# Comparison of Field and Laboratory Tests for Soil Suitability Assessment in Raw Earth Construction
## 3 Results and Discussions
### Laboratory Test Results  
This diversity in particle size distribution is crucial for determining the suitability of these soils for different earth construction techniques, because each method (such as rammed earth, adobe, or cob) requires specific particle size ratios for optimal performance. Figure 2 presents the particle size distribution analysis of 37 soils from various locations in the Nouvelle-Aquitaine region. Two soils were put aside because of their high OM content. The stacked horizontal bars show the relative proportions of five particle size categories: coarse gravel (>20 mm, shown in black), fine to medium gravel (2-20 mm, shown in gray), sand (0.063-2 mm, shown in beige), silt (0.002-0.063 mm, shown in orange), and clay-sized particles (<0.002 mm, shown in burgundy) [39]. The results reveal significant variations in soil composition across the sampling sites. Some locations, such as 4-St Selves and 17-Rebenac Scalpage, show high gravel content, while others like 1-BARP and 37-Fumel B are dominated by clay fractions. Several soils, including 6-Pessac and 23-Jumilhac, display more balanced particle size distributions. The analysis demonstrates the wide range of available soil compositions in the region, suggesting the potential for various earth construction applications, though some soils may require modification or mixing to achieve ideal particle size distributions for specific construction techniques. The presence of clay is essential in earnthen constructions because it acts as a binder of the other constituents. Low clay-sized particle content results in a lack of bonding between the particles, while high clay-sized particle content can cause significant shrinkage and consequently cracking. Several types of clays can be found, such as kaolinite, illite, and smectite [61]. Their molecular structure and stacking of the sheets involve strong interactions with water. The water content and plasticity are also of paramount importance during the implementation of raw earth construction. The PI is a fundamental soil property that reflects the range of water content over which a soil exhibits plastic behavior. Higher plasticity indices, such as 9% and 13%, have been found to have a negative effect on both soaked and unsoaked California Bearing Rate (CBR) values, which are a measure of a soil's strength and load-bearing capacity [62]. Moreover, the effect of soaking on strength reduction is more pronounced when the fine content is high and the dust ratio is greater than 0.6 [62]. Figure 2: Particle size distribution of soils. Figure 3 illustrates the plasticity characteristics of the soils, presenting both the plastic limit (PL) and PI for each tested soil. The data reveals considerable variation in soil plasticity properties. PL ranges from very low (2.8% for 5-StBazeille) to notably high values (58.4% for 25-StJdeCole), with several soils showing no measurable PL (31-Pessac Pontet and 33-Proissans). The plasticity index also demonstrates significant variation, from minimal values (1.3 for 6-Pessac) to substantial indices (49.9 for 13-StPdTrube). Soils like 3-LeFleix exhibit both high PL (16.2) and high PI (46.4), suggesting significant clay-sized particle content and activity. This wide range of plasticity characteristics has important implications for the soil's suitability in different earth construction techniques, as plasticity affects workability, shrinkage behavior, and final mechanical properties of the earthen materials. It is important to have soil classification systems in place to identify soils and predict how they will behave [63]. While standards based on particle size distribution are useful for coarse-grained soils, they are not sufficient for fine-grained soils. When dealing with fine-grained soils, it is important to consider their clay-sized particle content and mineralogy, as these factors significantly influence the soil's properties [64]. [PERSON] et al.