correlated with the laboratory results of previous studies that indicated that a curing time of 7-10 days was sufficient to reach a high value of strength. Although an increase in soil strength over time can be observed in Figures 1 and 2, the obtained points tend to fluctuate. This data variation can be attributed to the following reasons:  
* For each measurement, at least five pocket penetrometer tests were conducted, and the average value was used to estimate the soil surface strength. The PP measurements were performed in different parts of the soil samples. As the soil mass was not homogenous (due to the soil nature and experimental preparations), different values were sometimes obtained, depending on the point selected for the measurement. * For each experiment, the soil surface was sprayed with 100 mL of water twice per week to stimulate seed germination and growth, and changes in moisture on the soil surface contributed to the soil inhomogeneity. * Over time, the soil samples had a natural tendency to dry on the surface, initiating surface cracks. This process contributed to the soil inhomogeneity as well, and impacted the measured soil strength. Vane shear tests. The results of vane shear tests are summarized in Figure 3. Despite the soil type, similar results were obtained; that is, the specimens treated with lignin solutions produced greater strength than the untreated specimens. Interestingly, the vane shear strength of the black and brown soil specimens mixed with 1% lignin solution exhibited a greater strength than the specimens treated with the 3% lignin solution. Also, when the lignin solution was sprayed on the surface of all three soil specimens, the vane shear strength of the specimen with 3% lignin was always slightly greater than the strength of the specimens treated with 1% lignin solution.