Performance of Granular vs. Geotextilc filters in protecting cohesionless soils

Several researchers have drawn analogies between granular filters and geotextile filters. They have shown that both granular and geotextile filters can be designed using similar principles. However, these comparisons have been made on the basis of theory only. Limited experimental data is available to draw meaningful comparisons between granular and geotextile filters. In this paper, a comparison between granular and geotextile filters has been made on the basis of long-term, steady-state and hydrodynamic filtration tests. Under steady-state conditions, both granular and geotextile filters showed similar retention behavior. This was due to the formation of similar soil bridging zones above the granular and geotextile filters. Both the granular and geotextile filters acted as catalysts in the formation of soil filtration zones above the filters and were not directly involved in the filtering behavior of the system. It is believed that under hydrodynamic conditions with bi-directional flow, the filtration zone is not established behind the filter. Therefore, the retention behavior of the system under hydrodynamic conditions is a direct function of the physical properties of the filter. This finding is significant for the selection of filters under rip-rap, where the flow direction can change bi-directionally over time.