Comparative test on the anti-filtration effect of geotextile envelope material around subsurface drainage pipe using two kinds of soil

Abstract: Geotextiles are widely used as drainage pipe envelope materials in the construction of modern farmland drainage systems, due to the many product series, stable performance, and convenient transportation. Furthermore, the complex and diverse soil conditions have gradually promoted the mechanized manufacturing technology of subsurface drainage pipes in China. However, the unreasonable selection of geotextiles can lead to serious blockages of subsurface drainage pipes in many irrigation areas, and even premature loss of drainage functions. Therefore, it is urgent to select the geotextiles suitable for the local soil characteristics in the construction of farmland subsurface drainage engineering. The drainage pipe blockage can be effectively prevented while maintaining relatively stable water permeability. In this study, a systematic comparison was conducted to clarify the anti-filtration effect of geotextile envelope material around subsurface drainage pipes. An indoor hydraulic permeability test was also carried out using four kinds of hot-melt spun-bonded nonwoven geotextiles. Two kinds of soil samples were collected from Bengbu City, Anhui Province, and Pingluo County, Ningxia Hui Autonomous Region, China. The drainage flow, soil, and geotextile permeability were measured on the two kinds of soil samples under different geotextile protection measures. The permeability and anti-clogging performance of different geotextiles were then evaluated to analyze the drainage flow that increased with time (referred to as "hump") from the perspective of particle migration. The test results showed that the infiltration water flow induced the soil particles to rearrange, and then fill the pores in the early stage of drainage, particularly after the subsurface drainage pipe was laid and the soil was backfilled. Subsequently, the permeability of soil and geotextiles was reduced significantly, leading to a decrease in the drainage flow, as the soil compactness and the gradual silting of geotextiles increased with time. With the advancement of the test process, the geotextile suitable for the soil was discharged with the fine particles (such as the clay and powder particles) through particle screening, whereas, some coarse particles (such as sand particles) were intercepted with the fabric pores. As such, a well-permeable soil permeable skeleton was gradually formed around the drainage pipe, which in turn triggered the rise of drainage flow. At the same time, the test found that the equivalent pore size of geotextile failed to measure the permeability performance. Specifically, the equivalent pore size of the geotextile was too large to easily make the soil skeleton with the sand, as the construction material was damaged, causing the drainage flow. Excellent adaptability between the soil and geotextiles was observed in the silty sand loam in the Pingluo area of Ningxia and geotextiles with a mass of 68 g per unit area, while the silty loam in the Bengbu area of Anhui and geotextiles with a mass of 90 g per unit area. The characteristic particle size (d90) of soil above the geotextile increased by more than 20%, compared with the original soil. In addition, the experiment also successfully verified whether the soil and geotextile fit well, according to the "hump" phenomenon. Once the "hump" phenomenon occurred, it infers that the drainage system maintained the higher permeability for a long time, indicating the better water permeability and anti-sedimentation performance of geotextile. It was different from the continuous decline of the drainage flow caused by the geotextile siltation. Consequently, the geotextiles can be expected to screen the soil characteristics in the two target areas. The findings can provide a theoretical basis and technical support to optimize the subsurface drainage pipe envelope materials in similar irrigation areas.