## 2 Materials and Methodology  
### Materials  
For the purpose of testing, samples were formed with locally available soil (mixture M1) and by mixing locally available soil with a certain proportion of sand and gravel (mixtures M2-M11). The composition of the mixtures was chosen in accordance with the grading curves of the soil samples collected from traditional rammed earth houses from eastern Croatia (Figure 3; [28]) but also in accordance with the grading curves published in the literature related to research conducted in Portugal [29] and France [30]. Samples from eastern Croatia rammed earth houses were collected following the methodology described in Ref. [1]. Figure 4 shows the grading curves of the observed mixtures, which were determined by a combination of sieving and sedimentation in accordance with HRN EN ISO 17892-4 [31]. Table 2 introduces the grading parameters of traditional and reproduced mixtures observed in this paper. The granulometric curves of traditional earnthen materials from the Republic of Croatia used for rammed earth walls are introduced for the first time in this paper. With this, the global database of granulometric curves for earth materials used to build rammed earth houses is expanded. Since the largest grain size in the specimen for the direct shear test (according to Ref. [32]) should not be larger than \(1/5\) of the specimen height, the particle size distributions of the mixtures were formed with the largest grain of 4 mm. Nevertheless, deviations in the grading curves of mixtures with such corrections in relation to the grading curves of soil samples from rammed earth houses from the area of eastern Croatia are not significant, since, in the mentioned samples, the proportion of particles larger than 4 mm for most of the observed samples was less than 5% (Figure 3). In mixture M5, 4% of lime was added (by mass), while in mixture M6, 8% of lime was added (by mass). The proportion of lime in the mentioned mixtures was chosen based on the recommendations given by [PERSON] et al. [33], where the optimal proportion of lime for application in stabilized rammed earth was studied. In the aforementioned paper, an optimal proportion of 4% lime was recorded, while in the current research, twice the proportion (8%) was added in order to determine the influence on the shear strength. Lime was added to a mixture of locally available soil and gravel with the same particle size distribution as in mixture M4. Cereal straw was added to four samples, specifically, barley (mixture M7), oat (mixture M8), wheat (mixture M9) and rye (mixture M10); in one sample, industrial hemp fibers (mixture M11) were added. Cereal straw and industrial hemp fibers (Figure 5) were cut into lengths of approximately 2 cm and added at a mass fraction of 0.25% to a mixture of locally available soil and gravel (of the same particle size distribution as M4), thus creating \begin{table}
\begin{tabular}{c c c} \hline \hline
**Location** & **Latitude** & **Longitude** \\ \hline Aljmás & 45°31′44′′ N & 18°57°04′′ E \\ Bijelo Brdo & 45°31′02′′ N & 18°52′15′′ E \\ Karanac & 45°45′40′′ N & 18°41′02′′ E \\ Lug & 45°39′47′′ N & 18°46′23′′ E \\ Zmajevac & 45°47′53′′ N & 18°48′04′′ E \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Geographical coordinates of the study area. m mixtures M7 to M11. According to the reviewed literature, specifically [13; 16; 19], the mass fractions of cereal straw and industrial hemp are usually less than 1%, and in most cases, they are equal to 0.75%. With this research, the aim was to investigate whether smaller fractions of fibers also contribute to the shear strength of the material. A mass fraction of 0.25 percent was selected, which is less or equal to one-third of the mass fraction employed in the literature we studied. As part of the continuation of research, it is planned to test another mass fraction of fibers (in addition to 0.25%) in order to determine the optimal fraction for rammed earth walls. Figure 4: Grading curve of mixtures M1–M11.