Geotextiles are widely used in civil engineering, particularly in hydraulic engineering where they serve as filters and protect sand and clay from erosion. Their primary function is to prevent the washout of fine materials from the subsoil. Geotextiles are permeable fabrics and are usually made from plastics like polypropylene and polyethylene. However, due to environmental concerns surrounding synthetic geotextiles, there is a growing interest in moving away from plastics towards natural fibers. To facilitate this transition, understanding the engineering properties of these materials is essential. This study focuses on sandtightness, aiming to make applications more feasible.

Geotextiles can reduce the total thickness of a granular filter and thereby the costs. Because a geotextile can replace multiple granular filter layers, less material is required, which also results in lower emissions. Filters can be distinguished into two categories: geometrically open and geometrically closed filters. Sand particles cannot pass through a geometrically closed filter. The sandtightness is independent of the magnitude of the hydraulic load. Open filters prevent the erosion of the base material up to a certain hydraulic load. Open geotextile filters are not common used in hydraulic engineering.

Previous research has primarily focused on synthetic geotextiles, developing formulas to predict critical filter velocities based on their properties ((Van Der Knaap et al., 1986) and ( (Klein Breteler, 1988)). In contrast to these synthetic-focused studies, a study by Lemmens in 1996 on natural geotextiles, such as jute cloth, revealed a critical hydraulic gradient of 0.26, suggesting jute's potential as an alternative to synthetic materials. Despite these findings, stability criteria for natural geotextiles remain undefined, and newly developed geotextiles from natural materials have not yet been tested.
To address this gap, experiments were conducted in a flume set-up at the Fluid Mechanics Laboratory at TU Delft, using both woven and non-woven geotextiles made from natural materials such as jute, hemp, and wool. In the flow flume, a steady current was applied to a one meter long stretch of rock, closed at the top, with a geotextile-covered sand bed beneath it, allowing hydraulic gradients up to i = 1 to be exerted. This set-up enabled the determination of the critical load.

A total of 19 tests were conducted with 11 different configurations, involving variations with 7 different types of geotextile (4 woven jute geotextiles and 3 non-woven geotextiles made of hemp, jute, and wool). During all test steps of the 19 tests in the flow flume, measurements of water levels, pressures, and discharge were taken. Two endoscopes were installed at different locations in the filter layer, with holes drilled into two stones to secure the endoscopes. This allowed the camera to be positioned at the top of a pore, providing a view of the geotextile and detecting passing sand grains.

To determine the critical test step where the movement of base material begins, an innovative method of analyzing the erosion state was employed using endoscope images. These endoscopes are capable of detecting sand grains, allowing for the observation of erosion dynamics. In 4 of the 19 tests, the eroded sand was also suctioned after each test step to gain insight into the amount sediment transport. This indicates that even before the critical threshold level, there are small amounts of erosion immediately after increasing the hydraulic load. If the critical threshold level has not yet been reached, this erosion will return to zero during the rest of the test step. When determining the critical test step, two values were used for the critical load: a non-erosion criterion (Ho, 2007) and a criterion of 0.2 gr/s/m² (Klein Breteler, 1988), as used in previous studies to determine the start of movement. The non-erosion criterion is qualitative, whereas the Klein Breteler criterion is quantitative. It was found that these criteria are not equivalent, with a factor of 2 to 3 difference in the critical filter velocity.

This study concluded that the critical load for sediment transport is primarily determined by the filter velocity for open natural geotextiles. This contradicts previous studies that suggest a critical hydraulic gradient for open natural geotextiles. The critical filter velocity is influenced by the geotextile's characteristics, including thickness, opening size, and water permeability. Notably, the structure of the geotextile itself significantly affects the critical filter velocity. In this study, two woven jute geotextiles (J4: 422 gr/m² and O90 = 516.1 μm, J5: 518 gr/m² and O90 = 819.0 μm) with different structures were tested. The difference in opening size between the two geotextiles is approximately a factor of 1.5. However, the geotextile with much larger openings had a 1.75 times greater critical filter velocity with a start of movement criterion of 0.2 gr/s/m². Additionally, while the grading and size of the filter layer's grains influenced the critical hydraulic gradient, they had little to no effect on the critical filter velocity, emphasizing the significance of geotextile properties in determining their performance and stability under hydraulic load.

Ultimately, based on the tests, it can be concluded that the newly developed non-wovens could be a good alternative to synthetic geotextiles when considering sandtightness under parallel flow. All three non-wovens are reasonably stable at a hydraulic gradient of i ≈ 1, which is the maximum gradient that could be applied in our test set-up.

Recommendations for future research include testing with coarser sand, longer test durations, and synthetic counterparts for comparative analysis. Suggested improvements to the test set-up involve detailed height measurements of the sand bed and filter layer, and using high-resolution cameras for better sediment tracking. This sediment tracking by an endoscope can be improved by using a fixed pore for the endoscope for all tests. This can ensure that the pore volume is consistent across all tests and the size of the geotextile section captured in the image is also consistent. There is potential to use endoscope images for quantitative analysis of sediment transport. Further, the study emphasizes examining different flow conditions (perpendicular, and non-stationary) to enhance geotextile designs, particularly in dynamic environments like coastal revetments.