# Comparison of Field and Laboratory Tests for Soil Suitability Assessment in Raw Earth Construction
## 3 Results and Discussions
### Suitability Assessment  
The particle size distribution is a crucial and determining characteristic for earth construction as well as the nature of the clays determining their interactions with water [72]. Indeed, the granular skeleton composed of gravel, sand, and silts has a certain compactness, which confers resistance to the material. A wide particle size distribution can increase the density. Figure 7 shows the particle size analysis of some soils as well as the recommended particle size distributions for CEBs according to the NF XP P13-901 standard [73], for CEBs, adobe, and rammed earth according to the earth construction treatise [21], and for cob according to [PERSON] [74] who recommended; for example, a regular distribution with approximately a third gravel, a third sand, and a third silt and clay. The analyzed soils displayed diverse particle size distributions, soils dominated by particles >0.063 mm (sand and gravel-sized particles), soils with predominant sand-sized particles (0.063-2 mm), and soils with clay-sized particle content ranging from 5% to 75%. Some soils respect the range of recommended particle size distributions for earth construction, while others are outside this area. However, these limits are given as a guide  
Figure 6: Correlation between average cigar length and (a) PI and (b) MBV. in the references [21]. [PERSON] et al. [32] have shown that the classification with the texture envelopes is not suitable for selecting the appropriate soil characteristics because they do not reflect the diversity of soil compositions encountered in the field and do not consider the activity of the clay minerals, which is an important parameter for implementation, strength, and durability. These authors recommend carrying out experimental tests to evaluate suitability. These soils are not necessarily inappropriate if they do not adhere to the granular envelopes presented previously. Indeed, [PERSON] et al. [18] listed many scientific studies on soils outside of the recommended particle size distributions. Moreover, it is always possible to adapt the physical properties of soils by sieving them, or by mixing soils of different natures. Figure 8 presents the relationship between the PI and the liquid limit, as well as the correspondence of the soils to the plasticity recommendations for CEBs according to the NF XP P13-901 standard and for the CEBs, adobe, and rammed earth according to the Traite de Construction en Terre (TCT) [21]. The results show significant variability with plasticity indices ranging from 0% for sandy soils to 50% for silty clays according to the silt/clay ratio. According to the Casagrande chart, different types of soils can be distinguished according to the following categories: low plasticity clays, highly plastic clays, low plasticity silts, highly plastic silts, and organic soils [75]. Some soils in the recommended zones [21] are suitable for multiple earth construction techniques, while others may be suitable for only one technique or none at all. As mentioned earlier, this does not prevent their use, as the limits are given as a guide. The correlation analysis between particle size distribution, Casagrande plasticity classification, and the field test results reveal significant relationships in soil characteristics. Soils with high percentages of fine particles (passing <0.063 mm), such as 3-LeFleix, demonstrate both high plasticity indices in the Casagrande chart and superior performance in field tests,  
Figure 7: Particle size distribution curves of the studied earth and correspondence to recommendations. particularly in the cigar test (26.5 cm) and ring test (score of 5). This correlation is further supported by high MBV values and significant clay-sized particle content percentages. Conversely, well-graded soils like 6-Pessac, which show smooth distribution curves, and plot in the \"Low plastic clay\" or \"Low plastic silt\" zones exhibit poorer performance in field tests (cigar length 7.06 cm, ring test score of 0).