What are the quality indicators and testing methods for geotextiles?
1. Physical Properties
The physical properties of geotextiles primarily include the determination of mass per unit area and thickness. For mass per unit area measurement, ten test specimens, each with an area of 10,000 square centimeters, are cut. The cut specimens are then weighed individually on a balance in numerical order, with readings taken to 0.01 g. To measure thickness, ten representative specimens are cut, each with an area not smaller than that of the reference plate. The specimens are placed naturally between the reference plate and a press block, and the press block is gently lowered to apply a pressure of 2 MPa to the specimens. The reading is taken 30 seconds after contact begins, to an accuracy of 0.01 mm.
2. Mechanical Properties
The mechanical properties of geotextiles mainly encompass wide-width tensile tests, trapezoidal tear strength tests, CBR bursting strength tests, and puncture strength tests. The wide-width tensile test is a crucial quality indicator for geotextiles. For this test, at least five specimens, each measuring at least 200 mm in length and 200 mm in width, are cut in both the longitudinal and transverse directions. After conditioning in a standard environment for 24 hours, the tests are conducted using a constant-rate-of-extension (CRE) tensile testing machine with a gauge length of 100 mm and a testing speed of 20 mm/min. In the trapezoidal tear strength test, trapezoidal specimens are selected and conditioned in a standard environment for 24 hours before being clamped between the upper and lower clamps of the tensile testing machine. The external load is gradually increased, causing the short side of the trapezoidal geotextile to tear slowly towards the long side along the notch until complete rupture occurs. An CRE tensile testing machine is also used for this test, with a stretching speed maintained at 100 mm/min.
The CBR bursting strength test involves securing a test specimen with a specialized circular fixture, conditioning it in a standard environment for 24 hours, and then using a flat-headed cylindrical punch with a diameter of 50 mm to apply pressure to the specimen at a speed of 60 mm/min until rupture. An CRE tensile testing machine is employed for this test as well.
3. Hydraulic Properties
Hydraulic properties include effective pore size tests and vertical permeability tests. Effective pore size testing methods encompass wet sieving, dry sieving, digital image processing, and fluid filtration, with dry sieving being the most commonly used. In the dry sieving method, a sample of the geotextile is used to vibrate and sieve glass beads of a specific diameter, and the weight of the beads that pass through is measured to calculate the sieving rate. The test is repeated with beads of different diameters, and a pore size distribution curve is plotted to determine the equivalent pore size. The vertical permeability test is conducted using the constant head method, primarily to prevent soil particles from being carried away by water flow while allowing water to be filtered out. These characteristics are represented by the permeability coefficient. When the hydraulic gradient is 1, the thickness of the geotextile, as well as the water level difference and permeation velocity between the upstream and downstream, are measured to calculate the permeability coefficient.
4. Durability Properties
The durability properties of geotextiles mainly include resistance to oxidative aging, acid, and alkaline solutions. In engineering applications, geotextiles, which are typically composed of high-molecular polymers, are prone to aging, posing significant risks to the project. Therefore, durability testing is crucial. During testing, samples are subjected to treatments involving ultraviolet radiation, biological agents, low temperatures, acids, and alkalis, and the durability is expressed in terms of strength retention rate.
