# Influence of Xanthan Gum-Based Soil Conditioners on the Geotechnical Properties of Soils
## 4 Results and Discussion
### Drained Shear Strength  
Figure 8 (left) presents the results of direct shear tests for both untreated soil S1 and its mixtures prepared at a comparable initial water content and soaked during consolidation and shearing. Mohr--Coulomb failure envelopes for all investigated soils and mixtures exhibited a high degree of linearity (R2 \(\geq\) 0.98). Two repetitions of the tests were carried out for soil S1, mixed with SC_R H and SC_ZZC H, as shown in Figure 8 (right). The repeatability of the direct shear test results proved to be very good. The difference between the shear strength within the test range of effective vertical stresses is within 4% for SC_R H and even better for SC_ZZC H.  
Table 5 presents the measured values for the cohesion (\(c^{\prime}\)) and the friction angle (\(\varphi^{\prime}\)) for all untreated soils and their mixtures. Except for the mixtures of soils S2 or S3 with SC_R L, the friction angle of other mixtures is lower than the friction angle of the corresponding untreated soil. On the other hand, the \(c^{\prime}\) remains in the same range for all mixtures.  
It is interesting to note that mixtures with high dosages of the SCs have comparable values of the friction angles and the cohesions, irrespective of the soil type. A two-way ANOVA with a confidence level of \(\alpha\) = 0.05 was performed on all the data and is presented in Table 5. It was shown that there is no significant difference between the friction angles of the three soils, while the mixtures with SC_R H and SC_ZZC H exhibit significantly different friction angles. Due to the similar friction angles of the three soils, an additional one-way ANOVA analysis was performed on the data grouped into the following three groups by the SC type, regardless of the soil type: (1) S+SC_R H, (2) S+SC_CG H, and (3) S+SC_ZZC H, where S stands for the data of all the soil types. Within the three groups of soils treated with high dosages of three different SCs, the mixtures of soils with SC_CG H exhibited significantly higher friction angles (\(\alpha\) = 0.05). This could be associated with the longest average fiber length of the fillers in SC_CG.  
Despite the presence of oxide ash and cellulose fibers in the soil conditioners, the obtained results are comparable to the literature data for soil samples treated with XG and investigated in a wet or saturated state. For instance, in the investigation of low-plasticity  
Figure 8: Results of direct shear tests for the untreated soil S1 and its mixtures (**left**) and repeatability tests for two types of SCs mixed with soil S1 (**right**).  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**S1 1** **} & \multicolumn{2}{c}{**S2**} & \multicolumn{2}{c}{**S3**} \\ \cline{2-7}
**Specimen** & \(c^{\prime}\) **(kPa)** & \(\varphi^{\prime}\) (\(c^{\prime}\)) & \(c^{\prime}\) **(kPa)** & \(\varphi^{\prime}\) (\(c^{\prime}\)) & \(c^{\prime}\) **(kPa)** & \(\varphi^{\prime}\) (\(c^{\prime}\)) \\ \hline S* & 4.4 & 33.5 & 1.4 & 30.0 & 12.0 & 28.0 \\ S*+SC\_R L & 11.0 & 27.0 & 0.4 & 31.0 & 0 & 32.5 \\ S*+SC\_R H & 4.0/7.7 & 26.0/25.0 & 2.6 & 25.5 & 0 & 25.5 \\ S*+SC\_CG H & 1.6 & 26.5 & 1.7 & 27.5 & 0 & 27.0 \\ S*+SC\_ZZC H & 4.7/5.9 & 24.0/24.0 & 0 & 26.5 & 1.6 & 24.5 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Measured values of cohesion and friction angle for untreated soils and mixtures (* represents the identification number of the untreated soil).  
-clay (CL), treated with 1% XG, a 5% decrease in friction angle was reported [12]. For low-plasticity silt (ML), treated with the same dosage of XG, the decrease was more than 24% [8]. The impact of XG on cohesion depends on the cohesion of the untreated soil. For high-plasticity clay (CH) treated with 1% XG, the cohesion remains unaffected [15], while an approximately 4% increase was observed in the case of low-plasticity clay (CL) [12]. In partially saturated sandy materials treated with 1% XG, the increase in cohesion can be up to 35% [15].