Automatic System for the Study the Interaction Between Porous Media and Atmosphere, Clime Change and Environmental Conditions

1. Introduction

The study of the interaction processes between the atmosphere and the porous medium (soils, mine and metallurgical wastes, technosols, etc.) is one of the main lines of research in earth sciences [1,2,3,4,5,6,7,8,9,10,11,12]. Different authors have investigated the process of soil crack formation by desiccation due to interaction with the atmosphere [1,2,3,4,8]. There are works that have combined experimental study of drying crack formation under controlled environmental conditions and numerical simulation [8,9]. Different investigations have studied the preferential flow caused by the existence of shrinkage cracks in the porous medium [3,6,10]. The use of sensors for the experimental study of drying pores and the control of the spatio-temporal evolution of certain variables in the porous medium has been developed mainly in the last three decades [3,4,6,11,12,14].

The objective of this work is to present an automatic system for the study of the interaction between porous media and atmosfere. The results we analyze have been obtained with one of the variants in terms of shape and type of materials that can be studied with patent 2-241-477 [14].

2. Materials and Methods

2.1. Description the Main Component of Automatic System

The automated system for the study the interaction between porous media and environmental conditions has three main components (Table 1 and Figure 1,Figure 2,Figure 3,Figure 4,Figure 5).

I) Physical part, consisting of a column with a load cell (CC, Figure 1 and Figure 2), solenoid valve, fan, bulb, porous stone, filter and various sensors to measure relative humidity, temperature, suction, volumetric water content, etc.

II) An electronic interface (data acquisition card) which acts as a link between the physical part and the computer.

III) An automatic control and recording system for the different sensors programmed with a code developed in Visual Basic.

2.2. Materials

The material used in the experiment is silt-clay tailings. This is the result of the exploitation of a lateritic nickel deposit. More information in [3,13].

Table 2. Min physical caracteristic a layers of tailings (see localization in Figure 1).

Layers	H (cm)	A (cm2)	V (cm3)	Ws (g)	ρd (g/cm3)	η	Ww (g)	w
					N=3
1	2.55	637.94	1626.75	2300	1.41	0.64	1047.40	0.46
2	2.58	637.94	1645.89	2300	1.40	0.65	1066.54	0.46
3	2.59	637.94	1652.27	2300	1.39	0.65	1072.92	0.47
4	2.62	637.94	1671.41	2300	1.38	0.65	1092.06	0.47
5	2.63	637.94	1677.79	2300	1.37	0.65	1098.44	0.48
6	2.64	637.94	1684.16	2300	1.37	0.66	1104.82	0.48
7	2.65	637.94	1690.54	2300	1.36	0.66	1111.20	0.48
8	2.66	637.94	1696.92	2300	1.36	0.66	1117.58	0.49
9	2.67	637.94	1703.30	2300	1.35	0.66	1123.96	0.49
10	2.69	637.94	1716.06	2300	1.35	0.66	1136.72	0.49
11	2.68	637.94	1709.68	2300	1.34	0.66	1130.34	0.49
12	2.70	637.94	1722.44	2300	1.34	0.66	1143.10	0.50

*H: thickness of layers, A: area, V: volume, Ws: weight of solids, ρd: dry density, η: porosity, Ww: weight of water and w: gravimetric water content in saturated conditions.

Table 2. Min physical caracteristic a layers of tailings (see localization in Figure 1).

Layers	H (cm)	A (cm2)	V (cm3)	Ws (g)	ρd (g/cm3)	η	Ww (g)	w
					N=3
1	2.55	637.94	1626.75	2300	1.41	0.64	1047.40	0.46
2	2.58	637.94	1645.89	2300	1.40	0.65	1066.54	0.46
3	2.59	637.94	1652.27	2300	1.39	0.65	1072.92	0.47
4	2.62	637.94	1671.41	2300	1.38	0.65	1092.06	0.47
5	2.63	637.94	1677.79	2300	1.37	0.65	1098.44	0.48
6	2.64	637.94	1684.16	2300	1.37	0.66	1104.82	0.48
7	2.65	637.94	1690.54	2300	1.36	0.66	1111.20	0.48
8	2.66	637.94	1696.92	2300	1.36	0.66	1117.58	0.49
9	2.67	637.94	1703.30	2300	1.35	0.66	1123.96	0.49
10	2.69	637.94	1716.06	2300	1.35	0.66	1136.72	0.49
11	2.68	637.94	1709.68	2300	1.34	0.66	1130.34	0.49
12	2.70	637.94	1722.44	2300	1.34	0.66	1143.10	0.50

*H: thickness of layers, A: area, V: volume, Ws: weight of solids, ρd: dry density, η: porosity, Ww: weight of water and w: gravimetric water content in saturated conditions.

2.3. Methods

The paper investigates the drying and wetting process of a sample of tailings. The dry and weting tests are performed under boundary conditions of relative humidity and temperature imposed on the surface of the tailings sample. The sample is 32 cm in height and 29 cm in diameter. The saturation and drying process was carried out at 26 degrees Celsius at the sample surface, an air velocity of 2 m/s and a relative humidity of 60%. Saturation was performed with a recharge applied to the column surface was 0.3 cm³/cm²/minute (Q=190 cm³/minute).

In addition, in this case the test laboratory also controlled the temperature, an aspect that had not been done before in any of the previous tests [3,4,11]. The sample was prepared according to the methodology described by [3,4]. Information and method of calibration of the different sensors can be found in the following research works [3,6,8].

