Contribution to the Characterization of Lateritic Soils for the Manufacture of Compressed Stabilized Earth Bricks

The aim of this study is to contribute to the mastery of the physical characteristics of lateritic soils in order to improve their use for the manufacture of Compressed Stabilized Earth Bricks (CSEB) in the province of North Kivu in the Democratic Republic of Congo (DRC). The study of the physical characteristics of lateritic soils was carried out. Samples were subjected to experimental identification tests on the physical characteristics (water content, density characteristics, particle size distribution and consistency). The results of the laboratory analysis of soil samples show that the water content varies between 5.4% and 36.99%. The density of the solid grains has an arithmetic mean of 2.5 g/cm 3 . The apparent density varies from 0.83 to 1.35 g/cm 3 . As for the dry density, it is in the range of 0.61 to 1.25 g/cm 3 . These relatively low densities indicate that the material studied has a significant degree of defor-mability.


Introduction
The threat of global warming is real and the economic crisis is imminent. The so-called modern materials such as concrete and steel are almost inaccessible for a large part of the population, because of their high costs. The populations close to the forests have opted for wood construction, but this cannot be truly qualified as sustainable as long as its anarchic use accentuates deforestation and fire rates. The valorization of local materials in the construction of housing and public buildings is a necessity from a sustainable development perspective. Reducing the use of concrete in low-income countries has environmental and economic advantages [1]- [6]. Researchers have proposed tropical soils as a sustainable resource to be utilised in the construction of housing around the world. Laterite is a typical tropical soil that could fill the gap between a housing deficit and provision of modest housing for the growing populations in Africa [7] [8] [9] [10] [11]. However, the earth material is very little adopted, perhaps due to a lack of knowledge of its properties or ignorance of improved techniques of CSEB manufacturing are causes and inadequate implementation of Compressed Stabilized Earth Bricks in the buildings constructed resulting in a limited life cycle. The constructions in raw earth have a connotation of poor and bad quality habitats but this is indeed due to the ignorance of the thermal and hydrometric properties of this material. In addition, it is currently confirmed that the stabilization of CEB by binders and bitumen improves their mechanical strength and their sensitivity to water [12] [13]. The lateritic soils of North Kivu are used by certain social strata for the manufacture of Compressed and Stabilized Earth Bricks without mastering their physical characteristics, which leads to consequences such as the progressive crumbling of the walls, cracks, the poor performance of the plaster, and the discouragement of the use of the said technology. In addition, the CSEB technique is applied in other localities of North Kivu and elsewhere, but with the use of calcareous sand, river sand, sandy clay and not laterite, hence the study of the characteristics of these materials is important for the manufacture of CSEB. This study focuses on characterization and the classification of some lateritic soils on the basis of samples collected from Bweremana (in DRC) for their probable use in the manufacture of Compressed Stabilized Earth Bricks.

Sample Materials
The experimental studies were carried out on samples of lateritic soils collected  Table 1.

Experimentation Methods
Some preliminary tests as touch/smell/wash tests gave an apparent classification of the material and its organic content.  [15]. The density of the solid grains was determined using the pycnometer, according to the standard NF P 94-054 [16]. The apparent density and the dry density were determined by the cutting kit method according to the NF P 94-053 [17] standard. The wet sieves analysis of the material was carried out by sieving according to the requirements of the NF P 18-560 [18] standard. At the end of the sieving operation, the particles size less than 0.080 mm were collected. Their particle size analysis was carried out by sedimentometry according to the standard NF P 94-057 [19]. The particle size curves were generated with MATLAB software (Appendix 1 MATLAB code). The study of the terberg limits make it possible to characterize the clayness of a soil, thus determining the remarkable water contents located at the border between the different states (solid, plastic and liquid). The liquidity limit (W L ) was determined using the Casagrande apparatus and the plasticity limit (W P ) using the roller method, in accordance with standard NF P 94-051 [20]. The plasticity index (PI) was evaluated as the difference between the liquid limit and the plastic limit. Finally a classification of the samples soils was done according to the CLRB geotechnical classification system.

Results and Discussion
The study of the physical characteristics of lateritic soils was carried out on 12 samples. In this section we present the results of the experimental identification tests on the state of parameters (water content, different densities or density characteristics); particle size composition and consistency parameters.

Water Content
The water content values of samples are presented in Table 2.

Density Characteristics
The density characteristics of the studied soils are presented in Table 3. Table 3 shows that the solid grain density ρs has an arithmetic mean of 2.50 g/cm 3 . The bulk density ρ ranges from 0.83 to1.35 g/cm 3 , with an arithmetic mean of 1.13 g/cm 3 . The dry density ρ d is in the range of 0.61 to 1.25 g/cm 3 with an arithmetic mean of 1.00 g/cm 3 . These densities indicate that the material studied have a non-negligible degree of deformability.

Particle Size Distribution
Completing the wet particle size analysis with the particle size analysis by sedimentometry applied to particles smaller than 0.08 mm according to the AFNOR (2001) [21] standard, we obtained the particle size curves represented on Figures   3-14. These results show that the largest grain size for all the samples is 2 mm.
In general, the distribution of grains for the different materials is given in Table 4.

Consistency Parameters
The consistency parameters of the studied soils are presented in Table 5. Table 5 shows that the plasticity index (Ip) of materials BW-SM-01 to BW-SM-12 is between 17.15 and 19.36% with an arithmetic mean of 18.25%. The consistency index (Ic) is between 0.18 and 1.57 with an arithmetic mean of 1.08%, which gives the materials studied a plastic character. Similarly, the liquidity index (Il) is less than zero with an arithmetic average of −0.08, which gives these materials a character of hard plastic consistency.
The diagram on Figure 15 shows that the points of the studied material fall within the preferential plasticity range recommended by the AFNOR (2001) [22] standard for the manufacture of Compressed stabilized Earth Bricks (Appendix 3 Plasticity range indicated for soils used in earth construction).  [26] where the liquidity limit, the plasticity limit and the plasticity index to be considered must be respectively between 25% and 45%; 10% and 25%; and  7% and 25%, related to recommended material for the manufacture of CSEB.

Geotechnical Classification
The quick preliminary (smell) test attested that the sample materials have a poor Organic Matter Content. The particle size curves attested that the 12 samples materials have at least 70% of particles whose size is less than 0.08 millimetres.
According to the CLRB Classification System [27], those two parameters indicate that these samples materials are either Clay or Silt. The position of the studied samples in the Casagrande chart found the values of the plasticity index and the liquidity limit, show that the whole material of all the samples are classified as Ap (Plastic Clay Class).

Conclusion
The present work has shown that the studied soils are rich in fine particles. They are essentially plastic floors of class Ap of the CLRB classification system. The grain size curves of all the samples studied and the points that represent them lie within the preferential zones of good soils for the production of compressed earth blocks. It appears that in their natural state, the studied materials can be used for this purpose. The density of tropical soils can serve as an important index of their strength and durability. This is because density is mainly influenced by the types of minerals and oxides present in the soils. In order to obtain the acceptable properties for the materials for the production of stabilized earth blocks, the percentage of fine particles is an important parameter. But also the type of soil and the geographical position can play an important role in its properties. The methods used in the context of this article are standard and remain applicable for all similar studies.