Volume 15, Issue 3 (September 1992)
Long-Term Filtration Behaviour of Soil-Geotextile System
In addition to soil retention and permeability requirements for a geotextile filter, a geotextile should be able to function without having a significant number of its openings clogged. In order to evaluate the long-term filtration behaviour of a soil-geotextile system, a permeameter is developed in which both the long-term flow rates as well as the gradient ratio tests can be conducted simultaneously for different soils at different densities. Uniformly graded Ottawa sand mixed with various percentages of fine-grained silty soil is used to represent the unstable soil condition for internal soil migration. Two types of needle-punched nonwoven and one type of woven geotextiles are used whose opening sizes satisfy the soil retention criteria for the soil mixtures used.
An unstable flow behaviour is observed during the initial period of filtration. Fine soil particles move toward the geotextile and are either trapped within the geotextile or retained upstream, decreasing the flow rate of the soil-geotextile system. Continuous reduction in flow rate is observed which finally reaches a constant stabilized value. It is also observed that the fine soils adjacent to the geotextile pass through it and the flow rate increases. The duration of such unstable behaviour and time taken to attain a stabilized flow condition increase with fine contents.
The gradient ratio also attains a stable value in the long run after the formation of a soil filter cake in the soil-geotextile system. The initial unstable period does not indicate true clogging behaviour of the system. In view of this, it is recommended that a long-term stabilized gradient ratio be determined instead of the 24-h gradient ratio.