3. Results

We will now analyze some of the results of the experiment. Figure 5 shows an image of the experimental set-up in the laboratory. Figure 6 shows a 3-D example of the application of the drying and wetting experiment developed on a tailings column. The objective is to evaluate the spatio-temporal variation of some physical, hydraulic and environmental parameters and properties.

3.1. Espaciotemporal Evolution a Relative Humidity and Temperature

Figure 7 shows the results of the temperature evolution on the surface of the sample. The temperature shows very little variation range. However, the relative humidity of the air shows more variation than the temperature. It should be noted that the air velocity is always kept constant at 2 m/s and the temperature at the sample surface and in the laboratory. When the laboratory temperature is not controlled, it shows significant changes influenced by the temperature at the surface on the sample and in the laboratory [3].

3.2. Espaciotemporal Evolution a Saturation, Dry and Consolidation a Tailings Sample

The initial recharge applied to the surface of the column was 0.3 cm³/cm²/minute (Q=190 cm³/minute). This flow rate allowed a 2 mm high layer of water to form on the column 10 minutes after the saturation process had started. Subsequently, this height was kept constant with the water supply from the solenoid valve controlled by a water level sensor until 1000 minutes had elapsed. At this point, the position of the solenoid valve sensor was raised and the water level on the surface of the column was increased to 80 mm. As can be seen in Figure 4, the system developed sensor readings every 10 minutes during both the saturation and dry processes.

Figure 8a shows the spatial and temporal evolution of the increase in the degree of saturation during saturation and Figure 8b during drying. It can be seen that the tailings saturate quickly but the drying process is very slow. Consolidation during saturation begins when TDR2 starts to saturate at a depth of 15.6 cm. It starts to consolidate 0.8 days after the start of the recharge and ends after 2.4 days, after which time it stabilises. This behaviour is due to the presence of stratification and drying cracks in the sample. Studies by [3] show the existence of an increase in permeability of up to two orders of magnitude due to the effect of cracks and stratification.

The results show that the interaction processes of the porous medium with the atmosphere are complex. In the saturation process the response of the first sensors is very fast. After only two days the porous medium is saturated.

In the drying process the behaviour is different. A direct influence is observed in the first 20 cm of the soil in the first week. However, at greater depths the evaporation processes take more than 20 days to develop.

4. Discussion

In the following, we discuss on the basis of real results the main applications and uses that can be made with the automatic system (patent 2-241-477) [14].

a)- It allows the filling of the chosen or designed tank with solid material of any characteristic (natural or anthropogenic soils, waste, etc.). The filling of the column can be done all at once, in stages, in layers, by compacting the material or by pouring it wet and leaving it to dry and consolidate inside according to the objectives of the work [3,6,17].

- Flow and solute transport tests can be performed under saturated or unsaturated conditions. The porous medium can be homogeneous or heterogeneous. The hydraulic gradient of the column can be modified according to the objectives of the work. The saturated hydraulic conductivity is determined under steady state conditions and constant or variable hydraulic gradient. A solenoid valve controls the water level at the surface of the sample and waters deposit [3,6,17].

- It allows soil shrinkage (crack formation) tests with temperature control or imposition, with control of water mass loss by evaporation, relative humidity and surface wind speed. Soil shrinkage can only be measured continuously in the vertical direction. A computer continuously stores the data [3,6,17].

- It allows the measurement in depth of humidity, suction and temperature and their evolution in the vertical profile and over time [3,6,17].

- It measures the loss in weight as a function of time, which makes it possible to determine the evaporation of the sample tested [3,6,17].

- It allows the temperature of the desired test to be set [3,6,17].

- The desired wind speed can be set if a power regulator for the fan, motor is available [3,6,17].

- Relative humidity can be set with a variation of ±5percentage under ambient temperature conditions, this variation can be reduced by 1-2% if tests are carried out in a temperature-controlled laboratory [3,6,17].

- It allows a recharge (irrigation) to be applied or carried out on the soil sample under test provided that the total weight of solid and water does not exceed 100 kg, this can be solved by using a load cell with a higher capacity [3,6,17].

- The process of evolution of the volumetric water content can be controlled by the TDR and the suction by psychrometers and tensiometers [3,6,7].

- The saturation of the solid material can be carried out in two directions from bottom to top or from top to bottom [3,6,17].

- From this design, the height and capacity of the column, as well as the number of sensors can be varied according to the objectives of each research work [3,6,14,17].

The sensors used and calibrated for this study have been applied in other research in site [15,16] and doctoral thesis development [3,12].

More information and details in [3,14,16].

5. Conclusions

The results show the versatility of the utomatic system for the control of environmental variables and boundary conditions in one of the many test variants that can be performed.

With the data from the humidity, suction, relative humidity sensors and the physical properties of the porous medium, the water retention curve (WRC) is obtained. The WRC is obtained for the drying and wetting processes.

With the drying and wetting process, the spatio-temporal evolution of the consolidation experienced by the tailings is observed.

6. Patents

Patent: 2 241 477. Authors Rodríguez-Pacheco, R.L.; Lloret A.; Candela, L.; Pérez, T.; Cortés, F. Dispositivo para la caracterización de medios porosos sujetos a diferentes condiciones ambientales y de contorno, 2006. Patent: 2 241 477. DOI: 10.13140/RG.2.1.2012.4882