Experimental Investigation Anisotropy Permeability of Uniform Coarse Grain Soils

Investigation of water passing through soil is one of the most important problems in soil mechanics and environmental engineering. It is an important parameter for predicting the movement of water and contaminants dissolved in the water through the soil and measured on soil samples in the lab and sometimes tests carried out in the field. Soil permeability generally depends on two factors, the first one is soil Specifications contains an empty space of soil, surface roughness of solid particles, saturation, and another one is characteristics of the fluid (water) that passes through soil. Already few efforts have been made to recognize the characteristics of anisotropy in the geotechnical designs therefore this study has been done. Physical and mechanical properties of soils and sedimentary rocks are generally heterogeneous and hydraulic conductivity (k) is not an exception. The anisotropy of hydraulic conductivity of soils has a great influence on the fluid flow and the transmission of contamination. Knowing the hydraulic conductivity in cases such as flow through dams and dikes, internal erosion in soil masses, settling of consolidated clay levels, optimal design of water and oil wells, and the design of drainage systems for roads, airports and agricultural land. Generally, the hydraulic conductivity is more in the horizontal direction than the hydraulic conductivity in the vertical direction, and the hydraulic conductivity anisotropy is shown with a non-dimensional parameter rk which is equal to the ratio of the horizontal hydraulic conductivity to the vertical hydraulic conductivity. According to Chapuis et al. (1989), on more than 100 measurements of hydraulic conductivity along with the results of the experiments of Chapuis et al. (1990), Rice et al. (1970) and Leroueil et al. (1990), the anisotropy of the hydraulic conductivity of clays, sands and sedimentary rocks are almost like each other. The degree of anisotropy may depend on the shape of the particles, their arrangement, or the orientation of the free space among the particles of the soil, which appears to be less than 4. Due to the impossibility of preparing intact samples from grain materials, as well as the lack of suitable measuring instruments for grain samples, there are few valid results for non-sticky materials. As Chapuis et al. (1989) and Sferlazza et al. (2009) in accordance with most of the experimental results, the anisotropy of hydraulic conductivity increases with density, and also the degree of anisotropy decreases with increasing porosity ratio.
In order to conduct the present research, measurement device was designed and built. This device is a cube with 150 mm ×150 mm × 173 mm dimensions. The components of the device are: bleeding valves, inlet and outlet valves, porous plates and the size of the sample respectively. In this study, four uniform soil samples were selected for test. Samples are prepared in falling manner, with three porosity and under three different hydraulic gradient were tested. In Table 1 The general pattern of research experiments is presented. In this study, 36 tests were performed.
Table 1-Pattern of research experiments
Parameter Diameter particle Void ratio Water head
The number of test cases 4 3 3
To measure vertical permeability, due to large grains samples, according to ASTM D-2434 standard fixed-load test method has been used. First, the porous plate is placed on the bottom of the measuring device to prevent the soil from entering and exiting the water penetration then The soil is inserted from the fixed height into the device and the porous plate is placed on the sample. Then place should be located at the top of the device and close the screws so there should be no water leak. Then the weight of the soil should be measured and connect the system to the water. Then the outlet tap should be opened and water should be passed through the soil sample until the sample would be completely saturated and no air bubbles come out of the outlet pipe and fix water level. Then the water head and weigh the empty container and the duration of the outflow of water for a given water volume should be measured. After performing the test at a specified head, the elevation of water should be changed by reservoir adjustment and the permeability coefficient would be measured in other loads.
The effect of hydraulic loads on horizontal and vertical hydraulic conductivity coefficients for uniform samples
Horizontal and vertical hydraulic conductivity tests were performed on uniform samples including coarse aggregate materials with a diameter of 0.85, 2, 6.35, and 5.9 mm. In Figures (1) to (3), the effect of hydraulic load on horizontal and vertical hydraulic conductivity for uniform samples in minimum and maximum conditions is shown.
(B( (A(
(D( (C(
Figuer1- According hydraulic conductivity to hydraulic gradient for uniform samples with A) vertical hydraulic conductivity, minimal porosity B) Horizontal hydraulic conductivity, minimal porosity C) vertical hydraulic conductivity, maximum porosity D) horizontal hydraulic, maximum porosity
Investigations showed that in all cases, with increasing hydraulic load, the horizontal and vertical hydraulic conductivity decreased and then the process of change was almost constant.
Investigation of the effect of porosity on horizontal and vertical hydraulic conductivity of uniform samples
The results showed that the horizontal hydraulic conductivity coefficient for all samples was higher than the vertical hydraulic conductivity coefficient.
Also, the results showed that the minimum hydraulic conductivity (e = 0.46) and maximum porosity (e = 0.97) were about 34.33 and 0.35 percent higher than the hydraulic hydraulic conductivity, respectively.
Investigation of the effect of porosity on the anisotropy coefficient of hydraulic conductivity of uniform samples: The results showed that with increasing porosity, the coefficient of heterogeneity of hydraulic conductivity for uniform samples was reduced and this coefficient was for uniform samples in the range of 0.89 to 1.35.
The final results can be summarized as follow:1. The permeability in the horizontal direction is often greater than the permeability in the vertical direction.
2. The anisotropy permeability for uniform sample is between 0.85-1.35.
3. The anisotropy permeability decreases with increasing porosity.
4. In the uniform samples, maximum permeability occurs at higher hydraulic conductivity.
5. With increasing the uniformity coefficient, the amount of hydraulic conductivity decreases.