# Comparison of Field and Laboratory Tests for Soil Suitability Assessment in Raw Earth Construction
## 3 Results and Discussions
### Correlation Analysis Between Field and Laboratory Methods  
The comparison between field tests and geotechnical tests reveals several significant correlations that help validate the reliability of simpler field-testing methods. Figure 6 demonstrates a strong positive correlation between the field-based cigar test results and the laboratory-determined PI. For example, soil 3-LeFleix with a high PI of 46.4% achieved a cigar length of 26.5 cm, while soil 6 (Pessac) with a low PI of 1.3% only reached 7.06 cm. The high determination coefficient (R\({}^{2}\) = 0.8863) indicates that approximately 89% of the variation in cigar test lengths can be explained by PI, suggesting a robust relationship between these two parameters. This strong correlation is particularly significant as it validates the cigar test as a reliable field indicator of soil plasticity. The data points are well distributed along the regression line, with measurements ranging from low plasticity (PI \(\approx\) 0%) corresponding to short cigar lengths (\(\approx\)4 cm) to high plasticity (PI \(\approx\) 50%) corresponding to longer cigar lengths (\(\approx\)26 cm). This relationship provides practitioners with a valuable tool for quick field assessment of soil plasticity characteristics, potentially reducing the need for immediate laboratory testing during preliminary soil evaluations. In addition, the average length obtained with the cigar test showed a good correlation with the MBV. The MBV cannot be related at all to the percentage of clay. The same amount of montmorillonite, illite, or kaolinite in the soil will lead to different MBVs. The clay's mineral assemblage and particle size distribution governing the physical and chemical properties of the clay are likely key factors influencing the observed correlations [69]. While the cigar test provides a simple and qualitative assessment of soil cohesiveness, PI offers a more quantitative measure of soil behavior that can be correlated with other engineering properties, such as strength and compaction characteristics. Research has shown that resistance to soil penetration, an indicator of soil compaction, can be estimated using PI and other soil attributes [70]. The ring test results also demonstrate a strong correlation with clay-sized particle content percentages from particle size analysis and plasticity indices. Soils scoring high on the ring test (4-5) consistently showed higher clay-sized particle content in granulometric analysis and higher plasticity indices. For instance, soil 8-Haux scored 5 on the ring test and showed significant clay content in the particle size distribution, along with a high  
Figure 5: Comparison of the Methylene Blue Value and plastic limit. PI of 33.6%. Conversely, soil 4-St Selves scored 0 on the ring test and showed minimal clay-sized particle content with a relatively low PI of 13.7%. The ribbon test results parallel these correlations, showing strong alignment with both clay-sized particle content and plasticity indices. This reinforces the validity of both the ring and ribbon tests as field indicators of clay-sized particle content and plasticity. However, the dropping ball test showed less clear correlations with geotechnical parameters, suggesting it might be less reliable as a standalone indicator of soil properties. The pellet test resistance values show interesting correlations with both clay-sized particle content and plasticity indices. Soils with higher clay-sized particle content and higher plasticity indices generally achieved better resistance scores [71]. However, the shrinkage measurements from the pellet test do not show consistent correlations with geotechnical parameters, indicating that shrinkage behavior might be influenced by more complex combinations of soil properties such as mineral composition, and clay mineral activity, which were partially explored in this study.