Geotextile Pore Size Test and Analysis

The pore size of geotextiles is an important technical index for engineering applications. This paper introduces the basic principles of geotextile pore size testing and the test standards at home and abroad, and compares and analyzes the methods and related standards. Geotextile is a roll material for civil engineering made of synthetic fiber textile or non-woven process such as cementation and hot pressing needle punching, also known as geotextile or geomembrane. Geotextiles can be divided into woven geotextiles, knitted geotextiles, and non-woven geotextiles according to different processing methods [1]. The most commonly used is non-woven geotextile, which is a fibrous structural material formed by consolidating fiber webs by mechanical, chemical, thermal or other methods [2]. The unique fiber three-dimensional network structure of non-woven geotextile makes it have good drainage performance and sand retention performance, which can replace the traditional sand and gravel percolation layer, which can not only save investment but also shorten the construction period. The important basis for the design and selection of geotextile percolation layer is its water permeability and soil preservation performance, and the important characteristic index of these two properties is its pore size. Accurate determination of the pore size of geotextiles is conducive to more reasonable selection of geotextiles in engineering. Combined with practical work experience, this paper summarizes the testing methods of geotextile pore size as follows. 1. Pore size parameters Pore size parameters mainly include effective pore size, characteristic pore size, average pore size, maximum pore size, minimum pore size, bubble point pore size, pore size distribution, porosity, etc. [3]. 1.1 Effective aperture (Oe) JTG E50—The definition in 2006 'Highway Engineering Geosynthetics Test Regulations' is as follows: the approximate maximum particle diameter that can effectively pass through the geotextile, for example, O90 means that 90% of the pore sizes in the geotextile are lower than this value [4]. GB/T 14799—The definition in 2005 'Determination of Effective Pore Size of Geotextiles and Related Products' is as follows: The effective pore size is the approximate maximum particle diameter that can effectively pass through the geotextiles. For example, O90 means that 90% of the pore sizes in the geotextiles are lower than this value [5]. 1.2 Equivalent aperture EOS (or apparent aperture AOS) SL/T 235—The 1999 'Geosynthetics Test Procedures' is defined as follows: the sieve analysis of granular materials with geotextiles as sieve cloth, when the sieving rate of a granular material (the ratio of the weight of the granular material passing through the fabric to the total weight of the granular material) is 5%, the particle size is determined as the equivalent pore size of the geotextile [6]. GB 50290—1998 'Technical Specification for Application of Geosynthetics' and SL/T 225—The 1998 'Technical Specifications for the Application of Geosynthetics in Water Conservancy and Hydropower Engineering' is defined as follows: the maximum apparent pore size of the geotextile [7-8]. JTJ/T 019—The 1998 'Technical Specifications for Application of Highway Geosynthetics' is defined as follows: an index used to represent the pore size of fabric-type geosynthetics. Different equivalent aperture values ​​can be obtained by using different sieve ratio standards [9]. 1.3 Characteristic pore size The pore size of the geotextile is equivalent to the maximum particle size when 90% of the soil particles pass through the geotextile [10]. This definition is suitable for the wet sieve method in the determination of the effective pore size of geotextiles and related products. 1.4 Bubble point pore size The gas on one side of the filter cloth passes through the filter cloth and reaches the water on the other side to generate air bubbles. This method is used to calculate the pore size of the filter cloth [11]. 1.5 Maximum bubble point pore size The bubble point pore size when the gas passes through the filter cloth and reaches the water to generate the first series of bubbles [11]. 1.6 Pore size distribution For a given sample, according to the pore diameter distribution, the percentage of pores corresponding to a certain pore size can be calculated [12], which can be used to characterize the proportion of different pore sizes in the entire pore size distribution. 1.7 Porosity The ratio of the pore volume of the material to the total volume is an index reflecting the degree of voids in the geotextile, and it is an important factor affecting the hydraulic properties such as the permeability of the geotextile [13]. At present, in various standards, the definitions of equivalent pore size and characteristic pore size are basically the same, and the size of the particle is used to represent the size of the pore size. The bubble point pore size needs to be calculated according to the pressure difference when the bubble appears. 2. Pore size testing method Geotextile pore size testing methods are divided into direct method and indirect method. Direct methods include microscope method, image analysis method, etc.; indirect methods mainly include dry sieving method, wet sieving method, bubble point method, hydrodynamic method and mercury Press-in method, etc. [14]. The principle and evaluation of each method are shown in Table 1. Direct methods such as microscopy. This method is direct, intuitive and reliable. It can directly obtain the number and size of the pore size, without changing the original state of the sample, without contaminating and damaging the sample. It is especially suitable for thin fabrics, but the pore distribution on the projection surface cannot reflect the internal pores of the fabric. Therefore, this method is only suitable for regular fabrics, and the test results have certain randomness and are not representative enough. For geotextiles with irregular pores, the indirect method is generally used. Although the calculation method has certain rationality through the establishment and reasoning of mathematical models, the measurement of parameters cannot be separated from the experiment. The mercury intrusion method is toxic and harmful to the environment. The negative pressure drainage method is convenient and pollution-free, but the problem of hydrophilicity of the fabric has always been difficult to solve; the bubble point method can obtain a better pore size distribution curve, but it is not very It is a good simulation of actual use; the infiltration method is time-saving and reliable, but it cannot obtain a pore distribution curve.
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