# Small Dam Drainage with Nonwoven Geotextile after 40 Years of Exploitation
## 1 Introduction  
Currently, synthetic materials such as nonwoven geotextiles, mats, membranes or geogrids are commonly used in many branches of construction [1]. However, initially, they were used with some fears due to limited knowledge about their changes of properties over time. After about 10 years of geotextile use, the author of Reference [2] required whether the great interest in these materials in geotechnical engineering is just a popular or a lasting trend. The author immediately responds to himself that they are and will be widely used as substitutes for traditional building materials and will spearhead the development of innovative techniques appropriate only for geotextiles. After several decades, we can say that we know the methods of testing the physical properties of different geosynthetics [3; 4], we can determine the criteria for their selection depending on their function in construction [5; 6], and failures and bad solutions provide us with additional knowledge [7; 8]. However, it is important to know how the physical properties of these materials have changed over time. Initially, many years of geosynthetics works were simulated in laboratories, repeating changes of water level with a high frequency. [PERSON] et al. [9] described the changes occurring on the contact surface of the nonwoven fabric and soil, during several hours of observation reproducing the operation of the nonwoven fabric as a separation and filtration layer. The migration of the smallest particles causes a decrease of the water permeability coefficient of the nonwoven fabric and the formation of a reverse filter at the interface between the non-woven fabric and soil. As a result of the flow of water from the protected soil, particles with a diameter smaller than the pore diameters of thenonwoven fabric are carried out. Larger particles stop at the surface of the non-woven fabric, forming a reverse filter or, as the authors call it, a \"bridge network\". Depending on the properties of the geotextile and soil, changes in the ground-geotextile contact can follow one of three characteristics: I--the reverse filter is created and the motion of particles is stopped, II--continuous loss of protected soil particles, or III--the reverse filter is created while, simultaneously, the smallest particles flow out. The proper operation of geotextile as filtering layers requires that the processes should proceed according to the characteristics of I or II. [PERSON] et al. (2019) noted that due to the interaction between soil and geotextile, the most important period is the initial period of filtration when the processes of outflowing particles and stabilization take place the most intensively (in the study conditions of about 20 hours). During the operation of the geotextile as a filtering layer, due to changes occurring, we can distinguish five characteristic periods: migration of soil particles, flow of the finest particles, formation of filter/non-woven structure (filter cake), fine particle retention in geotextile (clogging), and stabilization of the protected ground. These processes usually are not linear and the duration of each of them depends on the volume flow of water flowing through the surface of geotextile Q/A (m\({}^{3}\) s\({}^{-1}\) m\({}^{-2}\)). However, it can be assumed that in a sufficiently long period of operation, the drainage work stabilizes (2019). Recently, it has been possible to find nonwovens research after a longer period of operation of several years. [PERSON] et al. (2019, 2020) have presented research on the nonwoven geotextile drainage of the earth dam in Bialobrzegi (side dam of the Zegrze Reservoir) after 7 and 22 years of operation. In relation to the nonwoven fabric, the water permeability coefficient decreased by 1.16 and 1.6 times respectively, after 7 and 22 years. The coefficient of water permeability of nonwoven geotextile, \(K_{n}\), was still more than 10 times higher than the permeability coefficient of the protected soil, \(K_{soil}\).