Geotextile Tips: Standardized Methods for Measure

Physical Properties

The physical properties of geotextiles mainly include the determination of mass per unit area and thickness.

For mass per unit area testing, ten specimens with an area of 10,000 cm² are cut. Each specimen is weighed one by one on a balance according to its serial number, with the reading accurate to 0.01 g.

For thickness measurement, ten representative specimens are selected. The specimen size shall not be smaller than the area of the reference plate. Each specimen is placed naturally and flat between the reference plate and the presser foot. The presser foot is gently lowered so that the pressure applied to the specimen is 2 MPa. Timing starts at the moment of contact, and the thickness reading is taken after 30 s, accurate to 0.01 mm.

Mechanical Properties

The mechanical properties of geotextiles mainly include wide-width tensile testing, trapezoidal tear strength testing, CBR puncture strength testing, and puncture resistance testing.

Among these, the wide-width tensile test is one of the most important quality indicators for geotextiles. For this test, at least five specimens are taken in both the machine direction and cross direction, with dimensions not less than 200 mm × 200 mm. After conditioning for 24 h under standard environmental conditions, the test is carried out using a gauge length of 100 mm. A constant-rate-of-extension tensile testing machine is used, with a loading rate of 20 mm/min.

For the trapezoidal tear strength test, trapezoidal specimens are selected and conditioned for 24 h under standard conditions. The specimens are clamped firmly by the upper and lower jaws of the tensile testing machine. The external load is gradually increased so that the short edge of the trapezoidal specimen slowly tears along the cut toward the long edge until complete rupture occurs. A constant-rate-of-extension tensile testing machine is used, with a tensile speed of 100 mm/min.

The CBR puncture strength test of geotextiles refers to fixing the specimen with a special ring clamp. After conditioning for 24 h under standard conditions, a flat-ended cylindrical plunger with a diameter of 50 mm is pushed against the specimen at a rate of 60 mm/min until the specimen fails. A constant-rate-of-extension tensile testing machine is also used for this test.

Hydraulic Properties

Hydraulic properties include the apparent opening size (AOS) test and the vertical permeability test.

Methods for the AOS test include the wet sieving method, dry sieving method, digital image processing method, and fluid filtration method, among which the dry sieving method is the most commonly used.

In the dry sieving method, the geotextile specimen is vibrated with glass beads of a specific diameter, and the mass of glass beads passing through the specimen is measured to calculate the passing percentage. The test is then repeated using glass beads of different diameters. Finally, the pore size distribution curve of the geotextile is plotted to determine the equivalent opening size.

The vertical permeability test adopts the constant head method. This test aims to prevent soil particles from being carried away by water flow while still allowing water to pass through effectively. These characteristics are expressed by the permeability coefficient. When the hydraulic gradient is 1, the thickness of the geotextile, the upstream and downstream water level difference, and the seepage velocity are measured to calculate the permeability coefficient.

Durability Properties

The durability properties of geotextiles mainly include resistance to oxidative aging, acid resistance, and alkali resistance.

In engineering applications, geotextiles generally remain in service permanently. Since they are mostly composed of polymer materials, aging is likely to occur, which can cause significant harm to engineering structures. Therefore, durability testing is of great importance.

During testing, specimens are subjected to ultraviolet radiation, biological exposure, low-temperature conditions, acidic solutions, and alkaline solutions. Durability performance is evaluated by the strength retention ratio.

Problems and Solutions in Geotextile Testing

In practical applications, geotextile testing was once governed by only nine national standards, and standardization and quality inspection often lagged behind product production and application. Later, the number of national standards increased to seventeen, covering sample selection and test preparation, mass per unit area determination, vertical permeability testing, wide-width tensile testing, trapezoidal tear strength testing, apparent opening size testing, resistance to oxidative aging, and resistance to acid and alkali solutions.

With the continuous improvement of standards, many enterprises have begun to pay close attention to testing work, actively implement new standards, and improve quality inspection systems. By using high-precision instruments and enhancing the technical and management capabilities of testing personnel, testing levels and overall product quality have been further improved. During production, several typical problems may arise and should be given due attention.

Contradictions in Quality Test Results

(1) Whether the material quality is uniform.

Geotextiles should not exhibit local uniformity while being globally non-uniform. Therefore, if different specimens taken from the same geotextile all show relatively uniform quality, the difference between their test results should not be significant. The coefficient of variation and standard deviation of test indicators can be used to evaluate material uniformity. When test results differ due to material non-uniformity, the lower result should be used as the basis for determining whether the geotextile meets qualification requirements.

(2) Consideration of test standardization.

This is related to the technical level of testing personnel and the precision of testing instruments. The deeper the understanding of the testing process, the higher the technical competence of the personnel, and the greater the precision of the instruments, the more accurate the test results will be. Geotextile testing procedures are not particularly complex. As long as the testers clearly understand the purpose, procedures, and workflow of the tests and carry them out carefully and conscientiously, reliable results can be obtained.

(3) Examination and analysis of specimens.

Even for geotextiles with uniform quality, repeated testing may not yield identical results. When material quality is non-uniform or testing errors exist, the results may fail to meet requirements. Increasing the frequency of testing and using the average value of test results can help determine whether the geotextile meets quality standards.