## 1 Introduction
\begin{table}
\begin{tabular}{c c c} \hline
**Biopolymer** & **Chemical Composition** & **Behavior with Soils** \\ \hline Agar Gum & C\({}_{14}\)H\({}_{24}\)O\({}_{9}\) & \begin{tabular}{c} Strengthening \\ Pore clogging \\ Erosion reduction \\ \end{tabular} \\ \hline \(\beta\)-glucan & C\({}_{18}\)H\({}_{32}\)O\({}_{16}\) & \begin{tabular}{c} Grouting \\ Strengthening \\ Superplasticizer in concrete \\ \end{tabular} \\ \hline Casein & C\({}_{81}\)H\({}_{125}\)N\({}_{22}\)O\({}_{39}\)P & \begin{tabular}{c} Strengthening \\ Water resistance \\ Hydraulic conductivity reduction \\ \end{tabular} \\ \hline Chitosan & C\({}_{18}\)H\({}_{35}\)N\({}_{3}\)O\({}_{13}\) & \begin{tabular}{c} Strengthening \\ Coagulant effects \\ Removal of heavy metals in water \\ \end{tabular} \\ \hline Dextran & H(C\({}_{6}\)H\({}_{10}\)O\({}_{5}\))\({}_{\lambda}\)OH & \begin{tabular}{c} Drilling muds \\ Conditioners \\ Erosion reduction \\ \end{tabular} \\ \hline Gellan Gum & C\({}_{24}\)H\({}_{37}\)O\({}_{20}\) & \begin{tabular}{c} Strengthening \\ Pore clogging \\ Erosion reduction \\ \end{tabular} \\ \hline Guar Gum & C\({}_{10}\)H\({}_{14}\)N\({}_{5}\)Na\({}_{2}\)O\({}_{12}\)P\({}_{3}\) & \begin{tabular}{c} Dust control \\ Strengthening \\ Grouting \\ \end{tabular} \\ \hline Starch & C\({}_{27}\)H\({}_{48}\)O\({}_{20}\) & \
\begin{tabular}{c} Adhesives for drilling fluids \\ Strengthening \\ Erosion reduction \\ \end{tabular} \\ \hline Xanthan Gum & C\({}_{36}\)H\({}_{58}\)O\({}_{29}\)P\({}_{2}\) &
\begin{tabular}{c} Drilling mud thickener \\ Strengthening \\ \end{tabular} \\ \hline \end{tabular}
\end{table} 
Table 1: Biopolymer additives used in geotechnical engineering [18]. pharmaceuuits [27], agriculture [28], winemaking [29] or energy storage [30,31] this natural resource hasn't been fully utilized. The biggest advantages of this polymer include availability, low cost, high biodegradability and ease of chemical modification [32, 33, 34, 35]. Polymer blending can also be performed to improve the mechanical properties of the bioproduct [25,32]. Application of biopolymers in geotechnical and geo-environmental engineering is relatively new. There are limited studies on the application of biopolymers for soil improvement. These studies were generally related with permeability [36], erosion control [37] and shear strength of the soils [21,38]. Experimental studies have shown CU Triaxial tests performed on compacted silt mixed with 1% chitosan solution indicate shear strength improvement of up to 30 percent within a week [39]. In another study [40], fine silica sand containing dextran showed 20 times increased critical shear stress (\(\tau_{c}\)) compared to the untreated condition. [PERSON] and [PERSON] [41] proved, the presence of biopolymers in soil might enhance shear stiffness (Kirchhoff modulus \(G\)). Various applications of chitosan are still studied. In geotechnical research there are still few results available that specifically concern soil-chitosan mixtures. Recent studies focus mainly on soil-chitosan composites shear strength or soil surface erosion prevention. To the authors' best knowledge there are yet no studies that focus on soil-chitosan mixtures' stiffness and dynamic properties. In this study the effect of chitosan on stiffness and dynamic properties of medium-grained low-cohesive soil was investigated. Soil-chitosan composited were subjected to cyclic slow-changing loading (torsional shearing test) and dynamic loading (resonant column). The specimens were prepared using two different chitosan solution concentrations. The obtained properties of soil-chitosan composites were compared with the properties of non-enhanced soil specimens with the same water content.