Soil samples are being taken from a locality in the Ouaddaï region, one of Chad’s 23 regions, Abeche, located in the Sahelian zone in the eastern part of the country. Situated in eastern Chad, the town of Abeche is the capital of the Ouaddaï region. It lies between latitude 13˚48'584'' North and longitude 20˚50'139'' East. The study area is subject to an intertropical climate with a 9-month dry season and a 3-month rainy season. The pattern of these two seasons is defined by fluctuations between dry air masses from the north (the harmattan) and humid maritime air masses from the southwest (the monsoon). The average annual rainfall is around 300 mm. The region’s temperature varies according to the season. The average annual temperature in Abeche is around 28˚C, varying between 16˚C and 35˚C in the cold season (December to February) and between 25˚C and 41˚C in the dry season (April and May). The soil sampling site, in the province of Abeche, is marked by a blue triangle on the map.

Figure 1 and Figure 2 show, respectively, the mapping of the locality and the sampling site for the samples studied.

Figure 2 shows the photograph of the soil sampling site to be studied.

a) Soil

The soil taken from the site was identified by geotechnical and physico-chemical methods in the article [12] . It resorts that these floors are clay and adapts for the manufacture of compressed earth bricks.

b) Dug cow

These are mainly local materials derived from animal biomass (cow’s dung in Figure 3 ), which are used to mix with the study material, clay. The choice of these local materials is justified by their abundance in Chad [13] . In the work [14] , the authors showed through energy dispersive spectrometry (EDS) analysis that cow dung mainly contained silica, alumina, calcium, potassium, magnesium, phosphorus, iron and sulfur. In addition, the authors mixed cow dung with soil and used it as coatings. In the work [15] , another study highlighted the effect of cow dung on microstructural changes of bricks (adobes). The authors showed that cow dung reacts with clay minerals such as kaolinite and quartz to produce an insoluble amino silicate, which sticks to isolated soil particles. Moreover, it has also been observed that the presence of fiber in cow dung prevents the propagation of cracks and strengthens the brick (increasing its resistance).

In Table 1 , we propose the formulation of the samples for the manufacture of the 4 × 5 × 8 cm³ test tubes.

At first, a mass of the sample is taken, sifted (2 mm), then pledged with the digital scale and mixes with the cow dung (0%, 2%, 4%, 6% and 8%) and water. This mixture (clay + cow’s dung + water) is kneaded and preserved for 20 minutes to have a perfectly homogeneous mixture. Figure 4 shows the different stages of the preparation of the samples.

The production of soil samples is ensured by a device ( Figure 5 ) set up on site allowing reproducible samples to be obtained.

Figure 6 presents the manufacturing steps for 4 × 5 × 8 cm³ test pieces to allow thermophysical characterization.

The determination of the thermophysical properties of these bricks are parameters which could be of great use for the evaluation of their effectiveness in the thermal insulation of buildings.

The thermophysical properties of these earth specimens to be studied in this paper are:

The device, shown in Figure 7 , consists of a thermal shock probe which will be placed between two identical samples of the material to be characterized, and an electronic. The probe principle and device were developed by CSTB. They are also based on ASTM D5930-97 and RILEM recommendation AAC 11-3 [16] .

The procedure consists of generating a low local temperature rise in the material (then considered to be in thermal equilibrium) and measuring the temperature rise over a very short time. The following equation relates thermal conductivity to temperature variation with respect to time.

$\Delta T=\frac{q}{\left(4\text{π}\lambda \right)}\ast \left(\ln \left(t\right)+cste\right)$(1)

With λ the thermal conductivity in W∙m⁻¹∙K⁻¹, q the linear flow injected in W/m, ΔT the temperature difference in K and t the duration of the test in s.

$\Delta T=q\left(RT-\frac{Cs}{Eff2}+2\frac{\sqrt{t}}{\left(Eff\sqrt{\text{π}}\right)}\right)$(2)

With Eff, the effusivity in J∙m⁻²∙K⁻¹∙S⁻¹, T the temperature in degrees K, q the injected flux density in W/m², t the duration in s, Rt the total resistance, i.e. the contact resistance in K∙m²/W and Cs an intrinsic characteristic of the probe in J/kg∙m²∙K.

The other thermophysical parameters are obtained by deduction through the different formulas which allow them to be linked.

$D=\frac{\lambda }{\rho \ast C\rho }\text{with CP, the specific heat}$(3)

$CP=\frac{E^2}{\left(\rho \ast \lambda \right)}$(4)

Figure 8 shows the variation in conduction as a function of the as a function of the percentage of cow dung used in the manufacture of the earth samples (5 × 4 × 8 cm).

From the result of Figure 8 , it can be observed that the thermal conductivity varies inversely with the increase in the percentage of cow dung. We notice a decrease in conductivity as the percentage of cow dung increases. In fact, cow dung contains part of the fiber and the presence of the fiber increases the void volumes in the test tubes, which reduces the thermal conductivity of clay matrices with more cow dung than that of 100% clay. For a sample without adding cow dung, the conductivity is around 0.715, rising to 0.420 W/m. K for a 2% sample of cow dung. This result is not very far compared to those in the literature in the work of [18] , we note that the sample with 100% clay, the conductivity is

of the order of 0.8 W/m∙K and comparing that of 5% of dung and 95% of clay, the conductivity is of the order of 0.3W/m∙K, on the other hand, can be explained by the porous structure of fiber itself increases the porosity of the composite which reduces and the absorption power of fiber will increase the humidity rate important to the fiber, which promotes the decrease in thermal conductivity. In the works of [19] , the authors found that the water absorption of date palm fibers increases with the fiber content. And also, in the works [20] , the water content of cocoa fibers increases with the quantity of these fibers.

By comparing the conductivity values found with other existing results in the literature, we obtain correct values.

Variation in thermal resistance as a function of cow dung percentage

There is a constant increase in thermal resistance with the increase in percentage of cow dung in Figure 9 . Because thermal resistance is inversely proportional to thermal conductivity. Indeed, Rth = e/λ with thickness e = 20 cm in this case. This increase, depending on the percentage addition of cow dung, can be explained by the simple fact that the quantity of cow dung in the mixture contains fiber which tends to increase the ports and therefore the thermal conductivity is slowed down.

Variation in density of the samples specimens (soils) as a function of the percentage of cow dung is given in Figure 10 bellow.

Figure 10 shows a decrease in density depending on the incorporation rates of 2%, 3%, 4%, 6% and 8% respectively of cow dung, compared to the 100% clay matrix. This can be explained by the fact that cow dung takes time to lighten the material. In the work of [19] , the author showed that the addition of 0.5% of rice straw lowers the dry apparent density from 1929 kg/m³ to 1916 kg/m³. Comparing our results with the other cited results, we obtain an acceptable variation in density.

Figure 11 shows the decreasing variation in thermal effusivity as a function of the cow dung incorporation rate.

Comparing the values of thermal effusivity obtained with that found by [18] varies from (1150 to 550 W/m²/K) for percentages of (0% to 5%). Our obtained values are acceptable.

According to the results in Figure 12 shows that the specific heat varies inversely with the increase of cow dung, which is very remarkable compared to the matrix without adding cow dung. The values obtained by [18] vary from (900 to 650 J/kg/K) for percentage of (0% to 5%) of cow dung. These values are in the same magnitude as our obtained values.

Figure 13 also shows a remarkable reduction in the addition of cow dung compared to the samples, i.e. 100% clay without the addition of cow dung.