The capillary flow porometry(CFP)technique, known as the bubble point method, has been applied to measure the entire pore size distribution (PSD)of nonwoven geotextiles.
The classical technique is based on the differential analysis of two test runs: a wet run, during which the specimen is saturated by a wetting fluid, and a dry run, carried out on a non-saturated specimen. In the wet run, a saturated specimen allows the passage of air through pores when the capillary attraction of the wetting fluid is exceeded by the air pressure. As the air pressure is increased, smaller and smaller pores will pass air. In the dry run, air will flow through all pores when any amount of air pressure is applied to one side of the specimen. Both runs are performed in a sealed chamber. The flow rate through the sample from zero pressure to a maximal pressure is measured during each run. Assuming a cylindrical shape of pores, the pore size distribution is then computed.
Needle-punched and heat-bonded, polypropylene continuous filament nonwoven geotextiles have been tested during the program. The geotextiles were selected based on manufacturing process and their basic physical properties, mass per unit area and thickness.
Results, in terms of indicative pore sizes obtained from the pore size distribution are compared to the selected properties of geotextile products (thickness, mass per unit area and mass per unit volume), and the filtration opening size (FOS), determined by the hydrodynamic sieving technique (CGSB 148.1, method 10–94).
Relationships between measured pore sizes and selected properties of geotextiles have been found.
Research is presently in progress to widen the application of the CFP technique to measure the pore sizes up to 300 micrometers by using a non-zero contact angle wetting fluid and to verify the CFP technique applicability for other types of geotextiles, as well.
Knowledge of the PSD will be helpful in filter design because it focuses on new structural parameters of geotextiles, not presently considered by conventional design procedures.