# Small Dam Drainage with Nonwoven Geotextile after 40 Years of Exploitation
## 2 Materials and Methods
### Method  
Our research was conducted in the field from which we took samples to properly calibrate and validate the numerical two-dimensional (2D) model using the HYDRUS (2D/3D) package. Use of software in water flow in porous media simulations is becoming a standard [19; 20; 21].  
The samples of nonwoven geotextile for testing were taken on May 17, 2018. The following procedure was applied during the sampling:  
* The sampling site was taken approximately 10 m above the well connecting the upper and lower sections of the drainage (place indicated as control cross-section in Figure 4);
* The samples were taken from a 2.0 m \(\times\) 1.5 m excavation from a depth of 1.65 m;
* A fragment of nonwoven geotextile with dimensions of 1.0 m \(\times\) 1.5 m was cut out, which was divided into six samples;
* The non-woven fabric was placed in a plastic bag, which allowed the moisture of the sample to be maintained until the water permeability test was carried out;
* The material taken was replaced by a nonwoven geotextile with similar characteristics;  
Figure 5: Drainage of the Piaski-Szczygliczka Dam: 1. Nonwoven geotextile WD-3; 2. Gravel; 3. Slotted pipe; A, B and C—nonwoven geotextile and soils sampling zones.  
* A 10 cm layer of soil directly covering the nonwoven geotextile was removed with particular care, simultaneously collecting three soil samples for grain size analysis.  
The appearance of the nonwoven geotextile surface taken from the drainage was not much different from the nonwoven fabric. However, two significant differences were observed: the nonwoven surface on the earth dam side covered a considerable amount of sand that was retained by the material fibers (the sand was not shedding), while on the nonwoven surface on the other side, there was extensive brown coloration caused by the elimination of iron trihydroxide. The side of nonwovens adjacent to the drainage was clean, without adherent grains of sand, and on the surface, permanently imprinted grains of backfill drainage were visible. From the downstream, visible spots of iron were concentrated on the curvature of the nonwoven fabric. Compared to the description of the appearance of WD-3 nonwoven fabric taken in 1987 and 1990 [13], no roots of plants or white spots of calcium carbonate were found.  
The water permeability coefficient was measured using a stand prepared in accordance with PN-EN ISO 11,058 for a method with a constant hydrostatic pressure. The laboratory set consists of a vertical Plexiglas column with diameter \(D\) = 194 mm. The tested nonwoven geotextile was placed on a round plate with holes 4 mm in diameter. The volumetric flow rate in the column was measured using the substitute vessel method. To measure hydraulic losses, open piezometers connected to the Siemens SISTRANS P DS III pressure transducer were used. The reading of the piezometric pressure difference was carried out with an accuracy of 0.1 mm.  
Nonwoven samples for water permeability tests were cut from the whole material taken in three zones: on the upper side of water A, on the drainage axis B and on the lower side of water C (Figure 5). Two samples from each zone were collected for water permeability tests (Figure 6).  
After the water permeability tests, the nonwoven geotextile samples were dried and additional parameters, i.e., surface density and the amount and size of the sand in the nonwoven pores, were determined. All samples of nonwoven geotextile were weighed, before and after shaking. During these measurements, it was noted that soil was spilled out only from the pores opened from the external surface of the nonwoven geotextile.  
Figure 6: Samples of nonwoven geotextile WD-3 from the drainage of the Piaski-Szczygliczka Dam; from A, B and C sampling zones.