## 4 Discussion  
Comparing results presented in Figures 9 and 10, increments of dynamic shear modulus \(G_{max}\) are noticeable. Maximum shear modulus values of fresh composite specimens F1 and F2 are 1.2 to 1.4 times higher than corresponding values for untreated, pure silica sand specimen P. Specimens curing resulted in increasing shear modulus values from 1.1 to 1.6 higher than corresponding values for fresh F1, F2 specimens.  
Influence of the chitosan binder and curing process on the stiffness of composites is significant. For cured composite specimens C1 and C2 maximum shear modulus values are 1.3 to 2.1 times higher than values for untreated, pure silica sand specimen P.  
It should be stressed that adding chitosan solution to soil enhances soil dynamic properties even before specimen curing. The beneficial effect of chitosan solution before curing was only observed in dynamic (RC) tests. It is an important observation, considering the fact that optimal curing conditions are easier to provide and control in laboratory, but might not be possible to provide on a construction site. Another important issue is obtaining a homogenous soil-chitosan mixture. While it is not a problem in the laboratory conditions, it can be quite difficult at the construction site. Additional research (including in situ tests of stabilized soil) should be conducted to propose and test appropriate mixing technology.  
In TS tests there was no significant difference between pure soil specimen and fresh composites' behavior.  
In the TS tests (as can be seen in Figures 13-20) the effect of chitosan on composite stiffness is substantial. Initial shear modulus \(G_{0}\) of cured specimens (C1 and C2) is 1.9 to 3.1 times higher compared to the fresh specimen (F1 and F2 respectively).  
Considering \(G\) values at the beginning of each of the four phases in full loading cycle (see Table 6), the cured specimens' stiffness is from 1.7 up to 3.3 times higher than for fresh mixtures (see Figures 21 and 22).  
Cured soil-chitosan composites exhibit better dynamic properties in response to high frequency and high amplitude loadings than non-enhanced soil. Is suggests chitosan can be considered as a soil stabilizer for temporary structures that are often in danger of stability loss due to, e.g., operation of construction machines.  
Stiffness (measured by the shear modulus \(G\)) of the composite specimen is several times greater than stiffness of the non-enhanced soil specimen.  
The composites exhibit slow degradation in a humid environment, especially when totally submerged in water. Some observable effects of water degradation were documented (Figure 23), but a precise determination of the rate of bond degradation is outside the scope of this study. As it is important to ensure geotechnical structures stability over time, further research is needed to assess the change in chitosan stabilization effect for large scale use of chitosan as a soil stabilizer.  
Figure 21: Comparison of back analysis results. Specimens F1-32551 and C1-30437.  
Figure 22: Comparison of back analysis results. Specimens F2-30608 and C2-23608.  
Considering the fact that both soil and chitosan are natural materials, the properties of the composite can be affected by many factors and can vary noticeably from specimen to specimen. Large number of tests should be performed to confirm the repeatability of the enhanced mechanical parameters. It should also be noted that the experiments were carried out in the controlled laboratory environment. The use of chitosan in geotechnical structures is a promising solution but it cannot be fully recommended without further studies on how soil-chitosan behavior is influenced by various environmental factors (e.g., seawater exposition). Future research should also focus on understanding the underlying binding mechanisms of chitosan stabilizer so that optimal conditions for binding can be clearly identified and recreated in engineering practice.